Estimating Nonpoint Source Pollution for the Twin Cities Metropolitan Area Using Landscape Variables

Abstract

Several stream monitoring programs aim to measure nonpoint source pollutant loading to the major river systems in the Twin Cities Metropolitan Area (TCMA) of Minneapolis and St. Paul, Minnesota. However, due to cost and logistical considerations, only a portion of the total nonpoint source load can be effectively monitored. Regression models were developed relating nonpoint source pollutant yields to landscape characteristics in order to estimate the total nonpoint source contributions of nutrients and suspended sediments to the Mississippi, Minnesota, and St. Croix Rivers from the region. The regression models were generally strong with r-squared values ranging from 0.57 for total suspended solids yield to 0.90 for nitrate yield. The model factors included both land cover variables, such as the percent row crop or the percent urban land as well as soil variables, such as the percent organic matter or the percent clay. These results highlight the importance of considering both land cover and soils when estimating nonpoint source pollutant loads. Using the fitted regression models, the estimated annual average total suspended solids, total phosphorus, nitrate, and total Kjeldahl nitrogen loads for the TCMA are 104,000 metric t/yr, 354 t/yr, 3,580 t/yr, and 1,760 t/yr, respectively.