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The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California


Abstract and Figures

Methylmercury (MeHg) is a bioaccumulative pollutant produced in and exported from flooded soils, including those used for rice (Oriza sativa L.) production. Using unfiltered aqueous MeHg data from MeHg monitoring programs in the Sacramento River watershed from 1996 to 2007, we assessed the MeHg contribution from rice systems to the Sacramento River. Using a mixed-effects regression analysis, we compared MeHg concentrations in agricultural drainage water from rice-dominated regions (AgDrain) to MeHg concentrations in the Sacramento and Feather Rivers, both upstream and downstream of AgDrain inputs. We also calculated MeHg loads from AgDrains and the Sacramento and Feather Rivers. Seasonally, MeHg concentrations were higher during November through May than during June through October, but the differences varied by location. Relative to upstream, November through May AgDrain least-squares mean MeHg concentration (0.18 ng L−1, range 0.15–0.23 ng L−1) was 2.3-fold higher, while June through October AgDrain mean concentration (0.097 ng L−1, range 0.6–1.6 ng L−1) was not significantly different from upstream. June through October AgDrain MeHg loads contributed 10.7 to 14.8% of the total Sacramento River MeHg load. Missing flow data prevented calculation of the percent contribution of AgDrains in November through May. At sites where calculation was possible, November through May loads made up 70 to 90% of the total annual load. Elevated flow and MeHg concentration in November through May both contribute to the majority of the AgDrain MeHg load occurring during this period. Methylmercury reduction efforts should target elevated November through May MeHg concentrations in AgDrains. However, our findings suggest that the contribution and environmental impact of rice is an order of magnitude lower than previous studies in the California Yolo Bypass.
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Methylmercury (MeHg) is a bioaccumulative pollutant produced
in and exported from ooded soils, including those used for
rice (Oriza sativa L.) production. Using unltered aqueous MeHg
data from MeHg monitoring programs in the Sacramento River
watershed from 1996 to 2007, we assessed the MeHg contribution
from rice systems to the Sacramento River. Using a mixed-eects
regression analysis, we compared MeHg concentrations in
agricultural drainage water from rice-dominated regions (AgDrain)
to MeHg concentrations in the Sacramento and Feather Rivers, both
upstream and downstream of AgDrain inputs. We also calculated
MeHg loads from AgDrains and the Sacramento and Feather Rivers.
Seasonally, MeHg concentrations were higher during November
through May than during June through October, but the dierences
varied by location. Relative to upstream, November through May
AgDrain least-squares mean MeHg concentration (0.18 ng L−1,
range 0.15–0.23 ng L−1) was 2.3-fold higher, while June through
October AgDrain mean concentration (0.097 ng L−1, range 0.6–1.6
ng L−1) was not signicantly dierent from upstream. June through
October AgDrain MeHg loads contributed 10.7 to 14.8% of the
total Sacramento River MeHg load. Missing ow data prevented
calculation of the percent contribution of AgDrains in November
through May. At sites where calculation was possible, November
through May loads made up 70 to 90% of the total annual load.
Elevated ow and MeHg concentration in November through May
both contribute to the majority of the AgDrain MeHg load occurring
during this period. Methylmercury reduction eorts should
target elevated November through May MeHg concentrations in
AgDrains. However, our ndings suggest that the contribution and
environmental impact of rice is an order of magnitude lower than
previous studies in the California Yolo Bypass.
The Contribution of Rice Agriculture to Methylmercury in Surface
Waters: A Review of Data from the Sacramento Valley, California
K. Christy Tanner,* Lisamarie Windham-Myers, Jacob A. Fleck, Kenneth W. Tate, Stephen A. McCord,
and Bruce A. Linquist
I anoxic soils, a portion of the inorganic mercury (Hg)
pool can be methylated, predominantly by sulfate- and iron-
reducing bacteria (Compeau and Bartha, 1985; Gilmour et
al., 1992; Fleming et al., 2006; Kerin et al., 2006), forming meth-
ylmercury (MeHg). Methylmercury binds strongly to thiols in
proteins (Ballatori, 2002) and is extremely toxic to organisms,
causing neurological problems and decreased reproductive suc-
cess (Crump and Trudeau, 2009). Environmental levels of MeHg
as low as 0.1 ng L−1 in freshwater ecosystems can have negative
eects on high trophic-level organisms via bioaccumulation and
biomagnication (Rudd, 1995; Watras et al., 1998; Chan et al.,
2003). Humans are exposed to MeHg primarily through the con-
sumption of sh and other wildlife from Hg-contaminated envi-
ronments, resulting in negative health eects (Chan et al., 2003).
Mercury and MeHg contamination of surface water is wide-
spread. In the United States, sh consumption advisories have
been issued in all 50 states and one US territory, including 1.8
million km of river and 6.6 million ha of lake (USEPA, 2011).
Wetlands provide anoxic soil conditions in which
Hg-methylating microbes thrive, resulting in elevated MeHg
production and bioaccumulation in a variety of wetland types
(Marvin-DiPasquale et al., 2003; Hall et al., 2008). Unlike many
crops, rice (Oriza sativa L.) is grown in ooded elds that are
eectively agricultural wetlands. Rice is grown on approximately
150 million ha globally (Czech and Parsons, 2002), comprising a
substantial portion of the worlds estimated 1.2 billion ha of wet-
lands (Finlayson et al., 1999) and serving as important wildlife
habitat (Czech and Parsons, 2002). Seasonal wet–dry cycles and
inputs of labile organic carbon from root exudates and rice straw
promote MeHg production and bioaccumulation in rice elds
(Windham-Myers et al., 2009, 2014a; Ackerman and Eagles-
Smith, 2010; Ackerman et al., 2010; Rothenberg and Feng,
2012). Many studies have found that MeHg can accumulate in
rice grain and that rice can be the primary route of MeHg expo-
sure for people living in Hg-contaminated inland rice-growing
Abbreviations: AgDrain, agricultural drainage; AgDrain-E, drainage from
agricultural areas east of the Sacramento River, also known as Sacramento Slough;
AgDrain-W, drainage from agricultural areas west of the Sacramento River, also
known as the Colusa Basin Drain; AgDrain-Woutfall, ows from AgDrain-W that enter
the Sacramento River via the Knight’s Landing Outfall Gates; AgDrain-Wdiversion,
ows diverted from AgDrain-W to the Yolo Bypass via Ridge Cut Slough; LSM,
least-squares mean; MeHg, methylmercury.
K.C. Tanner, K.W. Tate, and B.A. Linquist, Dep. of Plant Sciences, Univ. of California,
One Shields Ave., Davis, CA 95616; L. Windham-Myers, USGS, Western Region
Bureau of Regional Research, 345 Middleeld Rd., MS 480, Menlo Park, CA
94025; J.A. Fleck, USGS, California Water Science Center, 6000 J St., Placer Hall,
Sacramento, CA 95819; S.A. McCord, McCord Environmental, Inc., 759 Bianco Ct.,
Davis, CA 95616. Assigned to Associate Editor Karen Bradham.
Copyright © American Society of Agronomy, Crop Science Society of America, and
Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.
All rights reserved.
J. Environ. Qual.
This is an open access article distributed under the terms of the CC BY-NC-ND
license (
Supplemental material is available online for this article.
Received 11 July 2016.
Accepted 16 Oct. 2016.
*Corresponding Author (
Journal of Environmental Quality
Core Ideas
• We studied the impact of rice production on MeHg at the wa-
tershed scale.
• MeHg concentration was elevated in agricultural drainage wa-
ter during November through May.
• Watershed-scale MeHg loads were lower than expected based
on eld studies.
Published November 23, 2016
Journal of Environmental Quality
areas (e.g., Feng et al., 2008; Meng et al., 2010, 2011; Zhu et
al., 2015). In contrast, few studies have investigated export of
MeHg in rice-eld drainage water and its impact on downstream
ecosystems, with the exception of studies in the California Yolo
Bypass (Windham-Myers et al., 2014a). While it is clear that rice
elds produce and export MeHg (Alpers et al., 2014), their eect
has not been evaluated at a watershed scale.
MeHg concentration in stream channels and exposure to biota
in those channels result from the interaction of sources, sinks, and
transport processes in the watershed (Bradley et al., 2011). ough
oodplains may also produce MeHg when temporarily ooded
(Singer et al., 2016), at a watershed scale, the best predictor of MeHg
concentration in a water body is the percent wetland cover upstream
(Krabbenho et al., 1999). Methylmercury production may be
decoupled from export (Bachand et al., 2014), and processes such
as photodemethylation in open water (Seller et al., 1996) and par-
ticle settling can remove MeHg from the water column. As a result,
MeHg concentrations can be lower than would be predicted from
upstream MeHg inputs (Bradley et al., 2011).
e Sacramento River watershed (1.7 million ha) in
California provides a good opportunity to study how rice con-
tributes to watershed-scale MeHg concentrations and loads.
Rice is grown on more than 240,000 ha and is the main crop in
the low-lying central drainage area of the valley (USDA–NASS,
2016). Naturally enriched Hg conditions, plus a legacy of Hg
and gold mining (which used elemental mercury to amalgamate
the gold) in the mountains surrounding the valley, have resulted
in elevated Hg concentrations in river sediments, water, and sh
(Domagalski, 1998, 2001; Davis et al., 2008; Springborn et al.,
2011; Singer et al., 2013; Domagalski et al., 2016; Donovan et al.,
2016a, 2016b). Elevated MeHg concentrations in sh have led to
sh consumption advisories and enactment of a Total Maximum
Daily Load to reduce MeHg loads into the Sacramento–San
Joaquin Delta (Delta Mercury Control Program, 2010).
e overall objective of this study was to evaluate the con-
tribution from rice-producing areas to MeHg loads in the lower
Sacramento River by compiling and analyzing historic surface
water MeHg concentration and ow data. We evaluated MeHg
concentrations and loads in major agricultural drains from
rice-dominated areas and at mainstem sampling points in the
Sacramento and Feather Rivers, both upstream and downstream
of agricultural drain inputs. Furthermore, we evaluated seasonal
trends to identify the time of year in which MeHg loads exported
from rice elds may be of greatest concern, and long-term trends
to see if changes in postharvest rice straw management practices
have inuenced MeHg concentrations.
Materials and Methods
e Sacramento Valley covers 1.7 million ha and is situated in
the low-lying area between the Coast Ranges and Sierra Nevada
of California. It is 240 km long from north to south and ranges
from 32 km wide in the north to 72 km wide in the south (Bennett
et al., 2011). e Sacramento Valley has a Mediterranean climate,
with typically hot, dry conditions during April to September and
a cool, rainy season from October through March
e Sacramento River watershed has both natural and anthro-
pogenic Hg sources. Mineral springs (Youngs, 1994) and erodible
surcial soils (US Bureau of Mines, 1965) in the Coast Range to
the west and Cascade Mountains to the northeast are naturally
Hg enriched. Mercury mining occurred in the Coast Range to the
west (Jasinksi, 1995). Between 1846 and 1981, an estimated 34.5
million kg of Hg was released into the environment, partially as
Hg vapor during ore processing (Churchill, 2000). Most of the
Hg mining occurred in the Cache Creek watershed and farther
south; thus, runo from mine wastes in these areas enters the Yolo
Bypass downstream of the current study area (Domagalski, 1998).
Beginning in 1848, elemental Hg was used by the gold mining
industry in the Klamath and Sierra Nevada mountain ranges,
drained by the upper Sacramento River and Feather River, respec-
tively (Domagalski, 1998; Churchill, 2000). Combined Hg losses
from gold mining totaled 5.8 million kg , with 97% of losses occur-
ring before 1935 and 80 to 90% of losses occurring in the Sierra
Nevada (Churchill, 2000). While Hg losses were substantial, it is
unclear to what degree rice-growing areas in the central Sacramento
Valley have been contaminated with Hg. Reservoirs limit down-
stream transport of MeHg and total Hg (Slotton et al., 1995).
Domagalski (1998) measured total Hg in riverbed sediments: sites
in the central Sacramento River had 40 to 70 ng Hg g−1, while sites
within and downstream of the Feather River had 140 to 370 ng Hg
g−1. To our knowledge, there are no published Hg concentration
data for Sacramento Valley rice soils. Atmospheric deposition has
not been measured directly for the Sacramento Valley. However,
the San Francisco Estuary to the south received 19 mg Hg m−2 yr−1
as dry deposition and 4.2 mg Hg m−2 yr−1 as wet deposition in 1999
to 2000 (Tsai and Hoenicke, 2001).
Within the Sacramento Valley, there are two main waterways:
e Sacramento River on the western side of the valley and the
Feather River on the east. Additional watershed inows occur
along the eastern and western edge of the valley, including the
Yuba and Bear rivers. Both of the main rivers ow from north to
south and converge near the southern end of the Valleys primary
rice-growing region, north of the city of Sacramento (Fig. 1).
Aer conuence with the Feather River, the Sacramento is joined
by the American River and continues south to the Sacramento–
San Joaquin Delta. Surface water hydrology is highly manipu-
lated, with a number of constructed waterways used to direct
irrigation water (supply and drainage) and to mitigate ooding.
Drainage from rice agriculture (hereaer, “rice”), and to a lesser
extent nonagricultural wetlands, makes up the majority of water
in agricultural drainage canals. Rice is planted during late April
through May. Most elds are water-seeded, whereby pregermi-
nated rice seed is dropped from an airplane onto ooded elds
(Linquist et al., 2015). Water management during the rst month
of the growing season is highly variable, with the water level and
outow adjusted for seedling establishment and herbicide applica-
tion. Later in the growing season, maintenance ow is established
where irrigation water is applied to rice in excess of evapotrans-
piration demand in part to limit salinity build up (Grattan et al.,
2002; Scardaci et al., 2002). Excess water is exported from rice
elds as drainage water. Fields are drained in August or September,
3 to 4 wk before harvest. Irrigation of other crops in the water-
shed is managed to match evapotranspiration demand closely so
that little or no drainage water is produced. Drainage water from
managed wetlands is also present; however, wetland cover within
the drainage area is approximately one fourth that of rice (Fig. 1)
(US Fish and Wildlife Service, 2016). Water-export data from
wetlands managed for wildlife is limited, but rates of outow
Journal of Environmental Quality
are also managed to prevent salinity build up (this management
objective is similar to those of rice growers). Periods of high water
export from managed wetlands are the drawdown of seasonal wet-
lands (March–May) and areas ooded during the summer (July–
August) (S. Emmons, personal communication, 2016).
Drainage water from agricultural elds and other wetlands is
collected by two major agricultural drains, hereaer referred to
as “AgDrain-E” and “AgDrain-W.” AgDrain-E collects drainage
water from the eastern valley area (e.g., Sutter Bypass) between
the Sacramento and Feather Rivers and discharges into the
Sacramento River immediately upstream of the conuence of
the Sacramento and Feather Rivers (Fig. 1). Low ows during
summer in AgDrain-E are made up primarily of rice drainage
water; however, during high winter ows, large volumes of ood-
water from the Sutter Bypass oodplain are mixed with rice drain-
age water. AgDrain-W collects drainage water from the area west
of the Sacramento River. Flows from AgDrain-W are divided
immediately downstream of the sampling location, with some
water directed into the Sacramento River (AgDrain-Woutfall) and
the remainder diverted to the Yolo Bypass oodplain (AgDrain-
Wdiversion) (Fig. 1). Drainage from AgDrain-E and the Feather River
enter the Sacramento River 25 km downstream of AgDrain-Woutfall.
We categorized sampling sites into “site groups” based on
their location with respect to the two main AgDrains (Fig.
1): Sampling sites located at the mouths of AgDrain-E and
AgDrain-W are referred to as “AgDrain” sites. “Upstream-
Sacramento” and “Upstream-Feather” sites were located on the
Sacramento and Feather Rivers, respectively, upstream of where
AgDrains empty into the Sacramento River. Upstream sites serve
as control sites because they represent river water that has been
minimally inuenced by rice. All Upstream-Sacramento sites
were upstream of rice drainage water inputs, while Upstream-
Feather site 5 may have been inuenced by rice drainage water
from rice grown east of the Feather River and north of the Bear
River. Upstream sites also represent irrigation source water: sites 1
and 3 on the Sacramento River were located close to the Tehama–
Colusa and the Glenn–Colusa Canals, respectively, which are
major agricultural diversions that provide irrigation water to the
western side of the Sacramento Valley. “Downstream” sites were
located downstream of the conuence of the Sacramento River,
Feather Riverm and AgDrains, but upstream of the Sacramento
urban area and the conuence with the American River (Fig. 1).
Downstream sites represent Sacramento River water that has
been inuenced by Sacramento Valley rice drainage.
Data Sources
We used data from programs that monitored aqueous, unl-
tered MeHg concentrations in the Sacramento River water-
shed from 1996 to 2007. Programs include the USGS National
Water uality Assessment Program (Domagalski et al., 2000)
dataset from 1996 to 1998 (I); three programs conducted by the
Sacramento River Watershed Program (2005, 2008), including
data from 2000 to 2003 (II), 2004 (IV), and 2006 to 2007 (V);
and one dataset from the CALFED Bay-Delta Program (Foe et
al., 2008), including data from 2003 to 2006 (III) (Supplemental
Table S1). Samples were collected either as grab samples or as
depth-integrated samples at the midpoint of the channel cross-
section. Detection limits ranged from 0.0114 to 0.0234 ng L−1
(Supplemental Table S1). Methylmercury concentrations were
below the detection limit in 1.6% of samples. Nondetects were
equally distributed among Upstream-Sacramento, Upstream-
Feather, AgDrain, and downstream samples (n = 4, 2, 3, and
2, respectively). If the lab value was reported, it was used in the
analysis; otherwise undetected MeHg concentrations were treated
as 0.5 ´ detection limit in data analysis. Sampling sites varied
among programs, and sampling frequency ranged from monthly
to quarterly. A number of sites were omitted from the analysis due
to lack of relevance to the study questions or lack of data. ese
sites included small, seasonal creeks, sites on the Sacramento River
distantly upstream or downstream of rice, and sites with fewer
than 10 samples. For sites included in this analysis, the number of
samples available by site and program is shown in Table 1.
Both precipitation and burned rice area were tested as possible
explanatory variables. Precipitation from storm events may inu-
ence MeHg concentration by dilution or ushing MeHg from
areas where it is produced (Balogh et al., 2006). Precipitation
data were obtained from the California Irrigation Management
Information System (California Department of Water Resources,
2014b). Data from two weather stations that represented the
study area within the watershed were averaged for analysis (sta-
tion identications 12 and 3). Precipitation totals from 3, 5, and
Fig. 1. Map of the study area. Site labels correspond to site identi-
cation eld in Table 1. Rice area was obtained from the California
Department of Water Resources (2013). Wetland area is from the
National Wetland Inventory (US Fish and Wildlife Service, 2016). Water
ways are depicted according to the National Hydrography Dataset
(USDA–NRCS et al., 2013). AgDrain, agricultural drainage.
Journal of Environmental Quality
7 d before MeHg sample collection were tested as predictors
of MeHg concentration. Burning of rice straw removes carbon
from elds, possibly inuencing microbial activity (Windham-
Myers et al., 2014b; Zhu et al., 2015). Burned rice area data were
obtained from the California Rice Commission (2016).
Flow data for load estimations were obtained from the
California Data Exchange Center, the California Water Data
Library (California Department of Water Resources, 2014a,
2016), and the National Water Information System (USGS, 2016)
databases (Supplemental Table S2). Mean daily ow was used
when available; otherwise, hourly or quarter-hourly ow mea-
surements were averaged to obtain mean daily ow. Missing ow
data prevented load calculation at key sites, including AgDrain-
Wdiversion, AgDrain-Woutfall during the 2005 water year (October
2004–September 2005), and AgDrain-E during winter.
Data Analysis
Season was included in the analysis as a categorical variable
because seasonal variation occurs in rice management as well as
climate. We wanted to identify times of the year when concentra-
tions were elevated to inform future studies. We plotted MeHg
concentration data from all years by day and month and dened
two seasons based on periods with relatively higher or lower
MeHg concentrations.
Linear mixed-eects regression analysis was used to assess
dierences in MeHg concentration, allowing us to account
for the fact that sites were measured repeatedly but not always
concurrently. Site and year were used as random eects in the
model. Fixed eects tested included season (June–October or
November–May), site group (Upstream-Sacramento, Upstream-
Feather, AgDrain, or downstream), time (as a continuous vari-
able), fraction of rice area burned, and precipitation, as well as
site group ´ season, site group ´ time, and site group ´ fraction
of rice area burned interaction terms. Models were tted using
the lme4 package (Bates et al., 2015) in R (R Core Team, 2014).
We selected the nal model using backward stepwise regression.
Beginning with the fullest model, rst random eects, then xed
eects, were dropped stepwise if nonsignicant (p > 0.05). e
p-values for random eects were calculated using likelihood
ratio tests, whereas signicance of xed eects was determined
using F tests with denominator degrees of freedom, calculated
by Satterthwaite’s approximation. Post hoc Tukey-corrected
dierences of least-squares means (LSM) were used to identify
signicant dierences among categorical xed eects; dier-
ences signicantly dierent from zero (p < 0.05) were considered
signicant. Model selection and tests for dierences between
xed eects were done using the lmerTest package (Kuznetsova
et al., 2016). Assumptions of normalcy and homogeneity of vari-
ance were assessed using standard diagnostic plots. A natural log
transformation was used on MeHg, but not on other variables.
is transformation successfully corrected for heteroscedasticity
and normalized the residuals of the model.
Beale’s ratio estimator was used to calculate MeHg loads
(Beale, 1962; Richards, 1998) because it was found to be unbi-
ased and accurate in a number of studies comparing load calcu-
lation methods (Dolan et al., 1981; Beirman et al., 1988). e
mean daily load (concentration ´ ow) for days on which con-
centration was measured was multiplied by the ratio of average
ow during the period of interest to the average ow on days
when concentration was measured, then multiplied by a bias cor-
rection factor (Supplemental Eq. S1 and S2).
At each site, loads were calculated separately for monitor-
ing programs I, II, and V. Programs III and IV were combined
because samples were collected concurrently and program
IV did not have enough data for a separate load calculation.
Additionally, loads were calculated using data from all programs
combined. Separate calculations were done for the November
through May and June through October seasons.
Table 1. The number of methylmercury (MeHg) samples collected by each sampling program at sites used in this analysis. AgDrain, agricultural
drainage water from rice-dominated regions.
Site Site ID† Samples in each program‡ Total number
of samples
Sacramento River above Bend Bridge 1 17 30 4 18 69
Sacramento River at Woodson Bridge 2 31 31
Sacramento River at Hamilton City 3 17 31 4 18 70
Sacramento River at Ord Ferry Bridge 4 22 22
Sacramento River at Butte City 5 30 30
Sacramento River at Colusa 6 29 17 30 4 18 98
Feather River at Gridley 7 31 31
Yuba River at Marysville 8 18 31 4 18 71
Bear River below Wheatland 9 30 30
Feather River at Nicolaus 10 18 31 4 18 71
Colusa Basin Drain at Knights Landing W 25 16 31 4 18 94
Sacramento Slough at Karnack E 23 16 28 4 18 89
Sacramento River at Verona 11 27 – 27
Sacramento River at Veteran’s Bridge 12 17 17
† Site locations are denoted in Fig. 1 using site identication (Site ID).
‡ See Supplemental Table S1 for more information about the sampling programs.
Journal of Environmental Quality
Upstream-Sacramento and Upstream-Feather loads were calcu-
lated at sites 6 and 10, respectively. ese sites were located imme-
diately upstream of AgDrain inputs but downstream of other
known MeHg inputs, including the Bear and Yuba Rivers (Fig. 1).
Additionally, these sites had the most complete datasets (Table 1).
Results and Discussion
A total of 681 unltered water MeHg concentration mea-
surements were compiled from the various programs and used
in this study. Only three sites were sampled by all ve programs,
two of which were the main agricultural drainage canals that
discharge directly into the Sacramento River (AgDrain-E and
AgDrain-W), while the third was Upstream-Sacramento site 6.
Methylmercury concentrations ranged from below the detec-
tion limits (0.02–0.0114) to 1.97 ng L−1, and only three samples
in the dataset were above 1 ng L−1 (Fig. 2). Waterways with MeHg
concentrations above 0.1 ng L−1 are potentially negatively impacted
(Rudd, 1995), and 57% of samples in this study had concentrations
below this level. Twenty percent of samples were below 0.06 ng L−1,
the regulatory standard for the Sacramento–San Joaquin Delta.
Methylmercury Concentrations
Despite changes in rice straw management during the study
period, mixed-eects regression analysis did not detect a signicant
change in MeHg concentration over time (Fig. 2A), and there was
no interaction between time and location (e results of variable
selection for the mixed-eects model are shown in Supplemental
Table S3). During the study period, straw removal practices
changed as a result of the Connelly–Areias–Chandler Rice Straw
Burning Reduction Act of 1991 (California Environmental
Protection Agency, 1991), which mandated that rice straw burn-
ing in the Sacramento Valley be phased down starting in 1992 and
be allowed only under specied conditions for disease control by
2001. e practice of burning rice straw was replaced by incorpo-
rating rice straw into soil during winter, followed by ooding to
facilitate its decomposition (Linquist et al., 2006). e percentage
of area where rice straw was incorporated during winter increased
from <15% in 1992 to >80% by 2001 (California Air Resources
Board, 2003). Rice straw is a source of labile organic carbon that
may increase Hg methylation in rice elds (Windham-Myers et al.,
2014a; Zhu et al., 2015); thus, an increase in AgDrain MeHg con-
centration over time was expected due to this shi from burning
to incorporation and ooding during the study period. However,
the fraction of rice area burned did not signicantly aect MeHg
concentration and did not interact with site group. is result is
consistent with a recent controlled, replicated experiment test-
ing the eect of straw removal from rice elds (Eagles-Smith et
al., 2014). Other sources of variation in MeHg concentration may
obscure any eect of rice straw management on MeHg concentra-
tion at the valley scale.
We did not detect a signicant eect of precipitation on
MeHg concentrations. Elevated MeHg concentrations were
found in early 1997, 1 mo aer a rain on snow event that caused
major ooding throughout the region (samples were not col-
lected during the event) (Fig. 2A). Although Balogh et al. (2006)
reported elevated MeHg concentrations during high ows, con-
centration changes in response to storm events can be complex,
depending on the transported material, as well as watershed
characteristics (Richards and Holloway, 1987). e sampling
frequency in this dataset may be too low to detect any MeHg
concentration changes in response to precipitation.
Plotting all data by month and day revealed a seasonal pattern in
which MeHg concentrations were consistently lower from June to
October (range: less than detection limit to 0.3 ng L−1), while con-
centrations from November through May were higher and more vari-
able (less than detection limit to 1.98 ng L−1) (Fig. 2B). We hereaer
refer to these seasons as June to October (153 d) and November to
May (212 d), respectively. Mixed eects regression revealed a strong
seasonal eect, with November to May concentrations 65% higher
than June to October concentrations (F1, 644 = 62.3, p < 0.001); how-
ever, there was an interaction between site group and season (F3, 634
= 11.8, p < 0.001; Fig. 3). e nal model includes these signicant
xed eects, as well as site group because it was part of a signicant
interaction (Supplemental Table S3). Signicant random eects for
both site and year were also included.
Fig. 2. (A) Time series of methylmercury (MeHg) concentrations through-
out the study period. Bars at the top indicate when each sampling
program occurred (Supplemental Table S1). (B) MeHg concentrations
plotted by day of year. The gure shows data from the whole study
period. Bars at top show the rice-growing season and winter fallow, and
June to October and November to May seasons as dened in this study.
Journal of Environmental Quality
With respect to water management and inputs into these sys-
tems, June to October includes runo from rice elds in the form
of maintenance ow and the nal drain in preparation for har-
vest (August). November to May corresponds to the ooding of
rice and managed wetlands (October and November) and sub-
sequent runo and nal drains from rice (February) and natural
wetlands (April and May). For the years studied, 92 ± 6% of the
annual rainfall occurred in November to May. During high rain-
fall years or large storm events, regional ooding occurs, result-
ing in runo from other agricultural elds (and urban areas), and
river water may be diverted into bypasses, both resulting in com-
ingling of water sources, particularly for AgDrain-E.
Methylmercury concentrations did not dier between
Upstream-Sacramento and Upstream-Feather in November to
May (LSM = 0.079 and 0.077 ng L−1, respectively; p = 0.3) or June
to October (0.060 and 0.075 ng L−1, respectively; p = 0.9) (Fig. 3),
indicating that rice on the east and west sides of the Valley receive
similar MeHg inputs in irrigation water. Upstream-Feather MeHg
concentrations did not dier between seasons (p = 0.8) (Fig. 3),
but Upstream-Sacramento had slightly but signicantly higher
MeHg concentrations during November to May (p = 0.002).
During June to October, AgDrain MeHg concentrations
(LSM = 0.097 ng L−1) appeared elevated compared with other site
groups; however, there were no signicant dierences (p > 0.05) in
MeHg concentrations among site groups. In contrast, November
to May AgDrain MeHg concentrations (LSM = 0.18 ng L−1; p <
0.05) were signicantly higher than November to May Upstream-
Sacramento and Upstream-Feather concentrations. Previous studies
also reported elevated MeHg concentrations in rice drainage water
relative to irrigation water (Alpers et al., 2014; Zhao et al., 2016).
Similar to AgDrains, downstream MeHg concentrations
were signicantly higher than upstream during November to
May, and there was no signicant dierence between AgDrain
and downstream MeHg concentrations during either June to
October or November to May. However, the ows and loads of
AgDrain, upstream, and downstream sites must be considered
when determining the degree to which AgDrains were inuenc-
ing downstream MeHg concentrations.
In this study, Ag Drains exhibited a much stronger seasonal pat-
tern than upstream sites, suggesting that rice and wetlands inuence
concentrations. Studies in the Yolo Bypass reported a similar sea-
sonal pattern (Bachand et al., 2014; Marvin-DiPasquale et al., 2014;
Windham-Myers et al., 2014b). While MeHg is produced in rice
elds throughout the year (Marvin-DiPasquale et al., 2014), transpi-
ration during the growing season results in the downward movement
of surface water, causing MeHg to be transported into and stored in
the rootzone (Bachand et al., 2014). e absence of transpiration
during the fallow season allows MeHg to be released into surface
water via diusion (Bachand et al., 2014). Additionally, rice plants
can store a signicant amount of MeHg during the growing season
(Windham-Myers et al., 2014b). Other studies have reported that
MeHg stored in dry sediment may be quickly mobilized into the
water column on ooding (Kelly et al., 1997; Rumbold and Fink,
2006; Alpers et al., 2014) or proposed that methylation may occur
shortly aer inundation (Singer et al., 2016). us, the early grow-
ing season drainage events, in addition to the fallow season, may be
periods of MeHg export from rice. (e.g., Fig. 2B).
Methylmercury Loads
Using all available data, June to October MeHg loads for
AgDrain-Woutfall, AgDrain-E, Upstream-Sacramento, and
Upstream-Feather were (mean ± SD) 0.14 ± 0.03, 0.23 ± 0.03,
2.2 ± 0.3, and 0.88 ± 0.3 g d−1, respectively (Fig. 4). e down-
stream MeHg load is expected to be the sum of all upstream
Fig. 3. Least-squares mean ± SE of methylmercury (MeHg) concentra-
tions based on mixed-eects modeling for site groups and seasons.
There is a signicant interaction between site group and season.
Site groups that were not signicantly dierent during November
to May have the same letter. Site groups did not dier signicantly
during June to October. Asterisks (*) indicate a signicant dierence
between seasons at each site group. See Supplemental Table S4 for
p-values of pairwise comparisons. AgDrain, agricultural drainage.
Fig. 4. June to October methylmercury (MeHg) loads by sampling pro-
gram (indicated by x-axis groups and shading) and site (indicated by
color in legend). Loads from tributary sources are shown as a stacked
bar and are expected to equal downstream load. “All” represents
loads calculated using data from all sampling programs. Error bars
represent the standard deviation of sampling program estimates.
Data was not available for Upstream-Feather, so a placeholder (gray)
was used with the value from “All.” Color and shading is consistent
with Fig. 5. AgDrain, agricultural drainage.
Journal of Environmental Quality
tributary loads: Upstream-Sacramento, Upstream-Feather,
AgDrain-Woutfall, and AgDrain-E. e downstream load (2.5 ± 0.6
g d−1) was 73% of the total June to October tributary load (3.4 ±
0.4 g d−1). Although the downstream load was within the range
of variability of the tributary load, the downstream load was less
than the tributary load in both programs where it was measured (I
and V), possibly suggesting MeHg loss. June to October load esti-
mates diered among datasets by less than a factor of two (Fig. 4).
AgDrain loads accounted for 10.7% of the total June to October
upstream tributary load, or 14.8% of the downstream load.
November to May loads made up 70 to 90% of the total annual
load (within sites), and estimates diered among programs by
approximately a factor of four (Fig. 5). November to May MeHg
loads for AgDrain-Woutfall, Upstream-Sacramento, Upstream-
Feather, and downstream were 0.22 ± 0.1, 7.3 ± 4, 1.7 ± 0.8, and
24 ± 17 g d−1, respectively. November to May loads for AgDrain-E
were not possible to estimate due to gaps in ow data and oodwa-
ter being diverted into and out of the river at a number of locations.
Sampling frequency in this study was lower than recommended
for accurate load calculation (Dolan et al., 1981). Low sampling
frequency is problematic because it reduces the likelihood of accu-
rately capturing loads during storm events. is is particularly
evident for November to May downstream loads, where removal
of one elevated sample concentration (1.98 ng L−1, 13 Feb. 1997,
see Fig. 2A) decreases the downstream load estimate by 40%. is
sample was taken more than a month aer a major ood event,
during an extended period of high ow, and elevated MeHg con-
centrations were also observed at other sites (quality control data
did not suggest this was a result of contamination [Domagalski,
1998]). It is possible that higher concentrations occurred during
the peak of this or other storm events, but samples were not col-
lected then. erefore, uncertainty surrounding storm event loads
limits our condence in November to May loads. ere are typi-
cally no storm events and MeHg concentrations are less variable
during June to October (Fig. 2B), so the variation in MeHg con-
centration can be captured using fewer samples; thus, the sampling
frequency is likely adequate for June to October loads.
e true MeHg load at a site should be expected to show
considerable interannual variation, with larger loads occurring
in years with high ow (Hill, 1986). It is important to consider
whether the years measured in this study are representative of
historical ow regimes. Hill (1986) recommends using data from
6 to 7 yr to obtain robust estimates of average annual loads. With
the exception of downstream, loads presented here were based
on 7 to 9 yr of data, suggesting that this study adequately cap-
tures the interannual load variation at these sites. Flow during the
study period was greater than averages of historical average ows
at all sites except AgDrain-W in June to October (Supplemental
Table S5 and Supplemental Fig. S1), suggesting that loads during
this study may have been higher than historical average loads.
Full accounting of rice eld loads requires the total MeHg load
from AgDrain-W, which is the sum of AgDrain-Woutfall (reported
above) and AgDrain-Wdiversion (not calculated because of missing
ow data). However, AgDrain-Wdiversion ow data from 2007 and
2012 shows that AgDrain-Woutfall carried 36 ± 9% of November to
May ows and 73 ± 7% of June to October ows from AgDrain-W.
rough discussions with irrigation district managers, we deter-
mined that management of AgDrain-W ows have not changed
since the beginning of the study period (Bair, personal communi-
cation, 2016). Based on the fraction of ows carried by AgDrain-
Woutfall, the total AgDrain-W load was 0.61 ± 0.17 g d−1 in November
to May and 0.19 ± 0.02 g d−1 in June to October.
Windham-Myers et al. (2014a) reported MeHg loads exported
from rice elds in the Yolo Bypass. If these loads are extrapolated to
other rice-growing regions and multiplied by the total area of rice
in the Sacramento Valley (240,000 ha), the predicted MeHg loads
are 12± 7 and 26 ± 24 g d−1 in June to October and November to
May, respectively—an order of magnitude higher than the results
of this study. is substantial dierence may result from two
potential sources. First, there are known site dierences between
the eld-scale study site in the Yolo Bypass and typical Sacramento
Valley rice-growing areas. For example, MeHg concentrations in
irrigation source water were an order of magnitude higher in the
Yolo Bypass than in irrigation source water (Upstream-Sacramento
and Upstream-Feather) in this study. e Yolo Bypass is known to
accumulate Hg-laden sediment during storm events (Springborn
et al., 2011), while much of the rice land is not in the path of storm
ows that would deliver Hg-laden sediment. Secondly, dierences
may result from scale-dependent factors. Bradley et al. (2011)
found that MeHg loads exported from wetland sources overesti-
mated the watershed-scale load because MeHg was lost through
sink processes (including photo- and microbial demethylation and
particle settling) during transport. Windham-Myers et al. (2014a)
showed that rice elds are net MeHg sources; however, aque-
ous MeHg exported from rice elds in Sacramento Valley must
be transported through a network of canals before it reaches the
Sacramento River. Future research should seek to determine the
degree to which MeHg is lost during canal transport and to quan-
tify MeHg budgets for Sacramento Valley rice elds.
Rice Field Drainage Contribution
Elevated November to May downstream concentrations and
the large November to May downstream load relative to the
Fig. 5. November to May methylmercury (MeHg) loads by site (indi-
cated by color and x-axis groups) and sampling program (indicated
by shading in legend). Note the log scale y-axis. “All” represents loads
calculated using data from all sampling programs. Error bars repre-
sent standard deviation of sampling program estimates. Color and
shading is consistent with Fig. 4. AgDrain, agricultural drainage.
Journal of Environmental Quality
upstream tributary loads suggest that the uncalculated November
to May AgDrain-E was considerably larger than AgDrain-W.
However, AgDrain-E carries a mixture of oodwater and rice
drainage water, as well as drainage from wetlands. Determining
the MeHg load from rice requires further research on the eect of
oodplains on MeHg and the contribution of managed wetlands.
It is misleading to consider only MeHg loads exported from
rice because elds receive MeHg in irrigation water and atmo-
spheric sources. Irrigation source water (upstream) had a MeHg
concentration of 60 to 70% of that of AgDrain water (Fig. 3).
Due to evapotranspiration and percolation losses, the volume of
drainage water is £40% of irrigation water applied to a rice eld
during the growing season (Linquist et al., 2015). us, growing
season increases in MeHg concentrations between irrigation and
drainage water during the growing season were of a similar mag-
nitude to that expected from evapoconcentration alone. e June
to October AgDrain MeHg load contributed to the Sacramento
River may be similar to the MeHg load diverted from the river
for irrigation. During November to May (the fallow season),
upstream MeHg concentrations were 43% of AgDrain, while
drainage water exports are expected to be a larger fraction of irri-
gation water applied (no transpiration is expected in fallow rice
elds), resulting in increased MeHg exports relative to imports.
is is consistent with Bachand et al. (2014), who found that rice
elds might store MeHg during the growing season but release it
during the winter fallow. Wet and dry atmospheric deposition
may be important sources of MeHg and total Hg (Munthe et al.,
1995, Conaway et al., 2010) and have been shown to inuence
runo uxes from catchments (Hultberg and Munthe, 1995).
However, atmospheric deposition data is not available for the
Sacramento Valley. Deposition studies would help quantify the
degree to which rice elds are MeHg sources or sinks.
Elevated November to May AgDrain MeHg concentrations
and November to May AgDrain-W loads being fourfold higher
than June to October loads both indicate that November to May
is the period of higher concern for MeHg export from rice elds.
Without knowledge of the full annual cycle of MeHg production
in and export from a system, studies risk missing important peri-
ods of MeHg export. If the environmental management objective
is to reduce annual loads of MeHg, control eorts should focus
on the November to May period, when MeHg concentrations
in AgDrains are elevated. Finally, this study indicates that care
should be taken when extrapolating the impact of rice produc-
tion on MeHg in surface water at the eld to valley scale. While
the seasonal patterns of MeHg concentrations and exports were
similar to those observed at the eld scale, there were substantial
dierences in the magnitudes of concentrations and loads.
Graduate research assistantship funding for K.C. Tanner was provided
by the California Rice Research Board and the UC Davis Department
of Plant Sciences. L. Windham-Myers and J.A. Fleck were funded
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encouragement and support.
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... Two additional model structures were tested with the same model selection protocol described above. Water data were modeled as y = treatment + season + (treatment × season) + (1| block) + (1| plot), where season is either growing or fallow, because previous studies noted elevated water MeHg and THg during the fallow season (Tanner et al., 2017). A subset of rice grain and straw concentration and yield data collected at harvest, were modeled as y = treatment + year + (treatment × year) + (1| block) + (1| plot) where year is either 2014 or 2015. ...
... Both MeHg and THg concentrations in water were significantly higher during fallow season sampling events than in the growing season (Fig. 2). This is consistent with previous studies of MeHg and THg in California rice fields (Tanner et al., 2017). Although the reasons for this pattern are uncertain, elevated concentrations during the fallow season may be caused by the release of MeHg and THg temporarily stored in the soil or decomposing rice straw (Bachand et al., 2014;Windham-Myers et al., 2014). ...
... Although the reasons for this pattern are uncertain, elevated concentrations during the fallow season may be caused by the release of MeHg and THg temporarily stored in the soil or decomposing rice straw (Bachand et al., 2014;Windham-Myers et al., 2014). Another possibility is that decomposing straw may promote Hg(II) methylation by stimulating microbial activity (Marvin-DiPasquale et al., 2014); however, others have found that changes in straw management had complex (Zhu et al., 2015), insignificant (Tanner et al., 2017), or contradictory (Eagles-Smith et al., 2014) effects on MeHg concentrations. ...
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In flooded soils, including those found in rice (Oryza sativa L.) fields, microbes convert inorganic Hg to more toxic methylmercury (MeHg). Methylmercury is accumulated in rice grain, potentially affecting health. Methylmercury in rice field surface water can bioaccumulate in wildlife. We evaluated how introducing aerobic periods into an otherwise continuously flooded rice growing season affects MeHg dynamics. Conventional continuously flooded (CF) rice field water management was compared with alternate wetting and drying, where irrigation was stopped twice during the growing season, allowing soil to dry to 35% volumetric moisture content, at which point plots were reflooded (AWD-35). Methylmercury studies began at harvest in Year 3 and throughout Year 4 of a 4-yr replicated field experiment. Bulk soil, water, and plant samples were analyzed for MeHg and total Hg (THg), and iron (Fe) speciation was measured in soil samples. Rice grain yield over 4 yr did not differ between treatments. Soil chemistry responded quickly to AWD-35 dry-downs, showing significant oxidation of Fe(II) accompanied by a significant reduction of MeHg concentration (76% reduction at harvest) compared with CF. Surface water MeHg decreased by 68 and 39% in the growing and fallow seasons, respectively, suggesting that the effects of AWD-35 management can last through to the fallow season. The AWD-35 treatment reduced rice grain MeHg and THg by 60 and 32%, respectively. These results suggest that the more aerobic conditions caused by AWD-35 limited the activity of Hg(II)-methylating microbes and may be an effective way to reduce MeHg concentrations in rice ecosystems.
... Downstream in the Sacramento-San Joaquin Delta (hereafter referred to as the "Delta"), MeHg concentrations are elevated and negative impacts of Hg on wildlife have been documented . Both field-scale and watershed-scale studies report rice field drainage waters have elevated MeHg concentrations and/or loads during the fallow season Bachand et al., 2014;Eagles-Smith et al., 2014;Tanner et al., 2017), when fields are flooded to decompose rice straw. However, the field-scale studies Eagles-Smith et al., 2014) report concentrations that are considerably higher than observed at the watershed scale (Tanner et al., 2017). ...
... Both field-scale and watershed-scale studies report rice field drainage waters have elevated MeHg concentrations and/or loads during the fallow season Bachand et al., 2014;Eagles-Smith et al., 2014;Tanner et al., 2017), when fields are flooded to decompose rice straw. However, the field-scale studies Eagles-Smith et al., 2014) report concentrations that are considerably higher than observed at the watershed scale (Tanner et al., 2017). This may be because fieldscale studies were conducted in areas known to receive Hg-laden sediment from mining regions during flood events (Singer et al., 2013), whereas sediment transport from mining areas into the main Sacramento Valley rice-growing region is limited by dams (Slotton et al., 1995;Supplemental Fig. S1). ...
... Methylmercury and THg concentrations in outlets of neighboring fields were consistent with Butte and Yolo (Fig. 1). Butte and Yolo are located within the area studied by Tanner et al. (2017) who reported similar irrigation and drainage water MeHg concentrations and seasonal patterns. ...
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Few studies have considered how methylmercury (MeHg, a toxic form of Hg produced in anaerobic soils) production in rice (Oryza sativa L.) fields can affect water quality, and little is known about MeHg dynamics in rice fields. Surface water MeHg and total Hg (THg) imports, exports, and storage were studied in two commercial rice fields in the Sacramento Valley, California, where soil THg was low (25 and 57 ng g⁻¹). The median concentration of MeHg in drainage water exiting the fields was 0.17 ng g⁻¹ (range: < 0.007-2.1 ng g⁻¹). Compared with irrigation water, drainage water had similar MeHg concentrations, and lower THg concentrations during the growing season. Significantly elevated drainage water MeHg and THg concentrations were observed in the fallow season compared with the growing season. An analysis of surface water loads indicates that fields were net importers of both MeHg (76-110 ng m⁻²) and THg (1947-7224 ng m⁻²) during the growing season, and net exporters of MeHg (35-200 ng m⁻²) and THg (248-6496 ng m⁻²) during the fallow season. At harvest, 190 to 700 ng MeHg m⁻² and 1400 to 1700 ng THg m⁻² were removed from fields in rice grain. Rice straw, which contained 120 to 180 ng MeHg m⁻² and 7000-10,500 ng m⁻² THg was incorporated into the soil. These results indicate that efforts to reduce MeHg and THg exports in rice drainage water should focus on the fallow season. Substantial amounts of MeHg and THg were stored in plants, and these pools should be considered in future studies. © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.
... Furthermore, heavy metal accumulation and contamination of surface waters in rice systems is a cause of concern, particularly in the case of mercury. Studies in Central Valley rice systems have documented net ecosystem production and accumulation of mercury species in water and sediment, MeHg load, and bioaccumulation of toxic metals in aquatic invertebrates [52][53][54][55][56]. Dissolved organic carbon, total suspended solid, nitrogen, and phosphorus loads were assessed in rice systems [57][58][59], but these metrics were not included in the overall benefit/tradeoff analysis due in part to their specificity to land covers with an aquatic component. ...
Technical Report
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Executive Summary: The Central Valley of California is one of the most heavily modified landscapes in the world, with millions of acres of semi-arid grassland, desert, mesic, wetland, and riparian areas transformed into an irrigated crop production powerhouse through large-scale infrastructure and irrigation projects. Despite its reputation as an agricultural "sacrifice zone", it remains an area of conservation focus for its varied, unique, and vibrant ecosystems, from rare vernal pools and serpentine grasslands to the extensive networks of riverine systems, riparian forests, and wetlands that converge at the Sacramento-San Joaquin Delta. While the importance of these natural areas for human-valued functions such as water supply and quality regulation, biodiversity, culture, and recreation is well established, the dominance of agricultural land covers in the Central Valley underscores the need to understand to what extent they contribute to or detract from ecosystem functions beyond crop production. Much of the information that is available on the potential benefits from agricultural and natural land covers is not centralized. Instead, disparate reports from research activities that vary in geographic location, scope, and timeframe constitute the bulk of the literature. Furthermore, most studies implement a particular suite of metrics to characterize benefits or tradeoffs provided by a land cover depending on the objectives of the study. Therefore, a synthesis of information on multiple benefits that aggregates metrics into a single database with comparable units of measure is an important step towards incorporating multiple benefits research into concerted planning and policy-making efforts for a multifunctional Central Valley landscape. We performed a rapid evidence assessment following a consistent search strategy and predetermined inclusion/exclusion criteria. We limited the results of the literature search to peer-reviewed publications from 2010-2020 with a geographic focus on the Central Valley, including the Sacramento-San Joaquin Delta. We extracted published, quantitative estimates of benefits and/or tradeoffs associated with individual land covers and compiled a database consisting of metrics on: 1) climate regulation (e.g., greenhouse gas emissions, carbon storage/sequestration), 2) economy (e.g., livelihoods, production value), 3) environmental health (e.g., pollution, pesticide load), 4) water (e.g., water quality, water use), and 5) wildlife, specifically value for avian conservation. We also consulted expert panels in the fields of agricultural ecology and conservation to assess: 1) avian conservation value, and 2) vulnerability to the impacts of climate change of each of the land covers. Finally, we produced a spatially-explicit model using publicly-available datasets to visualize the distribution of ecosystem benefits and tradeoffs, including carbon storage potential, air and water quality, groundwater recharge, and socio-cultural benefits. We found that the agricultural land covers most likely to be associated with multiple benefits were alfalfa, rice, and rangelands/pastures (including shrublands and oak woodlands managed for grazing). Alfalfa was associated with benefits such as carbon sequestration and managed aquifer recharge potential, along with minor support for biodiversity, although tradeoffs such as nitrous oxide emissions from mature stands and high consumptive water use were also noted. Flooded rice systems were notable for their high value for wildlife, particularly waterfowl, shorebirds, and waterbirds, along with their economic value in the form of relatively high wages for agricultural labor, although methane emissions and consumptive water use were also a concern. As for orchard crops, which are notable for their large increase in planted area in recent years, their important contributions to agricultural production value and agricultural livelihoods were offset by potential tradeoffs in air quality metrics, nitrate leaching risk, and consumptive water use. Grasslands, including rangelands and pastures managed for livestock production as well as unmanaged grasslands, had high potential benefits for climate regulation via carbon storage and sequestration in soils and belowground biomass, along with high value for biodiversity and support of valuable agricultural pollination services. In contrast, annual field crops such as tomatoes, corn, and cotton were the most likely to be associated with tradeoffs such as greenhouse gas emissions, nitrate Multiple Benefits from Central Valley Land Covers iii Peterson et al. June 2020 leaching hazard, and heavy pesticide use. Natural land covers such as unmanaged grasslands, wetlands, and riparian areas were most widely associated with benefits such as support for wildlife populations, carbon storage (particularly in riparian areas) and pollutant mitigation (in the case of wetlands), while some tradeoffs in greenhouse gas emissions were noted. The spatial distribution of benefits and tradeoffs was highly heterogeneous, although in many cases a north-south trend was evident with areas in the northern Central Valley/Sacramento Valley exhibiting more relative benefits than areas in the southern Central Valley/San Joaquin Valley. The former is noted for the concentrated production of rice, along with a mixture of tomatoes, alfalfa, and orchard crops such as almonds and walnuts. The latter, on the other hand, is associated with most of the Central Valley's production of annual row crops (e.g., cotton), oranges and lemons, table grapes, and deciduous perennial tree crops such as pistachios, almonds, peaches, and prunes. Carbon storage patterns were particularly distinctive, with hotspots in the highly organic soils of the Sacramento-San Joaquin Delta and the former Tulare lakebed. However, the distribution of carbon storage potential was inversely related to carbon storage, in agreement with research showing the Delta and Tulare lakebed to be sites of carbon loss [1]. Our ability to draw general conclusions on the relative benefits or tradeoffs associated with Central Valley land covers was limited by the single-intervention nature of most of the quantitative research available on benefit/tradeoff related metrics. Experimental designs often must restrict activities to a single or few related land covers and investigate the impacts of an intervention on the metric of interest. For the purposes of cross-system comparisons, there were very few studies that addressed variability in benefit/tradeoff metrics across multiple land covers from a multiple benefits or multi-functional landscapes perspective. Many studies were focused on a few key metrics of known importance for a particular land cover, e.g., methane emissions in rice, rather than a broader survey of potential benefits and tradeoffs. Furthermore, most experimental analyses are spatially biased and not representative of the entire Central Valley landscape. These challenges highlight the need for more research on human-valued benefits across land covers from a multiple benefits perspective, preferably with a common set of metrics and indicators relevant to most or all of the land covers under consideration. The following report synthesizes the most recent, Central-Valley-specific literature available on multiple benefit and tradeoff metrics. Section I presents individual land cover profiles, with a compilation of published, quantitative estimates for benefit/tradeoff metrics relative to other land covers, and where relevant, discussion of additional metrics not included in benefit/tradeoff analysis. Section II provides further details on a benefit/tradeoff analysis across land covers using data extracted from the published literature, along with the results of expert panel scoring on relative avian conservation value and climate change vulnerability among land covers. Finally, Section III presents results for spatial models of benefits and tradeoff metrics, including carbon storage, air, water, and habitat quality, groundwater recharge potential, and socio-cultural benefits across the Central Valley. Appendices are included for detailed coverage of methods for the rapid evidence assessment, benefit/tradeoff analyses, and index development. The complete database and code in R script associated with this report are freely available on the Dryad repository under DOI:
... In actuality, there are a number of factors that impact the composition and amounts of DOC (and hence THM precursors) that are exported from agricultural systems into streams and rivers. For example, factors such as soil type, till vs. no till management, geomorphic features, riparian buffers, settling ponds, specific crop types, irrigation (flood/furrow/ spray) vs. non-irrigated lands, annuals vs. perennials, woody trees vs. highly vegetative crops, straw baling vs. burning vs. tilling, winter flooding to enhance decomposition, and of course the total area of cultivated land and volume of water exported (Tanner et al., 2017;Tanner et al., 2018). ...
To meet drinking water regulations, rather than investing in costly treatment plant operations, managers can look for ways to improve source water quality; this requires understanding watershed sources and fates of constituents of concern. Trihalomethanes (THMs) are one of the major classes of regulated disinfection byproducts, formed when a specific fraction of the organic carbon pool—referred to as THM precursors—reacts with chorine and/or bromine during treatment. Understanding the source, fate, timing and duration of the organic compounds that react to form THMs will allow identification of targeted and effective management actions. In this study we evaluated THM precursor contributions from multiple land use categories and hydrologic contexts, including novel data for urban land uses that demonstrate strong potential to release water with high THM formation potential (THMFP; median 618 μg L⁻¹): greater than storm runoff integrated across a mixed-use (1/3 natural, 2/3 agricultural) watershed (median 460 μg L⁻¹), irrigation runoff from agricultural systems (357 μg L⁻¹), or runoff from a natural forested (median 123 μg L⁻¹) and shrubland/grassland (median 259 μg L⁻¹) watersheds. While individual storm events released high THM precursor concentrations over short periods, dry season agricultural irrigation as well as urban landscapes have the potential to release water high in THM precursors for several months. Experimental bioassays and sampling along 333 miles of the California Aqueduct confirmed bioavailability and photooxidation potential of less than 10% for THM precursors, suggesting that rivers with residence times of days to weeks may act as THM precursor conduits, shuttling THM precursors from hundreds of miles away to drinking water intakes with minimal degradation. This finding has considerable implications for water managers, who may therefore consider THM precursor management strategies that target even sources located far upstream.
... In addition to the concern for MeHg exposure via rice consumption, rice agriculture can also be a source of MeHg to downstream ecosystems. Net export of MeHg from rice fields has been estimated in some locations, although this phenomenon may vary within each growing season (Bachand et al. 2014;Windham-Myers et al. 2014b;Tanner et al. 2017). ...
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The environmental cycling of mercury (Hg) can be affected by natural and anthropogenic perturbations. Of particular concern is how these disruptions increase mobilization of Hg from sites and alter the formation of monomethylmercury (MeHg), a bioaccumulative form of Hg for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of MeHg in the environment. The objective of this paper is to synthesize the scientific understanding of how Hg cycling in the aquatic environment is influenced by landscape perturbations at the local scale, perturbations that include watershed loadings, deforestation, reservoir and wetland creation, rice production, urbanization, mining and industrial point source pollution, and remediation. We focus on the major challenges associated with each type of alteration, as well as management opportunities that could lessen both MeHg levels in biota and exposure to humans. For example, our understanding of approximate response times to changes in Hg inputs from various sources or landscape alterations could lead to policies that prioritize the avoidance of certain activities in the most vulnerable systems and sequestration of Hg in deep soil and sediment pools. The remediation of Hg pollution from historical mining and other industries is shifting towards in situ technologies that could be less disruptive and less costly than conventional approaches. Contemporary artisanal gold mining has well-documented impacts with respect to Hg; however, significant social and political challenges remain in implementing effective policies to minimize Hg use. Much remains to be learned as we strive towards the meaningful application of our understanding for stakeholders, including communities living near Hg-polluted sites, environmental policy makers, and scientists and engineers tasked with developing watershed management solutions. Site-specific assessments of MeHg exposure risk will require new methods to predict the impacts of anthropogenic perturbations and an understanding of the complexity of Hg cycling at the local scale. Electronic supplementary material The online version of this article (10.1007/s13280-017-1006-7) contains supplementary material, which is available to authorized users.
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Mercury is a metal present in the Earth’s crust, but due to human contribution, its concentration can increase, causing environmental impacts to aquatic ecosystems, among others. The Reis Magos River Hydrographic Basin represents economic and socio-environmental importance for the state of Espírito Santo, Brazil. However, there are not many publications regarding the quality of water and sediments, so no data is reported concerning the total concentration of Hg. Thus, the present work aimed to evaluate the distribution of total Hg in water and sediments along this hydrographic basin. For a better inference, physicochemical parameters of the water were determined (temperature, pH, electrical conductivity, oxidation-reduction potential (ORP), turbidity, dissolved oxygen (DO), total dissolved solids (TDS), and salinity), and in the sediments, the contents of matter organic matter, pH, carbonates and granulometry. Mercury determination was performed by Thermodecomposition and Amalgamation Atomic Absorption Spectrometry (TDA AAS) with a DMA-80 spectrometer. The Hg determined in the water was lower than the limit of quantification, 0.14 µg∙L−1, which is lower than the maximum limits recommended by world reference environmental agencies. In the sediment samples, the Hg found were below 170 µg∙kg−1, values below which there is less possibility of an adverse effect on the biota. However, when the degree of anthropic contribution was evaluated using the Geoaccumulation index (IGeo), the contamination factor (CF), and the ecological risk potential index (EF), there was evidence of moderate pollution. Thus, this highlighted the need for monitoring the region since climatic variations and physical-chemical parameters influence the redistribution of Hg between the water/sediment interface.
To mitigate mercury (Hg) pollution and reduce Hg downstream transportation, a weir was designed by a river system that had been inflicted by leachate from the slagheap of the Yanwuping Hg mine in Wanshan Hg mining area. A whole year monitoring of Hg species was conducted, and the efficiency of Hg reduction by the weir application was evaluated. The Hg concentrations in the river water were significantly higher in the wet season than in the dry season. Waterflow was confirmed to be the main driving factor for Hg mobilization and transportation, and an episode study revealed that most Hg was released in times of storms. Increased monitoring and preventive maintenance measures need to be taken on barriers in advance of storms. A large proportion of the total Hg (THg) and methylmercury (MeHg) is associated to particles. During the study period, approximately 412 g THg and 4.04 g total MeHg (TMeHg) were released from the YMM slagheap, of which 167 g THg and 1.15 g TMeHg were retained by the weir. Annually, 40.4% THg and 38.4% TMeHg was retained by the weir. Weir construction is considered as a potential cost-effective measure to mitigate Hg in river water and should be promoted and extended in the future after optimization.
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Rice paddy plantation for human consumption is dominant land uses throughout Asia. Rice paddy fields have been identified as important sites for methylmerucry (MeHg) production in the terrestrial ecosystem, and a primary pathway of MeHg exposure to human in mercury (Hg) mining areas. We compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice-growing season at two different typical Hg-contaminated mining sites: an abandoned site with high Hg concentration in soil but low concentration in atmosphere, and a current-day artisanal site with low concentration in soil but high concentration in atmosphere. The contribution of new Hg to the ecosystem from irrigation and atmospheric deposition was insignificant relative to the pool of old Hg; the dominant source of MeHg to paddy soil is in situ methylation of inorganic Hg. Elevated MeHg concentrations jointly with the high proportion of Hg as MeHg in paddy water and the surface soil layer at the artisanal s ite demonstrated active Hg methylation at this site only. We propose that the in situ production of MeHg is dependent on elevated IHg in the atmosphere, and the deposition of new Hg into a low pH anoxic geochemical system. In contrast, the absence of depth-dependent variability in the MeHg concentration in soil cores collected from abandoned Hg mining site, consistent with the low concentration of Hg in atmospheric deposition and high pH of the paddy water/irrigation water, suggested that the net production of MeHg was limited. We also propose that the concentration of Hg in ambient air is an indicator for the risk of MeHg accumulation in paddy rice.
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We measured total mercury (THg) and monomethyl mercury (MMHg) concentrations and mercury (Hg) isotopic compositions in sediment and aquatic organisms from the Yuba River (California, USA) to identify Hg sources and biogeochemical transformations downstream of a historical gold mining region. Sediment THg concentrations and δ202Hg decreased from the upper Yuba Fan to the lower Yuba Fan and the Feather River. These results are consistent with the release of Hg during gold mining followed by downstream mixing and dilution. The Hg isotopic composition of Yuba Fan sediment (δ202Hg = -0.38 ± 0.17‰ and Δ199Hg = 0.04 ± 0.03‰; mean ± 1SD, n=7) provides a fingerprint of inorganic Hg (IHg) that could be methylated locally or after transport downstream. The isotopic composition of MMHg in the Yuba River food web was estimated using biota with a range of %MMHg (the percent of THg present as MMHg) and compared to IHg in sediment, algae and the food web. The estimated δ202Hg of MMHg prior to photodegradation (-1.29 to -1.07‰) was lower than that of IHg and we suggest this is due to mass-dependent fractionation (MDF) of up to -0.9‰ between IHg and MMHg. This result is in contrast to net positive MDF (+0.4 to +0.8‰) previously observed in lakes, estuaries, coastal oceans and forests. We hypothesize that this unique relationship could be due to differences in the extent or pathway of biotic MMHg degradation in stream environments.
Technical Report
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Groundwater quality in the Southern, Middle, and Northern Sacramento Valley study units was investigated as part of the Priority Basin Project of the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The study units are located in California’s Central Valley and include parts of Butte, Colusa, Glenn, Placer, Sacramento, Shasta, Solano, Sutter, Tehama, Yolo, and Yuba Counties. The GAMA Priority Basin Project is being conducted by the California State Water Resources Control Board in collaboration with the U.S. Geological Survey and the Lawrence Livermore National Laboratory. The three study units were designated to provide spatially-unbiased assessments of the quality of untreated groundwater in three parts of the Central Valley hydrogeologic province, as well as to provide a statistically consistent basis for comparing water quality regionally and statewide. Samples were collected in 2005 (Southern Sacramento Valley), 2006 (Middle Sacramento Valley), and 2007–08 (Northern Sacramento Valley). The GAMA studies in the Southern, Middle, and Northern Sacramento Valley were designed to provide statistically robust assessments of the quality of untreated groundwater in the primary aquifer systems that are used for drinking-water supply. The assessments are based on water-quality data collected by the USGS from 235 wells in the three study units in 2005–08, and water-quality data from the California Department of Public Health (CDPH) database. The primary aquifer systems (hereinafter, referred to as primary aquifers) assessed in this study are defined by the depth intervals of the wells in the CDPH database for each study unit. The quality of groundwater in shallow or deep water-bearing zones may differ from quality of groundwater in the primary aquifers; shallow groundwater may be more vulnerable to contamination from the surface. The status of the current quality of the groundwater resource was assessed by using data from samples analyzed for volatile organic compounds (VOC), pesticides, and naturally occurring inorganic constituents, such as major ions and trace elements. This status assessment is intended to characterize the quality of groundwater resources within the primary aquifers of the three Sacramento Valley study units, not the treated drinking water delivered to consumers by water purveyors. Relative-concentrations (sample concentrations divided by benchmark concentrations) were used for evaluating groundwater quality for those constituents that have Federal or California regulatory or non-regulatory benchmarks for drinking-water quality. A relative-concentration greater than 1.0 indicates a concentration greater than a benchmark. For organic (volatile organic compounds and pesticides) and special-interest (perchlorate) constituents, relative-concentrations were classified as high (greater than 1.0); moderate (equal to or less than 1.0 and greater than 0.1); or low (equal to or less than 0.1). For inorganic (major ion, trace element, nutrient, and radioactive) constituents, the boundary between low and moderate relative-concentrations was set at 0.5. Aquifer-scale proportions were used in the status assessment for evaluating regional-scale groundwater quality. High aquifer-scale proportion is defined as the percentage of the area of the primary aquifers that have a relative-concentration greater than 1.0 for a particular constituent or class of constituents; percentage is based on an areal rather than a volumetric basis. Moderate and low aquifer-scale proportions were defined as the percentage of the primary aquifers that have moderate and low relative-concentrations, respectively. Two statistical approaches—grid-based, which used one value per grid cell, and spatially-weighted, which used the full dataset—were used to calculate aquifer-scale proportions for individual constituents and classes of constituents. High and moderate aquifer-scale proportions were significantly greater for inorganic constituents than organic constituents in all three study units. In the Southern Sacramento Valley study unit, relative-concentrations for one or more inorganic constituents with health-based benchmarks (HBBs) were high in 30 percent (%), moderate in 30%, and low in 40% of the primary aquifer. In the Middle Sacramento Valley study unit, aquifer-scale proportions for inorganic constituents with HBBs were high in 24%, moderate in 38%, and low in 38% of the primary aquifer. Arsenic, boron, and nitrate were detected at high relative-concentrations in the Southern and Middle Sacramento Valley study units. In the Northern Sacramento Valley study unit, high, moderate, and low relative-concentrations of inorganic constituents relative to HBBs were 2.1, 12, and 86% of the primary aquifer, respectively. Arsenic was the only constituent detected at high relative-concentrations. The high aquifer-scale proportions for inorganic constituents with non-health-based benchmarks were 32, 27, and 4.6% of the primary aquifer for the Southern, Middle, and Northern Sacramento Valley study units, respectively. The high aquifer-scale proportions for organic constituents with HBBs were less than 1% in the Southern, Middle, and Northern Sacramento Valley study units. Organic constituents were detected at moderate relative-concentrations in about 3% of the Southern and Middle Sacramento Valley study units and in 1% of the Northern Sacramento Valley study unit. Of the 227 organic constituents analyzed for, 86 were detected, and of those detected, 56 have HBBs. Six organic constituents (atrazine, bentazon, chloroform, simazine, tetrachloroethene, and trichloroethene) were detected in 10% or more of the sampled wells in one or more of the three Sacramento Valley study units.
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Rice is a crop that is usually grown under flooded conditions and can require large amounts of water. The objective of this 3-year study was to quantify water use in water- (WS) and dry-seeded (DS) systems. In WS systems, the field is continuously flooded, while in DS systems the field is flush irrigated for the first month and then flooded. Research was conducted on commercial rice fields where the residual of the energy balance method using a sonic anemometer and the eddy covariance method were used to determine crop evapotranspiration (ETc) and crop coefficient (K c) values. In addition, inlet irrigation water and tailwater drainage were determined. Across years, there was no difference in ETc (averaged 862 mm), seasonal K c (averaged 1.07), irrigation water delivery (averaged 1839 mm) and calculated percolation and seepage losses (averaged 269 mm) between systems. An analysis of the first month of the season, when the water management between these two practices was different, indicated that K c and water use were lower in DS systems relative to WS systems when there was only one irrigation flush during this period, while two or three irrigation flushes resulted in similar values between the two systems.
Monomethyl mercury (MMHg) and total mercury (THg) concentrations and Hg stable isotope ratios (δ202Hg and Δ199Hg) were measured in sediment and aquatic organisms from Cache Creek (California Coast Range) and Yolo Bypass (Sacramento Valley). Cache Creek sediment had a large range in THg (87 to 3870 ng/g) and δ202Hg (− 1.69 to − 0.20‰) reflecting the heterogeneity of Hg mining sources in sediment. The δ202Hg of Yolo Bypass wetland sediment suggests a mixture of high and low THg sediment sources. Relationships between %MMHg (the percent ratio of MMHg to THg) and Hg isotope values (δ202Hg and Δ199Hg) in fish and macroinvertebrates were used to identify and estimate the isotopic composition of MMHg. Deviation from linear relationships was found between %MMHg and Hg isotope values, which is indicative of the bioaccumulation of isotopically distinct pools of MMHg. The isotopic composition of pre-photodegraded MMHg (i.e., subtracting fractionation from photochemical reactions) was estimated and contrasting relationships were observed between the estimated δ202Hg of pre-photodegraded MMHg and sediment IHg. Cache Creek had mass dependent fractionation (MDF; δ202Hg) of at least − 0.4‰ whereas Yolo Bypass had MDF of + 0.2 to + 0.5‰. This result supports the hypothesis that Hg isotope fractionation between IHg and MMHg observed in rivers (− MDF) is unique compared to + MDF observed in non-flowing water environments such as wetlands, lakes, and the coastal ocean.
Rice paddy plantation is the dominant agricultural land use throughout Asia. Rice paddy fields have been identified as important sites for methylmercury (MeHg) production in the terrestrial ecosystem and a primary pathway of MeHg exposure to humans in mercury (Hg) mining areas. We compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice-growing season at two different typical Hg-contaminated mining sites in Guizhou province, China: an abandoned site with a high Hg concentration in soil but a low concentration in the atmosphere and a current-day artisanal site with a low concentration in soil but a high concentration in the atmosphere. Our results showed that the flux of new Hg to the ecosystem from irrigation and atmospheric deposition was insignificant relative to the pool of old Hg in soil; the dominant source of MeHg to paddy soil is in situ methylation of inorganic Hg (IHg). Elevated MeHg concentrations and the high proportion of Hg as MeHg in paddy water and the surface soil layer at the artisanal site demonstrated active Hg methylation at this site only. We propose that the in situ production of MeHg in paddy water and surface soil is dependent on elevated Hg in the atmosphere and the consequential deposition of new Hg into a low-pH anoxic geochemical system. The absence of depth-dependent variability in the MeHg concentration in soil cores collected from the abandoned Hg mining site, consistent with the low concentration of Hg in the atmosphere and high pH of the paddy water and irrigation water, suggested that net production of MeHg at this site was limited. We propose that the concentration of Hg in ambient air is an indicator for the risk of MeHg accumulation in paddy rice.
Annual stream loads of mercury (Hg) and inputs of wet and dry atmospheric Hg deposition to the landscape were investigated in watersheds of the Western United States and the Canadian-Alaskan Arctic. Mercury concentration and discharge data from flow gauging stations were used to compute annual mass loads with regression models. Measured wet and modeled dry deposition were compared to annual stream loads to compute ratios of Hg stream load to total Hg atmospheric deposition. Watershed land uses or cover included mining, undeveloped, urbanized, and mixed. Of 27 watersheds that were investigated, 15 had some degree of mining, either of Hg or precious metals (gold or silver), where Hg was used in the amalgamation process. Stream loads in excess of annual Hg atmospheric deposition (ratio>1) were observed in watersheds containing Hg mines and in relatively small and medium-sized watersheds with gold or silver mines, however, larger watersheds containing gold or silver mines, some of which also contain large dams that trap sediment, were sometimes associated with lower load ratios (<0.2). In the non-Arctic regions, watersheds with natural vegetation tended to have low ratios of stream load to Hg deposition (<0.1), whereas urbanized areas had higher ratios (0.34-1.0) because of impervious surfaces. This indicated that, in ecosystems with natural vegetation, Hg is retained in the soil and may be transported subsequently to streams as a result of erosion or in association with dissolved organic carbon. Arctic watersheds (Mackenzie and Yukon Rivers) had a relatively elevated ratio of stream load to atmospheric deposition (0.27 and 0.74), possibly because of melting glaciers or permafrost releasing previously stored Hg to the streams. Overall, our research highlights the important role of watershed characteristics in determining whether a landscape is a net source of Hg or a net sink of atmospheric Hg.
The biogeochemical cycling of metals and other contaminants in river-floodplain corridors is controlled by microbial activity responding to dynamic redox conditions. Riverine flooding thus has the potential to affect speciation of redox-sensitive metals such as mercury (Hg). Therefore, inundation history over a period of decades potentially holds information on past production of bioavailable Hg. We investigate this within a Northern California river system with a legacy of landscape-scale 19th century hydraulic gold mining. We combine hydraulic modeling, Hg measurements in sediment and biota, and first-order calculations of mercury transformation to assess the potential role of river floodplains in producing monomethylmercury (MMHg), a neurotoxin which accumulates in local and migratory food webs. We identify frequently inundated floodplain areas, as well as floodplain areas inundated for long periods. We quantify the probability of MMHg production potential (MPP) associated with hydrology in each sector of the river system as a function of the spatial patterns of overbank inundation and drainage, which affect long-term redox history of contaminated sediments. Our findings identify river floodplains as periodic, temporary, yet potentially important, loci of biogeochemical transformation in which contaminants may undergo change during limited periods of the hydrologic record. We suggest that inundation is an important driver of MPP in river corridors and that the entire flow history must be analyzed retrospectively in terms of inundation magnitude and frequency in order to accurately assess biogeochemical risks, rather than merely highlighting the largest floods or low-flow periods. MMHg bioaccumulation within the aquatic food web in this system may pose a major risk to humans and waterfowl that eat migratory salmonids, which are being encouraged to come up these rivers to spawn. There is a long-term pattern of MPP under the current flow regime that is likely to be accentuated by increasingly common large floods with extended duration.
Waterbird use of agricultural wetlands has increased as natural wetlands continue to decline worldwide. Little information exists on waterbird use of wetland crops such as taro, hasu, and wild rice. Several reports exist on waterbird use of cranberry bog systems. Information exists on waterbird use of rice fields, especially by herons and egrets. Rice fields encompass over 1.5 million km2 of land and are found on all continents except Antarctica. Rice fields are seasonally flooded for cultivation and to decoy waterfowl, and drawn down for sowing and harvest. A wide variety of waterbirds including wading birds, shorebirds, waterfowl, marshbirds, and seabirds utilize rice fields for foraging and to a lesser extent as breeding sites. In some areas, especially Asia, waterbirds have come to rely upon rice fields as foraging sites. However, few reports exist on waterbird use of rice ecosystems outside of the Mediterranean Region. Species that are commonly found utilizing agricultural wetlands during the breeding season, migration, and as wintering grounds are listed. General trends and threats to waterbirds utilizing agricultural wetlands, including habitat destruction and degradation, contaminant exposure, and prey fluctuations are presented.