TMDL Executive summary / progress

Executive Summary

In 2002, the Minnesota Pollution Control Agency (MPCA) listed Lake Volney as impaired for aquatic recreation due to excess nutrients under section 303(d) of the Clean Water Act. Excessive phosphorus loading is the main cause of the impairment. The goals of the Lake Volney Excess Nutrients Total Maximum Daily Load (TMDL) study are to describe the nature and extent of the lake’s phosphorus impairment and determine source load allocations that consider major sources. Resources are currently being allocated to the Cannon River watershed (the major watershed that Volney lies in) to complete a comprehensive assessment, conduct stressor identification focused on biological impairments, construct a watershed model and complete additional TMDLs as necessary. These components of MPCA’s watershed approach, particularly the modeling, will allow for simulation of various management scenarios aimed at pollutant load reductions.

The Lake Volney watershed is at the western headwaters of the Cannon River watershed and covers 2,017 acres within the North-Central Hardwood Forest ecoregion. With an average depth of 22.7 feet and a maximum depth of 65 feet, deep lake standards for its ecoregion apply. It is located in Le Sueur County and is dominated by agricultural land use. At 277 acres, Lake Volney represents one of the smaller lake systems found within the Cannon River watershed.

This lake has been the subject of many investigations. The first known comprehensive study of water quality on Lake Volney occurred through MPCA’s Lake Assessment Program in 1986. The study results found average total phosphorus concentrations of 160 μg/L and also estimated that 90% of the phosphorus entering the lake was being retained. A Clean Water Partnership Diagnostic and Feasibility study followed by a successful implementation project was also completed on Lake Volney that spanned the period of 1995-2008. Summer mean total phosphorus concentrations improved from an average of 170 micrograms/liter (μg/L) in 1995 to 105 μg/L in 2002 to 87 μg/L in 2005 in response to several implementation activities within the watershed.

The focus and primary intent of this project is to better characterize phosphorus levels, probable sources, and estimate reductions required to meet the TMDL water quality goal. Watershed wide phosphorus loading was estimated to assess the magnitude of nonpoint and point sources and establish a cause-effect linkage of loading sources and subsequent in-lake phosphorus concentrations.

Samples were collected for the TMDL study between April and October 2009 and 2010 (note that only June-September results were used for TMDL calculation). Four monitoring stations were located throughout the watershed and lake. The resulting data illustrates a declining trend in water quality through the season due to watershed and internal phosphorus loading. The current total phosphorus load to Lake Volney is 807.6 kg/yr. A reduced total phosphorus load of 324.7 kg/yr would be required to reach the 5

water quality goal of 36 μg/l; the goal includes a 10 percent margin of safety. For some years, a reduction from watershed sources of up to about 482.9 kg/yr or 60 percent would be required to meet this goal. Over time, reductions in external loading should lead to reductions in internal loading.