A Comparative Water Quality Study of man-made drainage ditches and natural streams in south central Minnesota

The effects of man-made agricultural drainage ditch systems on the water quality of receiving bodies has not been documented. This study examined a four county area in south central Minnesota, dominated by agriculture. Thirteen drainage ditches and four natural streams in Blue Earth, Le Sueur, Nicollet, and Brown Counties were sampled during the ice free season of 1979. The study year was a wet year resulting in most of the water entering the drainage ditches as overland flow. The water was analyzed for total orthophosphate-phosphorus, total Kjeldahl-nitrogen, nitrate-nitrogen, and total dissolved solids. As flow increased the chemical load carried by the water increased but not in equal proportions for each parameter. Nitrate-nitrogen concentrations and total orthophosphate=phosphorus concentrations directly increased corresponding to rainfall events. Analysis of rivers and ditches according to Strahler's "stream order" revealed the greatest variability in water quality present in second order ditches. Third order ditches and third order rivers showed little variability.