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Scientific Investigations Report 2008–5222

Prepared in cooperation with the
New Hampshire Department of Environmental Services,
Coastal Program, and Geological Survey

Assessment of Ground-Water Resources in the Seacoast Region of New Hampshire

By Thomas J. Mack

Abstract

Thumbnail of and link to report PDF (10.3 MB) Numerical ground-water-flow models were developed for a 160-square-mile area of coastal New Hampshire to provide insight into the recharge, discharge, and availability of ground water. Population growth and increasing water use prompted concern for the sustainability of the region’s ground-water resources. Previously, the regional hydraulic characteristics of the fractured bedrock aquifer in the Seacoast region of New Hampshire were not well known. In the current study, the ground-water-flow system was assessed by using two different models developed and calibrated under steady-state seasonal low-flow and transient monthly conditions to ground-water heads and base-flow discharges. The models were, (1) a steady-state model representing current (2003–04) seasonal low-flow conditions used to simulate current and future projected water use during low-flow conditions; and (2) a transient model representing current average and estimated future monthly conditions over a 2-year period used to simulate current and future projected climate-change conditions.

The analysis by the ground-water-flow models indicates that the Seacoast aquifer system is a transient flow system with seasonal variations in ground-water flow. A pseudosteady- state condition exists in the fall when the steady-state model was calibrated. The average annual recharge during the period analyzed, 2000–04, was approximately 51 percent of the annual precipitation. The average net monthly recharge rate between 2003 and 2004 varied from 5.5 inches per month in March, to zero in July, and to about 0.3 inches per month in August and September. Recharge normally increases to about 2 inches per month in late fall and early winter (November through December) and declines to about 1.5 inches per month in late winter (January and February). About 50 percent of the annual recharge coincides with snowmelt in the spring (March and April), and 20 percent occurs in the late fall and early winter (November through February). Net recharge, calculated as infiltration of precipitation minus evapotranspiration, can be negative during summer months (particularly July).

Regional bulk hydraulic conductivities of the bedrock aquifer were estimated to be about 0.1 to 1.0 feet per day. Estimated hydraulic conductivities in model areas representing the Rye Complex and the Kittery Formation were higher (0.5 to 1 foot per day) than in areas representing the Eliot Formation, the Exeter Diorite, and the Newburyport Complex, which have estimated hydraulic conductivities of 0.1 to 0.2 foot per day. A northeast-southwest regional anisotropy of about 5:1 was estimated in some areas of the model; this pattern is parallel to the regional structural trend and predominant fracture orientation. In areas of the model with more observation data, the upper and lower 95-percent confidence intervals for the estimated bedrock hydraulic conductivity were about half an order of magnitude above and below the parameter, respectively, and the estimated confidence intervals for estimated specific storage were within an order of magnitude of the parameter. In areas of the model with few data points, or few stresses, confidence intervals were several orders of magnitude. Estimated model parameters and their confidence intervals are a function of the conceptual model design, observation data, and the weights placed on the data.

The amount of recharge that enters the bedrock aquifer at a specific point depends on (1) the location of the point in the flow field; (2) the hydraulic conductivity of the bedrock (or the connectivity of fractures); and (3) the stresses within the bedrock aquifer. In addition, ground water stored in unconsolidated overburden sediments, including till and other fine-grained sediments, may constitute a large percentage of the water available from storage to the bedrock aquifer. Recharge into the bedrock aquifer at a point can range from zero to nearly all the recharge at the surface depending on regional hydrogeologic and anthropogenic factors. In a setting with few ground-water withdrawals, a larger portion of the recharge in the ground-water-flow system remains in the unconsolidated aquifers or upper bedrock than moves through the deeper bedrock aquifer, even in a setting with conductive bedrock, at any given time. With increased withdrawals in the bedrock aquifer, a larger proportion of the recharge in the aquifer system will move into the deeper areas of the aquifer system at any given time.

Ground-water residence time estimated by chlorofluorocarbon age-dating methods ranged from near zero (recently recharged) to more than 50 years. Ground water was oldest in areas with little water use, a low head gradient above the point of interest, and at a point of discharge in the flow system. At such locations, ground water may have flowed a considerable distance in the watershed. Where water use was high, or at an area of recharge, the ground-water age may be younger. At large ground-water withdrawal points, ground water withdrawn includes a mix of water from recently recharged to that with residence times 30 years or more. The ground-water flow to large withdrawals includes ground water in the immediate area of the well and older water from greater distances. The age of water captured by recently installed large ground-waterwithdrawal wells may become younger with time as the effects of the withdrawal on the flow system become established and the flow system reaches a new equilibrium.

Simulated effects to the Seacoast hydrologic system caused by increasing future water use include stream base flows declining by about 7 percent; fresh ground-water discharges to tidal bays, estuaries, and the ocean declining by about 2 percent; and lowered ground-water levels. Changes in ground-water levels were subtle but were greatest near large ground-water withdrawals with increasing demands and in developing rural areas. On the basis of the simulations, the hydrologic system will be most affected during periods of low flow, which may result in longer annual low-streamflow periods. Simulations show that the effects of increased demand will likely become apparent within the next 10 years (before 2017). Simulations of a hypothetical increase in sewering result in further declines in base flow (13 percent) and discharge to bays, estuaries, and the ocean (5 percent).

Climate change in New England is forecast to include more frequent and intense precipitation events, with a slight decrease to little change in total precipitation, and increasing temperatures. The effects of this potential future climate change on the Seacoast hydrologic system would likely include reduced base flows and fresh ground-water discharges to tidal areas and lowered ground-water levels. The effects of these climate changes by 2025 were estimated to be greater than the potential effects of increased water demands. The analyses indicated that there are potential issues of concern for future use of water resources in the Seacoast region. The models developed and demonstrated in this investigation can provide water-resource managers and planners tools with which to assess future water resources in this region. The findings regarding the effects of increasing water demand and potential climate change on ground-water availability may be transferrable to other regions of the Nation with similar hydrogeologic and climatic characteristics.

 

Revised February 2012

First posted June 22, 2009

  • Appendixes 1–10 and
    model files
    Revised model files are provided at the link above.
    The ten appendixes are presented as ten individual PDF files. DVD users can find them in the folder named pdfs. Web users can reach them by the link above: click to open each individually, or click on the link below for one large PDF of all ten apdix01-10all.pdf (15.9 MB).

For additional information contact:
Director
U.S. Geological Survey
New Hampshire-Vermont Water Science Center
361 Commerce Way
Pembroke, NH 03275
(603)226-7800

http://nh.water.usgs.gov

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Suggested citation:

Mack, T.J., 2009, Assessment of ground-water resources in the Seacoast region of New Hampshire: U.S. Geological Survey Scientific Investigations Report 2008–5222, 188 p., available online at http://pubs.usgs.gov/sir/2008/5222.



Contents

Abstract

Introduction

Purpose and Scope

Description of the Study Area

Geologic and Geographic Setting

Background and Previous Studies

Methods

Hydrogeologic Characteristics of the Seacoast Region

Hydraulic Properties

Surficial Deposits

Bedrock

Ground-Water Flow

Recharge

Water Levels

Surface Water

Ground-Water-Flow Simulation

Model Design and Spatial Discretization

Simulated Recharge, Discharge, and Storage

Storage

Future Water Availability

Potential Future Water Use

Potential Climate Change

Discussion

Model Limitations

Summary and Conclusions

References Cited


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