Table of Contents

Introduction
Objectives
Approach
Products and Benefits
Timeframe
Selected References

INTRODUCTION

Ground water withdrawn from fractured bedrock provides 25 percent of the total drinking water and 85 percent of the water for private domestic wells in the State. It also accounts for 8 percent of the drinking water supplied by public systems and 5 percent of the water used for commercial, industrial, and agricultural purposes.

Demand for ground water from the bedrock aquifer is continuously increasing as new sources of surface water decrease and the cost of surface-water treatment increases. In addition, not all communities have sand and gravel aquifers that are sufficient for public water supply or for commercial or industrial demands. Thus, evaluation of potential water availability from the bedrock aquifer is a major part of a comprehensive assessment of the State's ground-water resources.

The statewide bedrock aquifer resource assessment was initiated in 1994. The purpose of the study is to identify potential high-yielding sources of ground water and analyze the quality of water from this source. The results of this study will provide information useful to communities, as well as to regional and State planners, in the development of water supplies and the management of aquifer and well-head-protection programs.

OBJECTIVES

The major objectives of the bedrock aquifer resource assessment are to:

• Identify geologic and other factors, such as rock type and fracture characteristics, that affect the yields of bedrock wells in New Hampshire.

• Develop predictive tools and data needed by communities for evaluating the ground-water-development potential of bedrock aquifers in their jurisdictions.

• Produce statewide maps that identify the relative potential ground-water yields of bedrock aquifers.

• Describe the ambient quality of ground water in bedrock throughout the State and identify areas of potential bedrock water-quality concerns such as high iron and manganese.

Assessment of potential water supplies from the bedrock aquifer is complex because of the wide variation, from one location to another, in bedrock fracture characteristics. A method that has worked well in New Hampshire is based on the identification of fracture traces on the land surface using aerial photographs and other remotely sensed imagery.

Four types of imagery are being used to analyze the State's land surface for bedrock fractures. These include Landsat imagery (obtained from satellites that orbit the Earth at an altitude of about 570 miles), side-looking radar (obtained from aircraft flown at an altitude of about 33,000 feet), high-altitude aerial photography (flown about 40,000 feet above the land surface), and low-altitude photography (flown about 20,000 feet above the land surface). Observations in the field will also be used to supplement fracture-characteristic data in some locations.

Fracture data obtained from the aerial and satellite imagery and field observations will be compared with data on water yields for more than 18,000 bedrock wells that are in the New Hampshire Department of Environmental Services well data base. From this comparison, relationships will be developed between yields of bedrock wells, in gallons per minute, and such factors as the proximity of the fracture traces on the land surface to the wells, bedrock type and age, thickness of the sediment overburden, physiography of the area, and types of well construction.

By using the established relationships, maps will be prepared that show areas of potential development of ground-water supplies in bedrock aquifers. Maps will also be prepared to show areas with various background water-quality characteristics.

The compilation and production of the mapping will be done with the aid of a computerized Geographic Information System (GIS). A GIS is a system where maps are stored, analyzed, and plotted by use of a computer.

Subsurface analyses using geophysical techniques will be employed at some of the highest yielding bedrock well sites in the State. These analyses will identify and demonstrate new tools and procedures for identifying high-yielding zones in bedrock.

PRODUCTS AND BENEFITS

The New Hampshire Bedrock Aquifer Resource Assessment program plans to produce a number of products that will benefit development and management of ground-water resources in the State.

• Statewide maps providing the locations of major fracture traces on the land surface at a scale of 1:48,000 (1 inch = 4,000 feet).

• Predictive information that can be used for assessing the water-supply development potential of bedrock aquifer sites throughout the State.
  (Assessing High-Yielding Bedrock Aquifers in New Hampshire)

• Statewide maps showing zones of relative potential water yield at a scale of 1:48,000.

• An evaluation of the effectiveness of geophysical tools and procedures used in locating potential high-yield zones for specific bedrock well sites.

• Reconnaissance-level maps and assessment of the quality of water from the bedrock aquifer that can be used to identify potential need for treatment.

• Map products will be available in computerized GIS format.

The statewide bedrock aquifer assessment was initiated in February 1994 and is underway. The initial product, statewide fracture-trace maps, will be produced by the end of 1997. Final maps of potential water yield and reports providing predictive relations, water-quality data, and an assessment of geophysical techniques are planned for publication in 2001.

--- Richard Bridge Moore, Stewart F. Clark, Jr., and James R. Degnan

SELECTED REFERENCES:

Chormann, Frederick H., 1990, Bedrock water wells in New Hampshire: a statistical summary of the 1984-1990 inventory: Concord, N.H., New Hampshire Department of Environmental Services, NHDES-WRD-90-3, 26 p.

Daniel, C.C., III, 1989, Statistical analysis relating well yield to construction practices and siting of wells in the Piedmont and Blue Ridge provinces of North Carolina: U.S. Geological Survey Water-Supply Paper 2341-A, 27 p.

Haeni, F.P., Lane, J.W., Jr., and Lieblich, D.A., 1993, Use of surface-geophysical and borehole-radar methods to detect fractures in crystalline rocks, Mirror Lake area, Grafton County, New Hampshire, in Banks, Sheila, and Banks, David, eds., Hydrogeology of hard rocks, International Association of Hydrologists, 24th Congress, Oslo, Norway, June 1993, Proceedings: Oslo, Norway, International Association of Hydrologists, p. 577-587.

Lattman, L.H., and Parizek, R.R., 1964, Relationship between fracture traces and the occurrence of ground water in carbonate rocks: Journal of Hydrology, v. 2, p. 73-91.

Lyons, J.B., Bothner, W.A., Moench, R.H., and Thompson, J.B.,Jr., eds., 1986, Interim geologic map of New Hampshire: Concord, N.H., New Hampshire Department of Resources and Economic Development, Open-File Report 86-1, 1 sheet, scale 1:250,000.

Related USGS links:
Geology -- Water -- Mapping