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A novel approach for direct estimation of fresh groundwater discharge to an estuary

DOI:10.1029/2011GL047718
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Neil Ganju at United States Geological Survey
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1] Coastal groundwater discharge is an important source of freshwater and nutrients to coastal and estuarine systems. Directly quantifying the spatially integrated discharge of fresh groundwater over a coastline is difficult due to spatial variability and limited observational methods. In this study, I applied a novel approach to estimate net freshwater discharge from a groundwater‐fed tidal creek over a spring‐neap cycle, with high temporal resolution. Acoustic velocity instruments measured tidal water fluxes while other sensors measured vertical and lateral salinity to estimate cross‐sectionally aver-aged salinity. These measurements were used in a time‐ dependent version of Knudsen's salt balance calculation to estimate the fresh groundwater contribution to the tidal creek. The time‐series of fresh groundwater discharge shows the dependence of fresh groundwater discharge on tidal pump-ing, and the large difference between monthly mean dis-charge and instantaneous discharge over shorter timescales. The approach developed here can be implemented over time-scales from days to years, in any size estuary with dominant groundwater inputs and well‐defined cross‐sections. The approach also directly links delivery of groundwater from the watershed with fluxes to the coastal environment. Citation: Ganju, N. K. (2011), A novel approach for direct estima-tion of fresh groundwater discharge to an estuary, Geophys. Res. Lett., 38, L11402, doi:10.1029/2011GL047718.
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A novel approach for direct estimation of fresh groundwater
discharge to an estuary
Neil K. Ganju
1
Received 8 April 2011; revised 26 April 2011; accepted 27 April 2011; published 4 June 2011.
[1] Coastal groundwater discharge is an important source of
freshwater and nutrients to coastal and estuarine systems.
Directly quantifying the spatially integrated discharge of
fresh groundwater over a coastline is difficult due to spatial
variability and limited observational methods. In this study, I
applied a novel approach to estimate net freshwater discharge
from a groundwater‐fed tidal creek over a spring‐neap cycle,
with high temporal resolution. Acoustic velocity instruments
measured tidal water fluxes while other sensors measured
vertical and lateral salinity to estimate cross‐sectionally aver-
aged salinity. These measurements were used in a time‐
dependent version of Knudsen’s salt balance calculation to
estimate the fresh groundwater contribution to the tidal creek.
The time‐series of fresh groundwater discharge shows the
dependence of fresh groundwater discharge on tidal pump-
ing, and the large difference between monthly mean dis-
charge and instantaneous discharge over shorter timescales.
The approach developed here can be implemented over time-
scales from days to years, in any size estuary with dominant
groundwater inputs and well‐defined cross‐sections. The
approach also directly links delivery of groundwater from
the watershed with fluxes to the coastal environment.
Citation: Ganju, N. K. (2011), A novel approach for direct estima-
tion of fresh groundwater discharge to an estuary, Geophys. Res.
Lett.,38, L11402, doi:10.1029/2011GL047718.
1. Introduction
[2] Quantifying fresh groundwater discharge to the coastal
margin is confounded by spatial and temporal variability
and the inherent difficulty of observing the discharge. While
many eutrophic estuaries receive the majority of their fresh-
water and nutrient loads from rivers, the effects of coastal
groundwater discharge can also be large depending on bio-
geochemistry and the transformation of nutrients during
transit time in the aquifer. Valiela et al. [1990] highlighted
the importance of coastal groundwater discharge as a large
overlooked source of nutrients to coastal ecosystems and
addressed the potential biogeochemical significance. Slomp
and Van Cappellen [2004], highlighting the higher nitro-
gen/phosphorus ratio in groundwater, explored possible
shifts in nutrient limitation and primary productivity as the
ratio of groundwater to riverine water is changed. There are
instances in which the riverine freshwater and nutrient load
is rivaled or exceeded by the coastal groundwater portion.
In Tampa Bay, Kroeger et al. [2007] found that submarine
groundwater discharge accounted for up to 33% of the fresh-
water discharge, and 50% of nutrient loads. Valiela et al.
[1990] estimated that over 70% of the nitrogen load to eight
New England bays resulted from direct coastal groundwater
discharge.
[3] Several methods to measure fresh coastal groundwater
discharge have been used with varying success dependent
on the system and the assumptions. Radiochemical tracer
methods [e.g., Moore, 1996; Cable et al., 1996] which sample
estuarine water for constituents such as radon and radium,
trace total groundwater discharge (i.e. fresh and saline) rather
than specifically freshwater discharge. Seepage meters [Lee,
1977] can be used to estimate flows but spatial variability
in groundwater seepage confounds extrapolation of those
measurements to entire basins. Advanced techniques such as
the eddy‐correlation method [Crusius et al., 2008] avoid
some complications of seepage meters and cover somewhat
larger footprints. Remote sensing methods can identify
spatial variability [Portnoy et al., 1998], but cannot readily
resolve the total mass transport. Watershed water‐balance
methods [Kroeger et al., 2006] yield whole system estimates
of fresh groundwater discharge over longer time‐periods (e.g.
annual to decadal), but cannot resolve the temporal variability
that may be caused by tidal fluctuations, discrete rainfall
events, or enhanced evapotranspiration. Lee and Kim [2007]
used intensive salinity sampling to estimate freshwater bud-
gets and the submarine groundwater discharge in a coastal
system where the oceanic end member was relatively con-
stant. There is a clear need for independent measurements of
fresh groundwater discharge, separate from saline discharge,
as only the fresh discharge carries new land‐derived materials
including nutrient loads.
[4] In contrast to the difficulty in measuring groundwater
discharge, quantifying tidal water fluxes through estuarine
cross‐sections is relatively straightforward. Simpson and
Bland [2000] first detailed the use of shipboard ADCPs
to calculate tidally varying discharge in estuarine channels;
Ruhl and Simpson [2005] further described methods to gen-
erate continuous time‐series of tidal flows in tidally affected
channels. While these methods are accurate (±5%) for
instantaneous tidal flows, extracting the mean (residual flow
due to freshwater) can be difficult due to the small ratio
of freshwater flow to instantaneous tidal flow. Ganju and
Schoellhamer [2006] found that a tidal velocity bias of
0.01 m/s was enough to reduce net freshwater flow estimates
by 50% through a large channel in San Francisco Bay. Quan-
tifying net freshwater discharge with flow measurements can
be improved by measurement of mean salinity in the channel:
as a conservative constituent, the flux of salt through the
cross‐section must balance. This article presents direct esti-
mates of fresh groundwater discharge to a groundwater‐fed
tidal creek and seaward estuary using acoustic measurements
of velocity, spatially intensive salinity measurements, and salt
1
Woods Hole Coastal and Marine Science Center, U.S. Geological
Survey, Woods Hole, Massachusetts, USA.
This paper is not subject to U.S. copyright.
Published in 2011 by the American Geophysical Union.
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L11402, doi:10.1029/2011GL047718, 2011
L11402 1of6
balance calculations. The mechanisms and implications of the
temporal variability are discussed as well as the sensitivity of
the salt balance calculation.
2. Methods
2.1. Site Description
[5] Mashapaquit Creek (Figure 1) is a small tidal creek
(∼1.5 m deep at mean sea level) that drains into West
Falmouth Harbor, Massachusetts, located on Upper Cape
Cod. The creek is bordered by marsh and residential areas.
Groundwater is the largest source of freshwater to coastal
margins on Cape Cod [Valiela et al., 1992]; the largest lens of
the aquifer is centered 15 km northeast of West Falmouth
Harbor. Prior estimates of freshwater loading to Mashapaquit
Creek by Kroeger et al. [2006] used a mass‐balance approach
over delineated water table contours (Figure 1), using average
annual hydrological conditions. In that study the ground-
water discharge to Mashapaquit Creek was estimated to be
0.019 m
3
/s. During the summer of 2010 a suite of instruments
was deployed in Mashapaquit Creek, with the goal of mea-
suring tidal water fluxes, cross‐sectionally averaged salinity,
and fresh groundwater discharge over a spring‐neap cycle.
2.2. Tidal Water Fluxes
[6] Computing water fluxes in tidally affected channels
requires a continuous record of index velocity (v
i
) and
water level (h) and a less‐frequent record of channel‐average
velocity (v
ca
) and channel area (A) over some representative
period [Ruhl and Simpson, 2005]. A complete record of v
ca
is computed using the correlation between v
i
and v
ca
, and a
complete record of Ais computed using hand the channel
geometry. The product of v
ca
and Afrom the complete record
yields a continuous record of tidal water fluxes (Q). A Nortek
Aquadopp ADCP and Seabird 39 pressure/temperature (PT)
sensor were deployed approximately 0.1 m above the bed
to measure v
i
and hrespectively from 22 July 2010 to
9 September 2010. The package was deployed in the center of
the channel just landward of the outlet to West Falmouth
Harbor (Figure 1), approximately 400 m seaward of the ter-
mination of the creek. Measurements of v
ca
and Awere col-
lected on 11 August 2010 using a 1200 kHz RD Instruments
Rio Grande ADCP operated from an OceanSciences River
Surveyor catamaran with radio modems and a tagline secured
from bank‐to‐bank (Figure 1). At all sites U.S. Geological
Survey protocols [Mueller and Wagner, 2009] were followed
for ADCP settings, compass calibration, and edge estimates
(due to the inability to measure near banks). The survey was
performed on a spring tide (11 August 2010) when the largest
range of conditions was expected. The index velocity method
relies on the assumption that the relationship between the
index measurement and cross‐sectionally averaged value
during the tidal‐cycle survey is steady over the entire period.
This is true for the index salinity method described below.
2.3. Cross‐Sectionally Averaged Salinity
[7] Generating a continuous record of cross‐sectionally
averaged salinity (s
ca
) requires an index salinity (s
i
) mea-
surement and a less‐frequent record of cross‐sectionally
averaged salinity over some representative time period;
the correlation between the two can be used to estimate con-
tinuous cross‐sectionally averaged salinity. A YSI 6‐series
multi‐parameter sonde was deployed from a floating dock
(depth approximately 1.25 m at mean high water), with the
measurement volume approximately 0.1 m below the water
surface. The instrument was deployed in this fashion due
to prior observations of strong vertical stratification in the
shallow, groundwater‐fed creek. Vertical stratification was
Figure 1. Mashapaquit Creek and the landward portion of West Falmouth Harbor, Massachusetts. Dashed lines indicate
watershed delineation reported by Kroeger et al. [2006], and arrows denote general direction of groundwater flow.
GANJU: ESTIMATION OF GROUNDWATER DISCHARGE L11402L11402
2of6
accounted for by deploying a vertical array of three Seabird
Microcat conductivity/temperature (CT) sensors adjacent to
the dock at 0.33, 0.55, and 0.77 mab. The dock is located
approximately 30 m landward of the ADCP/PT package
(Figure 1). The multi‐parameter sonde was downloaded
and serviced weekly following the guidelines of Wagner
et al. [2006]. The vertical salinity profiles obtained from
the floating dock and vertical array were interpolated to a
uniform depth coordinate (due to variable locations in the
water column relative to each other) and averaged to yield
a depth‐averaged salinity at the edge of the creek. Cross‐
sectionally averaged salinity was estimated by vertical pro-
filing with a multi‐parameter sonde at five equally spaced
locations in the cross‐section. The sonde sampled at 1 Hz,
and downcast data were interpolated to a uniform vertical
coordinate, weighted by total depth of the profile, and aver-
aged. Profiles along a transect (Figure 1) were collected from
a canoe secured to the tagline, intermittently during the ADCP
surveys on 11 August 2010.
2.4. Time‐Dependent Salt Balance and Freshwater
Discharge Calculation
[8]MacCready and Geyer [2010] describe the steady salt
balance of Knudsen [1900] as
Qout ¼sin
DsQRand Qin ¼sout
DsQRð1Þ
where Q
out
is the net water flux on ebb tides, s
out
is the cross‐
sectionally averaged salinity on ebb tides, Q
in
is the net
water flux on flood tides, s
in
is the cross‐sectionally averaged
salinity on flood tides, Dsis the difference between s
in
and
s
out
, and Q
R
is the total freshwater discharge to the estuary.
Equation (1) is typically applied as a spatially varying
description of tidally averaged quantities; MacCready [2011]
derived the analogous time‐dependent version of equation (1)
Qout tðÞ¼sin tðÞ
DsQRtðÞþ 1
DsVds
dt and
Qin tðÞ¼sout tðÞ
DsQRtðÞþ 1
DsVds
dt
ð2Þ
where the last term on the right hand sides accounts for
storage of salt or freshwater within an estuary of volume V.
Rearranging the first relationship to solve for the freshwater
discharge gives
QRtðÞ¼ Ds
sin tðÞ
Qout tðÞ 1
sin tðÞ
Vds
dt ð3Þ
Assuming zero net salt flux eliminates the last term in
equation (3) and allows for the calculation of Q
R
using
instantaneous tidal water flux and cross‐sectionally averaged
salinity data. The tidal water flux and salinity merely need
to be separated based on whether the water and salt flux are
landward or seaward.
3. Results
3.1. Tidal Water Fluxes
[9] An index velocity relationship between v
i
and v
ca
was successfully developed with the continuous and cross‐
sectional ADCP data (Figure S1 of the auxiliary material); the
stage‐area relationship was constructed with a high‐water
channel geometry cross‐section and the continuous water
level data (Figure S2 of the auxiliary material and Figure 2).
1
Figure 2. Time‐series measurements of water level, tidal water flux, cross‐sectionally averaged salinity, and calculated fresh
groundwater discharge (over varying averaging windows). Negative freshwater fluxes may arise from error or more landward
transport of fresh groundwater from other areas of the harbor.
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL047718.
GANJU: ESTIMATION OF GROUNDWATER DISCHARGE L11402L11402
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The computed tidal water flux (Figure 2) shows a maximum
tidal water flux of 5 m
3
/s, with a pronounced spring‐neap
signal. Maximum neap tidal water flux was about half the
spring tide magnitude. The time‐mean of this tidal water flux
is 0.2 m
3
/s (in the seaward direction), which equates to a
representative water velocity of 0.007 m/s (using the average
channel area) and is approximately the same as the error of the
index velocity relationship. The average tidal excursion at
the site was approximately 1 km, larger than the distance to
the creek terminus.
3.2. Cross‐Sectionally Averaged Salinity
[10] The continuous vertical profile data showed that
the creek was strongly stratified on ebb tides, with vertical
salinity gradients exceeding 25 m
−1
on neap tides. Stratifi-
cation was less on spring tides, due to greater tidal energy
and mixing. The relationship between the creek‐edge,
depth‐averaged salinity and the cross‐sectionally aver-
aged salinity during the 11 August 2010 survey was linear
(Figure S3 of the auxiliary material).The time‐series of
computed continuous cross‐sectionally averaged salinity
(Figure 2) shows the largest fluctuations in tidal‐timescale
salinity occur during the spring tide (8 August 2010 –
15 August 2010), when the salinity difference between flood
and ebb tide is maximized.
3.3. Freshwater Discharge Calculation
[11] The tidal water flux and cross‐sectionally averaged
salinity data were used in equation (3) by separating the data
depending on whether the current was flooding or ebbing
(to separate Q
in
from Q
out
, and s
in
from s
out
). This separation
was first performed over the entire time‐series resulting in a
groundwater flux of 0.02 m
3
/s; separation was then applied in
moving windows of 30 and 120 h, to evaluate the temporal
variation of groundwater discharge over varying timescales
(Figure 2). With the shortest 30‐h averaging window (which
essentially represents a “tidally averaged”window), the peak
fresh groundwater discharge was 0.85 m
3
/s during the spring
tide, which is an order of magnitude larger than the period
mean. During neap tides groundwater flux was closer to the
period mean. Rainfall during this period was minimal (http://
www.emc.ncep.noaa.gov) and cannot account for the large
variations in freshwater discharge. Small negative values may
be due to error or landward transport of fresh groundwater
from elsewhere in the harbor.
4. Discussion
4.1. Sensitivity of Salt Balance
[12] The salt balance calculation is most sensitive to sys-
tematic bias in tidal water flux measurements (Table 1), and
is largely insensitive to random errors and biases in salinity
measurement. The sensitivity to tidal water flux bias is due to
the tight coupling between near‐slack tide (i.e. zero tidal
water flux) and maximum fresh groundwater input to the
creek. Synthetically biasing the tidal water flux measure-
ments with a sensitivity analysis essentially shifts the phasing
between slack tide and minimum salinity such that ebb tide
carries fresher water seaward, thus resulting in larger calcu-
lated fresh groundwater discharge. However, identifying slack
tide with acoustic velocity measurements is relatively reliable
in a small channel. Biases in salinity are less critical due to
the Dsterm in equation (3) which is unaffected by bias.
Random errors are minor as they do not shift the phasing of
tidal water flux and salinity, nor alter the net water transport
on flood and ebb tides.
4.2. Fresh Groundwater Discharge and Tidal Pumping
[13] The temporal variability in fresh groundwater discharge
is highly correlated with tidal range (Figure 3). Correlations
were tested between the fresh groundwater discharge and
tidal range (r
2
= 0.79), low‐tide level (r
2
= 0.51), and time
below a range of arbitrary tide levels (r
2
< 0.4). The 120‐h
mean fresh groundwater discharge was used to reduce scatter
in the relationship and highlight the lower frequency varia-
tion with tidal range. The stronger correlation with tidal range
as opposed to low tide level supports the “tidal pumping”
mechanism elucidated by others [Nielsen, 1990; Moore,
1999] whereby increased tidal action extracts more ground-
Table 1. Sensitivity Analyses for Salt Balance Calculation
a
Perturbed Variable Time‐mean Q
R
(m
3
/s) Error Max Q
R
(m
3
/s) Error
Q
in,out
+ 5% random error 0.020 0% 0.85 0%
Q
in,out
+ 5% seaward bias 0.045 125% 0.90 6%
s
in,out
+ 5% random error 0.020 0% 0.84 1%
s
in,out
+ 5% fresh bias 0.021 5% 0.89 5%
Q
in,out
,s
in,out
+ 5% random error 0.021 5% 0.85 0%
Q
in,out
,s
in,out
+ 5% seaward/fresh bias 0.047 135% 0.95 12%
a
Original value of Q
R
was 0.02 m
3
/s, maximum Q
R
was 0.85 m
3
/s.
Figure 3. Relationship between daily tidal range and
mean fresh groundwater discharge over a full spring‐neap
cycle.
GANJU: ESTIMATION OF GROUNDWATER DISCHARGE L11402L11402
4of6
water from the aquifer. Those studies apply to the total sub-
marine groundwater discharge (i.e. fresh and recirculated
saline groundwater); the results of this study show that the
mechanism applies for the fresh groundwater component
alone as well. The large temporal variability over spring‐neap
timescales (from virtually no net discharge to peak discharge
in a few days) has major implications for ecosystems. The
delivery of “new”nutrients in fresh groundwater fluctuates
over short timescales and may confound ecological observa-
tions (e.g. primary production estimates) that are not as tem-
porally resolved.
4.3. Steady Salt Balance Assumption
[14] The steady salt balance assumption which simplifies
equation (3) is supported by two calculations. Firstly, the salt
storage term in equation (3) can be calculated using repre-
sentative values of estuarine volume landward of the cross‐
section (V), the variation in subtidal salinity (ds/dt), and flood
tide salinity (s
in
) for comparison with Q
R
.Vis calculated as
6.25 × 10
4
m
3
assuming an area that includes the channel and
adjacent marsh plain (1.25 × 10
5
m
2
) and an average depth
of 0.5 m; over the neap‐to‐spring transition from 4 August
2010 to 11 August 2010 ds/dt is approximately 1 d
−1
, and s
in
is approximately 20, giving a value of 3.125 × 10
3
m
3
/d or
0.036 m
3
/s. This is an order of magnitude less than the cal-
culated peak Q
R
using either the 30‐h or 120‐h averaging
windows. Secondly, the increased fresh groundwater dis-
charge on spring tides is not caused by storage of freshwater
during neap tide periods (when flushing is reduced) and
subsequent export on spring tides with greater flushing
ability. The total volume of exported freshwater between
6 August 2010 and 17 August 2010 was 1.4 × 10
5
m
3
; over
the area of Mashapaquit Creek’s channel and marsh plain
(∼1.25 × 10
5
m
2
) this would require a subtidal water level
increase of 1 m which is not supported by the data (Figure 2).
In systems with larger embayments, however, storage of
freshwater on neap tides could be an important mechanism
controlling the temporal variability of freshwater export at
the estuary mouth. In this case the data support storage within
the coastal aquifer on neap tides, and enhanced extraction on
spring tides.
5. Conclusion
[15] This study presents a new robust approach to quan-
tifying the fresh portion of coastal groundwater discharge
to estuarine systems. The methods require careful acoustic
velocity and salinity surveys, but rely on a simple time‐
dependent salt balance which can be applied over multiple
timescales. The approach was implemented in a groundwater‐
fed tidal creek and quantified the large temporal variability
in fresh groundwater discharge over the spring‐neap time-
scale. The variability was highly correlated with tidal range,
supporting the tidal pumping mechanism for sequentially
increasing and decreasing pressure gradients within the coastal
aquifer leading to increased discharge of fresh groundwater.
The method is most sensitive to the phasing of tidal water
flux and salinity, though this sensitivity ultimately depends
on the nature of the tidal wave (i.e. standing vs. progressive)
at the estuarine cross‐section. This approach can be used
to complement other methods (gas tracer, high‐resolution
models) by quantifying specifically the fresh portion of
coastal groundwater discharge. The approach can be applied
in any estuarine cross‐section where groundwater is the
dominant source of freshwater, and provides a foundation
for estimating loads of watershed‐derived constituents to
coastal margins.
[16]Acknowledgments. Funding was provided by the USGS Coastal
and Marine Geology Program. Access to Mashapaquit Creek was allowed
by Alan Rottenberg and Michael Jackson. Patrick Dickhudt designed and
fabricated the vertical CTD array. Jon Borden, Jennifer Thomas, Lane Boyer,
and Alex Nunez performed tidal‐cycle surveys with support from Marinna
Martini and Christine Sabens. Christopher Sherwood retrieved precipitation
data and provided motivation for the study. Kevin Kroeger, David Ralston,
and John Warner provided indispensable feedback on this study and manu-
script. Finally, Rocky Geyer provided the critical guidance needed to apply
the methods in a robust manner and was extremely generous with his time.
Any use of trade, product, or firm names is for descriptive purposes only
and does not imply endorsement by the U.S. Government.
[17]The Editor thanks John Largier and an anonymous reviewer.
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GANJU: ESTIMATION OF GROUNDWATER DISCHARGE L11402L11402
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... This comparison yields a less clear picture of DOC sources, but this is to be expected considering the aforementioned complexity of surrounding land uses, potential DOC inputs, and limited mixing at West Falmouth Harbor. Furthermore, spatial representation of δ 13 C values at West Falmouth Harbor (Fig. 4c) show 13 C-depleted samples in the northeastern corner of the harbor, the location of a freshwater culvert discharging groundwater (Ganju, 2011). On the whole, the conservative mixing models used in this study may not be appropriate for a system as complex as West Falmouth Harbor. ...
... West Falmouth Harbor in particular showed a different absorption coefficient to fDOM ratio as compared to the general trend for Barnegat and Chincoteague Bays (Fig. 3). We ascribe this difference to groundwater inputs, which have been shown to have lower CDOM (Shen et al., 2015;Chen et al., 2010;Huang and Chen, 2009) and are substantial in West Falmouth Harbor (Ganju, 2011). Additionally, the extremes of CDOM variability in this study can be explained by differing DOC sources within the estuaries. ...
Colored dissolved organic matter in shallow estuaries: Relationships between carbon sources and light attenuation
Article
Full-text available
  • Feb 2016
  • BG
Light availability is of primary importance to the ecological function of shallow estuaries. For example, benthic primary production by submerged aquatic vegetation is contingent upon light penetration to the seabed. A major component that attenuates light in estuaries is colored dissolved organic matter (CDOM). CDOM is often measured via a proxy, fluorescing dissolved organic matter (fDOM), due to the ease of in situ fDOM sensor measurements. Fluorescence must be converted to CDOM absorbance for use in light attenuation calculations. However, this CDOM–fDOM relationship varies among and within estuaries. We quantified the variability in this relationship within three estuaries along the mid-Atlantic margin of the eastern United States: West Falmouth Harbor (MA), Barnegat Bay (NJ), and Chincoteague Bay (MD/VA). Land use surrounding these estuaries ranges from urban to developed, with varying sources of nutrients and organic matter. Measurements of fDOM (excitation and emission wavelengths of 365 nm (±5 nm) and 460 nm (±40 nm), respectively) and CDOM absorbance were taken along a terrestrial-to-marine gradient in all three estuaries. The ratio of the absorption coefficient at 340 nm (m−1) to fDOM (QSU) was higher in West Falmouth Harbor (1.22) than in Barnegat Bay (0.22) and Chincoteague Bay (0.17). The CDOM : fDOM absorption ratio was variable between sites within West Falmouth Harbor and Barnegat Bay, but consistent between sites within Chincoteague Bay. Stable carbon isotope analysis for constraining the source of dissolved organic matter (DOM) in West Falmouth Harbor and Barnegat Bay yielded δ13C values ranging from −19.7 to −26.1 ‰ and −20.8 to −26.7 ‰, respectively. Concentration and stable carbon isotope mixing models of DOC (dissolved organic carbon) indicate a contribution of 13C-enriched DOC in the estuaries. The most likely source of 13C-enriched DOC for the systems we investigated is Spartina cordgrass. Comparison of DOC source to CDOM : fDOM absorption ratios at each site demonstrates the relationship between source and optical properties. Samples with 13C-enriched carbon isotope values, indicating a greater contribution from marsh organic material, had higher CDOM : fDOM absorption ratios than samples with greater contribution from terrestrial organic material. Applying a uniform CDOM : fDOM absorption ratio and spectral slope within a given estuary yields errors in modeled light attenuation ranging from 11 to 33 % depending on estuary. The application of a uniform absorption ratio across all estuaries doubles this error. This study demonstrates that light attenuation coefficients for CDOM based on continuous fDOM records are highly dependent on the source of DOM present in the estuary. Thus, light attenuation models for estuaries would be improved by quantification of CDOM absorption and DOM source identification.
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... We plotted groundwater temperature data for SGT5W3 with the POI mud and water temperature data to see if groundwater was a possible source of bottom heat. We also plotted groundwater levels and tidal stage in the canal as an indicator of differences in hydraulic head and possible tidal pumping (Ganju 2011). Discharge calculations for the FU-1 weir at POI were obtained from DBHY-DRO (SFWMD website). ...
... The correlation analysis also showed that larger head differences created by low tide cycles were associated with larger temperature inversions. These are good indicators that tidal pumping (Ganju 2011) was advecting warm groundwater into the bottom water during low tidal periods of winter 2009−2010, allowing bottom water to warm independently of the colder surface water. Similar bottom-warming, surface-cooling trends can be seen during the first half of winter 2008−2009, but are absent during the second half, when salinity stratification was no longer present (Fig. 3). ...
Passive thermal refugia provided warm water for Florida manatees during the severe winter of 2009−2010
Data
Full-text available
  • Oct 2015
Haloclines induced by freshwater inflow over tidal water have been identified as an important mechanism for maintaining warm water in passive thermal refugia (PTR) used by Florida manatees Trichechus manatus latirostris during winter in extreme southwestern Florida. Recordsetting cold during winter 2009−2010 resulted in an unprecedented number of manatee deaths, adding to concerns that PTR may provide inadequate thermal protection during severe cold periods. Hydrological data from 2009−2010 indicate that 2 canal systems in the Ten Thousand Islands (TTI) region acted as PTR and maintained warm bottom-water temperatures, even during severe and prolonged cold periods. Aerial survey counts of live and dead manatees in TTI during the winter of 2009−2010 suggest that these PTR were effective at preventing mass mortality from hypothermia, in contrast to the nearby Everglades region, which lacks similar artificial PTR and showed high manatee carcass counts. Hydrological data from winter 2008−2009 confirmed earlier findings that without haloclines these artificial PTR may become ineffective as warm-water sites. Tidal pumping of groundwater appears to provide additional heat to bottom water during low tide cycles, but the associated thermal inversion is not observed unless salinity stratification is present. The finding that halocline-driven PTR can maintain warm water even under extreme winter conditions suggests that they may have significant potential as warm-water sites. However, availability and conflicting uses of freshwater and other management issues may make halocline-driven PTR unreliable or difficult to manage during winter.
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... This comparison yields a less clear picture of DOC sources, but this is to be expected considering the aforementioned complexity of surrounding land uses, potential DOC inputs, and limited mixing at West Falmouth Harbor. Furthermore, spatial representation of δ 13 C values at West Falmouth Harbor (Fig. 4c) show 13 C-depleted samples in the northeastern corner of the harbor, the location of a freshwater culvert discharging groundwater (Ganju, 2011). On the whole, the conservative mixing models used in this study may not be appropriate for a system as complex as West Falmouth Harbor. ...
... West Falmouth Harbor in particular showed a different absorption coefficient to fDOM ratio as compared to the general trend for Barnegat and Chincoteague Bays (Fig. 3). We ascribe this difference to groundwater inputs, which have been shown to have lower CDOM (Shen et al., 2015;Chen et al., 2010;Huang and Chen, 2009) and are substantial in West Falmouth Harbor (Ganju, 2011). Additionally, the extremes of CDOM variability in this study can be explained by differing DOC sources within the estuaries. ...
Colored dissolved organic matter in shallow estuaries: the effect of source on quantification
Article
Full-text available
Light availability is of primary importance to the ecological function of shallow estuaries. For example, benthic primary production by submerged aquatic vegetation is contingent upon light penetration to the seabed. A major component that attenuates light in estuaries is colored dissolved organic matter (CDOM). CDOM is often measured via a proxy, fluorescing dissolved organic matter (fDOM), due to the ease of in situ fDOM measurements. Fluorescence must be converted to CDOM absorbance for use in light attenuation calculations and models. However, this fDOM-CDOM relationship varies among and within estuaries. We quantified the variability in this relationship within three estuaries: West Falmouth Harbor (MA), Barnegat Bay (NJ), and Chincoteague Bay (MD, VA). Land use surrounding these estuaries ranges from urban to developed, with varying sources of nutrients and organic matter. Measurements of fDOM and CDOM absorbance were taken along a terrestrial-to-marine gradient in all three estuaries. The ratio of the absorption coefficient at 340 nm (m−1) to fDOM (QSU) was higher in West Falmouth Harbor (1.22) than in Barnegat Bay (0.22) and Chincoteague Bay (0.17). The fDOM-CDOM absorption ratio was variable between sites within West Falmouth Harbor and Barnegat Bay, but consistent between sites within Chincoteague Bay. Stable carbon isotope analysis for constraining the source of dissolved organic matter in West Falmouth Harbor and Barnegat Bay yielded δ13C values ranging from −19.7 to −26.1‰ and −20.8 to −26.7‰, respectively. Stable carbon isotope mixing models of DOC in the estuaries indicate contributions from marine plankton, terrestrial plants, and Spartina cordgrass. Comparison of DOC source to fDOM-CDOM absorption ratio at each site demonstrates the influence of source on optical properties. Samples with a greater contribution from marsh (Spartina) organic material had higher fDOM-CDOM absorption ratios than samples with greater contribution from terrestrial organic material. Applying a uniform fDOM-CDOM absorption ratio and spectral slope within a given estuary yields errors in modeled light attenuation ranging from 11–33% depending on estuary. The application of a uniform absorption ratio across all estuaries doubles this error. These results demonstrate that continuous monitoring of light attenuation in estuaries requires some quantification of CDOM absorption and source to refine light models.
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... The method assumes that turbulent mixing is the dominant vertical transport in the water column near the water-sediment interface. Head or salinity steady-state mass balances can be used to determine heat or salinity eddy flux, which can be subsequentially transformed to a groundwater flux if there is enough temperature and salinity contrast between groundwater and the surrounding water (Crusius et al., 2008;Ganju, 2011;Hu and Hemond, 2020). ...
Improving the use of radium isotopes and radon as tracers of submarine groundwater discharge
Thesis
Full-text available
  • May 2022
Natural tracers – any substance present in the environment in small but measurable amounts – are a central tool in many scientific disciplines, providing information about processes and systems. In hydrology and oceanography, these scientific objects have become fundamental for understanding processes that are invisible or have disparate temporal scales, some of them differing greatly from the human time scale. One of the research topics that has been most closely linked to the use of natural tracers is the assessment of submarine groundwater discharge (SGD). This process, which involves the discharge of terrestrial and marine groundwater from coastal aquifers to the coastal ocean, has been recognized as an important process modulating the chemical budgets of the coastal ocean, controlling coastal ecosystems, and providing significant ecosystem services to society. The radioactive tracers of radium isotopes and radon represent the most extensive and widespread tool for investigating the magnitude and implications of this process in a wide variety of environments, from small coves to the entire ocean. However, reporting SGD estimates by means of these tracers is complex and requires profound knowledge regarding fundamental steps in the process of quantifying SGD using these radionuclides, from the tracer measurement techniques to the estimation of groundwater and solute fluxes. This Thesis explores the use of radium isotopes and radon as tracers of SGD by addressing a set of research gaps dealing with (1) the analytical techniques for measuring and quantifying these radionuclides, (2) their geochemical behavior in groundwater systems, and (3) their applications as tracers for both groundwater systems and the coastal ocean. Regarding the analytical techniques, this Thesis includes an assessment of the quantification systematics of the RaDeCC system, the most widely used counter for quantifying short-lived Ra isotopes, providing quantification limits and guidelines. The work represents a significant advance in pursuing better and more precise SGD estimates, as well as for any hydrological and oceanographic application of these tracers. Additionally, the Thesis presents a comprehensive analysis of the behavior of Ra isotopes and Rn in groundwater through a novel transport model of radionuclides. This model enables the use of these tracers for identifying SGD pathways, constraining the tracer concentration in the discharging groundwater, and evaluating the groundwater flow characteristics. Finally, this dissertation presents one of the first works evaluating the variations of SGD and associated nutrient fluxes induced by extreme precipitation events using Ra isotopes as tracers. The work emphasizes the relevance that these episodic events may have for coastal ecosystems, their relative significance for annual SGD estimates, and their implications in future climate change scenarios. Overall, the works presented in this Thesis contribute to improving the current knowledge both about the use of radium isotopes and radon as tracers of environmental processes and about the magnitude and implications of submarine groundwater discharge.
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... A salt balance 228 calculation (Ganju, 2011) was used to evaluate the potential contribution of groundwater inputs.229Annually averaged groundwater inputs were 0.07 m 3 s -1 , with a peak of ~1.8 m 3 s -1 (data not 230 shown), similar to previous observations(Ganju, 2011), and below the uncertainty of the 231 discharge measurement itself, and thus too small to affect flux calculations. This salt balance 232 approach further confirms that effects of ET were minimal, and there is no major underlying bias 233 in our modeling of the hydrologic cycle in the system. ...
Hydrologic Export Is a Major Component of Coastal Wetland Carbon Budgets
Article
Full-text available
Coastal wetlands are among the most productive habitats on Earth and sequester globally significant amounts of atmospheric carbon (C). Extreme rates of soil C accumulation are widely assumed to reflect efficient C storage. Yet the fraction of wetland C lost via hydrologic export has not been directly quantified, since comprehensive budgets including direct estimates of lateral C loss are lacking. We present a complete net ecosystem C budget (NECB), demonstrating that lateral losses of C are a major component of the NECB for the largest stable brackish tidal marsh on the U.S. Pacific coast. Mean annual net ecosystem exchange of CO2 with the atmosphere (NEE = −185 g C m² year⁻¹, negative NEE denoting ecosystem uptake) was compared to long‐term soil C burial (87–110 g C m² year⁻¹), suggesting only 47–59% of fixed atmospheric C accumulates in soils. Consistently, direct monitoring in 2017–2018 showed NEE of −255 g C m⁻² year⁻¹, and hydrologic export of 105 g C m⁻² year⁻¹ (59% of NEE remaining on site). Despite their high C sequestration capacity, lateral losses from coastal wetlands are typically a larger fraction of the NECB when compared to other terrestrial ecosystems. Loss of inorganic C (the least measured NECB term) was 91% of hydrologic export and may be the most important term limiting C sequestration. The high productivity of coastal wetlands thus serves a dual function of C burial and estuarine export, and the multiple fates of fixed C must be considered when evaluating wetland capacity for C sequestration.
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... Quantifying the magnitude of SGD is confounded by its spatial and temporal variabilities and the inherent difficulty of observing the discharge, as it occurs at the subsurface (Crusius et al., 2005;Ganju, 2011). Thus far, three basic approaches have been developed to quantify SGD: 1) water balances or hydrogeological models (Kim et al., 2003;Shimizu et al., 2009;Park et al., 2014); 2) seepage measurements (Lee, 1977;Taniguchi et al., 2006); and 3) geochemical tracers (Lambert and Burnett, 2003;Burnett and Dulaiova, 2006;Swarzenski et al., 2007a;Burnett et al., 2008;Peterson et al., 2008;Santos et al., 2009b;Garcia-Solsona et al., 2010;Mejías et al., 2012;Povinec et al., 2012;Cerdà-Domènech et al., 2017;Liu et al., 2017). ...
Evaluation of the spatial distribution of submarine groundwater discharge in a small island scale using the 222Rn tracer method and comparative modeling
Article
Submarine groundwater discharge (SGD) is an important source of freshwater, nutrients, and other chemicals to the coastal water, and has significant impacts and implications for the coastal environment and ecology. Here, we combined geochemical tracers and hydrologic modeling to investigate the spatial patterns and quantities of SGD and submarine fresh groundwater discharge (SFGD) at a small island of western Japan. The results reveal large spatial variability in SGD and SFGD, significant discharge in areas with steep topography, and much lower discharge from low-lying areas. Topographic influences are likely to be a major driver of spatial variability in SFGD. The ²²²Rn mass balance model and two-end-member mixing model were used to estimate the SGD and SFGD rates. The values were ranged from 5.43 to 25.4 cm·d⁻¹ with an average value of 16.2 cm·d⁻¹, and 0.39 to 1.98 cm·d⁻¹ with a mean value of 1.17 cm·d⁻¹, for SGD and SFGD, respectively. The total flux of SFGD was calculated to be 8.49 × 10⁶ m³·yr⁻¹, with 4.37 × 10⁶ and 4.12 × 10⁶ m³·yr⁻¹ in the northern and southern regions of the island, respectively. The results were consistent with the values estimated by the topographic-based model. The ratio of the average annual SFGD to average annual precipitation (3.69 × 10⁷ m³) reached up to 23%, and the magnitude of annual SFGD was found to be similar to the total discharge from nine large rivers. Since the island faces a risk of water shortage, the results of the study can provide a useful insight into developing appropriate groundwater management strategies in this island's habitats.
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... Assuming the annual net tidal flow should be zero, the mean water flow over the study period was shifted to match to the net groundwater flow as described in further detail in Wang et al. (2016). The annual net groundwater flow was estimated to be 0.00024 m 3 s −1 or 7570 m 3 yr −1 calculated using the isohaline method from MacCready (2011) and the salt balance application from Ganju (2011). The correction shift to match the mean flow to the net groundwater flow was 0.0017 m 3 s −1 (5.36 × 10 4 m 3 yr −1 ). ...
Deciphering the dynamics of inorganic carbon export from intertidal salt marshes using high-frequency measurements
Article
Full-text available
The lateral export of carbon from coastal marshes via tidal exchange is a key component of the marsh carbon budget and coastal carbon cycles. However, the magnitude of this export has been difficult to accurately quantify due to complex tidal dynamics and seasonal cycling of carbon. In this study, we use in situ, high-frequency measurements of dissolved inorganic carbon (DIC) and water fluxes to estimate lateral DIC fluxes from a U.S. northeastern salt marsh. DIC was measured by a CHANnelized Optical Sensor (CHANOS) that provided an in situ concentration measurement at 15-min intervals, during periods in summer (July – August) and late fall (December). Seasonal changes in the marsh had strong effects on DIC concentrations, while tidally-driven water fluxes were the fundamental vehicle of marsh carbon export. Episodic events, such as groundwater discharge and mean sea water level changes, can impact DIC flux through altered DIC concentrations and water flow. Variability between individual tides within each season was comparable to mean variability between the two seasons. Estimated mean DIC fluxes based on a multiple linear regression (MLR) model of DIC concentrations and high-frequency water fluxes agreed reasonably well with those derived from CHANOS DIC measurements for both study periods, indicating that high-frequency, modeled DIC concentrations, coupled with continuous water flux measurements and a hydrodynamic model, provide a robust estimate of DIC flux. Additionally, an analysis of sampling strategies revealed that DIC fluxes calculated using conventional sampling frequencies (hourly to two-hourly) of a single tidal cycle are unlikely to capture a representative mean DIC flux compared to longer-term measurements across multiple tidal cycles with sampling frequency on the order of tens of minutes. This results from a disproportionately large amount of the net DIC flux occurring over a small number of tidal cycles, while most tides have a near-zero DIC export. Thus, high-frequency measurements (on the order of tens of minutes or better) over the time period of interest are necessary to accurately quantify tidal exports of carbon species from salt marshes.
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... Due to the complex temporal and spatial variations in environmental conditions, the estimation of the groundwater-surface water exchange flux is difficult, particularly in estuarine tidal flats (Sakamaki et al., 2006). Previous studies showed that nutrient concentrations are often higher in coastal groundwater than in rivers (Valiela et al., 1990;Moore, 1999;Charette and Buesseler, 2004;Kroeger et al., 2007;Ganju, 2011). In some estuaries and bays, nutrient fluxes carried by SGD exceed those from rivers, greatly influencing coastal marine nutrient cycling and primary productivity (Moore et al., 2002;Garrison et al., 2003;Slomp and Van Cappellen, 2004). ...
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... Improvements in technology have made direct field measurement at tidal inlets possible. Acoustic Doppler Current Profiler (ADCP) instruments are used widely to measure discharge in tidal inlets (e.g., Bartholoma et al., 2009;Chen et al., 2009;Ganju, 2011;Martinez-Marrero et al., 2008;Mueller and Wagner, 2009;Qiao et al., 2008;Yang, Kim, and Hwang, 2016). The tidal inlet is an important site at which to conduct ADCP measurements. ...
Field Measurements for Investigating the Dynamics of the Tidal Prism during a Spring-Neap Tidal Cycle in Jiaozhou Bay, China
Article
  • Apr 2018
Xiao, K.; Li, H.; Song, D.; Chen, Y.; Wilson, A.M.; Shananan, M.; Li, G., and Huang, Y., 0000. Field measurements for investigating the dynamics of the tidal prism during a spring-neap tidal cycle in Jiaozhou Bay, China. Journal of Coastal Research, 00(0), 000-000. Coconut Creek (Florida), ISSN 0749-0208. The tidal prism is a crucial environmental index for evaluating the water self-purification capacity in semi-enclosed bay systems, but quantitative studies linking spring-neap tidal variations are few. In this paper, field measurements of the tidal level and tidal flux were conducted in Jiaozhou Bay (JZB) to estimate the hydrodynamic and tidal prism variations during a spring-neap tidal cycle. JZB is located on the south coast of Shandong Peninsula, China, with a narrow inlet of 3.1 km width, which was accessible for continuous shipboard Acoustic Doppler Current Profiler measurements over 15 days. Unfortunately, due to the violent sea induced by typhoon ''Chan-hom,'' the measurements were suspended for 2 days. However, the fluxes during the temporary suspension were reproduced using the significant linear relationship between the inlet fluxes and the changes in tidal level (r 2 ¼ 0.94, P , 0.001), without considering the storm surge. The relationship between tidal range and tidal prism was also linearly correlative (r 2 ¼ 0.69, P , 0.001), indicating that the tidal prism was primarily controlled by the tidal range in JZB. These linear relationships are well verified by historical data in JZB. Compared with previous studies in JZB, the tidal prism has decreased 26% from 1928 to 2008. The decreased intertidal areas, caused by large-scale land reclamations, were likely responsible for the decrease in the tidal prism, which in turn weakened the self-purification capability and triggered the chronic pollutant enrichment. However, the tidal prism increased 4% from 2008 to 2015 as the restoration of water area through protective policies formulated by government. These findings will contribute to designing maintenance strategies for reducing human activity impacts on the health of JZB in the future. ADDITIONAL INDEX WORDS: Water exchange, shipboard ADCP measurement, land reclamation, historical change, tidal flux.
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... A small marsh creek within SLP was selected for this study to produce a relatively wellconstrained tidal flooding and drainage basin area, water budget, and area-normalized constituent flux estimates. The groundwater input to the drainage area is relatively insignificant with an annual flux of 6310 m 3 estimated based on the isohaline method of MacCready (2011) and salt balance as applied by Ganju (2011). SLP has a small, forested watershed with relatively low nutrient load (12 kg N ha 21 yr 21 ) (Kroeger et al. 2006). ...
Intertidal salt marshes as an important source of inorganic carbon to the coastal ocean: Marsh lateral export of inorganic carbon
Article
Full-text available
  • Jul 2016
Dynamic tidal export of dissolved inorganic carbon (DIC) to the coastal ocean from highly productive intertidal marshes and its effects on seawater carbonate chemistry are thoroughly evaluated. The study uses a comprehensive approach by combining tidal water sampling of CO2 parameters across seasons, continuous in situ measurements of biogeochemically-relevant parameters and water fluxes, with high-resolution modeling in an intertidal salt marsh of the U.S. northeast region. Salt marshes can acidify and alkalize tidal water by injecting CO2 (DIC) and total alkalinity (TA). DIC and TA generation may also be decoupled due to differential effects of marsh aerobic and anaerobic respiration on DIC and TA. As marsh DIC is added to tidal water, the buffering capacity first decreases to a minimum and then increases quickly. Large additions of marsh DIC can result in higher buffering capacity in ebbing tide than incoming tide. Alkalization of tidal water, which mostly occurs in the summer due to anaerobic respiration, can further modify buffering capacity. Marsh exports of DIC and alkalinity may have complex implications for the future, more acidified ocean. Marsh DIC export exhibits high variability over tidal and seasonal cycles, which is modulated by both marsh DIC generation and by water fluxes. The marsh DIC export of 414 g C m−2 yr−1, based on high-resolution measurements and modeling, is more than twice the previous estimates. It is a major term in the marsh carbon budget and translates to one of the largest carbon fluxes along the U.S. East Coast.
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Guidelines and Standard Procedures for Continuous Water-Quality Monitors: Station Operation, Record Computation, and Data Reporting
Technical Report
Full-text available
  • Apr 2006
The U.S. Geological Survey uses continuous water- quality monitors to assess the quality of the Nation’s surface water. A common monitoring-system configuration for water-quality data collection is the four-parameter monitoring system, which collects temperature, specific conductance, dissolved oxygen, and pH data. Such systems also can be configured to measure other properties, such as turbidity or fluorescence. Data from sensors can be used in conjunction with chemical analyses of samples to estimate chemical loads. The sensors that are used to measure water-quality field parameters require careful field observation, cleaning, and calibration procedures, as well as thorough procedures for the computation and publication of final records. This report provides guidelines for site- and monitor-selection considerations; sensor inspection and calibration methods; field procedures; data evaluation, correction, and computation; and record-review and data-reporting processes, which supersede the guidelines presented previously in U.S. Geological Survey Water-Resources Investigations Report 00 – 4252. These procedures have evolved over the past three decades, and the process continues to evolve with newer technologies.
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Estimating submarine discharge of fresh groundwater from a volcanic island using a freshwater budget of the coastal water column
Article
Full-text available
  • Mar 2007
The magnitude of submarine fresh groundwater discharge (SFGD) was estimated using a freshwater budget of the coastal water, which was calculated from salinity anomalies off a volcanic island, Jeju, Korea. We obtained high-resolution profiles of salinity from the entire coastal area of Jeju Island using accurate CTD sensors. Lower salinity anomalies relative to the pristine offshore seawaters were observed and assumed to be a result of SFGD on the basis of negligible surface runoffs from the island and the relatively high levels of 222 Rn in the low-salinity waters. The SFGD flux during the study period calculated from a freshwater budget of the water column was 2.5À3.8 Â 10 6 m 3 d À1 , which is close to the long-term average based on the inland water balance. Our study shows that SFGD can be accurately and easily measured using a freshwater budget of the coastal waters off a highly permeable coastal zone with little surface runoff. Citation: Lee, J.-M., and G. Kim (2007), Estimating submarine discharge of fresh groundwater from a volcanic island using a freshwater budget of the coastal water column, Geophys. Res. Lett., 34, L11611, doi:10.1029/ 2007GL029818.
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Calculating Estuarine Exchange Flow Using Isohaline Coordinates
Article
Full-text available
  • Jun 2011
A method for calculating subtidal estuarine exchange flow using an isohaline framework is described, and the results are compared with those of the more commonly-used Eulerian method of salt flux decomposition. Concepts are explored using a realistic numerical simulation of the Columbia River estuary. The isohaline method is found to be advantageous because it intrinsically highlights the salinity classes in which subtidal volume flux occurs. The resulting expressions give rise to an exact formulation of the time-dependent Knudsen relation, and may be used in calculation of the saltwater residence time. The volume flux of the landward transport, which can be calculated precisely using the isohaline framework, is of particular importance for problems in which the saltwater residence time is critical.
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Tidal Dynamics of the Water Table in Beaches
Article
  • Sep 1990
Tidal motions of the water table height inside a sloping beach are investigated via field measurements and theoretical considerations. Only the movements forced by the tide are considered, so a beach with negligible wave activity was chosen for the field measurements. The data show that even in the absence of precipitation the time averaged inland water table stands considerably above the mean sea level. Also the water table at a fixed point inside the beach is far from sinusoidal even though its variation is forced by an essentially sinusoidal tide. This latter effect is due to the boundary condition along the sloping beach face which acts as a highly nonlinear filter. The observed behavior of the water table is explained in terms of perturbation extensions to the classical "deep aquifer solution." One extension deals with the nonlinearity in the interior, the other with the boundary condition at the sloping beach face.
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The discharge of nitrate-contaminated groundwater from developed shoreline to marsh-fringe estuary
Article
  • Nov 1998
As residential development, on-site wastewater disposal, and groundwater contamination increase in the coastal zone, assessment of nutrient removal by soil and sedimentary processes becomes increasingly important. Nitrogen removal efficiency depends largely on the specific flow paths taken by groundwater as it discharges into nitrogen-limited estuarine waters. Shoreline salinity surveys, hydraulic studies, and thermal infrared imagery indicated that groundwater discharge into the Nauset Marsh estuary (Eastham, Massachusetts) occurred in high-velocity seeps immediately seaward of the upland-fringing salt marsh. Discharge was highly variable spatially and occurred through permeable, sandy sediments during low tide. Seepage chamber monitoring showed that dissolved inorganic nitrogen (principally nitrate) traversed nearly conservatively from the aquifer through shallow estuarine sediments to coastal waters at flux rates of 1-3 mmolm-2h-1. A significant relationship between pore water NO3-N concentrations and NO3-N flux rates may provide a rapid method of estimating nitrogen loading from groundwater to the water column.
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Couplings of Watersheds and Coastal Waters: Sources and Consequences of Nutrient Enrichment in Waquoit Bay, Massachusetts
Article
  • Dec 1992
Human activities on coastal watersheds provide the major sources of nutrients entering shallow coastal ecosystems. Nutrient loadings from watersheds are the most widespread factor that alters structure and function of receiving aquatic ecosystems. To investigate this coupling of land to marine systems, we are studying a series of subwatersheds of Waquoit Bay that differ in degree of urbanization and hence are exposed to widely different nutrient loading rates. The subwatersheds differ in the number of septic tanks and the relative acreage of forests. In the area of our study, groundwater is the major mechanism that transports nutrients to coastal waters. Although there is some attenuation of nutrient concentrations within the aquifer or at the sediment-water interface, in urbanized areas there are significant increases in the nutrient content of groundwater arriving at the shore’s edge. The groundwater seeps or flows through the sediment-water boundary, and sufficient groundwater-borne nutrients (nitrogen in particular) traverse the sediment-water boundary to cause significant changes in the aquatic ecosystem. These loading-dependent alterations include increased nutrients in water, greater primary production by phytoplankton, and increased macroaglal biomass and growth (mediated by a suite of physiological responses to abundance of nutrients). The increased macroalgal biomass dominates the bay ecosystem through second- or third-order effects such as alterations of nutrient status of water columns and increasing frequency of anoxic events. The increases in seaweeds have decreased the areas covered by eelgrass habitats. The change in habitat type, plus the increased frequency of anoxic events, change the composition of the benthic fauna. The data make evident the importance of bottom-up control in shallow coastal food webs. The coupling of land to sea by groundwater-borne nutrient transport is mediated by a complex series of steps; the cascade of processes make it unlikely to find a one-to-one relation between land use and conditions in the aquatic ecosystem. Study of the process and synthesis by appropriate models may provide a way to deal with the complexities of the coupling.
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Transport of Groundwater-Borne Nutrients from Watersheds and Their Effects on Coastal Waters
Article
  • Jul 1999
Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers. The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn, increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the characteristically high respiration rates found in enriched waters.
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