United States
Environmental Protection
Agency
Environmental
Research Laboratory
Athens, GA 30605-2720
Research and Development
EPA/600/SR-93/048 May 1993
Project Summary
South San Francisco Bay
Water Quality Modeling and
Waste Load Allocation Study
Pei-Fang Wang, Robert B. Ambrose, Jr., and Kevin J. Novo-Gradac
A waste load allocation modeling
study was conducted in South San
Francisco Bay, California. Relatively
numerous reports on hydrodynamics
and less complete data for water qual-
ity, especially sediment levels, in the
Bay were reviewed for use in the study.
Simulations were based on the premise
that sediments maintain equilibrium
over long periods of time. Copper con-
centrations were simulated using dif-
ferent loading conditions describing
different scenarios. Nontidal transport
results were obtained for suspended
solids, copper, nickel, and lead. The
wide ranges of historical water quality
data were addressed through sensitiv-
ity analysis of unsteady nonpoint
source loads. For demonstration pur-
poses, the domain for tidal simulations
covered only the regions south of
Dumbarton Bridge. The effects of the
reduction of point-source loads over
the past few years and of the droughts
that began in 1987 were simulated us-
ing appropriate loading conditions.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Athens, GA, 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).
Background
In response to a. request from the Cali-
fornia Regional Water Quality Board, San
Francisco Ekty Region, the U.S. Environ-
mental Protection Agency's Center for Ex-
posure Assessment Modeling conducted
a water quality study of the South Bay
portion of San Francisco Bay from April
1991 to December 1992 to support the
determination of waste load allocations
for copper. South Bay, which receives rela-
tively little fresh water, has near-oceanic
salinity characteristics for much of the year
and usually is well mixed. The embayment
is generally shallow with a deep channel
in the middle.
South Bay has very irregular geometry
and bathymetry and poses great model-
ing problems in the representation of tidal
flats. Hydrodynamics data were sufficient
for calibration of tidal and nontidal mod-
els. More solids data were needed for the
calibration of sediment transport and wa-
ter quality models. Both point and non-
point source loading data were available
for simulations predicting copper concen-
trations in South Bay. It is uncertain, how-
ever, what fraction of the nonpoint source
loads is delivered from different source
areas in the Bay.
Because most usable data were not
collected concurrently, extrapolation was
required for modeling purposes. Extrapo-
lation may cause inaccuracies in the setup
of input data and make interpretations cf
the modeling results difficult. While some
of the data can be used for nontidal water
quality simulations, the information is net
sufficient for the verification of more so-
phisticated sediment transport models.
The speciation of copper in the South
Bay was of interest because of increased
concentrations of the metal and its poten-
tial bioavailability. The geochemical model
MINTEQA2 was employed, using histori-
cal data, to assess the degree of impor-
tance of competing factors on dissolved
copper levels and to determine an effec-
Prmted on Recycled Pap<>:
-------
live partition coefficient for the transport
modeling.
Results
MINTEQA2 modeling indicated adsorp-
tion-based control of dissolved copper lev-
els, controlled primarily by iron
oxyhydroxide phases on sediments. A rela-
tively constant partition coefficient of
14,000 L/kg was observed for copper
throughout South Bay. Dissolved organic
carbon may not play an important role in
copper speciation. Small changes in pH,
however, can cause larger changes in the
sorptive properties of iron oxyhydroxide in
South Bay. Any small pH changes, such
as those related to runoff or phytoplank-
ton blooms, could alter the copper parti-
tioning behavior. Better datasets were
needed on pH, the binding capacity of
dissolved organic carbon, and the readily
extractable iron in the sediments.
Water quality in South Bay was investi-
gated under historical flow and loading
conditions using WASP4 to simulate sedi-
ment and contaminant transport. A nontidal
approach was employed to elucidate the
large-scale response of the system to cop-
per, nickel, and lead loadings. A coarse
network of ten water column and ten un-
derlying benthic segments was set up be-
tween Oakland Bay Bridge and the sloughs
south of Dumbarton Bridge.
The modeling strategy involved using
previously determined dispersion coeffi-
cients along with net advective flows from
point and nonpoint sources. Suspended
solids were simulated based on observa-
tions that sediments are in long-term equi-
librium in South Bay, with little net
deposition or erosion. A constant deposi-
tion velocity was specified based on an
earlier study. Resuspension velocities were
calibrated to match historical data on sus-
pended solids in the water column. Parti-
tion coefficients were specified based on
historical data and geochemical consider-
ations.
Simulations of annual average water
quality parameters were conducted using
the annual mean loading and flow data.
Results wera compared with various
datasets spanning the period 1976 through
1990. Calibrated suspended solids lie, in
general, between the medians for dry and
wet weather data collected between 1969
and 1975. The field data exhibit wide varia-
tions that the steady-state simulation does
not capture.
Simulated total copper concentrations
lie within the ranges of field data from the
Oakland Bay Bridge to the area just south
of Dumbarton Bridge. Simulated results,
however, overestimate the Discharger's
Two-Year Study data in the southern-most
portion of the Bay. Simulated dissolved
copper tends to overpredict measured val-
ues, particularly in the southernmost re-
gions. Observed differences could have
been caused by the model parameteriza-
tion for sediment transport, partitioning, or
loading.
Similar simulations were conducted for
nickel and lead. While simulated total nickel
concentrations slightly underpredict ob-
served data, simulated dissolved nickel
follows the data desirably. Simulated total
lead concentrations overpredict observed
data by a factor of about two, whereas
simulated dissolved lead concentrations
overpredict by a large margin. More study
is needed on lead sorption and sediment
transport,
A simulation was conducted using an-
nually varying flow and copper loading
data from 1977 through 1990 in order to
understand the concentration response
dynamics. Observed data are available at
the beginning and end of this period. The
model prediction of very little change in
the northern and middle reaches of South
Bay during this period is confirmed by
data. The historic data south of Dumbarton
Bridge seem to indicate net constant or
perhaps slightly declining copper concen-
trations. The model simulates increasing
concentrations until 1986, and declining
concentrations thereafter, with a small net
increase over the period. Sensitivity stud-
ies indicate that more vigorous sediment
exchange or net deposition in this region
would cause the model to predict a mini-
mal net increase during this period.
Because of the prolonged drought from
1987 through 1991, the model was set up
to simulate this period using greatly re-
duced nonpoint source loads. Steady state
results were compared with data taken
during 1989 through 1991. In the south-
ern reaches of South Bay, reduced flow
and loads resulted in lowered concentra-
tions of sediment, copper, and nickel, and
improved modeling results as compared
with field datn.
Field data in the southern reaches of
South Bay exhibit large temporal variabil-
ity. Tc better understand possible causes
for the high variability, a study of concen-
tration responses to different runoff load-
ing conditions was conducted.
Hypothetical 13.5-hour runoff events
composed of 1/10 and 1/29 of the total
wet weather period flows and loads were
simulated. In some simulations, resuspen-
sion velocities during post-storm periods
were reduced to 75% and 50% of the
annual means for 6 days.
The analysis showed most sensitivity to
changes in loading and sedimentation
south of Dumbarton Bridge. In this area,
runoff loading produced small short-term
concentration pulses. The predicted wet
weather dissolved copper concentration
variations were smaller than the sus
pended solids and total copper responses
which is consistent with the field data
Regions north of Dumbarton Bridge are
not very sensitive to the runoff loadings
due to their very large volumes. Sus
pended solid concentrations in South Bay
are primarily a result of the resuspension
deposition processes.
Tidal variations south of Dumbarton
Bridge were investigated using the three
dimensional estuarine circulation model
HYDRO3D linked with WASP4. The hy
drodynamic model network was composed
of vertically integrated rectangular cells
measuring 200 m by 200 m. Tidal eleva
tions were specified at Dumbarton Bridge
Simulations compared well in ranges and
trends with data at Coyote Creek in the
extreme southern end of the bay.
Tidal flows and volumes were linked
with a 3-segment and a 15-segment
WASP4 model The tidal analysis predicts
the water transport in extreme South Bay
within several tidal cycles. Results show
that mass is conserved through the link
ing process. Variations of concentrations
over a tidal cycle reach more than plus or
minus 15% of the annual mean value.
Possible copper load reductions were
studied for South Bay based on the cali
brated model. To gain perspective on the
gross response of South Bay to waste
loads, future concentrations following elimi-
nation of various loads were examined
under annually averaged conditions. Fol
lowing the elimination of all loads, ap-
proximately 3 years would be required to
bring total copper concentrations in the
southernmost segment down by half. The
decrease of copper in interior segments
proceeds at a much slower rate; the ei
fective half-life in this area is as much as
16 years.
Next, we examined the response of
South Bay to the elimination of various
classes of waste loads, assuming as a
base the calibrated average profile for
1977-1989. Elimination of all loads north
of Dumbarton Bridge has little effect on
the extreme South Bay, whereas elimina-
tion of all loads south of Dumbarton Bridge
should reduce concentrations throughout
South Bay to below 2 u,g/L. Although con-
centrations would decline considerably fol-
lowing the elimination of all point source
loads, nonpoint sources would continue to
cause concentrations above 4 u,g/l_ south
of Dumbarton Bridge.
If drought conditions are taken as a
base, then the response of South Bay to
the elimination of loadings changes. In
-------
this case, the largest contribution is from
point source loads south of Dumbarton
Bridge. Nonpoint source loads from south
of Dumbarton Bridge contribute much less.
If nonpoint source loads during drought
conditions could be maintained, the elimi-
nation of all point source loads would lead
to concentrations below 3 u.g/L throughout
South Bay.
Finally, calculations of copper concen-
trations were made for eight loading sce-
narios of management interest. Given the
numerous assumptions that were made in
order to compensate for lack of data, the
model results are better viewed as predic-
tions of the general type, extent, and term
of response rather than as precise nu-
merical predictions. Following realistic load
reductions, copper concentrations are ex-
pected to decrease at slow rates. The
slowly decreasing copper concentrations
demonstrate that sediment deposition and
resuspension constitute important pro-
cesses controlling the long term transport
of metals in the water column of South
Bay.
Conclusions
With the reduction of point source efflu-
ent concentrations to 2.9 u,g/L and the
reduction of nonpoint source loads by 50%,
the concentrations of copper decrease
most rapidly. After 20 years following load
reduction, total copper concentrations de-
crease by about half, falling below the 4.9
u.g/L standard in regions north of Dumbar-
ton Bridge. Regions south of Dumbarton
Bridge are expected to maintain average
concentrations above this standard. Su-
perimposing the effects of tidal fluctua-
tions and storm pulses, even higher con-
centration excursions are expected.
The effects of reducing the copper loads
from the publicly owned treatment works
discharges south of Dumbarton Bridge
were examined to identify conditions re-
flecting both average and drought non-
point source loads. Reducing the effluent
concentrations to 4.0 u.g/L had about the
same effect as reducing the flows and
loadings by 30%. During drought condi-
tions, average copper concentrations
should fall below the 4.9 u.g/L standard
except in extreme South Bay, where slight
exceedances are expected. During aver-
age nonpoint source years, concentrations
north of Dumbarton Bridge should still re-
main below the standard. Exceedances
below Dumbarton Bridge, however, would
be higher and more widespread.
'U.S. Government Printing Office: 1993 — 750-071/6024 ?
-------
The EPA author, Robert B. Ambrose, Jr. (also the EPA Project Officer, see below)
is with the Environmental Research Laboratory, Athens, GA 30605-2720.
Pel-Fang Wang and Kevin J. Novo-Gradac are with AScI Corporation, Athens,
GA 30605.
The complete report, entitled "South San Francisco Bay Water Quality Modeling and
Waste Load Allocation Study," (Order No. PB93-166981/AS; Cost: $27.00;
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens, GA 30605-2720
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-93/048
------- |