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Significant Escherichia coli Attenuation by Vegetative Buffers on Annual Grasslands
Tate, Kenneth W, University of California, Davis
Atwill, Edward R, University of California Davis
Bartolome, James, University of California Berkeley
Postprints, UC Davis
E. coli, filter strips, rangeland, California, best management practice, conservation practice, water
A study was conducted to estimate the retention efficiency of vegetative buffers for Escherichia coli
deposited on grasslands in cattle fecal deposits and subject to natural rainfall-runoff conditions.
The study was conducted on annual grasslands in California's northern Sierra Nevada foothills,
a region with a distinct wet–dry season Mediterranean climate. We used 48, 2.0- by 3.0-m runoff
plots to examine the efficacy of 0.1-, 1.1-, and 2.1-m buffers at three land slopes (5, 20, and
35%) and four dry vegetation matter levels (225, 560, 900, and 4500 kg/ha) across 27 rainfall-
runoff events during two rainfall seasons. Buffer width treatments were implemented by placement
of cattle fecal material containing known loads of E. coli 0.1, 1.1, or 2.1 m upslope of the plot
runoff collector. Mean total runoff to total rainfall ratio per plot ranged from 0.014:1 to 0.019:1 and
reflected the high infiltration capacity of these soils. Approximately 94.8 to 99.995% of total E. coli
load applied to each plot appears to be either retained in the fecal pat and/or attenuated within 0.1
m downslope of the fecal pat, irrespective of the presence of a wider vegetated buffer. Relative
to a 0.1-m buffer, we found 0.3 to 3.1 log10 reduction in E. coli discharge per additional meter of
vegetative buffer across the range of residual dry vegetation matter levels, land slope, and rainfall
and runoff conditions experienced during this project. Buffer efficiency was significantly reduced
as runoff increased. These results support the assertion that grassland buffers are an effective
method for reducing animal agricultural inputs of waterborne E. coli into surface waters.
Significant Escherichia coli Attenuation by Vegetative Buffers on Annual Grasslands
Kenneth W. Tate,* Edward R. Atwill, James W. Bartolome, and Glenn Nader
A study was conducted to estimate the retention efficiency of veg-
etative buffers for Escherichia coli deposited on grasslands in cattle
fecal deposits and subject to natural rainfall-runoff conditions. The
study was conducted on annual grasslands in California’s northern
Sierra Nevada foothills, a region with a distinct wet–dry season Medi-
terranean climate. We used 48, 2.0- by 3.0-m runoff plots to examine
the efficacy of 0.1-, 1.1-, and 2.1-m buffers at three land slopes (5, 20,
and 35%) and four dry vegetation matter levels (225, 560, 900, and
4500 kg/ha) across 27 rainfall-runoff events during two rainfall seasons.
Buffer width treatments were implemented by placement of cattle
fecal material containing known loads of E. coli 0.1, 1.1, or 2.1 m
upslope of the plot runoff collector. Mean total runoff to total rainfall
ratio per plot ranged from 0.014:1 to 0.019:1 and reflected the high
infiltration capacity of these soils. Approximately 94.8 to 99.995% of
total E. coli load applied to each plot appears to be either retained in
the fecal pat and/or attenuated within 0.1 m downslope of the fecal pat,
irrespective of the presence of a wider vegetated buffer. Relative to a
0.1-m buffer, we found 0.3 to 3.1 log
reduction in E. coli discharge per
additional meter of vegetative buffer across the range of residual dry
vegetation matter levels, land slope, and rainfall and runoff conditions
experienced during this project. Buffer efficiency was significantly re-
duced as runoff increased. These results support the assertion that
grassland buffers are an effective method for reducing animal agri-
cultural inputs of waterborne E. coli into surface waters.
ATHOGENIC bacteria and protozoa, such as Escheri-
chia coli O157:H7 and Cryptosporidium parvum,
are waterborne zoonotic infectious diseases of public
health concern found on watersheds with intensive and
extensive cattle production systems (Atwill et al., 1999,
2003; Jones, 1999; Renter et al., 2004). Water quality and
public health protection agencies commonly utilize fecal
coliform and generic E. coli as indicators of pathogen
contamination in freshwater systems. Waterborne trans-
port of microbial pathogens and indicators, and resulting
public health risk, is governed by (i) processes that load
a watershed with microbial pollutants, (ii) processes that
attenuate or inactivate microbial pollutant load, and (iii)
the efficiency of hydrologic transport processes which
connect terrestrial pollutants to aquatic components of
the watershed. One strategy for minimizing the transport
of microbial pathogens and bacterial indicators from
animal agricultural operations to surface water is to
create vegetated buffer strips between animal manure
sources and vulnerable surface water supplies (Young
et al., 1980; Dillaha et al., 1989; Castelle et al., 1994;
Younos et al., 1998; Schmitt et al., 1999; Dosskey, 2002).
The attenuation efficiency of vegetative buffers varies
by pollutant (e.g., sediment, nitrate) and depends on site
specific factors such as runoff volume, soil properties, and
buffer management (Castelle et al., 1994; Schmitt et al.,
1999; Bharati et al., 2002; Bedard-Haughn et al., 2004).
Several studies determined that relatively short veg-
etated buffers can remove substantial amounts of water-
borne bovine genotypes of the protozoa C. parvum from
overland flow generated under simulated or real rainfall
conditions (Mawdsley et al., 1996; Tate et al., 2000, 2004a;
Atwill et al., 2002; Davies et al., 2004; Trask et al., 2004).
C. parvum oocysts (eggs) are spherical with a diameter of
4to6mm. Trask et al. (2004) reported recovery in runoff
of 0.6 to 27.2% of C. parvum applied to grass covered soil
chambers with recovery increasing as simulated rainfall
rate increased. Davies et al. (2004) reported similar pat-
terns of C. parvum transport and retention for vegetated
and bare soil conditions on intact soil blocks during
simulated rainfall. Atwill et al. (2002) and Tate et al.
(2004a) reported 1.0 to 3.0 log
reductions in C. parvum
transport per meter of grass buffer under soil box and
simulated rainfall conditions. Results reported from a
series of simulated rainfall, irriga tion, and/or soil–
vegetation condition experiments are mixed on the effi-
cacy of vegetative buffers to attenuate bacteria in runoff
downslope of animal manure application. Enteric bacte-
ria of concern are rod-shaped microorganisms ranging in
length from 2 to 5 mmand0.5to1.5mminwidth.Buck-
house and Gifford (1976) reported a negative association
between distance (m) downslope of cattle fecal material
deposits (fecal pats or fecal pies) and surface runoff fecal
coliform concentrations on rangeland in Utah. Larsen
et al. (1994) reported 83 and 95% reduction in fecal
coliforms in surface runoff from 0.61- and 2.13-m grass-
sod buffers below fresh cattle fecal material. Coyne et al.
(1995) examined the effectiveness of 9.0-m grass buffer
strips to attenuate fecal coliforms entrained in surface
runoff following poultry waste application, and reported
maximum reductions of 43 to 74% for fecal coliform
transport. Coyne et al. (1998) reported trapping efficien-
cies of 75 and 91% for 4.5- and 9.0-m grass vegetated
buffers for fecal coliforms in runoff from poultry waste–
amended soil. Chaubey et al. (1994) found that 3.0- and
9.0-m grass vegetated buffers failed to significantly re-
duce fecal coliform in runoff from plots treated with liq-
uid swine manure. Entry et al. (2000) examined the effect
K.W. Tate, Department of Plant Sciences, Mail Stop 1, One Shields
Avenue, University of California, Davis, CA 95616-8515. E.R. Atwill,
Veterinary Medicine Teaching and Research Center, School of Vet-
erinary Medicine, University of California, 18830 Road 112, Tulare,
CA 93274. J.W. Bartolome, Department of Environmental Science
Policy and Management, 151 Hilgard Hall #3110, University of Cali-
fornia, Berkeley, CA 94720-3114. G. Nader, University of Califor-
nia Cooperative Extension, 142A Garden Highway, Yuba City, CA
95991-5593. Received 28 Apr. 2005. *Corresponding author (kwtate@
Published in J. Environ. Qual. 35:795–805 (2006).
Technical Reports: Surface Water Quality
ª ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
Abbreviations: RDM, residual dry matter; SFREC, Sierra Foothill
Research and Extension Center.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Published online April 3, 2006
of several combinations of vegetation type on total and
fecal coliform concentrations in surface and subsurface
water following direct pulse flow application of swine
wastewater to 4-m-wide 3 30-m-long riparian filter strips
in winter, spring, summer, and fall. This study differed
from other studies referenced in that the buffer received
direct application of the pollutant of concern. The au-
thors concluded that concentrations in the applied waste-
water pulse did not decline as the pulse moved downslope
in the filter strip regardless of vegetation type or season.
Surface runoff was sufficient to overcome infiltration
capacity and discharge from study plots only in winter
Microbial pollution of surface drinking water sources
is a significant management and regulatory issue on Cal-
ifornia’s approximately 7 million ha of annual grass-
lands. These grasslands are used extensively for cattle
grazing during the winter rainfall–growing season, pro-
viding approximately 75% of the forage produced on
California’s rangelands. Approximately two-thirds of
the state’s drinking water reservoirs are located within
annual grasslands (Griffin, 1977; Forest and Rangeland
Resources Assessment Program, 1988). The Mediterra-
nean climate of California’s annual grasslands creates
distinct rainfall and dry seasons. As a result, these grass-
land watersheds generate runoff and transport land-
based contaminants during the period November
through April (Tate et al., 1999; Lewis et al., 2000).
Cattle densities reach their annual peak on these winter
foraging areas during this same time period. The ob-
jective of this project was to assess the efficacy of annual
grassland buffers to attenuate E. coli released from cattle
fecal material deposits and entrained in surface runoff
under natural rainfall-runoff and hillslope conditions.
MATERIALS AND METHODS
The study site was located on the University of California
Sierra Foothill Research and Extension Center (SFREC)
100 km north of Sacramento, CA (398149220 N, 1218179460 W).
Elevation at the study site is 350 m. Climate is Mediterranean
with cool rainfall seasons (November through April) and hot,
dry summers (May through October). Average annual precip-
itation is 650 mm, with approximately 90% falling October
through March (Lewis et al., 2000). Located in the foothills of
the Sierra Nevada mountains, topography at SFREC is hilly
with land slopes ranging from 5 to 45%. Study site soil is a
Sobrante (fine-loamy, mixed, active, thermic Mollic Haplo-
xeralfs)–Timbuctoo (fine, parasesquic, thermic Typic Rhodo-
xeralfs) gravelly loam complex formed over basic metavolcanic
(greenstone) bedrock and classified as Haploxeralfs (Lytle,
1998). This soil extends to a depth of 1.0 to 1.5 m and overlies
relatively massive bedrock. Vegetation on the study site is
composed of annual grasses and forbs such as annual ryegrass
(Lolium multiflorum Lam.), wild oats (Avena fatua L.), soft
chess (Bromus molli s L.), and redstem filaree [Erodi um
cicutarium (L.) L’Her].
Runoff Plots and Surface Runoff Collection
During September 2001 through February 2002, 48 runoff
plots designed to capture surface runoff (overland and litter
flow) during natural rainfall-runoff events were established at
the study site (Fig. 1). Each runoff plot was 2 m in width
(parallel to slope) and 3 m in length (perpendicular to slope),
and was bordered with metal flashing inserted 5 cm into the
soil, with 5 cm of flashing extending above the soil surface to
isolate the plot from external upslope and/or sideslope surface
runoff. A 1-m distance was maintained between boundaries of
all adjacent plots (16 plot blocks at same land slope) to
minimize cross-plot flow and microbial movement. Aluminum
and polyvinylchloride runoff collectors were positioned across
the bottom of each plot to capture surface runoff and collect it
in a sealed polypropylene container, allowing measurement of
discharge volume (L) and enumeration of E. coli discharge
concentration and load. Collectors were designed, tested, and
modified accordingly in the field to eliminate incidental col-
lection of rainfall. Collectors were installed to collect all sur-
face runoff occurring above the litter–mineral soil surface
interface. There was a 0.1- to 4.0-cm depth of litter across the
surface of all plots depending on residual dry vegetation mat-
ter treatment. Surface water discharged from each plot during
each rainfall event occurring during trials (n 5 2) conducted 12
Mar. 2003 to 4 May 2003 (2002–2003 rainfall season) and 14
Dec. 2003 to 2 Mar. 2004 (2003–2004 rainfall season) was
collected, volume recorded, and E. coli discharge determined
for each storm event and rainfall season. Runoff samples were
stored at 48C on collection from the field. An automatic re-
cording tipping bucket rainfall gauge was established at the
site to record the occurrence, duration, rate, and amount of
each rainfall event realized during the study.
Land slope treatments were implemented by establishing
three blocks of runoff plots (16 plots per block) at three lo-
cations approximating 5, 20, and 35% slope, respectively
(Fig. 1). Actual land slope (%) of each plot was measured
following final installation of the plot boundary (metal flash-
ing) and runoff collector. Mean (minimum, maximum, SE
mean) plot slope (%) at each block was 6.1 (4.0, 7.5, 0.2), 20.4
(18.0, 24.0, 0.3), and 33.9 (29.0, 42.0, 0.6) for the 5, 20, and 35%
land slope blocks, respectively.
Residual Dry Matter
One of the predominant grazing management recommen-
dations and/or standards on California annual range is the
achievement of end of dry season (October) residual dry mat-
ter (RDM) levels to promote sustainable forage production
and quality levels, soil surface protection from erosion, and
other benefits (Bartolome et al., 1980, 2002). Residual dry
matter treatments were 225, 560, 900, and 4500 kg/ha. This
range mimicked heavy (225 kg/ha) to no grazing (4500 kg/ha)
intensity for California annual grasslands. The 225, 560, and
900 kg/ha treatments were implemented annually by hand
cutting and removal of vegetation in October. The 4500 kg/ha
treatment represented residual dry matter levels without cut-
ting and removal (no grazing). The RDM treatments were
implemented annually for 5 yr before this study, as part of an
existing experiment examining plant community response to
long-term, consistent RDM treatments. Four replicates of each
RDM treatment were randomly established and maintained
annually in each slope block (Fig. 1).
Within each block of plots, three of the four replicate plots
(plots with same slope and residual dry matter) were spiked
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
796 J. ENVIRON. QUAL., VOL. 35, MAY–JUNE 2006
with cattle fecal material containing generic E. coli at either
0.1, 1.1, or 2.1 m upslope from the collector to evaluate the
effect of buffer width on E. coli pollutant discharge (Fig. 1).
The 0.1-m buffer treatment represented almost direct hydro-
logic linkage between fecal material and the runoff collector
(no buffer). A no fecal material application treatment was
included on the fourth residual dry matter replicate in each
block as a background control, and is considered in this paper
as one level of the buffer treatment (0.1 m, 1.1 m, 2.1 m, and no
feces control). Thus, each combination of RDM and buffer
treatment was present in each slope block. Buffer treatments
were allocated in a stratified random manner (random among
RDM treatment replicate plots within each block).
Fecal Material and E. coli Load Application
There were two trials conducted during the course of this
experiment. The 2002–2003 rainfall season trial occurred from
the period 12 Mar. 2003 to 4 May 2003, and the 2003–2004
rainfall season trial occurred from the period 14 Dec. 2003 to 2
Mar. 2004 (Fig. 2). During the 2002–2003 rainfall season trial
fresh cattle fecal material was applied to all plots on 12 Mar.
2003 (2002–2003 rainfall season spike). During the 2003–2004
rainfall season trial fresh cattle fecal material was applied to all
plots on 14 Dec. 2003, 14 Jan. 2004, and 8 Feb. 2004 (2003–2004
rainfall season spike) (Fig. 2). Generic E. coli is commonly
present in high concentrations in fresh cattle fecal material.
For each application event, a sufficient volume of fresh cattle
fecal material was hand collected directly from the rectums of
cattle into several large containers and thoroughly mixed, and
1-kg loads were randomly allocated for application to each
plot. Mean E. coli concentration in feces used for each
application was determined for every other 1.0-kg spike (n =
36) by dispersing randomly collected aliquots of 1.0 g of feces
in 39 mL of phosphate buffered solution (PBS) using a
rotational mixer for 5 min. The feces–PBS solution was then
serially diluted (10
). The E. coli concentration in
diluted feces–PBS solution was determined by direct mem-
brane filtration and culturing onto CHROMagar EC (Chro-
magar Microbiology, Paris, France) at 44.5 8Cfor24h
(American Public Health Association, 1989).
Table 1 reports total E. coli load (cfu) applied to plots for
each rainfall season. Livestock were excluded from the study
site to eliminate incidental fecal deposition within or upslope
of plots. With the exception of the no fecal material application
plots, each plot received two 1-kg doses (approximately 15-cm-
diameter by 7-cm-deep fecal pats) of fresh cattle fecal material
on each application date. Fecal material was placed in the
center of the plot 0.1, 1.1, or 2.1 m upslope of the collector.
Multiple fecal material applications were required during the
2003–2004 rainfall season to maintain observable concentra-
tions of E. coli across the entire runoff season. The single
n.f. – no fecal material
RDM Treatments (kg/ha)
receiving 1 of 4
buffer, and 1 of 4
5% land slope block, randomized buffer and RDM treatment layout (16 runoff plots)
35% land slope block, randomized buffer and RDM treatment layout (16 runoff plots)
20% land slope block, randomized buffer and RDM treatment layout (16 runoff plots)
Fig. 1. Runoff plot design and layout of buffer and residual dry matter (RDM) treatments for 5, 20, and 35% land slope treatment blocks.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
797TATE ET AL.: E. COLI ATTENUATION BY VEGETATIVE BUFFERS
application late in the 2002–2003 rainfall season (12 Mar. 2003)
was sufficient to maintain observable concentrations until the
end of that rainfall season (Fig. 2).
Enumeration of E. coli Concentration in Runoff
The E. coli concentration (cfu/100 mL) in surface runoff
water was determined by direct membrane filtration and cul-
turing the membrane onto CHROMagar EC at 44.5 C for 24 h
(American Public Health Association, 1989). We observed
that runoff water turbidity (ntu) was closely linked to transport
of suspended and dissolved solids (and thus microbial pol-
lutants) from feces on these plots. Thus, we estimated sample
dilution requirements before filtration based on sample tur-
bidity. Aliquots of several volumes ranging from 1 to 100 mL
were directly filtered for samples with turbidity of ,100 ntu.
Turbid samples (.100 ntu) were serially diluted (10
) and all dilutions filtered and cultured.
Adjustment of E. Coli Concentration for
Variable Hold Times
Variation in time elapsed from surface runoff sample col-
lection and sample processing (hold time) was inherent in this
project due to differences in time of collection and time of
processing caused by storm to storm variation in storm dura-
tion and storm end time. Standard methods require either (i)
sample processing within 6 to 24 h post collection or (ii) de-
velopment of a quantitative method to adjust sample concen-
trations for inconsistency in sample hold time (American
Public Health Association, 1989). We developed a quantitative
relationship between hold time (6, 24, 48, 72 h) and E. coli
concentration (cfu/100 mL) to adjust all concentrations to a
24-h hold time from the mid-point of each storm event. Sample
hold times in this study ranged from 4 to 72 h.
To develop this relationship, surface runoff water was
collected from six plots (two from each block) with a no fecal
material application treatment and thoroughly mixed into a
single container. Background E. coli concentration was deter-
mined via direct membrane filtration and culturing as de-
scribed for runoff water analysis. Following culturing, several
E. coli colonies were randomly selected from the membranes,
suspended, and dispersed in 40 mL of PBS via rotational mix-
ing for 15 min. The concentration of this 40-mL E. coli–PBS
solution was then determined via serial dilution and direct
membrane filtration. Following enumeration of the E. coli–
PBS solution concentration, we used serial dilution of aliquots
from this solution to generate aliquots with concentrations of
, and 10
cfu/100 mL. These con-
centrations represent the range observed during the course of
the study. Five replicates of each concentration were devel-
oped for each of four hold times (6, 24, 48, and 72 h) for a total
of 80 aliquots. Within a concentration group, each aliquot was
randomly assigned to a hold time group. Individual aliquots
were then held at 48C and processed (appropriately diluted,
filtered, and cultured at 44.58C) at 6, 24, 48, or 72 h post gen-
eration according to their hold time assignment.
Analysis for E. coli Hold Time Adjustment
A linear mixed effects analysis (Pinheiro and Bates, 2000)
was used to generate the time-dependent decay coefficient(s)
for E. coli in our source water. The log
concentration of E.
coli was used as the outcome variable, dose and time (i.e.,
duration in hours between water collection in the field and
E. coli enumeration in the laboratory) were set as fixed effects
or covariates, and replicates within a group were set as re-
peated measures or a group random effect to control for lack
Table 1. Total duration, storm number, rainfall, and E. coli load-discharge statistics for the 2002–2003 (12 Mar. 2003 to 4 May 2003) and
2003–2004 (14 Dec. 2003 to 2 Mar. 2004) rainfall seasons.
season Duration† Storms Total rainfall Load‡ Discharge§
2002–2003 53 11 205 1.27 3 10
2.27 3 10
2003–2004 79 16 349 2.88 3 10
4.36 3 10
† Days elapsed between placement of fecal material on plots and last storm sample collected for each year.
‡ E. coli load applied to each plot in cattle fecal material during each rainfall season.
§ Calculated as the mean E. coli discharge of all 48 plots for each season.
3/10/03 3/24/03 4/7/03 4/21/03 5/5/03
Cumulative Rainfall (cm)
12/17/03 1/7/04 1/28/04 2/18/04 3/10/04
line = cummulative rainfall (cm)
dot = runoff collection event
date = fecal application event
Fig. 2. Cumulative rainfall (cm), storm runoff collection timing, and fecal material application dates during the portions of the 2002 rainfall season
(12 Mar. 2003 to 4 May 2003) and 2003 rainfall season (14 Dec. 2003 to 2 Mar. 2004) included in the study period.
798 J. ENVIRON. QUAL., VOL. 35, MAY–JUNE 2006
of independence of E. coli concentration within replicate
samples. A group was comprised of five replicate samples for
each dosage and time. In addition, given marked heterosce-
dasticity of the error term across different strata of the E. coli
dosage, different variance terms were fitted for each dosage
(Pinheiro and Bates, 2000). Level of significance for the
covariate terms was set at a P value of ,0.05, based on a
conditional t test.
To adjust the E. coli concentration in each water sample
tested x hours (t 5 x) after initial time of collection (t =0)toa
single 24-h standard (t 5 24), we first assumed the following
) 5 log
) 1b(t 5 x) 
) was the observed log
E. coli determined x hours (t 5 x) after initial time of col-
) was the modeled log
E. coli at the initial time of collection (t 5 0), and b(t 5 x) was
the fitted decay coefficient(s) generated by the linear mixed
effects model described above. The term b(t 5 x) was allowed
to be a univariate or polynomial term depending on whether
the raw data signified a first or second-order time-dependent
decay process for the E. coli concentration in our source water.
The decay process was for water samples held at approxi-
mately 48C. Once b(t 5 x) was obtained, Eq.  was used to
adjust each sample to a single 24-h standard (t 5 24):
was the fitted or expected concentration of
E. coli at a 24-h standard, EC
was the observed concen-
tration of E. coli determined x hours (t 5 x) after initial time of
collection, and 10
was the expected decay coefficient
adjustment factor raised to the power of 10 which allowed us to
model concentrations of E. coli directly instead of log
Analysis of Treatment Effect
Linear mixed effects analysis was used to determine the
effect of land slope, residual dry vegetative matter, runoff pa-
rameters, and buffer width on E. coli discharge and thus buf-
fer attenuation efficacy (log
reduction) per rainfall season
(n 5 2). This is an analysis approach we have employed suc-
cessfully in similar experiments to evaluate vegetated buffer
efficacy for attenuating C. parvum (Atwill et al., 2002) and
nitrogen (Bedard-Haughn et al., 2004). A forward stepping
approach was used to develop a final model, with P , 0.10 set
as the criterion for inclusion of a variable in the final model.
Final model coefficients were estimated using restricted max-
imum likelihood, and P value for each coefficient was esti-
mated using the conditional t test (Pinheiro and Bates, 2000).
Plot identity was set as a group effect to account for potentially
correlated data induced by repeated measures on the same
experimental unit (plot).
The dependent variable was total E. coli colony forming
units (cfu) discharged per plot per rainfall season. The raw
discharge data were transformed [log
(1 + cfu)] to help nor-
malize and account for heteroscadiscity in residuals. Sample
size was 96 (48 plots 3 2 rainfall seasons). Total rainfall season
E. coli discharge per plot was calculated as the sum of dis-
charges for each storm event of the rainfall season for each
plot. Storm event E. coli discharge per plot was calculated as
the product of concentration (cfu/100 mL) and storm event
runoff (L) for each plot. All calculations and analysis were
conducted on data adjusted for hold time. Independent
variables introduced in the forward stepping analysis were
land slope (%), residual dry matter (kg/ha), buffer treatment
(0.1 m, 1.1 m, 2.1 m, no fecal material application), rainfall
season (2002–2003, 2003–2004), total runoff (L) per plot per
rainfall season, and maximum event runoff (L) per plot per
rainfall season. The quadratic form of residual dry matter
), land slope (land slope
), total runoff (total runoff
and maximum runoff event (maximum runoff event
also tested for inclusion into the final model. All two-way
interactions of each independent variable were tested for
inclusion in the final model.
Adjustments to the Observed
E. coli Concentrations
Figure 3 illustrates the relationship between log
coli concentration and hold time (sample age) for hold
times ranging from 6 to 72 h. The log
E. coli followed a first order decay process, such that the
decay coefficient b from the expected decay coefficient
adjustment factor raised to the power of 10 [i.e., 10
in Eq. ] was 20.00356 (95% CI, 20.0054, 20.0017),
with units of time in hours. The P value was .0.05 for
an interaction term between time and E. coli concen-
tration, indicating that a single decay coefficient could
be used for adjusting E. coli concentrations at t 5 x to a
24-h standard (t 5 24) for all samples collected in this
study, regardless of concentration. The value for this
decay coefficient (20.00356) signifies that for each ad-
ditional hour of holding time at 48C, the concentration of
E. coli in our source water declined by about 0.8%.
Weather Conditions, Storm Events, and Runoff
Figure 2 reports surface runoff collection dates and
cumulative rainfall for the2002–2003 and 2003–2004 rain-
fall seasons from first fecal application date until the last
runoff generating storm of the season. Eleven rainfall-
Sample Age (hr)
E. coli (cfu/100ml)
Fig. 3. Change in mean concentration of E. coli (cfu/100 mL) in
surface water runoff samples held for 6, 24, 48, and 72 h from
collection (time 5 0 h) before processing. Data points represent the
mean concentration of five replicates with an initial concentration
, and 10
cfu/100 mL, respectively. Error bars report
one standard error of the mean.
799TATE ET AL.: E. COLI ATTENUATION BY VEGETATIVE BUFFERS
runoff events were realized and collected following fecal
application on 12 Mar. 2003 for the 2002–2003 rainfall
season, and 16 events were captured over the course of
the 2003–2004 rainfall season following the 14 Dec. 2003
fecal application (Table 1). Table 2 reports summary
statistics for storm event rainfall totals, durations, and
rates realized during the study period. Figure 4 displays
monthly rainfall and maximum daily air temperature
realized during the 2002–2003 rainfall season, the 2003–
2004 rainfall season, and the intervening 2003 dry season
which occurred during this study. Mean monthly rainfall
and maximum daily air temperature from the long-term
record (1989–2004) at SFREC are also included for
reference. Rainfall during April 2003, December 2003,
and February 2004 was above average, while rainfall
during January 2004 was below average. Air temperature
realized during the study period was representative of
During the 2002–2003 rainfall season total surface
runoff ranged from 0.4 to 50.0 L/plot, with a mean of
16.7 L/plot. Maximum single storm event runoff ranged
from 0.3 to 14.1 L/plot, with a mean of 5.2 L/plot. During
the 2003–2004 rainfall season total surface runoff ranged
from 9.7 to 116.2 L/plot, with a mean of 40.4 L/plot;
maximum single storm event runoff ranged from 2.6 to
17.0 L/plot, with a mean of 13.2 L/plot. On average, the
maximum single storm event runoff volume accounted
for 37.0 and 39.3% of total plot runoff during the 2002–
2003 and 2003–2004 rainfall seasons, respectively. On
average, each plot received a cumulative rainfall volume
of 1230 (200 3 300 3 20.5 cm) and 2094 L (200 3 300 3
34.9 cm) during the 2002–2003 and 2003–2004 rainfall
season, respectively. Thus, the mean total runoff to total
rainfall ratio per plot for each season was 0.014:1 and
0.019:1, respectively. These relatively low runoff-to-
rainfall ratios reflect the high infiltration capacity of
these soils, the predominance of shallow subsurface flow
paths and variable source areas in stream flow genera-
tion on these watersheds, and the relatively low intensity
(mm/h) of frontal storm events typical of this region of
California (Lewis et al., 2000).
E. coli Discharge Patterns
Table 1 reports overall mean E. coli discharge per plot
per rainfall season, calculated as the mean discharge of
all 48 plots for each rainfall season. During the 2002–
2003 rainfall season the percent of total E. coli load
Table 2. Storm event duration, rainfall amount, rainfall rate
statistics for the storms occurring during the 2002–2003 (12 Mar.
2003 to 4 May 2003) and the 2003–2004 (14 Dec. 2003 to 2 Mar.
2004) rainfall seasons.
Rainfall season Statistic† Total‡ Duration§ Rate¶
mm h mm/h
2002–2003 minimum 2.29 0.28 0.33
mean 18.24 14.42 2.71
maximum 36.58 39.48 16.14
2003–2004 minimum 1.52 0.22 0.32
mean 21.48 8.90 4.31
maximum 76.96 21.93 12.24
† Minimum, mean, and maximum observed across 11 and 16 storm events
during the 2002–2003 and 2003–2004 rainfall seasons, respectively.
‡ Total storm event rainfall amount.
§ Duration of rainfall during individual storm events in hours.
¶ Mean storm event rainfall calculated as rainfall amount (mm) divided by
rainfall duration (h).
Air Temperature (
2002-03 rainfall season
2003 dry season 2003-04 rainfall season
Fig. 4. Total rainfall (mm) and mean maximum daily temperature (°C) observed for each month of the 2002–2003 rainfall season, the 2003 dry
season, and the 2003–2004 rainfall season with long term monthly means for the period of record (1989 through 2004) at the U.C. Sierra Foothill
Research and Extension Center official weather station (California Irrigation Management Information System, Station #84).
800 J. ENVIRON. QUAL., VOL. 35, MAY–JUNE 2006
applied per plot that discharged as surface runoff ranged
from 0.00 to 5.20%, with an arithmetic mean of 0.22%.
During the 2003–2004 rainfall season the percent of total
E. coli load applied per plot that discharged as surface
runoff ranged from 0.00 to 0.23%, with an arithmetic
mean of 0.02%. These numbers indicate that a signif-
icant portion, conservatively .90%, of the microbial
pollutant load applied to each plot was either retained
within the cattle fecal material, filtered by vegetative
litter and soil surface organic matter, or entered the soil
profile via infiltration.
Figure 5 reports mean E. coli discharge per plot per
season for buffer, RDM, and land slope treatments.
Buffer treatment means reported in Fig. 5 were cal-
culated as the mean of all 48 plots by combining rainfall
season, residual dry matter treatment, and land slope
treatment. Residual dry matter treatment means re-
ported in Fig. 5 were calculated as the mean of all 48
plots by combining rainfall season, buffer treatment, and
land slope treatment. Land slope treatment means
reported in Fig. 5 were calculated as the mean of all 48
plots by combining rainfall season, buffer treatment, and
residual dry matter treatment. The overall treatment
means reported in Fig. 5 indicate an apparent reduction
in total E. coli discharge as buffer width increases from
0.1 to 1.1 to 2.1 m. E. coli were discharged from plots
with no cattle fecal application, indicating that back-
ground levels were not zero. E. coli discharge also ap-
pears to increase as land slope increases from 5 to 35%,
but these mean raw-data values are weighted heavily
toward the data generated by the 0.1-m plots. E. coli
discharge decreases as RDM level increases from 225 to
900 kg/ha, but E. coli discharge then appeared to in-
crease as RDM level rises to 4500 kg/ha (no vegetation
Analysis of Effects of Buffer Width and Associated
Covariates on E. coli Discharge
Table 3 reports the final linear mixed effects analysis
identifying significant relationships between E. coli dis-
reductions, and the various plot treatments.
Figure 6 plots observed E. coli discharge [log
(cfu 1 1)
transformed] against values predicted by the linear
mixed effects model reported in Table 3, allowing eval-
uation of model fit. Figures 7 and 8 graphically display
E. coli discharges and log
reductions as a function of
buffer width, total plot runoff, land slope, and RDM.
Due to the log
transformation, coefficients reported in
Table 3 can be directly interpreted as log
annual E. coli discharge associated with each factor.
Water quality data from the 0.1-m buffer plots in-
dicates that 94.8 to 99.995% of total E. coli load applied
Table 3. Linear mixed effects model for the effect of buffer width
and associated covariates on discharge of E. coli from 2.0-m-
wide by 3.0-m-long annual grassland runoff plots across 11 and
16 storm events during the 2002–2003 and 2003–2004 rainfall
Factor Coefficient† 95% CI P value‡
2002–2003§ 0.00 – –
2003–2004 21.06 (21.76, 20.37) 0.003
0.1 m§ 0.00 – –
1.1 m 22.79 (23.97, 21.60) ,0.001
2.1 m 23.60 (24.86, 22.34) ,0.001
Control¶ 24.55 (25.81, 23.50) ,0.001
RDM (kg/ha) 20.002 (20.0034, 20.0004) 0.017
(kg/ha) 3.8 3 10
(6.9 3 10
, 6.9 3 10
Land slope (%) 0.03 (0.008, 0.059) 0.01
Total runoff (L) 20.004 (20.032, 0.024) 0.76
0.12 (0.046, 0.2) 0.002
Buffer 3 total
0.1 m 3 RO§ 0.00 – –
1.1 m 3 RO 0.037 (0.003, 0.073) 0.04
2.1 m 3 RO 0.031 (20.003, 0.065) 0.07
Control 3 RO 0.034 (0.004, 0.064) 0.02
Intercept 6.32 (5.02, 7.62) ,0.001
† Total cfu of E. coli discharged per plot across all storms occuring during
each rainfall season of the study period (2002–2003 and 2003–2004) was
set as the dependent variable; year, buffer, residual dry matter, land slope,
and runoff were set as fixed and independent effects; plot identity set as
a group effect to account for repeated measures. Coefficients are for
transformed total E. coli discharge [log
(cfu 1 1)].
‡ Significance (coefficient not equal to 0) was determined at a P value of
#0.10, using a conditional t test.
§ Referent condition to which other levels of the categorical factor were
¶ Negative control plots to which no cattle fecal material with E. coli was
b) residual dry matter treatment (kg/ha)
225 560 900 4500
Total E. coli Discharged (cfu)
a) buffer treatment and no fecal control
0.1 m 1.1 m 2.1 m control
c) slope (%)
Fig. 5. Mean total E. coli (cfu) discharged from 48, 2.0-m-wide by 3.0-m-long annual grassland runoff plots across the 2002–2003 and 2003–2004
rainfall seasons following application of a total of 41.5 3 10
E. coli (cfu) in cattle fecal material. (a) Mean discharge for buffer treatments of 0.1-m
buffer, 1.1-m buffer, 2.1-m buffer, and a no fecal application negative control; calculated by combining rainfall season, land slope, and residual dry
matter treatments. (b) Mean discharge for residual dry vegetation matter in October treatments of 225, 560, 900, and 4500 kg/ha; calculated by
combining rainfall season, land slope, and buffer treatments. (c) Mean discharge for land slope treatments of 5, 20, and 35%; calculated by
combining rainfall season, buffer, and residual dry matter treatments. Error bars report one standard error of the mean.
801TATE ET AL.: E. COLI ATTENUATION BY VEGETATIVE BUFFERS
to each plot appears to be either retained in the fecal pat
and/or in the narrow 10 cm of soil for the duration of the
storm season, irrespective of the presence of a wider
vegetated buffer. The significant interaction between
buffer width and total runoff per plot per rainfall season
indicates that the effect of buffer width on E. coli dis-
charge was in part dependent on total runoff (Table 3).
E. coli attenuation for 1.1- to 2.1-m buffer widths com-
pared to 0.1-m buffer width ranged from an additional
2.22 to 0.31 log
reduction, respectively, as total plot
runoff increased from 15 to 65 L (Fig. 6). E. coli dis-
charge from no feces control plots ranged from 4.15 to
lower than 0.1-m buffer plots as total plot
runoff increased from 15 to 65 L. Figure 7 illustrates that
as total runoff increases, the log
reductions in water-
borne E. coli for wider buffers are substantially re-
duced and begin to approach the microbial water quality
from plots having no effective buffer (0.1 m). E. coli
discharge was also significantly associated with maxi-
mum single storm event runoff volume such that each
1-L increase in maximum runoff event runoff resulted
in a 0.12 log
(approximately 32%) increase in E. coli
discharge (Table 3).
E. coli discharge was significantly related to RDM
levels. The relationship is quadratic in nature as evident
by the significance of the quadratic residual dry matter
) in the model. Figure 8 illustrates that
Total Runoff (L)
10 20 30 40 50 60 70
Total E. coli Discharge (cfu)
0.1 m buffer
1.1 m buffer
2.1 m buffer
no feces control
Total Runoff (L)
10 20 30 40 50 60 70
Fig. 7. Relationship of total E. coli discharge (cfu) and log
reduction in discharge due to buffer width and total runoff for storm series observed
during portions of the 2002–2003 and 2003–2004 rainfall seasons contained in the study period. Slope and residual dry matter level set to 20% and
900 kg/ha, respectively.
slope = 0.70
intercept = 1.35
Fig. 6. Observed total E. coli [log
(cfu 1 1)] discharged from annual
grassland runoff plots versus values predicted by linear mixed
effects model which contained year, buffer width, residual dry
vegetative matter, land slope, and runoff as independent factors.
802 J. ENVIRON. QUAL., VOL. 35, MAY–JUNE 2006
E. coli discharge decreased as RDM increased from 225
to 900 kg/ha, but that E. coli discharge increased as
RDM then increased to 4500 kg/ha. E. coli discharge
increased 0.03 log
(approximately 7%) with each
percent increase in land slope. E. coli discharge was
less (91% reduction) during the 2003–2004
rainfall season compared to the 2002–2003 rainfall sea-
son (Table 3).
These results demonstrate the significant capacity of
vegetative buffers to attenuate waterborne E. coli de-
posited in cattle fecal material (fecal pats) on annual
grasslands under natural rainfall and hillslope condi-
tions. Relative to the 0.1-m buffer, it is reasonable to
expect 0.3 to 3.1 log
reduction in E. coli discharge per
additional meter of vegetative buffer across the range of
residual dry vegetation matter, land slope, and rainfall
and runoff conditions experienced during this project.
Moreover, water quality data from the 0.1-m-wide buf-
fer plots demonstrate that the majority of E. coli (94.8
to 99.995%) appears to be retained in the fecal material
for the duration of the rainfall season, irrespective of
the presence of a vegetated buffer. Hence, technical risk
analyses regarding minimum buffer widths for expected
environmental loading rates of these microorganisms
from livestock operations should be adjusted accord-
ingly (i.e., .90% of E. coli deposited on the terrestrial
component of the landscape in cattle fecal pats will not
be entrained in overland flow, presenting minimal water-
borne disease risk to downstream communities).
Under natural hillslope and rainfall-runoff conditions
we found grassland buffers to be relatively more efficient
at attenuating E. coli than reported in much of the exist-
ing literature on bacterial indicators (Larsen et al., 1994;
Chaubey et al., 1994; Coyne et al., 1995, 1998; Entry et al.,
2000). This is not unreasonable given that many of the
existing studies purposely simulated worst case rainfall-
runoff and microbial transport scenarios. We found that
buffer efficacy for E. coli attenuation declined as total
runoff volume increased (i.e., approaching potential
worst case microbial transport conditions where buffers
can fail). Many of the published studies examined liquid
and solid animal waste product application at rates sig-
nificantly higher than typically found on the extensively
grazed annual grasslands represented in our study. Var-
iable reporting of load versus concentration discharge
reductions also complicates direct comparison of study
results. The results of this project agree with recent
Residual Dry Matter (kg/ha)
0 1000 2000 3000 4000 5000
Total E. coli Discharge (cfu)
Residual Dry Matter (kg/ha)
0 1000 2000 3000 4000 5000
Fig. 8. Relationship of total E. coli discharge (cfu) and log
reduction in discharge due to residual dry vegetation matter and land slope for storm
series observed during portions of the 2002–2003 and 2003–2004 rainfall season contained in the study period.
803TATE ET AL.: E. COLI ATTENUATION BY VEGETATIVE BUFFERS
studies of the transport and attenuation of the pathogenic
protozoa C. parvum conducted under simulated rain-
fall and soil–vegetation conditions (Atwill et al., 2002;
Davies et al., 2004; Tate et al., 2004a; Trask et al., 2004).
Atwill et al. (2002) reported a 2 to 3 log
tion in surface and shallow subsurface waterborne
C. parvum oocysts using a 1-m-long soil box with 85
to 99% grass cover and under conditions of 5 to 20%
land slope, repacked soil from the same study site where
this project was conducted (i.e., SFREC site, loam soil),
and rainfall rates of 15 or 40 mm/h. In a soil box exper-
iment using soil and vegetation typical of southern Sierra
Nevada annual grasslands, Tate et al. (2004a) observed
reductions of C. parvum per meter of veg-
etated buffer of 1.44, 1.19, and 1.18 for buffers set at 5,
12, and 20% land slope, respectively. Rainfall application
rate (mm/h) in this soil box study was strongly associated
with oocyst discharge from these vegetated buffers,
resulting in a decrease of 2 to 4% in the log
per meter buffer for every additional mm/h of rainfall
applied to the soil box.
Runoff volume and associated hydrologic transport
capacity is an important factor determining vegetative
buffer efficiency and microbia l discharge on these
annual grasslands. Due to the inherently high infiltration
capacity of these soils and the low intensity frontal sys-
tem storm events typical of the region, the ratio of total
runoff to total rainfall is exceedingly low. Surface run-
off is generated on these plots when sufficient rainfall
has occurred to create saturated antecedent soil condi-
tions. Once these conditions are achieved, significant
runoff and nonpoint-so urce pollutant transport can
occur during single storm events (Tate et al., 1999;
Lewis et al., 2000). Maximum storm event runoff per
rainfall season accounted for almost 40% of total plot
runoff per season, and was a significant factor deter-
mining E. coli discharge and conditions when buffers
can be expected to fail. Total plot runoff per rainfall
season was positively associated with E. coli discharge.
The significant interaction between buffer width and
total runoff volume in determining E. coli discharge
illustrates that buffer efficiency (log
diminished as hydrologic transport capacity increases
(Fig. 7). Surface hydrologic transport capacity is a lim-
iting factor in the discharge of microbial pollutants from
these annual grassland sites, with episodic large trans-
port events likely accounting for the majority of micro-
bial discharge across a rainfall season. This limited
surface hydrologic transport potential is also likely a
major mechanism for the relatively high efficiency of
these annual grasslands to buffer microbial pollutants in
surface runoff during the majority of storm events.
Lower overall E. coli discharge in 2003–2004 com-
pared to 2002–2003 is potentially due to differences in
timing of fecal material application relative to the
subsequent occurrence of storm events (Fig. 2). Kress
and Gifford (1984) report significant reductions in E.
coli release from cattle fecal pats as pat age increased
from 2 to 100 d. Preliminary examination of storm event
E. coli discharge data from this project (data not shown)
indicates that E. coli discharge decreased as fecal mate-
rial age increased. Differences in event rainfall intensity
and duration also exist between years (Table 2).
Residual dry vegetation matter was a significant fac-
tor in determining E. coli discharge (Fig. 8). Recom-
mended RDM levels for the study site would range from
500 to 900 kg/ha depending on slope. E. coli transport
decreased as RDM increased from 225 to 900 kg/ha, but
increased as RDM increased to 4500 kg/ha. The re-
duction in E. coli transport as RDM increased from 250
to 900 kg/ha could be due to reductions in soil surface
infiltration capacity for low RDM level treatments ap-
plied over the five years previous to this study. In annual
grasslands in the southern Sierra Nevada foothills, Tate
et al. (2004b) found that soil surface bulk density, a
measure of soil compaction and inversely correlated to
infiltration capacity, was significantly increased by 0.08,
0.18, and 0.21 g/cm
at sites with long-term (15 yr) RDM
levels of .1100, 670 to 900, and ,450 kg/ha compared to
sites not grazed by cattle for .26 yr. It is further possible
that the mechanism behind the increase in E. coli as
RDM levels increase from 900 to 4500 kg/ha is
ecological rather than hydrological. At the 4500 kg/ha
RDM level there is significant accumulation of vegeta-
tion and litter that moderates fluctuations in tempera-
ture and moisture, thereby providing a relatively moist,
cool, nutrient rich environment in which E. coli could
survive and multiply. Increased survival and replication
of bacteria in high RDM plots could offset increased
infiltration capacity and lead to an increase in E. coli
that discharges off the site.
The results of this and several experiments indicate
the potential microbial pollution risk reduction benefits
of physically establishing vegetated buffers around
drinking water storage reservoirs and their primary trib-
utaries (Mawdsley et al., 1996; Atwill et al., 2002; Davies
et al., 2004; Trask et al., 2004; Tate et al., 2004a). Our
results indicate that light to moderate cutting and re-
moval of vegetation in buffers may reduce E. coli dis-
charge relative to high RDM conditions. Concerns exist
about buildup of excessive wildfire fuel loads, and sub-
sequent public safety risk and liability, on grasslands
along California’s expansive urban–grassland interface
(Fried et al., 2004). Public health concerns exist about
the excessive buildup of natural organic carbon levels
near surface drinking water sources due to potential
subsequent formation of disinfection by-products at
drinking water treatment facilities (Bull et al., 2001;
Jassby and Cloern, 2000; Krasner et al., 1989). The
potential for buffers to serve as sources rather than sinks
for some nutrients is dependent on vegetation harvest
and removal (Bedard-Haughn et al., 2005; Mendez
et al., 1999; Jackson et al., 1988). Collectively, these
issues indicate that prudent management of vegetation
within any fixed buffer system is warranted to optimally
achieve multiple public health and safety benefits near
critical drinking water surface supplies.
This work was supported in part by a grant from Interna-
tional Life Sciences Institute (ILSI) North America Technical
Committee on Food Microbiology. The opinions expressed
804 J. ENVIRON. QUAL., VOL. 35, MAY–JUNE 2006
herein are those of the authors and do not necessarily repre-
sent the views of ILSI North America or the committee. We
thank Donna Dutra, Nikolai Schweitzer, Dean Bedard-Haughn,
Betsy Huang, Maria Das Gracas C. Pereira, Yukako Sado,
Dustin Flavell, Dave Labadie, Martin Beaton, Gary Childers,
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