Tracy H. Dorgan | USGS Science

USGS1995

Results of geophysical surveys of glacial deposits near a former waste-disposal site, Nashua, New Hampshire

Geophysical investigations were done near a former waste-disposal site in Nashua, New Hampshire to determine the thickness and infer hydraulic characteristics of the glacial sediments that underlie the area. Approximately 5 miles of ground- penetrating radar (GPR) data were collected in the study area by use of dual-80 Megahertz antennas. Three distinct radar-reflection signatures were evident from the data and are interpreted to represent (1) glacial lake-bottom sediments, (2) coarse sand and gravel and (or) sandy glacial till, and (3) bedrock. The GPR signal penetrated as much as 70 feet of sediment in coarse-grained areas, but penetration depth was generally less than 40 feet in extensive areas of fine-grained deposits. Geologic features were evident in many of the profiles. Glacial-lake-bottom sediments were the most common features identified. Other features include deltas deposited in glacial Lake Nashua and lobate fans of sediment deposited subaqueously at the distal end of deltaic sediments. Cross-bedded sands were often identifiable in the deltaic sediments. Seismic-refraction data were also collected at five of the GPR data sites. In most cases, depths to the water table and to the till and (or) bedrock surface indicated by the seismic-refraction data compared favorably with depths calculated from the GPR data. Test holes were drilled at three locations to determine the true depths to radar reflectors and to determine the types of geologic material represented by the various reflectors.

Open-File Report↗

USGS1995

Geohydrology of the Flints Pond Aquifer, Hollis, New Hampshire

Flints pond has been subjected to accelerated eutrophication as a result of watershed development (building of new homes and conversion of summer cottages into permanent homes) since the 1930's. Ground-water flow is the primary recharge and discharge mechanism for Flints Pond. The saturated thickness, transmissive properties, and altitude of the water table were determined by use of surface geophysics, test drilling, and aquifer-test data. Information on the geohydrology of the adjacent Flints Pond aquifer can be used in developing a water and nutrient budget for the pond-aquifer system. Ground-penetrating-radar surveys were done over more than 4 miles of the study area and on Flints Pond. Three distinct reflection signatures were commonly identifiable on the radar profiles: (1) thin, relatively flat-lying, continuous reflectors that represent fine-grained lacustrine sediment; (2) subparallel to hummocky and chaotic, coarse-grained reflectors that possibly represent coarse-grained ice-contact deposits or deltaic sediments in a lacustrine environment; and (3) sharply diffracted, fine-grained, chaotic reflectors that represent till and (or) till over bedrock. The saturated thickness of the aquifer exceeds 90 feet in the northern end of the study area and averages 30 to 50 feet in the southern and eastern parts. The saturated thickness of the western part is generally less than 10 feet. Test borings were completed at 19 sites and 13 wells (6 of which were nested pairs) were installed in various lithologic units. A water-table map, constructed from data collected in November 1994, represents average water-table conditions in the aquifer. Horizontal hydraulic conductivities calculated from single-well aquifer test data for stratified drift range from 2.8 to 226 feet per day. Hydraulic conductivities were quantitatively correlated with the reflector signatures produced with ground-penetrating radar so that transmissivities could be inferred for areas where well data were unavailable but where ground- penetrating-radar surveys were done. A saturated- thickness and transmissivity map for the aquifer shows that transmissivities exceeds 3,000 feet squared per day in the southern and east-central parts of the aquifer. Transmissivity ranges from 1,000 to 2,000 feet squared per day in the northern part of the aquifer and is generally less than 1,000 feet squared per day in the western part.

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