During the last decade of the twentieth century, McHenry County had the fastest-growing population in Illinois. Just north of the Chicago metropolitan area, land use in the eastern half of the county changed from row-crop agriculture to urban sprawl. Water supplies are from shallow sand and gravel aquifers and are highly vulnerable. We evaluated the change of groundwater quality in McHenry County during most of the twentieth century and identified the degree and extent of contamination, and sources, using available historic water-quality data. To evaluate historic data, we calculated background concentrations of selected ions using cumulative probability plots to identify the presence of anthropogenic contamination. Timing of groundwater contamination coincides with that of population growth and the onset of utilization of artificial N-fertilizer and road salt. Groundwater from urban areas showed greater Na+ and Cl contents than rural areas, which reflect more extensive applications of road salt beginning in the early 1960s. Groundwater was collected for chemical and isotope analyses from selected shallow wells with historically elevated NO3 concentration as well as from farms with livestock. The isotope data suggest N-fertilizer and soil nitrogen are the predominant sources for NO3 in shallow groundwater. Animal waste was also a source for NO3 near farms with livestock. Spatial analysis suggested that the source of NO3 in the groundwater was from surface-borne contaminants. The permeable soils and near-surface sand and gravel aquifer found in most of McHenry County provide pathways for surface contaminants to migrate into shallow groundwater.

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