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

Abstract

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 dependin