IMPROVING DRINKING WATER QUALITY BY BETTER UNDERSTANDING GROUND-WATER/SURFACE-WATER INTERACTION

Pumping wells in close proximity to a surface-water body can induce surface water infiltration, potentially compromising ground-water quality. In order to protect public water supplies, the U.S. Environmental Protection Agency (US EPA) established a ground water under direct influence of surface water (GWUDI) designation in 1986. If a well is classified as GWUDI, installation of filtration and disinfection systems is required according to the US EPA Surface Water Treatment Rule. In Rhode Island, GWUDI classification is determined based on distance between the well and nearest surface water body, well construction, and historical fecal bacteria data. Reliance on fecal coliform data highlights possible pathogen impacts but does not consider possible solute transport from the surface-water body. As a result, wells that are hydraulically connected to surface water but currently not contaminated by fecal bacteria are not classified as GWUDI and may be at risk in the event of a contaminant spill.

This study focuses on improved assessment of the degree of hydraulic connection and contribution of surface water to well discharge using a combination of chemical, physical and biological indicators at a public-supply well site located within 100 feet of a stream-fed pond, not currently classified as GWUDI. Hydraulic head data confirm the development of a pronounced downward gradient in the pond bottom when the well is activated. Temperature profiles, measured in the pond sediments, suggest that infiltration is occurring, however, some of the temperature signature may be attributable to heat conduction rather than water flow. Chemical indicators including chloride, sodium, and calcium suggest the potential for surface water infiltration at this site. A mass-balance mixing model, using oxygen isotopes, confirms that a small percentage of the well water is derived from surface water. Results from two microscopic particulate analyses reveal that surface-water organisms are present in the well water. All together, if the surface water body became contaminated, this well's water quality would be impacted. Hence, the conjunctive use of these methods provides a better understanding of the degree and nature ground-water/surface-water interaction and would further strengthen the approach to public-supply protection.