Nitrate concentrations that exceed U.S. Environmental Protection Agency drinking-water standards have been reported in Jackson Purchase Region groundwater since the 1960\u27s. More recently, other nonpoint-source chemicals such as pesticides and volatile organic compounds have also been found. The sources of these chemicals and the pathways by which they reach domestic groundwater supplies must be determined to protect human health and to design effective land-management practices.
To investigate the occurrence of nonpoint-source chemicals in Jackson Purchase groundwater, 60 wells were selected on the basis of geographic location, hydrostratigraphic unit penetrated, and well type (bored or drilled); distances to row crops, known septic systems, and active and abandoned feedlots were recorded for each well. Groundwater was sampled in the spring and fall of 2005 for field measurements, water properties, major and minor solutes, nutrients, metals, pesticides, volatile organic compounds, caffeine, and bacteria.
Nitrate concentrations exceeded EPA maximum contaminant levels in three wells, and exceeded half the MCL in three other wells. High nitrate concentrations were found mainly in samples from shallow wells in the Continental Gravel hydrostratigraphic unit near row crops. The combination of the permeable gravel and proximity to fertilizer applications makes these wells highly susceptible to nitrate contamination. Pesticides and caffeine were most commonly detected in Continental Gravel samples, but also in other units throughout the area. Total coliform bacteria were found throughout the region from all hydrostratigraphic units and all well types. High nitrate concentrations and detectable caffeine and pesticides are most often found in shallow wells regardless of whether the well was drilled or bored. Shallow wells, bored wells, and those near septic systems were most likely to produce water containing total coliform bacteria, however.
The occurrence of high nitrate concentrations and detectable amounts of pesticides and caffeine shows that groundwater contamination is occurring in the Jackson Purchase. Land management practices that take into account the different hydrologic properties of hydrostratigraphic units and bored wells\u27 greater susceptibility to annular leakage are needed to protect groundwater quality from further degradation. The occurrence of total coliform bacteria in groundwater samples indicates contamination of either the well or water delivery system (or both) or the water-producing aquifer. More extensive testing is needed to determine exactly where the bacteria reside.
This study was one of the first to consider well types and distances to potential sources of nonpoint groundwater contaminants with water-chemistry investigations. Future studies of nonpoint-source groundwater contamination must first establish that pollutants are not entering the well via local pathways such as leaky well casings or near-wellhead surface sources of chemicals, but rather that the local or regional groundwater system is affected. Second, future studies must carefully document distances from wells to potential sources of contaminants. In many cases this will require historical investigations to locate abandoned confined-animal feeding operations or on-site waste-disposal systems
