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The latest TMDL (Total Maximum Daily Load) by the Minnesota Pollution Control Agency, which measures Dissolved Oxygen in the Minnesota River. Click on pic for the full report.

Total phosphorus by source during low-flow periods (from TMDL report). Click to see full chart.

new tmdl measures health of the lower minn — and suggests remedies

A May 2004 report by the Minnesota Pollution Control Agency (MPCA) lays out ways to decrease polluting phosphorus and increase dissolved oxygen (DO) in the lower Minnesota River.

This MPCA report establishes a TMDL for phosphorus — or Total Maximum Daily Load. According to report co-author Larry Gunderson, the report recommendations are “like putting the Minnesota River on a diet for phosphorus, so the amount in the water decreases.”  The study itself is also called a TMDL.

What's an impaired water body? and what's a TMDL?

In the December 2002 News, we reported on the federal government’s requirements for monitoring and restoring the health of our lakes and streams.

Both the Clean Water Act of 1973 and Environmental Protection Agency rules require that every two years states furnish the EPA with a list of impaired water bodies. 

An impaired body of water is one that cannot fulfill its designated uses, whether for recreation, fishing, drinking, agriculture, or industry.

In Minnesota, the MPCA measures water body impairment via TMDLs of specific pollutants, including phosphorus, fecal coliform bacteria, turbidity (excess sediment), PCBs, and mercury. (For suggestive list of pollutants, go to the EPA web site.)

TMDLs determine the maximum amount of a pollutant that a water body can receive and still meet water quality standards.

But what does a TMDL do?

TMDLs are designed to measure pollution loads … and also to suggest what kinds of reductions are necessary to restore the water body to its designated uses.

In the mid-1980s, the lower Minnesota River (the lower 22 miles of the river below Shakopee) was added to the state’s list of impaired waters. The problem was, specifically, that DO levels during the river's summertime low flow were too low to sustain aquatic life comfortably. And they would have remained too low unless phosphorus loads were reduced.

When phosphorus from urban yards and farm fields enters the river, it nourishes algae blooms; the blooms are devoured by bacteria; the bacteria consume oxygen; the depletion of oxygen threatens aquatic life like fish, amphibians and more desirable plant forms. During low flow in the river, too much phosphorus causes excessive algae blooms.

Is it true that all good things come in phases?

The TMDLs that measure and aim to remedy the lower Minn’s water quality problems do indeed come in phases.

Phase I. The first TMDL, issued in 1985, focused on concentrated or “point” sources like the Blue Lake and Seneca wastewater treatment plants on the lower Minnesota; this report resulted in significant upgrades in equipment at the plants and corresponding reductions in biochemical oxygen demand (BOD) on the lower Minn.

Phase II (2004 report). The TMDL report, just released, concentrates on the Minnesota River Basin upstream of Jordan. Whereas the discharges from the big metro wastewater treatment plants are easy to spot and thus regulate, diffuse, “non-point” sources like lawns, farm fields, and septic systems add to the load in ways that aren’t always easy to spot and stop.

In the words of the 2004 MPCA report, authored by Larry Gunderson and Jim Klang, the new TMDL “focuses on achieving [a] 40 percent BOD reduction goal by reducing the high phosphorus loading upstream of the metropolitan area.” In other words, now that improvements have been made at the metro treatment plants, upstream sources too must be controlled.

Gunderson explains the kind of data that his agency works with:

1. Daily measurements of pollutants in the lower Minnesota River taken by the Metropolitan Council, whose river probes communicate with computers and give read-outs. If the DO daily average falls below 5 milligrams per liter (mg/l), the MPCA must investigate the causes and suggest remedies. 

2. Computer modeling studies, which the MPCA and other agencies conduct to simulate real-world conditions in the river and suggest ways to improve pollutant loads.

To date, DO violations have occurred only during summer low flows,  when algae die and accumulate in the dredged and slow-moving downstream part of the river basin. “The lower Minnesota River,” says Gunderson, “acts kind of like a lake, where water slows down, so there’s more time to produce algae and where bacteria use the oxygen in the water.”

Phase III. In phases I and II, the MPCA looked at the big plan: first, at the point sources of pollution in the lower Minn, then to the whole Minnesota River Basin.

In Phase III, the last, multi-year phase of the plan, the agency will focus on individual major watersheds throughout the Minnesota River Basin and suggest how they might lower their phosphorus at both point sources, like wastewater plants, and non-point sources, like farm fields.

Phase III will also factor in other studies, as on Minnesota River turbidity and excess nutrients in Lake Pepin, in order to address pollution during low-flow periods.

What solutions does the new Phase II report suggest?

Over the next 10 years, Gunderson indicates, the MPCA will seek to increase the amount of oxygen available in the river. As a result of extensive computer modeling and conversations with stakeholders — including farmers, conservation districts, counties, municipalities, and citizens —the agency will address the dissolved oxygen problem by enlisting stakeholders to help

1. Decrease the amount of phosphorus that enters the river and
2. Increase flows so that low flows happen less frequently and for shorter periods 

Decreasing phosphorus

The MPCA's computer models indicate that 1,240 pounds of phosphorus per day is generated in the Minnesota River Basin at low flow. To reach the new DO standard will require reducing phosphorus to 752 pounds per day, the Total Maximum Daily Load.  To achieve this goal, the MPCA is drafting a watershed permit that will require the 40 largest continuously discharging wastewater treatment plants to lower phosphorus.

The agency proposes working with both treatment plants and other stakeholders to reduce phosphorus in these specific ways:

• Reducing phosphorus loads from these 40 largest wastewater treatment facilities collectively by 51 percent in 10 years
• Requiring wastewater treatment facilities to conduct feasibility studies for reducing phosphorus in their effluent by 30 to 50 percent
• Point-point trading to reduce phosphorus loads by 35 percent in 5 years (this means that a point source without high-tech equipment, in a smaller community, might pay a larger plant to assume some of its phosphorus reduction load)
• Working with septic system owners to reduce noncompliance by 90 percent
• Reducing phosphorus in stormwater runoff, whether or not the community’s storm sewers are separated from waste water sewers

Increasing flow

Dissolved oxygen decreases during low flow periods when agricultural runoff is at a minimum. Still, farmers can help improve ground water quality by instituting various Best Management Practices (BMPs) like

• Leaving crop residue on the topsoil, like soybean stubble and cornstalks
• Making sure the openings of surface drainage tiles remain open

If farmers observe these BMPs, then rain water will not run off directly into the river. Rather, it will percolate into the soil and be temporarily stored as groundwater. During future low flow periods in the river, this groundwater will seep into the river via springs and thus increase the low flows.

With the cooperative activities of cities, farmers, industry, and the general public, we can reduce phosphorus and BOD enough to meet the dissolved oxygen water quality standard.