USGS
U.S. GEOLOGICAL SURVEY
Open-File Report 03-278

Approximate Potentiometric Surface of the Bedrock Aquifer at Great Bay, Southeastern New Hampshire, 2001

Prepared in cooperation with the
UNIVERSITY OF NEW HAMPSHIRE and the
COOPERATIVE INSTITUTE FOR COASTAL AND ESTUARINE ENVIRONMENTAL TECHNOLOGY

By R.M. Roseen, J.R. Degnan, L.K. Brannaka, T.P. Ballestero, and T.J. Mack


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ABSTRACT

Ground-water elevations (heads) in 265 wells were used to map the potentiometric-head surface in the fractured-bedrock aquifer adjacent to Great Bay, N.H.  Remotely sensed data, spring-water temperatures and specific conductances, land-surface elevations, and simulated heads were also used to guide construction of the potentiometric surface.  Heads in the bedrock aquifer locally varied more than 20 ft (feet) between adjacent wells as little as 200 ft apart.  The depth to water below land surface was greater on the western and northwestern sides of the bay (about 20 ft) than on the southern and eastern sides (about 8 ft).  Topography has a greater effect on heads than does temporal variation or measurement accuracy.  Head gradients are steeper at the southern side of Great Bay, where the topographic relief is greatest, than at the northwestern and northeastern sides.

INTRODUCTION

Great Bay, a 9.1-square-mile estuary near Portsmouth, N.H., is part of the National Estuarine Research Reserve System designated by the National Oceanic and Atmospheric Administration (NOAA).  The U.S. Geological Survey (USGS), in cooperation with the Civil Engineering Department at the University of New Hampshire (UNH) and the Cooperative Institute for Coastal and Estuarine Environmental Technology (CICEET), is assessing ground-water flow to the bay (Brannaka and others, 2002) to locate potential discharge sites, flow paths, and source areas of nutrients and other ground-water contaminants.  The approximation of the potentiometric-head surface in the bedrock aquifer presented in this report characterizes regional ground-water flow to Great Bay. 

Study Area

The regional ground-water-flow system consists of a till- or marine-sediment-covered crystalline bedrock aquifer.  Till, an unsorted mixture of sand, silt, and gravel, and marine sediments consisting of fine-grained sand, silt, and clay, are generally less than 20 ft thick.  Coarse-grained sand and gravel aquifers are generally small and discontinuous with the exception of a large ice-contact deposit beneath the former Pease Air Force Base, in Newington, N.H.  Crystalline bedrock consists of three main units (Lyons and others, 1997): the Kittery Formation, a metasandstone on the western side of the bay; the Eliot Formation, a phyllite along the eastern and southern sides of the bay; and the Exeter Diorite, inland west of the bay.Most bedrock wells are domestic wells cased from the land surface to a depth of at least 10 ft into bedrock.  Below the casing, the wells generally consist of an open borehole drilled from tens to hundreds of feet into the bedrock.

Acknowledgments

This project was conducted in cooperation with the National Oceanic and Atmospheric Administration (NOAA)/UNH Cooperative Institute for Coastal and Estuarine Environmental Technology  [NOAA grant numbers NA87OR512 and NA07OR0351].  Special thanks are extended to the homeowners who volunteered access to their land and wells, thereby making this study possible.

The authors wish to thank Peter Forbes, of the U.S. Air Force, and Julia Widman, of Montgomery Watson Herzog, Inc., for providing bedrock water-level data collected at the former Pease Air Force Base.  The authors thank the Federal, State, and municipal officials, residents, and well contractors who provided data for this study.

APPROACH AND METHODS

A generalized (20-ft contour) potentiometric-head surface was constructed by contouring bedrock ground-water elevations (heads) measured at 165 wells between June 11 to June 15, 2001.  The June 2001 head survey was augmented with heads measured in 86 bedrock wells at the former Pease Air Force Base in the spring of 2001.  Historical records from 14 bedrock well completion reports in the USGS's ground-water-site inventory database and additional qualitative data also were used to construct the head surface.

The vertical accuracy of the June 2001 heads and those measured at the former Pease Air Force Base is within 0.1 ft or less, because measurements were made from surveyed top-of-casing elevations.  The accuracy of the historical data is +/- one-half the topographic-map contour interval, or about 10 ft.  Annual and seasonal variations in heads were generally about 3 ft or less during 2001 and 2002 at the former Pease Air Force Base.  Tidal effects, measured in some bedrock wells in the study area (Montgomery Watson Herzog, Inc., 2002), were about 0.5 ft.  Water-level-measurement error, which is generally less than 1 ft, was not a factor in constructing the potentiometric surface.  Domestic-well owners in the study area were requested not to use their wells heavily during the June 2001 head survey to minimize the temporal effects of pumpage.  Residual drawdowns were estimated to be less than a few feet, but heads that appeared to be affected were removed from the database.

Qualitative data were used to indicate areas where the potentiometric surface in the bedrock aquifer may be above the surface of the bay.  Zones of ground-water discharge, interpreted from analysis of thermal infrared anomalies (Roseen and others, 2001; Roseen, 2002), and freshwater springs near bedrock outcrops (Degnan and Clark, 2002) with summer water temperatures less than surface-water temperatures (Roseen, 2002; Stewart Clark, Jr., U.S. Geological Survey, written commun., 1999) were used to approximate the location of the 0-ft head contour.  In areas where head data were particularly limited or unavailable, head contours were approximated from simulated heads (Mack and others, 2002) and digital-elevation-model data.  In some areas, contours are above the land-surface elevations shown on the topographic map.  These contours were modified where they were considered to be unrealistic, but were retained where they were considered to be reasonable and to contribute to a hydrologically sound representation of the regional potentiometric surface of the bedrock aquifer in the study area.

POTENTIOMETRIC SURFACE

Regional heads are depicted by a generalized potentiometric head surface because of inherent variations caused by the following:  (1) heads were measured in open bedrock boreholes and do not reflect head variations that may be present at individual fractures in a borehole; (2) heads in the bedrock wells show large differences (greater than 20 ft) over short distances (as little as 200 ft); and (3) temporal effects, which include tidal, seasonal, and annual variations, as well as residual drawdowns from domestic well use, are not accounted for.  Topographic relief in the study area ranges from 0 to nearly 300 ft and is probably the primary factor affecting the potentiometric surface.

Based on limited data, the average depth to water was about 20 ft on the northwestern and western sides of the bay and 8 ft on the southern and eastern sides.  The configuration of the potentiometric surface generally follows the land-surface topography.  Head gradients are steeper along the southern side of Great Bay, where the topographic relief is more pronounced, than at the northeastern and northwestern sides. 

REFERENCES CITED

Brannaka, L.K., Ballestero, T.P.,  Mack, T.J., and Roseen, R.M., 2002, Inflow loadings from ground water to the Great Bay Estuary:  CICEET Progress Reports through July 31, 2002: Durham, N.H., University of New Hampshire, 2 p.

Degnan, J.R., and Clark, S.F., Jr., 2002, Fracture-correlated lineaments at Great Bay, southeastern New Hampshire:  U.S. Geological Survey Open-File Report 02-13, 1 pl., 14 p.

Lyons, J.B., Bothner, W.A., Moench, R.H., and Thompson, J.B., 1997, Bedrock geologic map of New Hampshire:  U.S. Geological Survey Special Map, 1:250,000, 2 sheets (transverse mercator projection).

Mack, T.J., Degnan, J.R., and Moore, R.B., 2002, Regional simulation of ground-water flow in a fractured-bedrock aquifer, New Hampshire, in Fractured-Rock Aquifers 2002, Abstracts with Programs:  Denver, Colo., National Ground Water Association, p. 147-151.

Montgomery Watson Herzog, Inc., 2002, Summary results of the April 2002 Haven Well safe yield test, former Pease Air Force Base, Portsmouth, New Hampshire:  Malvern, Penn., 22 p.

Roseen, R.M., 2002, Quantifying groundwater discharge using thermal imagery and conventional groundwater exploration techniques for estimating the nitrogen loading to a meso-scale inland estuary:  Durham, N.H., University of New Hampshire, Ph.D. Dissertation, 188 p.

Roseen, R.M., Brannaka, L.K., and Ballestero, T.P., 2001, Assessing estuarine groundwater nutrient loading by thermal imagery and field techniques verified by piezometric mapping:  A methodology evaluation, in Geological Society of America Meeting, Boston, Mass., November 5-8, 2001:  Geological Society of America, session No. 17.


DOWNLOAD OPTIONS

The entire Open-File Report 03-278, which is comprised of the above text and the potentiometric surface map of the bedrock aquifer at Great Bay, may be downloaded from either of the following links. It is recommended that you right-click the link and use "save target as" or "save link as" to download the file to your computer because of the large file sizes. The dimensions of the map report are 27 x 32 inches, so it will require a large-format plotter for printing.

1:24,000-scale Map (27" x 32") available as a jpg image (5,883 KB)

1:24,000-scale Map (27" x 32") available as a PDF file (22,790 KB) Best viewed using Adobe Reader 6.

To open the PDF version, Adobe Acrobat Reader is required. If you do not have the latest version of this software installed on your computer, it can be downloaded for free from the Adobe website.


CONTACTS

For technical questions regarding this report, contact:

Tom Mack, tjmack@usgs.gov
phone: 603-226-7805
U.S. Geological Survey
361 Commerce Way
Pembroke, NH 03275

To order a paper copy of this report, contact:

USGS Information Services
Box 25286
Denver Federal Center
Denver, CO 80225-0046
Tel: 888-ASK-USGS; Fax 303-202-4693

For more information about USGS activities in New Hampshire,
visit the USGS New Hampshire-Vermont District home page.


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