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GROUNDWATER MODEL
Assessment
of Water Resources in the Seacoast Region of New Hampshire by Groundwater-Flow
Simulation
| Wells are being installed, and drilled progressively deeper, throughout the seacoast to meet the increasing demand for water. Considerable amounts of hydrologic data are being collected at sites of proposed large groundwater withdrawals; however, there currently are no comprehensive means to evaluate the cumulative hydrologic effects of new withdrawals, in conjunction with existing and proposed water uses, on the water resources of a region. Regional groundwater-flow models are tools used for evaluating the cumulative effects of multiple, and potentially competing, uses on water resources in a region. A portion of the seacoast study area (figure 1) was selected to apply a regional groundwater-flow model (figure 2). This area encompasses approximately 170 mi2 and has a population of more than 80,000. |
Figure 1.
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STUDY OBJECTIVES
| The objective of the groundwater-flow model is to provide a quantitative assessment of ground-water resources in the seacoast region (figure 2). The analyses will be conducted at the regional scale, and results reported at regional, watershed, subwatershed, and town scales. The model will be a valuable tool for (1) estimating groundwater recharge, discharge, and storage at these spatial scales; (2) assessing the cumulative effects of existing and proposed water resource uses and developments; and (3) evaluating the cumulative effects on water resource of various water management options. The model is not intended for use in site-specific hydrologic analyses without additional site-specific data and enhancements, but will be useful in providing a regional context for evaluating water resources. |
Figure 2. |
Specific objectives
of the groundwater-flow model include:
- Quantify the groundwater
resources of the seacoast model area.
- Estimate the hydrologic
effects of 10- and 20-year water-use projections on the water resources
of the model area.
- Estimate the hydrologic
effects of alternative planning and water-resources management options.
- Provide and document
the use of a groundwater-flow model for regional water-resources planning
and management.
SEACOAST GROUND-WATER-FLOW SYSTEM
The seacoast model area is largely bounded by saltwater bodies (figure 2) including the Atlantic Ocean, Great Bay, and the Piscataqua, Squamscott, and Merrimack Rivers. The saltwater bodies and rivers are used to form the hydrologic boundaries of a regional groundwater flow system. The Powwow River, a freshwater tributary to the Merrimack River, forms the southwest boundary of the regional flow system.
The groundwater-flow system in the model area consists of a fractured crystalline bedrock aquifer, of varying regional transmissivity, overlain by a relatively thin overburden (surficial) aquifer. A diagrammatic cross section between the Squamscott River and the Atlantic Ocean, presented in figure 3, indicates the general directions of groundwater flow. Groundwater flow in the bedrock aquifer likely follows short flowpaths and supply is largely supported by storage in the overburden aquifer, surface-water bodies or wetlands, and streams. However, where transmissivity of the bedrock aquifer is relatively high, groundwater flowpaths may be longer, crossing into adjacent watersheds, and storage may be higher than in other areas of the crystalline bedrock-aquifer.
A preliminary simulation of groundwater levels in the seacoast bedrock aquifer are shown in figure 4. A complete conceptual discussion of the seacoast groundwater-flow system, and preliminary model design and analysis is provided by Mack (2003). |
Figure 3.
Figure 4.
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APPROACH
The investigation
will provide an assessment of seacoast groundwater resources (groundwater
recharge, discharge, and storage) through an integrated analysis of geohydrologic
and water-use data using the U.S. Geological Survey's groundwater-flow
model software MODFLOW-2000. This investigation builds upon the results
and ongoing data collection activities by
the NH Geological Survey,
streamflow data collection by the USGS, and a detailed water use investigation
by the USGS. Specific tasks required for accomplishing the objectives
of the groundwater-flow simulation are described in the project work
plan.
PRODUCTS
Two products are planned
from the groundwater modeling component of the seacoast study:
- A USGS report
documenting the application of the groundwater model to evaluate water
resources, growth scenarios, and alternative management practices in
the seacoast region.
- Copies of the
groundwater-flow model datasets that will allow others to use the model.
BENEFITS
Results from this
study will contribute to a better understanding of surface water and groundwater
hydrology in the seacoast region of New Hampshire. The regional groundwater-flow
model will provide a valuable tool for (1) estimating groundwater
recharge, discharge, and storage at regional, watershed, and town scales;
(2) assessing the cumulative effects of existing and proposed water-resources
developments; and (3) evaluating alternative, large-scale planning and
management options that will be of assistance to planners at state, regional,
and town levels. The proposed model will provide a regional context for
more detailed site-specific hydrologic analyses by the private sector
in their assistance to local planners, developers, managers and regulators
in evaluating individual water-resources developments.
Specific benefits
of this investigation include the following:
- Establishment
of town-wide and subwatershed scale water balances.
- Quantification
of the immediate and long-term groundwater resource conditions in the
seacoast model area.
- Identification
of the hydrologic effects of water-resource development and of alternative
water-resource management scenarios.
- Documentation
of a groundwater-flow model for regional water-resource decision-making.
REFERENCES
Mack, T.J.,
2003, Preliminary ground-water-flow model of a coastal bedrock-aquifer
system, Southeastern New Hampshire: in MODFLOW and More 2003: Understanding
through Modeling, September 17-19, 2003, Abstracts with Programs: International
Ground Water Modeling Center, Denver, Co. [PDF
document, 1.16MB]
For technical questions, contact: Tom Mack, tjmack@usgs.gov
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U.S. Department of the Interior |
U.S. Geological Survey
