ADVANCED EUTROPHICATION MODELING OF THE UPPER MISSISSIPPI RIVER.  

Edward J. Garland 1, James J. Szydlik 1, Catherine E. Larson 2, and
Dominic M. Di Toro 1, 3.HydroQual, Inc., 1 Lethbridge Plaza, Mahwah,
NJ 07430; 2 Metropolitan Council Environmental Services, 230 East
Fifth Street, St. Paul, MN 55057; 3 Manhattan College, Manhattan
College Parkway, Bronx, NY 10471.  

An advanced mathematical model has been developed to evaluate the
effectiveness of point and nonpoint source controls for achieving
water quality objectives in Pools 2, 3, and 4 of the Upper
Mississippi River.  The three-dimensional, time variable model
includes linked hydrodynamic, sediment transport, and
eutrophication components.  The eutrophication component is a
descendent of the Chesapeake Bay eutrophication model and
includes a sediment flux submodel, which calculates nutrient
fluxes to the water column in response to the deposition of
particulate organic matter.  In this study, particulate inorganic
phosphorus was added to the eutrophication kinetics to account
for the transport, settling, and resuspension of sorbed inorganic
phosphorus.  Resuspension and settling rates are calculated in
the sediment transport component.  The model was calibrated with
data collected over a range of hydrological conditions during a
12-year period, from 1985 through 1996.  Approximately 90 percent
of phosphorus inputs to the study area enter the Upper
Mississippi River in Pool 2.  The Metropolitan Wastewater
Treatment Plant (Metro Plant), a 220-mgd facility operated by
Metropolitan Council Environmental Services, accounted for
approximately 22 percent of the total phosphorus (TP) discharged
to the study area between 1985 and 1996.  In the low flow year of
1988, however, the Metro Plant contributed almost 50 percent of
the TP load.  Nonpoint sources contributed approximately 50
percent of the TP loads, on average.  In high flow years,
nonpoint sources contribute nearly two-thirds of the TP loads to
the study area; however, in low flow years nonpoint sources
contribute less than 20 percent of the TP loads.  In order to
evaluate the response to nutrient controls, the model needed to
be capable of determining the effect of high phosphorus inputs
from nonpoint sources under highflow conditions, such as 1986, on
subsequent low flow years (i.e., 1987 through 1989).  Sources of
TP to the sediment include settling organic matter and phosphorus
sorbed to nonvolatile suspended solids.  Model results indicate
that the average solids trapping efficiencies of Pools 2, 3, and
4 are 23, 7, and 69 percent, respectively.  Phosphorus trapping
in Lake Pepin is computed in 10 of the 12 years modeled.  In the
low flow years of 1987 and 1988, however, vertical stratification
produced hypoxic or anoxic conditions in the lower waters of the
lake, resulting in substantial releases of inorganic phosphorus
(PO4) from the sediment.  Model results reproduce the dramatic
increase in PO4 through Lake Pepin that was observed during the
summer of 1988.  The calibrated model was used to simulate future
water quality conditions for the next 24 years under several
scenarios of nutrient loading reductions from point and nonpoint
sources.  Three scenarios of point source phosphorus controls
were evaluated, representing point source TP reductions of
between 67 and 89%.  Two nonpoint source control scenarios,
developed by the Minnesota Pollution Control Agency, were run in
combination with the point source reductions.  The nonpoint
source scenarios were developed on a seasonal basis and specified
as a function of river flow.  The magnitude of the nonpoint
source TP reductions ranged from 0 to 43%.  During the summer
months in Lake Pepin, these combinations of point and nonpoint
source phosphorus reductions produce decreases in ambient PO4 of
as much as 50 to 70%; however, decreases in average summer
chlorophyll-a concentrations of more than 15% are only achieved
in the low flow years, equivalent to 1987-1989, and never exceed
26%.  In most years, the projected PO4 levels do not impose
significant nutrient limitations on algal growth.  By imposing a
strict mass balance on both the water column and the sediment,
the modeling analysis includes the effect of current nutrient
inputs, as well as inputs that reached the sediment in prior
years.  The multi-year simulation allows the response of the
system to be computed over a range of hydrologic and point and
nonpoint source loading conditions.  

Keywords:  Upper Mississippi River, eutrophication model,
phosphorus, point source, nonpoint source