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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 ground water. Recent events have shown that these aquifers are highly susceptible to contamination that results from a number of sources, such as seepage from wastewater lagoons, leaking petroleum-product storage tanks, and road salting. To protect the quality of water pumped from supply wells in these aquifers, it is necessary to ensure that potentially harmful contaminants d not enter the ground in the area that contributes water to the well. A high degree of protection can be achieved by application of appropriate land-use controls within the contributing area. However, the contributing areas for most supply wells are not known. This report describes the factors that affect the size and shape of contributing areas to supply wells and evaluates several methods to delineate contributing areas of wells in glacial-drift, river-valley aquifers. Analytical, two-dimensional numerical and three-dimensional numerical models were used to delineate contributing areas. These methods of analysis were compared by applying them to a 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. Analytical methods proved easiest to apply but gave results that are considered to be significantly less accurate than those obtainable by means of numerical-model analysis. 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 glacialdrift valley aquifers. For "average" conditions in the hypothetical aquifer, the analytical method predicts a contributing area that is limited to the well side of the river. This is caused by using a constant-head boundary simulated with image wells in the analytical model. For typical glacial-drift, river-valley aquifers, this is an unrealistic simulation because drawdowns caused by a pumping well and the contributing area of the well can extend beneath and beyond a river or stream. On the basis of results obtained with the twodimensional numerical model, for which a wide range of hydrologic conditions were simulated, it seems that the contributing area in a typical glacialdrift, river-valley setting for a well pumped at a rate of 1.0 million gallons per day--a common pumping rate--may be expected to range from about 0.9 to 1.8 square miles. 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 three-dimensional delineation of the zone of contribution for a pumped well. For the relatively thin (100 feet or less) unconfined aquifers considered in this analysis, the three-dimensional model showed that the zone of contribution extended throughout the entire saturated thickness of aquifer. Because of this, the two-dimensional simulations were considered adequate for delineating contributing areas in this particular hydrologic setting. For thicker aquifers (greater than 100 feet), this may not be the case. Values for several of the factors that affect the size and shape of contributing areas generally are not known with a high degree of certainty. Therefore, determination of the recharge area that contributes water to a pumped well by any method is an approximation at best. Nevertheless, in river valleys where a reasonable amount of accurate geohydrologic data are available, methods described in the report can be used to estimate the extent of contributing areas with sufficient accuracy to be of use to planners and water-resource managers. Control of land-use activities by State and local governments within the contributing areas to supply wells alone will not guarantee that water pumped from a well is of a quality suitable for human consumption. Many supply wells in river-valley aquifers derive substantial portions of their water from induced infiltration of streamflow. Thus, if the quality of streamllow entering the contributing area of a well is unsuitable for human consumption, the quality of water pumped from the well also may be unsuitable.

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