Long Prairie River watershed TMDL project - Lessons learned from phase I

Long Prairie River drains a predominantly agricultural watershed in central Minnesota. Designated uses of the River are aquatic life, recreation, industrial consumption, agriculture, wildlife, aesthetic enjoyment, and navigation. Monitoring in recent years has shown dissolved oxygen (DO) concentrations in portions of the river intermittently below the state water quality standard of 5 mg/L. As a result, Minnesota Pollution Control Agency (MPCA) placed four lower-river reach segments on the agency's 1998 "303(d) list". The main objectives of the project are to define the extent, persistence, and severity of the DO depletion problem, quantify point and non-point pollutant sources contributions to water quality impairments, and develop a TMDL for the watershed. The TMDL project has three phases. Phase I is an analysis of existing monitoring data to: determine the technical issues to be addressed within the scope of the project, outline a modeling strategy, define data gaps and additional monitoring needs, and prepare a monitoring plan to address the additional data needs. Review of the existing water quality data for the Long Prairie River indicates substantial seasonal and inter-annual variability but no significant long-term trends. Longitudinal total phosphorus profiles show a consistent pattern of gradual increase from the headwaters to the City of Long Prairie, followed by a steep increase downstream from the City. Longitudinal nitrogen series profiles indirectly suggest significant nitrification in the reach between the cities of Long Prairie and Browerville. Diurnal DO variations in the vicinity of City of Long Prairie indicate substantial primary productivity.