Identifying a source of contamination based on sampling results from drinking water and monitoring wells can be confusing. Evaluations of groundwater sampling can lead to conclusions that are seemingly contradictory. The investigation notes key factors that may be useful to those studying volatile organic chemical (VOC) movement in sand and gravel outwash. Density of the contaminants, depth of well screens, and the varying hydraulic conductivities associated with heterogenous sediments must be considered before interpreting the results of groundwater samples. In this case, VOCs were detected in seventeen drinking water wells in Blackhawk Subdivision near the village of Rockton. The subdivision is located at the confluence of the ancestral Pecatonica and Rock River Bedrock Valleys which are filled with thick outwash deposits. Groundwater movement in this area is complex due to the thickness and variety of sediments. The Illinois Environmental Protection Agency conducted an investigation to dettermine the source area of the contamination.