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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

By Allan D. Randall and Anne J. Finch

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Appendix A
Appendix B

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.

Contents

Abstract

Introduction

Geohydrologic Setting

Bedrock

Unconsolidated Deposits

Data Collection

Well Inventory

Water-Level Measurements

Measurements in 1988

Measurements in 2002

Adjustments of Water-Level Measurements

Measurements in 2005

Water-Quality Data

Field Measurements

Chemical Analyses

Reported Experience of Residents

Ground-Water Levels and Flow Directions

Patterns of Ground-Water Flow in 2002

History of Ground-Water Development at Averill Park Since 1930

Changes in Water Levels, 1988–2002 and 2002–2005

Effect of Fluctuations in Precipitation and Recharge

Water-Level Changes as a Function of Resident Population or Business Activity

Water-Level Declines as a Function of Withdrawals and Natural Discharge

Water-Level Declines as Cause and Result of Well Deepening

Quality of Water in Bedrock

Patterns of Chemical-Quality Distribution

Moderately Hard Water in Shallow Bedrock Beneath Thick Till

Soft Water in Deep Bedrock

Very Hard, Mineralized Water near Highways Where Depth to Bedrock is Slight

Localized Nuisance Conditions

Hydrogen Sulfide (“Sulfur Water”)

Suspended Clay or Silt

Iron and Manganese

Quantity of Water Obtainable from Bedrock

Well Yields

Possible Effect of Bedrock Lithology on Well Yield

Possible Effect of Water-Level Declines on Well Yield

Potential for Augmenting Well Yield by Deepening the Well

Recharge Rate as a Limit on Ground-Water Withdrawals from Bedrock

Delineation of Capture Area

Estimation of Ground-Water Withdrawals

Computations of Recharge to Bedrock and Population Density Supported by that Recharge

Uncertainty in Estimate of Recharge

Recharge Rate as a Function of Till Thickness

Contrast with Recharge in Other Upland Areas as Estimated from Streamflow Records

Probable Effect of Increased Withdrawals at Averill Park

The Wynants Kill as a Possible Source of Recharge

Need for Additional Studies of Rates of Recharge to Bedrock

Summary and Conclusions

Acknowledgments

References Cited

Appendix A. Records of Wells at Averill Park

Appendix B. Measured Water Levels in Wells, 1987–2005

Appendix C. Procedures by which Measured Water Levels in Wells Were Converted to a Common Datum

Appendix D. Calibration of conductivity meter

Appendix E. Estimation of Annual Average Per-Capita Water Use

Appendix F. Effect of a Decline in Water Levels Below the Bedrock Surface on Well Yield

Plates

1. Bedrock-surface topography
2. Locations of wells
3. Water levels in bedrock on August 15, 2002

Figures

E–1. Graph showing frequency distribution of average daily per-capita water use among residences surveyed in East Greenbush and North Greenbush, N.Y.

Tables

1. Changes in depth to water, resident population, and well depth since 1970 and 1988 in several localities at Averill Park, N.Y.
2. Depth to water in three wells and calculated amount of water available for recharge during antecedent time periods.
3. Net change in water level from May 1999 to August 2002 in wells along Prospect Avenue at Averill Park.
4. Chemical analyses of ground-water samples collected from 21 wells at Averill Park.
5. Evidence of decrease in water hardness at several properties at Averill Park.
6. Well yield at Averill Park, as a function of location, well depth, and depth at which water enters the well.
7. Estimated domestic use of ground water within the part of Averill Park that is encompassed by a ground-water divide.
8. Net change in flow of the Wynants Kill in a 3,500-foot reach at Averill Park, August–September 2001.
   

Suggested citation

Randall, A.D., and Finch, A.J., 2008, 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: U.S. Geological Survey Scientific Investigations Report 2008–5087, 78 p., 3 pls., scale 1:24,000, online only.

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For further information, contact:

Director
U.S. Geological Survey
New York Water Science Center
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Troy, NY 12180

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