United States
Environmental Protection
Agency
Chesapeake Bay
Program
Annapolis MD 21401
Research and Development
EPA-600/S3-83-036 Sept. 1983
&ERA Project Summary
Evaluation of Management
Tools in the Occoquan Watershed
Barron Weand and Tom Grizzard
From May, 1979 to May, 1981, nine
water quality monitoring stations were
operated in small catchments in the
Occoquan Watershed of Northern
Virginia. The study sites incorporated
different land uses (pasture land, corn
croplands, suburban developments,
and forest) as well as contrasting
management approaches (heavy versus
light grazing, no-till versus minimum-
till cropping, detention ponds). Water
samples were routinely analyzed for
total suspended solids, ammonia
nitrogen, total Kjeldahl nitrogen,
oxidized nitrogen, ortho-phosphorus,
total soluble phosphorus and total
phosphorus. Meteorological records
were also kept during the study period,
and collections of dryfall and
precipitation were routinely analyzed.
Loading rates, calculated as
kilograms per hectare per centimeter
precipitation, indicated that the heavily
grazed pasture site generally exhibited
the highest pollutant concentrations.
The forested site and lightly grazed
pasture typically generated the least
pollutant export. Significant differ-
ences were observed between the no-
till and minimum-till croplands. The
greatest differences were in the
transport of soluble nutrient forms.
Results of measurements at the
stormwater pond were sometimes
ambiguous, but evidence was found
that proper maintenance of such a
structure greatly improves its
efficiency.
Measurements of atmospheric
pollutant loadings indicated that the
greater proportion generally came from
wetfall. Annual loadings for various
constituents were calculated. The
existence of acid rain in the study area
was confirmed repeatedly, and based
upon indirect evidence, the source was
hypothesized to be sulfur oxides.
This Project Summary was devel-
oped by EPA's Chesapeake Bay
Program. Annapolis, MD. to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
This study was designed to character-
ize nonpoint source pollution from
various land-use areas within the
Occoquan Watershed and to provide a
basis for the comparison of selected
management practices. The data base
was also intended to be used in the
calibration of a mathematical pollutant
transport model. Actual data collection
began on May 15, 1979, and continued
through May 31, 1981.
Procedures/Methodology
Nine monitoring stations were
established in small watersheds,
primarily in agricultural areas. Drainage
areas appropriate to the established
criteria were selected with the assistance
of the U.S. Cooperative Extension (CES),
the U.S. Soil Conservation Service (SCS),
and the Virginia Division of Forestry
(VDF).
Site one was a heavily grazed pasture,
as evidenced by visible erosion and loss of
vegetative cover. The soils in this 12.7-ha
drainage area are moderately well
drained and relatively inefficient in
producing runoff. Whereas the upper
reaches of the drainage showed a four
percent average slope and better cover
vegetation, the lower drainage was
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characterized by a six percent average
slope and thin, poorly established
vegetation.
Site two was a no-till corn field of 10.8
ha, which was located on the same farm
as sites one, three, and four. Soils similar
to those of site one were predominant,
and a relatively uniform slope of 8.5
percent was evidenced over the drainage
area. Drainage from this area entered the
pond. After thefirst season of monitoring,
the management practice at this location
was changed to a minimum-till approach.
Site three was a small area of heavily
grazed pasture immediately adjacent to
the site two drainage area and upstream
of a farm pond. The drainage area here
was 4.5 ha, with a uniform slope of about
10 percent. The primary function of this
station was to provide data on input to the
farm pond.
Site four was located immediately
below a farm pond and received runoff
from sites two and three. Water flowed
from the five-ha pond by means of a 10-
cm diameter riser pipe. Pool height in the
pond varied seasonally according to
precipitation, and at times even the
emergency spillway was overtopped.
Total drainage area to this lower.station
was 20.8 ha.
Site five was a lightly grazed pasture of
7.6 ha. The average slope was 3.5
percent, and canopy heights of over 60
cm were observed late in the growing
season. This site was paired with site one
in an attempt to evaluate the effects of
different management practices on
pollutant loadings.
Site six was originally set up at a no-till
corn cropland but had to be abandoned
shortly thereafter because of a change in
management practices and problems
with landowner cooperation. No data
from this site were recorded.
Site seven was a 27.6-ha drainage that
collected runoff from a suburban
townhouse development via a 183-cm
(72-inch) corrugated metal pipe. The area
was approximately 90 percent
townhouse development and 10 percent
open land. Flow from this drainage
proceeded to a dry stormwater manage-
ment pond.
Site eight provided a measure of the
outflow from the stormwater
management pond. Drainage to the pond
included an area not measured at site
seven, for a total drainage area of 35.7 ha.
A perforated riser pipe provided for
detention in the pond, with the effluent
passing through a concrete conduit.
Together, sites seven and eight provided
the potential for compiling a pollutant
transport mass balance for the
stormwater pond. These two sites thus
form a management pair.
Site nine contained 30.6 ha of
hardwood forest. Because the area was
relatively undisturbed, this site was
selected to provide data to represent pre-
development conditions. The average
slope in the watershed was 9.4 percent.
Good under-canopy vegetation and a
thick layer of litter reduced runoff
potential at this site.
Site ten contained 10.4 ha of corn,
representing minimum-till management.
The average slope here was about 3.4
percent. This site was paired with site two
in order to compare effects of
management practices on pollutant
loadings.
Each monitoring station was
instrumented to provide data on
precipitation and runoff and to collect
runoff samples during storm events. At
one station, samples of atmospheric
fallout were collected. Meteorological
parameters — including solar insolation,
mean wind speed, net evaporation,
temperature, and relative humidity --
were also measured at this station.
Most of the monitoring sites were fitted
with a type-H flume for primary flow
control. Continuous stage measurements
were recorded using pressure transducer
type flowmeters. In addition to this, each
site also contained an automated sampler
for the collection of discrete samples and
a tipping bucket raingage, which
recorded rainfall in increments of 0.25
mm (0.01 inches) of precipitation.
The focus of the analytical efforts was
on nutrient forms. The following
determinations were routinely made:
total Kjeldahl nitrogen (TKN); soluble
Kjeldahl nitrogen (SKN); ammonia
nitrogen (NH3-N); oxidized nitrogen (ox-
N), or combined nitrate and nitrite
nitrogen; total phosphorus (TP); total
soluble phosphorus (TSP); and total
suspended solids (TSS). Other analyses
were also performed but with less
frequency than those identified above.
These included analyses for biochemical
oxygen demand (BOD), chemical oxygen
demand (COD), lead, zinc, pesticide and
herbicide concentrations, and various
soil parameters.
Results/Conclusions
During the study period (May 1979
through May 1981), a total of 245 storm
events were monitored. The distribution
of monitored events was uneven due to
varying precipitation patterns and differ-
ing hydraulic efficiencies at each site.
Both median values for pollutant
conce, itrations and loadings measured at
the various sites were used for
comparison. The use of whisker and box
plots was incorporated to provide a better
interpretation of the data distribution.
It was determined that the cropland
sites (sites two and ten) produced
relatively high nutrient concentrations in
stormwater runoff. Pollutant loads were
also found to be higher at site ten. The use
of commercial fertilizers and animal
manure on such lands obviously
contributed to the observed nutrient
levels.
As expected, the hardwood forest (site
nine) and the lightly grazed site (site five)
showed both the lowest pollutant
concentrations and loads. The forest
probably showed the lowest levels
because of its abundant ground cover and
canopy. At both sites, the general lack of
soil disturbance within the catchments is
probably basic to the low concentrations
of pollutants.
The concentrations of TSS, TKN, and TP
were generally higher at the heavily
grazed pastureland (site one). This
reflects the erodable nature of the soils in
this catchment. As was the case with the
cropland site (site ten) site one was found
to have high median loadings. The lowest
of these concentrations was at the lightly
grazed pasture site (site five) and at the
forested site (site nine).
Site seven, the suburban site, showed
high levels of both pollutants and
loadings. The median loading of total
nitrogen at this site is as high as or higher
than those levels at the cropland sites.
This of course may reflect the use of
fertilizers in the suburban environment.
The loading rates for total phosphorus
and total suspended solids show a similar
pattern.
The data were variable among the
parameters and from site to site. For
example, site seven was sampled most
often and showed relatively small varia-
bilities. This may reflect the unchanging
nature of the site; the cropland sites (two
and ten), on the other hand, undergo
more intermittent disturbances resulting
in greater variabilities. Total suspended
solids were highest at site seven and
lowest at site two. Of these two sites, site
seven may be indicative of a more
constant input from impervious surfaces,
whereas the agricultural catchmnets
may undergo more intermittent
disturbances resulting in greater
variability.
Some water quality data were collected
irregularly. These data included zinc and
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lead concentrations, biochemical and
chemical oxygen demand measurements,
and pesticide analyses.
Lead was detected only in the suburban
catchment and even then infrequently.
Zinc appeared to be much more
prevalent. The highest concentrations of
total zinc were found at sites one and two,
which were located on the same farm.
Zinc was most consistently detected at
the suburban site but was also found in
all samples analyzed.
Median values of COD were nearly the
same at all sites with the exception of
heavily grazed pasture (site one), where
values were much higher. The forest site
exhibited the lowest COD concentrations.
In general, the BOD values measured
were low. This may be due in part to
constituents in the runoff (e.g., heavy
metals or pesticides), which could inhibit
bacterial growth. Analysis of filtered and
unfiltered samples indicated that more
than half of the BOD measured was
soluble. Analysis of inhibited and
uninhibited samples indicated
carbonaceous BOD represented 70 to 90
percent of the total.
During the course of this study two sets
of samples were collected specifically for
analysis of pesticides and herbicides. The
initial results were of some concern
because relatively high concentrations of
polychlormated biphenyls (PCBs) were
reported.
A broader scan for pesticides and
herbicides was carried out seven months
later. PCB levels in these samples were
noticeably lower. None of these samples
was filtered, so that the variations
observed in duplicate analyses might be
due to differences in the suspended
matter included in each individual sample.
Comparison of management practices
indicated that the heavily grazed pasture
consistently produced greater pollutant
concentrations than the lightly grazed
pasture. Statistical differences in
pollutant concentrations were found in
the TN, TSN, TKN, ox-N, TP, and TSS. The
similarity of soils and hydraulic efficien-
cies at these two sites underscored the
effect of management practices on
pollutant transport.
In comparing the no-till and minimum-
till cropland site, statistically significant
differences in observed concentrations
were indicated with TN, TSN, ox-N, OP,
and TSP. These differences were related
primarily to soluble nutrient forms.
During the study, site two was converted
from no-till management to a minimum-
till approach. The data suggest that both
pollutant concentrations and hydraulic
efficiencies increased under minimum-
till management.
Initial study of the farm pond indicated
removal efficiencies for suspended solids
and nutrients to be over 85 percent.
However, allowance was not made for
pond storage capacity in these estimates.
After a survey was conducted to establish
that capacity, an ensuing drought
precluded additional estimates.
Concentration data alone, however,
indicated high removal efficiencies for
TSS and TP (85 percent and 86 percent,
respectively) and a relatively low removal
efficiency of 34 percent for TN.
Although 27 paired storms were
monitored at the suburban detention
pond site, the results were often
contradictory. Because a satisfactory
water balance could not be routinely
made between the monitoring stations
used, it was impossible to compare
pollutant loadings with adequate confi-
dence.
Evidence regarding the cleaning of the
stormwater pond suggests that removal
of most pollutants was greater after the
maintenance activity was completed.
Atmospheric loadings were measured
using a wetfall/dryfall collector located in
an agricultural setting. Annual pollutant
loadings from wetfall were commonly
found to exceed those from dryfall. More
solids deposition resulted from dryfall--
over 60 percent of the total load of 96.1
kg/ha/yr. Nutrient loadings were found
to be 16.82 kg/ha/yr for TN and 0.651
kg/ha/yr for TP.
The pH of precipitation measured
during this study was ordinarily in the
range to warrant the use of the term acid
rain. The range of pH values observed
was from 3.2 to 6.1, with a median value
of 3.8. Indirect evidence suggested
that sulfates were the causative factor.
The small catchments studied were all
within the Occoquan Watershed. Similar
studies of larger basins within the
watershed were simultaneously
performed under the continuing mission
of the Occoquan Watershed Monitoring
Program. Comparison of these results
indicated generally higher pollutant
concentrations at the small catchments.
Unit area loads, however, were generally
higher at the stream sites, due to the
greater hydraulic efficiencies of the
larger basins.
Barron Weand and Tom Grizzard are with the Virginia Polytechnical Institute.
Manassas, VA 22111.
James Smullen is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Management Tools in the Occoquan
Watershed." (Order No. PB 83-255 687; Cost: $ 19.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. MA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Chesapeake Bay Program
U.S. Environmental Protection Agency
2O83 West Street, Suite 5G
Annapolis, MD 21401
irUS GOVERNMENT PRINTING OFFICE 1983-659-017/7188
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