EFFECT OF LAND USE ON GROUNDWATER QUALITY: A STUDY USING GIS AND ISOTOPE TOOLS

McHenry County, located in northeast Illinois near Chicago, has the fastest growing population in Illinois. The majority of the public and private water supplies in McHenry County come from shallow sand and gravel aquifers, which are vulnerable to contamination. Elevated nitrate levels have been detected in many shallow wells in the county. The main objectives of this research are to evaluate the change of groundwater quality throughout the history of urban development in McHenry County and to identify the source of nitrate in the groundwater. Approximately 38,000 historical groundwater quality records for McHenry County, from 1913 to 2002, have been obtained from the Illinois State Water Survey and the Health Department of McHenry County. Computer software, ArcGIS, was used to display the chemical data from the two databases together on a county map to delineate temporal and spatial trends. Temporal analysis of the database revealed that total dissolved solids, chloride, and nitrate concentrations in groundwater increased from the 1970s, indicating deterioration in groundwater quality. This timing coincides with that of population growth in the county. Spatial analysis of nitrate data shows a reverse relationship between nitrate levels and depth, suggesting the source of nitrate is from the surface. A map of land coverage revealed a correlation between elevated nitrate concentration and areas of crop land, and near lakes and rivers. Such correlation suggests that nitrogen compounds applied or produced in association with agricultural activities are the major sources of nitrate in groundwater. Groundwater from selected wells with high nitrate records has been collected for isotope analysis. Preliminary nitrate isotope data confirms that fertilizer is a major source for nitrate in shallow groundwater. Greater nitrate concentrations also correlated well with areas of greater leaching potential on an aquifer sensitivity map. The permeable soil, and near surface sand and gravel aquifer in many areas of McHenry County provide unobstructed pathways for surface contaminants such as nitrate to migrate from surface to shallow groundwater.