Water Quality Analysis of the Lower Minnesota River and Selected Tributaries: River (1976-1991) and Nonpoint Source (1989-1992) Monitoring Volume I

During the 1970's and 1980's the water quality standards for dissolved oxygen (DO) were not being consistently met in the Lower Minnesota River. This section of MRAP was designed to study this and other water quality parameters in the Lower Minnesota River. This study first determined the Metro Area tributaries provided more loading to the system than did the two Metropolitan Waste Commission waste water treatment plants. The results indicated that agricultural watersheds in the Metro Area had higher concentrations and loadings of nitrate (NO3), total phosphorus (TP), total suspended solids (TSS) and carbonaceous oxygen demand (CBOD) per acre than urban watersheds. High lead levels were shown in the most urban watershed. CBOD and chlorophyll-a (Chl) concentrations, as well as the ratio of suspended volatile solids to TSS in the Lower Minnesota River were highest during low flow. Analysis reviled that 72% of the BOD variability at a Jordan site can be accounted for by changes in Chl concentrations. High Chl concentrations relate to high TP levels. Thus it was determined that control of CBOD concentrations may require regulation of TP levels. Flow level was also demonstrated to have an effect on the system. During a low flow year (1988) nonpoint sources contributed 90%. Concentrations and loadings of TP were highest in the Minnesota River, with TP concentrations being 5.6-17 times greater than the Mississippi and St. Croix Rivers respectively. A core sample from Lake Pepin indicated that deposition of phosphorus and sediment have increased 3-5 times since the 1860's. Herbicide levels were also tested and showed the highest levels during the one to two weeks following spring application. High concentrations of TSS were also associated with this portion of the hydrograph