Recharge to Shale Bedrock at Averill Park, an Upland Hamlet in Eastern New York - An Estimate Based on Pumpage within a Defined Cone of Depression

Abstract

Water levels beneath parts of Averill Park, a residential hamlet in an upland area of till-mantled shale bedrock in east-central New York, have declined in response to increased withdrawals from new wells. Similar experiences in many upland localities in the northeastern United States have resulted in awareness that the rate of recharge to bedrock can be an important constraint on the density of new development in uplands. Recharge at Averill Park was calculated on the basis of careful estimation of pumpage within a defined cone of depression. The data-collection and recharge-estimation procedures documented herein could be applied in a variety of upland localities in support of community-planning studies. Static water levels measured in 145 wells at Averill Park during the late summer of 2002 defined a 0.54-square-mile cone of depression within which ground-water discharge took place entirely as withdrawals from wells. Rates of withdrawal were estimated largely from surveys in similar neighborhoods a few miles away served by public water supply. Comparison of the water-level measurements in 2002 with measurements on other dates revealed localized declines that could be attributed to new housing developments or commercial demands, but also demonstrated that water levels in 2002 within the cone of depression had stabilized and were not declining persistently over time. Therefore, the current withdrawals were equated to recharge from infiltrating precipitation. Recharge within this area was estimated to average 104 gallons per day per acre, equivalent to 1.4 inches annually, and was sufficient to sustain a residential population of 1.9 persons per acre. This recharge rate is much lower than rates estimated from streamflow records for upland watersheds elsewhere in the northeastern United States. This rate is an average of an unknown larger rate in the 30 percent of the study area where bedrock is discontinuously overlain by less than 30 feet of till and an unknown smaller rate in the remainder of the area where bedrock is overlain by thick till in the form of drumlins. The spatial variation in rate of recharge is inferred from the fact that high heads and strong downward gradients in bedrock, and very hard water with high chloride concentrations caused by winter highway runoff, are largely restricted to the area of discontinuous, thin till. Wells less than 180 feet deep and distant from highways typically yield water of moderate hardness (50-170 milligrams per liter as calcium carbonate) that is caused by dissolution of limestone fragments in the till. Some wells that are more than 180 feet deep yield very soft water (0-50 milligrams per liter) with high pH and high sodium concentrations resulting from ion exchange within the bedrock. Nearly all wells in some areas of thick till yield very soft water. Most wells near the center of Averill Park yield less than 3 gallons per minute. The likelihood of obtaining an additional 2 gallons per minute or more by drilling deeper than 200 feet is calculated to be about 25 percent. Most wells west and southwest of the center yield at least 3 gallons per minute, and the liklihood of obtaining an additional 2 gallons per minute or more by drilling deeper than 200 feet is about 50 percent.