Public-Supply Well Vulnerability to Contamination—A Function of Aquifer Vulnerability and the Interaction Between the Well and Aquifer

Placement, construction, and operation of a well dictate the part of an aquifer sampled by the well. Differences among wells can result in differences in vulnerability to contamination, even for wells within a single aquifer. Similarly, differences in aquifer properties, sources and sinks of water, and physical stresses contribute to differences in well vulnerability among aquifer systems, even where similar contaminant sources exist. A multi-aquifer investigation of the vulnerability of public-supply wells (PSWs) to contamination underscores some of these differences and provides insight into monitoring strategies. For example, on the basis of chemical constituents, including the mass ratio of conservative tracers chloride and bromide, a significant relation was found between median percentage of houses with septic tanks within 500 meters of shallow monitoring and domestic wells and water quality. A similar relation was not observed for PSWs, even when the analysis was repeated using numbers of houses with septic tanks in contributing recharge areas from groundwater flow models. PSWs with chloride/bromide ratios of 400-1100 (potentially affected by septic tanks), however, had significantly shorter simulated travel times from the water table to the wells compared to PSWs with ratios outside this range. Thus, the time lag between contaminant loading and arrival at the PSWs diminished the relation between activities near land surface and water quality in these wells. Simulated groundwater age mixtures at PSWs in several settings were used to further explore the response of PSWs to non-point sources of contamination. Results provide insight into monitoring for long-term trends. Interestingly, seasonal nitrate fluctuations were observed in one PSW where simulated ages ranged from years to thousands of years. This unexpected finding was attributed to flow within the wellbore during periods of low pumpage—rather than changes within the aquifer—and highlights the need to monitor during different modes of PSW operation.