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A study was done by the U.S. Geological Survey, in cooperation with the State of New Hampshire Department of Environmental Services, Water Resources Division, to describe the geohydrology and water quality of stratified-drift aquifers in the lower Connecticut River basin in southwestern New Hampshire. Stratified-drift aquifers discontinuously underlie 116 mi2 (square miles) of the lower Connecticut River basin in southwestern New Hampshire, which has a total drainage area of 1,163 mi2. Saturated thicknesses of stratified drift are locally greater than 400 feet, but generally are less. Transmissivities locally exceed 4,000 ft2/d (feet squared per day), but are generally much less. Only 20.8 mi2 are identified as having a transmissivity greater than 1,000 ft2Id. Characteristics of stratified-drift materials that affect ground-water storage and flow are related to the original glaciofluvial environment in which they were deposited. Deglaciation occurred by a systematic process of retreat that resulted in deltas and other lake deposits, primarily along the Connecticut and Ashuelot River valleys in the western part of the study area, and the deposition of eskers, kames, kame terraces, and outwash within some of the eastern upland valleys. Glacial Lake Hitchcock, which existed at the time of deglaciation, extended all the way from the lake outlet at what is now New Britain, Connecticut, northward up the Connecticut River valley, through the western part of the study area. The lake probably extended as far north as Burke, Vermont. The presence of this lake and other smaller lakes greatly affected deglaciation and the depositional processes that formed many of the stratified-drift aquifers found within the study area. The geohydrology of stratified-drift aquifers was investigated by focusing on basic aquifer properties, including aquifer boundaries; recharge, discharge, and direction of ground-water flow; saturated thickness and storage; and transmissivity. Surficial geologic mapping assisted in the determination of aquifer boundaries. Data from more than 2,300 wells, test borings, and springs were collected and stored in the U.S. Geological Survey's Ground-Water Site Inventory (GWSI) data base. More than 650 of these sites were from areas of stratified drift. These data were used to produce maps of water-table configuration, saturated thickness, and transmissivity of stratified drift. Seismic-refraction profiles were completed at 29 locations in the study area. These profiles aided in the construction of the water-table and saturated-thickness maps. Seismic-reflection data also were collected along 60 miles of the Connecticut River and were used in constructing the saturated-thickness maps. A stratified-drift aquifer in southern Croydon and northern Newport was analyzed for aquifer yield by transient simulations and use of a two-dimensional, finite-difference flow model. The objective was to estimate the yield after a 180-day period of pumping. Two hypothetical wells were pumped simultaneously at 1.0 Mgal/d (million gallons per day) and 0.5 Mgal/d for 180 days. The maximum amount of water, 1.97 cubic feet per second, was taken from the stream to sustain this pumping rate. The amount of water available for infiltration was the limiting factor. If another management plan were used or if more water were available for infiltration, increased pumping rates would be possible. Water samples from 22 wells and 8 springs were collected and analyzed to assess background water quality in the aquifers. Known areas of contamination were avoided. Results of the sampling program show that water in the stratified-drift aquifers meet U.S. Environmental Protection Agency primary and secondary drinking-water regulations, with some exceptions: water from 1 well had elevated concentrations of chloride and water from 3 wells had elevated concentrations of sodium; approximately half of the water samples had elevated concentrations of iron and manganese (a common and presumably natural phenomenon in New Hampshire); water from 11 of the wells had small but detectable concentrations of volatile organic compounds.

Geohydrology and water quality of stratified-drift aquifers in the lower Connecticut River basin, southwestern New Hampshire