The quality of our Nation's waters: factors affecting public-supply-well vulnerability to contamination: understanding observed water quality and anticipating future water quality

Abstract

A better understanding of public-supply-well vulnerability to contamination can lead to a reduced need for treatment of drinking water. Well screen Well casing Public-supply well Recharge Monitoring wells Cone of depression Domestic well Water table Groundwater flow pathways Chapter 1-Major Findings and Implications 3 dispersion of contaminants as they move through an aquifer. Third is the ease with which water and contaminants can travel to and through an aquifer, also referred to as intrinsic susceptibility; for example, a thick layer of dense clay can reduce groundwater vulnerability by acting as a barrier to the movement of water and contaminants. The vulnerability of a public-supply well 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. For example, the location of a well determines whether a particular contaminant source is in the area that contributes water to the well. The placement of the well screen determines which chemical and physical processes in the aquifer 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. The depth of the well screen and the pumping rate of the well determine how quickly water and contaminants can travel from the water table to that particular well, and from what distance. Because well design, construction, and operation directly influence water quality, water produced by different types of wells (public-supply, domestic, and monitoring wells) might not contain the same concentrations of contaminants, even if the wells are completed at similar depths within the same aquifer. What Measures of Vulnerability are Useful for Individual Public-Supply Wells? Study findings indicate that information on contaminant input, contaminant mobility and persistence, and intrinsic susceptibility within the area that con tributes water to a well can help answer the question, "Which contaminants in an aquifer might reach the well, and when, how, and at what concentration might they arrive?" Study-team scientists found that the following measures-each related to a different aspect of public-supply-well vulnerability-are particularly useful for understanding the quality of water pumped from individual publicsupply wells: (1) the sources of recharge that contribute water to a well, and the contaminants associated with the recharge-a measure of contaminant input; (2) the geochemical conditions encountered by water drawn into a well-a measure of contaminant mobility and persistence; and (3) the groundwater-age mixture of different waters that blend (or mix) in a well-a measure of intrinsic susceptibility. These measures of public-supply-well vulnerability and examples illustrating their utility for decisionmaking are discussed herein. Related implications for public-supply-well water quality are noted. Wate...

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