Estimation of the recharge area contributing water to a pumped well in a glacial-drift, river-valley aquifer

Abstract

The highly permeable, unconfined, glacial drift aquifers that occupy most New England river valleys constitute the principal source of drinking water for many communities that obtain part or all of their public water supply from groundwater. Analytical , two-dimensional numerical and three-dimensional numerical models were used to delineate contributing areas of groundwater pollution. These methods of analysis were compared by applying them to hypothetical aquifer having the dimensions and geometry of a typical glacial drift, river valley aquifer. In the model analyses, factors that control the size and shape of a contributing area were varied over ranges of values common to glacial drift aquifers in New England. These controlling factors include the rate of well discharge, rate of recharge to the aquifer from precipitation and from adjacent till and bedrock uplands, distance of a pumping well from a stream or other potential source of induced recharge, degree of hydraulic connection of the aquifer with a stream, horizontal hydraulic conductivity of the aquifer, ratio of horizontal to vertical hydraulic conductivity, and degree of well penetration. Numerical models of valley aquifers are deemed best suited to determine the approximate contributing area of a well because of their capability to simulate more accurately the variable geohydrologic conditions typical of glacial drift valley aquifers. On the basis of results obtained with the two-dimensional numerical model, for which a wide range of hydrologic conditions were simulated, the contributing area in a typical glacial drift, river valley setting for a well pumped at a rate of 1.0 million gal/day--a common pumping rate--can be expected to range from about 0.9 to 1.8 sq mi. Model analysis also shows that the contributing area of pumped wells may be expected to extend to the opposite side of the river and to include significant areas of till uplands adjacent to the aquifer on both sides of the valley. Simulations with the three-dimensional model allow a full delineation of the zone of contribution for a pumped well. For the relatively thin (100 ft or less) unconfined aquifers considered in this analysis, the model showed that the zone of contribution extended throughout the entire saturated thickness of aquifer. (Lantz-PTT)