The impact of wetlands and drainage on water quality in an agricultural watershed in south central Minnesota

To form a baseline study of a proposed county ditch, nutrient levels (PO4-P, TKN, and NO3-N), conductivity, and turbidity were monitored during 1980 at eight sites along the stream proposed to be channelized. Tributaries from three partly drained wetlands and four drainage ditches were also sampled to compare wetland and ditch outputs and to access their impact on the stream's water quality. The predominantly agricultural watershed (4554 acres) of the stream was dissected into the tributary subsheds and the percentages of different land use categories, the numbers of animals, drainage histories, and soil types were compiled for these watersheds and examined for relationships to water quality differences in the tributaries. Significant differences in nutrient concentrations were found among the wetland and drainage ditch outputs ranging from the most distinct wetland (PO4-P, TKN, and NO3-N means of 0.919, 3.42, and 0.24ppm) to the most distinct drainage ditch (PO4-P, TKN, and NO3-N means of 0.112, 1.32, and 8.75 ppm). It was possible to form a wetland-to-ditch continuum of all the tributary sites based primarily on gradients in nitrate concentrations which were significantly lower in wetland outputs and higher in ditch outputs. Also, of the three wetlands studied, one wetland in the spring and two wetlands in the summer had significantly higher mean concentrations of PO4-P and TKN than the drainage ditches. One wetland showed greater similarities to the water quality of ditches. All three wetlands shared seasonal patterns in nitrate concentrations and conductivity that were clearly different form the drainage ditches. Land use in the watersheds was shown to be a likely factor contributing to the differences in tributary water quality. A lower cropland to wetland ratio in watershed of a drainage ditch corresponded to lower No3-N and, to some degree, higher PO4-P and TKN concentrations in the outputs, while a higher ratio in the watershed of a wetland corresponded to higher NO3-N and lower PO4-P and TKN concentrations in the output. Watersheds that terminated in wetlands showed greater wetland influence on water quality than watersheds dominated by drainage ditches yet containing higher proportions of wetland area. Due to greater flow, the master stream was found to be generally unresponsive to the tributary inputs despite the large differences in their water quality. It was concluded from this study that the drainage of wetlands may involve a trade in water quality values (in this case higher NO3-N for lower PO4-P concentrations) rather than adversely affecting all parameters.