Minnesota Water Resources Conference - Water Resources Center ...

Concurrent Sessions V 1:15–2:45
Track A: Lower Minnesota TMDL
BOOK OF ABSTRACTS
Wednesday, Tuesday, October 24 28 23
Bridge Over Troubled Waters: Lower Minnesota River Model
Catherine Larson, Metropolitan Council, Cathy.Larson@metc.state.mn.us
The Minnesota River carries high nutrient and sediment loads as it flows through one of only four national
wildlife refuges in a metropolitan area. The lower reach experiences stress from a large agricultural watershed
to the west and rapidly expanding urban area to the east. Two large modeling efforts are addressing
impairments upstream of Jordan and downstream in the Mississippi River, leaving a 40-mile gap in the lower
Minnesota River. The Metropolitan Council coordinated a six-year project with federal, state, and local partners
to develop a water-quality model to fill the gap. The partners designed a comprehensive monitoring program
to support the model, including biweekly sampling of the river, tributaries, and discharges over three years.
Intensive monitoring was added at low summer flows to capture critical conditions. To support specific model
settings, we conducted special studies of oxygen, phytoplankton, nutrient, and sediment dynamics. An overview
and results will be presented.
Phosphorus Dynamics and Loading in the Turbid Minnesota River (USA): Controls and Recycling
Potential
William James, U.S. Army Engineer Research and Development Center, William.F.James@erdc.usace.army.mil; Catherine E.
Larson, Metropolitan Council
Phosphorus dynamics were examined in the lower 40-mile reach of the Minnesota River in relation to hydrology,
loading sources, suspended sediment, and chlorophyll to identify biotic and abiotic controls over soluble
phosphorus (SRP) and the recycling potential of particulate phosphorus (PP) during transport to the Upper
Mississippi River. Wastewater treatment plant contributions as SRP were greatest during low flows and declined
with increasing flow and nonpoint source phosphorus loading. SRP declined during low flow in conjunction
with increases in chlorophyll, suggesting transformation to PP via phytoplankton uptake. During higher flows,
SRP was constant at 0.115 mg/L and coincided with an independently measured equilibrium P concentration
for suspended sediment in the river. PP accounted for ~66% while redox-sensitive PP, estimated using extraction
procedures, represented 43% of the annual PP load. Recycling potential of this load via anoxic diffusive
phosphorus flux was estimated as ~17 mg m -2 d -1 using published regression equations.
Modeling the Hydrodynamics and Water Quality of the Lower Minnesota River Using CE-QUAL-W2
David Smith, U.S. Army Engineer Research and Development Center, David.L.Smith@usace.army.mil; Tammy Threadgill, U.S.
Army Engineer Research and Development Center; Barry Bunch, U.S. Army Engineer Research and Development Center;
Catherine Larson, Metropolitan Council
The U.S. Army Engineer Research and Development Center with assistance from the Metropolitan Council
and partners developed a water-quality model of the lower Minnesota River, Jordan to the mouth, using the
2-D laterally averaged CE-QUAL-W2 model (version 3.5). The objective was to produce a water-quality model
that could be used to forecast changes in water quality as a function of management actions. Our approach
was to develop a comprehensive database of hydrodynamic (discharge, elevation, and temperature) and waterquality
(total dissolved solids, inorganic suspended solids, phosphate, ammonia, nitrate, silica, biological
oxygen demand, three algal groups, and dissolved oxygen) data to assess model performance. Performance was
measured with correlation coefficients, mean error, root mean square error, and absolute mean error statistics
at eight longitudinal locations in the river. Model performance demonstrates that the CE-QUAL-W2 model is a
sound tool for assessing and forecasting water-quality impacts of management actions in the watershed.
Minnesota Water Resources Conference, October 27–28, 2008 68