Factors Affecting Vulnerability of Public-Supply Wells to Contamination

Public-supply-well vulnerability to contamination starts with groundwater vulnerability to contamination. Even wells within a single aquifer, however, may not be equally vulnerable to contamination because individual wells produce unique mixtures of the groundwater from different parts of the aquifer.

A study done from 2001 to 2011 (Transport of Anthropogenic Contaminants, or TANC study) sheds light on factors that affect the vulnerability of water from public-supply wells to contamination. The study also identified measures that can be used to determine which factor (or factors) plays a dominant role at an individual public-supply well. Case-study examples show how such information can be used to improve water quality.

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In general, the vulnerability of the water from public-supply wells to contamination depends on contaminant input in the area that contributes water to a well, the mobility and persistence of a contaminant once released to the groundwater, and the ease of groundwater and contaminant movement from the point of recharge to the well. The following measures are particularly useful for indicating which contaminants in an aquifer might reach an individual public-supply well and when, how, and at what concentration they might arrive:

• Sources of recharge—Information on the sources of recharge for a well provides insight into contaminants that might enter the aquifer with the recharge water and potentially reach the well.

• Geochemical conditions—Information on the geochemical conditions encountered by groundwater traveling to a well provides insight into contaminants that might persist in the water all the way to the well. Use the Redox Framework Tool to determine the redox conditions associated with water from a public-supply well.

• Groundwater-age mixtures—Information on the ages of the different waters that mix in a well provides insight into the time lag between contaminant input at the water table and contaminant arrival at the well. It also provides insight into the potential for in-well dilution of contaminated water by unaffected groundwater of a different age that simultaneously enters the well. Use the web-based Groundwater Age Mixtures and Contaminant Trends Tool to explore the effects of basic aquifer properties and well configurations on groundwater age mixtures in groundwater discharge and on contaminant trends from different nonpoint-source contaminant input scenarios.

Preferential flow pathways—pathways that provide little resistance to flow—can influence how all other factors affect public-supply-well vulnerability to contamination. For example, preferential flow pathways can influence whether a contaminant source is physically linked to a well, whether contaminant concentrations are substantially altered before contaminated groundwater reaches a well, and whether contaminated groundwater can arrive at a well within a timeframe of concern to the well owner. Methods for recognizing the influence of preferential flow pathways on the quality of water from a public-supply well are presented in this circular and can provide opportunities to prevent or mitigate the deterioration of a water supply.

Knowing what water-quality variables to measure, what spatial and temporal scales on which to measure them, and how to interpret the resulting data makes it possible for samples from public-supply wells to provide a broad window into a well’s past and present water quality—and possibly future water quality. Such insight can enable resource managers to prioritize actions for sustaining a high-quality groundwater source of drinking water.

Learn more about the USGS National Water Quality Assessment Project and research on the Nation’s groundwater and surface-water quality.