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Delineation of Water Sources for Public-Supply Wells in Three Fractured-Bedrock Aquifer Systems in Massachusetts

Water-Resources Investigations Report 02-4290

By Forest P. Lyford, Carl S. Carlson, and Bruce P. Hansen

 

ABSTRACT

Fractured-bedrock aquifer systems in West Newbury, Maynard, and Paxton, Massachusetts, were studied to advance methods of data collection and analysis for delineating contributing areas to public-supply wells completed in fractured rock and for determining the effects of pumping on streams and wetlands. Contributing areas, as defined for this study, include all areas through which ground water flows from recharge areas to wells.

In West Newbury, exploratory public-supply wells at two locations were completed in phyllite of the Eliot Formation. Aquifer testing indicated that subhorizontal and steeply dipping fractures that parallel two sets of foliation form elongated transmissive zones in the bedrock aquifer near the two well locations and also form a vertical hydraulic connection to surficial materials consisting of till at one location and marine clay at the other location. Recharge to bedrock is largely through a thin veneer of till over bedrock, but leakage through thick drumlin tills also recharges bedrock. Simulated contributing areas for the three supply wells pumped at a combined rate of 251 gallons per minute encompass about 1.3 square miles and extend to ground-water divides within most of a subbasin of the Artichoke River. Pumping likely would reduce streamflow in the Artichoke River subbasin by approximately the pumping rate. Pumping is likely to affect wetland areas underlain by till near the wells because of the vertical hydraulic connection to surficial materials.

In Maynard, three exploratory public-supply wells were completed in coarse-grained schist of the Nashoba Formation. Aquifer testing indicated that a dense network of fractures in bedrock forms a laterally extensive transmissive zone in bedrock that is well connected vertically to surficial materials consisting of sandy till, lacustrine silts, sand and gravel, and wetland deposits. The simulated contributing area for the three supply wells pumped at a combined rate of 780 gallons per minute encompasses about 1.8 square miles of the Fort Pond Brook drainage area. Pumping likely would reduce streamflow in Fort Pond Brook by about the same amount as the pumping rate, and wetland-water levels within a 2,000-foot radius from the wells are likely to be lowered below the land surface by pumping.

In Paxton, three existing supply wells are completed in granofels and schist of the Paxton and Littleton Formations. Aquifer testing demonstrated that a shallow bedrock well completed to a depth of 150 feet is closely connected hydraulically to overlying till. Two deep wells, however, receive much of their water from fractures at depths below 500 feet. Ground-water flow in bedrock appears to be mostly through parting fractures along a foliation set that dips gently (10 degrees) eastward. These parting fractures at depth are poorly connected vertically to shallow bedrock

CONTENTS

Abstract

Introduction

Purpose and Scope 

Acknowledgments

Study Methods

Well Installation

Geologic Mapping

Borehole Geophysics

Hydrologic Data Collection

Aquifer Testing

Positioning of Data-Collection Points and Location Identifiers

Numerical Modeling

Knowles and Andreas Well Sites, West Newbury, Massachusetts

Geology

Bedrock

Surficial Geologic Units

Ground-Water-Flow Patterns and Water-Level Fluctuations

Ground-Water Recharge

Aquifer Testing and Observed Hydrologic Responses to Pumping

Hydraulic Properties of Geologic Units

Numerical Model

Areal Extent and Boundary Conditions

Horizontal and Vertical Discretization

Model Stresses

Hydraulic Properties

Alternative Models for Uncertainty Analysis

Model-Simulated Heads

Simulated Contributing Areas to Wells

Simulated Effects of Pumping on Streamflow and Wetlands

Rockland Avenue Well Site, Maynard, Massachusett

Geology

Bedrock

Surficial Geologic Units

Ground-Water-Flow Patterns and Water-Level Fluctuations

Ground-Water Recharge

Aquifer Testing and Observed Hydrologic Responses to Pumping

Hydraulic Properties of Geologic Units

Numerical Model

Areal Extent and Boundary Conditions

Horizontal and Vertical Discretization

Model Stresses

Hydraulic Properties

Alternative Models for Uncertainty Analysis

Model-Simulated Heads

Simulated Contributing Areas to Wells

Simulated Effects of Pumping on Streamflow and Wetlands

Leicester Well Site, Paxton, Massachusetts

Geology

Bedrock

Surficial Geologic Units

Ground-Water-Flow Patterns and Water-Level Fluctuations

Ground-Water Recharge

Aquifer Testing and Observed Hydrologic Responses to Pumping

Hydraulic Properties of Geologic Units

Numerical Models

Areal Extent and Boundary Conditions

Horizontal and Vertical Discretization

Model Stresses

Hydraulic Properties

Model-Simulated Heads

Simulated Contributing Areas to Wells

Simulated Effects of Pumping on Streamflow and Wetlands

Summary and Conclusions

References Cited

Appendix 1. Considerations for Determining Contributing Areas to Wells and Aquifer System Responses to Pumping in Crystalline Bedrock

Information Sources and Data-Collection Methods

Water-Level Data

Transmissivity of Geologic Units

Bedrock Surface

Recharge and Leakage Rates

Storage Properties

Transient Simulation of Contributing Areas

Streamflow and Wetland Responses to Pumping

Appendix 2. Definitions of Geologic Terms


Continued on Page 2

AVAILABILITY

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PDF version of report:
        title page, table of contents, West Newbury site; pp. 1 to 39
        Maynard site, pp. 40 to 63
        Paxton site, pp. 63 to 113

The citation for this report, in USGS format, is as follows:

Lyford, F.P., Carlson, C.S. and Hansen, B.P., 2003, Delineation of Water Sources for Public-Supply Wells in Three Fractured-Bedrock Aquifer Systems in Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 02-4290, 114 p.

 For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.




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Last modified: Friday, September 16 2005, 04:23:02 PM
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