Determination of the contributing area to six municipal ground-water supplies in the Tug Hill glacial aquifer of northern New York, with emphasis on the Lacona-Sandy Creek well field

Abstract

The contributing areas to six municipal ground-water supplies (Adams, Mannsville, Lacona-Sandy Creek, Pulaski, Orwell, and Camden) that tap the Tug Hill aquifer were estimated from surficial geologic maps and potentiometric-surface maps. Contributing areas to the individual water supplies ranged from 0.01 to 1.0 square mile (mi2) but may include as much as 17 mi2 of adjacent upland areas that contribute recharge to the aquifer through streambed infiltration and direct runoff. The potential for contamination within the contributing area is low because the region is predominantly rural.The contributing area to the Lacona-Sandy Creek well field was calculated by several methods for purposes of comparison. A finite-difference ground-water flow model and a post-processing particle-tracking program were used for a range of pumping, recharge, and hydraulic conductivity values. Ground-water budgets computed from steady-state simulation indicate that most of the water pumped by the wells is water that would be lost to springs and as evapotranspiration in the western flank of the aquifer. High pumping rates combined with low recharge rates may induce minor infiltration from Little Sandy Creek. Results of flow-path analysis indicate that (1) the size and shape of the contributing area differs significantly from the area of influence (the surface expression of the cone of depression), (2) flow paths from the eastern edge of the aquifer are less than 1 mile long, and (3) travel times to the supply well are generally between 500 and 1,000 days.Two modified analytical techniques also were used the Dupuit uniform-flow method and the Theis nonequilibrium method. Analytical methods are easier to apply than numerical methods but are constrained by limiting assumptions that, if not satisfied, can result in large errors. The Dupuit method, modified for a sloping water table, indicated a contributing area of 0.04 mi2 for a production well pumped at 200 gallons per minute and a horizontal hydraulic conductivity of 1,200 feet per day. This is smaller than the 0.13-mi2 contributing area obtained by numerical techniques for similar hydraulic properties, and its position differs also. The Theis method modified for partial penetration of the pumped well, dewatering of the aquifer, and a single linear impermeable boundary indicated a contributing area of 0.12 mi2, the size and shape of which is similar to the contributing area obtained by the numerical simulation.The selection of a technique for delineating a contributing area ultimately depends on the resources available for the analysis and the degree of accuracy required. Despite the uncertainties and incomplete information on the factors that affect the size of the contributing area, the four methods used in this study provide a more reliable estimate than the commonly used fixed-radius method.