In a recent study of public-supply-well water quality in the United States, nearly two-thirds of all samples from 932 public wells tapping 30 regionally extensive aquifers contained detectable amounts of drinking-water contaminants that originated entirely or primarily from manmade sources. Twenty-two percent of the samples contained at least one contaminant (manmade or naturally occurring) at concentrations greater than drinking-water standards or other human health benchmarks (Toccalino and Hopple 2010). These findings imply that water from nearly one in five public-supply wells in the United States may need to be treated or blended with more dilute water sources to decrease concentrations of drinking-water contaminants before delivery to the public. Removing contaminants from water intended for drinking is difficult and expensive. It also is becoming increasingly necessary (Job 2011). Consequently, understanding factors affecting public-supply-well vulnerability to contamination is important. Public-supply-well vulnerability to contamination is not the same as groundwater vulnerability, but the two cannot be decoupled. Groundwater vulnerability is the tendency or likelihood for contaminants to reach a specified position in the groundwater system (National Research Council 1993). Groundwater vulnerability depends on three factors: (1) the presence of manmade or natural contaminant sources, (2) the combination of chemical and physical processes in the subsurface that affect contaminant concentrations, and (3) the ease of water and contaminant movement to and through an aquifer, or its intrinsic susceptibility (Focazio et al. 2002). When mapping groundwater or “resource” vulnerability, the target is frequently the water table or top of the aquifer (Frind et al. 2006); however, trace elements and radionuclides derived from aquifer solids may account for the majority of drinking-water standard or other human health benchmark exceedances in raw water from public wells (Toccalino and Hopple 2010). Public-supply-well vulnerability depends on all of the above factors (contaminant input, contaminant mobility and persistence, and intrinsic susceptibility) but is further affected by the location, design, construction, operation, and maintenance of the well. This is because groundwater vulnerability, and thus water quality, is not uniform throughout an aquifer, and wells “sample” only part of an aquifer. In other words, well location determines whether a particular contaminant source is in the capture zone for the well. Screen placement determines which chemical and physical processes in the aquifer will have influenced the water before it is pumped from the well and, therefore, which contaminants might be present in the water as it enters the well, and at what concentrations. Well depth and pumping rate determine how quickly water and contaminants can travel from the water table to the well, and from what distance. The interaction of a well with the surrounding aquifer determines whether the well intercepts water moving along preferential flow pathways, which can affect the relative importance of each of the other factors contributing to its vulnerability. Finally, the pumping schedule determines when poor quality water can migrate between aquifer units by way of wellbore flow, thereby influencing the mix of contaminated and uncontaminated water that enters the well at different points in time (Eberts et al. 2013). To summarize, a well affects its own vulnerability by drawing in water from a unique combination of flow pathways that are associated with different combinations of water and contaminant sources, aquifer geochemical conditions, and travel times, giving rise to the flux-averaged contaminant concentrations that occur in the produced water. Thus, the vulnerability and water quality of every well is unique to itself. This is true even for wells within the same aquifer and wellfield (Eberts et al. 2013).
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Apr 13, 2015
Tomoya Konishi + 2
Open Access