Chemical and Isotopic Database for McHenry County Study on Groundwater Quality and Land Use Completed

Geologists examine a groundwater sample.This investigation was conducted to evaluate the effect of changing land use on groundwater quality. McHenry County possesses unique features for such a comparison. During the last few decades, McHenry County has been the fastest growing population in Illinois. Originally an agriculturally based county, many areas have transitioned into urban environments with fast-growing housing demands; however, croplands still dominant the rural areas. The majority of water resources in the county comes from shallow sand and gravel aquifers, which are vulnerable to surface-borne contamination. For the study, approximately 38,000 groundwater quality records from McHenry County were retrieved from the Illinois State Water Survey and the McHenry County Health Department. GIS software was used to analyze this large database. In addition, 30 groundwater samples were collected from private wells (both drilled and dug) for chemical and isotopic analyses. These data were analyzed to identify the distribution and sources of elevated nitrate concentrations and to evaluate the effect of changing land use on shallow groundwater quality.

Temporal analysis of historical groundwater data revealed that total dissolved solids, chloride, and nitrate concentrations in groundwater has increased since the mid-1960s, indicating deterioration of groundwater quality in the previous century's last decades. This timing coincides with rapid population growth in McHenry County and the use of road salt. Spatial analysis revealed that the highest concentrations in all three categories were found in shallow wells less than 100 feet deep, strongly suggesting that the contaminant sources were surface-borne. Chemical and isotopic data from our groundwater samples indicate that the type of land use controlled the type and concentrations of ions present. For example, groundwater samples from urban areas had higher sodium and chloride concentrations, which we attributed to the application of road salt. Samples from rural and near-livestock facilities had higher nitrate and total nitrogen concentrations, which we attributed to the application of fertilizer on croplands. Based on isotopic results, the predominant sources of nitrate in shallow groundwater samples collected from both rural and urban areas were fertilizer and soil organic matter due to the widespread fertilization of croplands surrounding isolated urban areas in McHenry County. Overall, the results of this investigation revealed that (1) groundwater quality in McHenry County has deteriorated significantly since the mid-1960s due to the increasing application of road salt and fertilizer and that (2) land use in an area such as McHenry County, where sand and gravel dominate the near surface geology, can rapidly and adversely affect the local groundwater quality. The study results were published as ISGS Open File Series 2007-6.