Knowledge of Where and How Contamination-Susceptible Water Enters Public- Supply Wells Can Be Used To Improve Monitoring Strategies and Protection Plans

A USGS investigation into the transport of contaminants to public-supply wells (PSWs) in four distinctly different aquifers demonstrates how differences in the chemistry of water produced by PSWs and nearby monitoring wells can reveal where and how contamination-susceptible water enters PSWs. This knowledge can be translated into improved monitoring strategies and protection plans. Samples were collected from multiple depths during pumping in selected PSWs. The chemistry of these depthdependent samples, along with composite samples from the PSWs, was compared to that of adjacent nested monitoring wells. Four examples are given here: o In the Central Valley aquifer system, comparisons revealed that concentrations of uranium above the drinking water standard are a result of cross contamination between the shallow and deeper parts of the aquifer as a result of downward well-bore flow in the PSW during periods of low or no pumping. Improved water quality at this well probably could be achieved by changes in the PSW’s pumping schedules. Similar consideration of the influence of vertical hydraulic head gradients that exist because of regional pumping and reapplication of irrigation water may be useful for improving monitoring and protection strategies throughout this setting. o In the High Plains aquifer, comparisons revealed that wells and boreholes open to multiple aquifer layers create short-circuit pathways vertically across a confining unit and are responsible for anthropogenic and natural compounds entering the PSW. Strategies for improved protection of PSWs in this setting include locating and altering the open interval of or destroying such multi-layer wells. o In the Floridan aquifer system, comparisons revealed that the PSW produces water that is more similar to water in the overlying surficial aquifer system than to water from the Upper Floridan aquifer itself due to short-circuiting that results from the co-occurrence of sinkholes and high pumping stresses. As a result, monitoring and protection efforts could largely focus on the surficial aquifer system and the most transmissive zones within the Upper Floridan aquifer in areas where breaches in the confining unit and high pumping stress co-occur. o In a glacial aquifer, potential sources and pathways throughout the contributing area were consistent with observations near the PSW; the aquifer contains young ground water and is affected by point-source contaminants. Protection plans that focus on determining the distribution and density of point sources of ground-water contamination would substantially contribute to the understanding of the vulnerability of PSWs in this setting. Although the mechanisms for transport of contaminants to the PSWs vary among these diverse hydrogeologic settings, effective protection strategies require an understanding of the dominant pathways of water contributed to the wells.