"Site-Specific" Water Standards for Lake Pepin and South Metro Mississippi
The Minnesota Pollution Control Agency (MPCA) has developed state-wide water quality standards for lakes and streams that are used as targets for TMDLs, watershed restoration projects, and sometimes for setting wastewater treatment plant effluent limits. Statewide standards are designed to support beneficial uses of lakes and rivers, such as swimming, fishing, and the health of aquatic species or ecosystems. Occasionally, when working with lakes or rivers with unusual characteristics, the MPCA needs to set "site-specific" standards. This is the case with the South Metro Mississippi and Lake Pepin.
The South Metro Mississippi River is unique in that it bears an enormous burden of sediment from the Minnesota River -- 10 times the load that was discharged in pre-settlement times and 7 times as much phosphorus. Upstream of the confluence with the Minnesota River, the Mississippi River meets the standard for clarity -- the statewide turbidity standard -- with ease. Also, downstream of Lake Pepin, where most of the sediment particles settle out, the Mississippi River meets the state standard and supports a healthy aquatic ecosystem. Between the Minnesota River and Upper Lake Pepin, the river's aquatic ecosystem is severely constrained by turbidity. Extensive research has shown that the statewide turbidity standard is not adequate to support a healthy ecosystem. Hence the need for a site-specific standard.
The Minnesota Pollution Control Agency (MPCA) has developed state-wide water quality standards for lakes and streams that are used as targets for TMDLs, watershed restoration projects, and sometimes for setting wastewater treatment plant effluent limits. Statewide standards are designed to support beneficial uses of lakes and rivers, such as swimming, fishing, and the health of aquatic species or ecosystems. Occasionally, when working with lakes or rivers with unusual characteristics, the MPCA needs to set "site-specific" standards. This is the case with the South Metro Mississippi and Lake Pepin.
The South Metro Mississippi River is unique in that it bears an enormous burden of sediment from the Minnesota River -- 10 times the load that was discharged in pre-settlement times and 7 times as much phosphorus. Upstream of the confluence with the Minnesota River, the Mississippi River meets the standard for clarity -- the statewide turbidity standard -- with ease. Also, downstream of Lake Pepin, where most of the sediment particles settle out, the Mississippi River meets the state standard and supports a healthy aquatic ecosystem. Between the Minnesota River and Upper Lake Pepin, the river's aquatic ecosystem is severely constrained by turbidity. Extensive research has shown that the statewide turbidity standard is not adequate to support a healthy ecosystem. Hence the need for a site-specific standard.
The draft standard that has been developed would reduce suspended solids in the water by about half, and double the density of beneficial rooted vegetation. Technically, the standard calls for 32 parts per million of Total Suspended Solids at Locks and Dams 2 and 3 as a summer average. The U.S. Geological Survey's Long-Term Resource Monitoring Program's bimonthly water quality monitoring data will be used to measure this water quality statistic.
Lake Pepin is very different from most of the lakes for which water quality standards were developed -- glacially sculpted lakes with small watersheds and a very slow rate of water exchange, allowing months or years for the processing of nutrients by aquatic organisms. Lake Pepin was created by an alluvial dam of sand at the mouth of the Chippewa River some 11,000 years ago. It has an enormous watershed, covering half the state of Minnesota, and a very short flushing rate -- ranging from 6 to 60 days, depending on whether the river is flowing high and fast or slow and low.
A site-specific standard is needed to define the concentration of phosphorus, the limiting nutrient for algae growth, for the range of flows at which the potential for severe algal blooms arises. This standard has been challenging to develop because of the mixed river/lake nature of Lake Pepin, and because the usual signal of nuisance algal blooms -- chlorophyll a -- is affected by the presence of tiny organisms called diatoms, as well as non-nuisance species such as green algae.
By contrast, most lakes are dominated by nuisance blue-green algae in mid- to late summer, and chlorophyll a is a reliable indicator of nuisance algae. Citizen surveys have determined that for Lake Pepin nuisance conditions are present when chlorophyll a levels reach 50 parts per billion (compared to 30 parts per billion for normal lakes in the same type of region). MPCA scientists have determined that a summer average of 32 parts per billion chlorophyll a -- just a bit above the standard for normal lakes -- will minimize the number of days when nuisance conditions are experienced. A phosphorus concentration of 100 parts per billion, far below the current average of 180 pbb, will be needed to achieve this standard, according to a computer model constructed to characterize the Mississippi River from Lock and Dam 1 to the discharge point of Lake Pepin.
After developing the actual numerical standards, the MPCA is required to go through an administrative process to promulgate the site-specific standards. This includes a period of public notice, usually extending at least 30 days, approval by the MPCA Commissioner, and finally approval by the U.S. Environmental Protection Agency Region 5 in Chicago.
During the public notice period, citizens may send written comments on the standards to the MPCA. Citizens may also request that the MPCA hold a public information meeting on one or both of the site-specific standards, or ask the Commissioner of the MPCA for a contested case hearing.
- Norm Senjem, MPCA