Drought helps scientists study river water quality

Low-flow river data is critical to water quality studies

While the lack of rain this summer has not been good news to farmers, it’s been helpful to scientists working on water quality studies on the Minnesota and Mississippi Rivers.

Diver with equipment entering the Minnesota river

Diver Philip Murphy prepares to plunge into the very murky Minnesota River to check the operation of a chamber used to measure the rate at which microorganisms in sediment at the river bottom consume oxygen.

Flow levels on the Mississippi River at Prescott first hit the target level of 8,000 cubic feet per second (dashed line) for the low-flow study in mid-July.

LARGER CHART

Springs on the banks of the Minnesota River near Jordan MN

Numerous groundwater seeps or springs on the banks of the Minnesota River near Jordan are exposed during low flows.

Scientists at several agencies, including the Metropolitan Council, are working cooperatively to develop computer models that will allow the Minnesota Pollution Control Agency (MPCA) to determine pollutant discharge limits necessary to maintain the health of both rivers. The Council also needs the models to plan for potential wastewater treatment plant expansions or new plant location and size. To develop accurate models, the scientists need data collected during a variety of flow conditions.

The Council has historical data from its ongoing river monitoring program, including data from the drought of 1988-89. But more detailed data were needed to develop reliable models of the interaction of the wide variety of parameters and processes that impact water quality. Since the data collection for the new computer models began, flows have been average or higher.

Agencies had been waiting for low flows

“The low-flow period couldn’t have come at a better time for us,” said Cathy Larson, an environmental scientist with the Council. Monitoring for the Mississippi River model began in 1990, shortly after the 1988 drought. The Council has since kept funds in reserve for low-flow monitoring, with plans and teams in place to take advantage of low flows when they occurred. Data collection started on July 24 and is scheduled to end on September 30.

The Council, with assistance from the MPCA, moved into high gear in July when flows on the Mississippi at Lock and Dam No. 3 in Red Wing started to move toward the target of 8,000 cubic feet per second (cfs) and flows in the Minnesota River approached 2,000 cfs. Water samples are being drawn at roughly double the number of sites on the two rivers. The samples are being collected more often, and more pollutants and other parameters are being measured.

Lake Pepin: where the pollutants settle

Excess nutrients and sediment are major problems for water quality in the Minnesota River, and those problems become the Mississippi’s when the two rivers join in the Twin Cities. The urbanized metropolitan area adds its own share of pollutants to the river. And those pollutants end up in Lake Pepin, a 40-square-mile stretch of the river centered at Lake City on the Mississippi.

The lake is slowly filling up due to sedimentation, and the sediments release phosphorus into the lake. Phosphorus also continues to flow into the lake from upstream. Phosphorus contributes to more frequent and severe algae blooms, which deplete oxygen needed by fish and other aquatic life.

Major sources of phosphorus include agricultural and urban runoff, failing or nonconforming septic systems, and wastewater treatment plant effluent. Between 2004 and 2006, the Council reduced the phosphorus being discharged from the massive Metro Plant by 80 percent. Region-wide, the Metropolitan Council’s treatment plants are discharging half of the phosphorus they did two years ago. All the plants meet their current state and federal permit limits.

Agencies engaged in concurrent studies

The low-flow study is one of several special studies being conducted in order to refine the models and ultimately, the pollutant limits. Other studies on the Minnesota River are the impact of groundwater inflows, the annual sediment and nutrient budget of the Minnesota River, the rates of oxygen gain and loss in the river, and other factors that affect water quality. In Lake Pepin, the release of phosphorus from the sediment bed will be measured. The MPCA is planning additional research projects in the Lake Pepin watershed, such as identifying sources of sediment to the rivers.

The effort has been a model of interagency cooperation, Larson said. Participating in data collection and/or analysis in one or more of the studies are the Council, MPCA, Army Corps of Engineers, Minnesota Department of Natural Resources, Lower Minnesota Watershed District, Metropolitan Airports Commission, and U.S. Geological Survey, among others.

The MPCA will use the models to determine the pollutant limits, known technically as Total Maximum Daily Loads, or TMDLs, that will be placed on a variety of pollutant sources in both watersheds. The Council plans to use the lower Minnesota River model for wastewater facility planning for future growth in the southwestern communities of the metro area.