Agricultural Chemicals, Land Use, and Their Impacts on Stream and Ground Water Quality in the Little Plover River Watershed

Abstract

The Little Plover River watershed is located in Central Wisconsin just southeast of Stevens Point. It is an area of glacial till and outwash with some Cambrian sandstone outcrops overlying a Precambrian crystalline bedrock basement. The dominant land use in the area is agriculture, with irrigated agriculture playing a major role in the growing of potato and vegetable crops. Ground and surface water quality in the watershed have been significantly affected by land use practices. This project examines pesticide and long-term nitrate, chloride, and other constituent concentrations in twenty monitoring wells sampled since 1980. This study also examines the spatial distribution of nitrate and chloride concentrations in a number of private water supply wells located in and around the Little Plover River watershed. Pesticide data collected from the twenty monitoring wells since July 1995 show the presence of atrazine, deethylatrazine, deisopropylatrazine, simazine, metribuzin, alachlor, metolachlor, and hexazinone. There were 25 exceedences of the NR 140 preventative action limit (PAL) for atrazine compounds as total chlorinated residue (TCR). Other compounds showing PAL exceedences were alachlor ( 4 occurrences) and metolachlor (1 occurrence). The enforcement standard for alachlor was exceeded on two instances. Pesticide data collected from seven different Little Plover River stream sites indicate the presence of atrazine, deethylatrazine, deisopropylatrazine, metribuzin, alachlor, and metolachlor. A survey completed along the streambed in 1996 using minipiezometers show triazine concentrations in the 0 - .18 ppb range. Long-term monitoring data indicate that eleven of the twenty monitoring wells in the watershed have median nitrate concentrations above the enforcement standard. Median nitrate varies unsystematically according to well depth and varies in relation to land use. Median concentrations for most wells are well above background. Results of the geographic information system (GIS) study indicate that average nitrate (and chloride) concentrations per ¼, ¼ section increase moving from the recharge area in the northeast toward the Little Plover River in the southwest. This same database was then separated into three distinct regions according to dominant land use and a comparison was made in nitrate and chloride concentrations between these different land uses. The section dominated by irrigated agricultural land use shows both high nitrate nitrogen and chloride concentrations. Long-term mass flux (discharge multiplied by concentration, QC) for nitrate, chloride, and sulfate trend upward for the Little Plover River. Total phosphorus trends downward. No significant trends to mass flux for sodium, potassium, or reactive phosphorus are evident in the stream data for the Hoover A venue sampling site. Average nitrate nitrogen mass flux by month for the whole site six data set (1967-1997) is highest in April and lowest in July, indicating mass flux to be more dependent on stream discharge than on nitrate concentration. Land use in the watershed changed considerably in the 1960's and 1970's. Many acres of land in nonirrigated agriculture switched over to irrigated agriculture. All stream data collected is higher in nitrate than what would be considered background for unaffected water. The Mann Kendall test for trend indicates 53% of monitoring wells in the watershed have a significant upward trend for nitrate, 16% show a significant downward trend, and 31% show no significant trend. Significant trends, both upward and downward, are present for chloride, sulfate, potassium, sodium, pH, conductivity, alkalinity, total hardness, calcium hardness, and reactive phosphorus.