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Bedrock Regional Aquifer Systematics Study


The goal of BRASS work in Connecticut is to determine the primary hydrogeologic characteristics of the crystalline bedrock formations in the State of Connecticut and, in combination with the surficial geology (glacial deposits), develop effective methodologies for determining source-water and wellhead protection criteria for bedrock and surficial aquifer ground-water wells throughout the state. This is a cooperative research project with the USGS-WRD Connecticut District and the Connecticut Geological and Natural History Survey.  Several areas in the state are receiving or will receive intensive study, including 1:24,000-scale bedrock and surficial mapping, well-data compilation and statistical analysis, and aquifer testing.  In conjunction with the bedrock mapping, U-Pb and 40Ar/39Ar radiometric dating of lithologic units by John Aleinikoff and Michael Kunk, respectively, have shed new light on the tectonic history of New England.  One of the goals in Connecticut is to produce a statewide map depicting dominant bedrock fracturing trends, in order to predict anisotropies in zones of contributing areas for wellfields.

1. Southern Connecticut focus area.  1:24,000-scale bedrock and surficial geologic maps are being made for the adjoining Old Lyme and Essex 7.5-minute quadrangles by Gregory Walsh, Robert Scott, Janet Stone, and Robert Wintsch (Indiana University).The two quadrangles will initially be published separately as open-file maps and eventually combined as a Miscellaneous Investigations bedrock-surficial hydrogeologic map. Well data are being compiled for the area, and aquifer tests and ground-water modeling are being conducted by Jeffrey Starn.  An abstract has been published on new U-Pb ages for rocks in the study area.

2. Western Connecticut focus area. The New Milford 7.5-minute quadrangle bedrock geologic map has been completed and will be used in conjunction with well data to evaluate bedrock geologic controls on well yield. A paper was published on the geology and geochronology of the area.

3. Pomperaug basin.
A new 1;24,000-scale geologic map is planned for the Pomperaug basin, an early Mesozoic basin covering parts of four 7.5-minute quadrangles that is an outlier of the much larger Hartford basin to the east. This basin comprises part of the Pomperaug River watershed, an area with significant ground-water supply issues, and it was identified as the area in greatest need of new geologic mapping by the State Geologist of Connecticut.

4. Statewide fracture characterization.  A statewide map is planned that depicts fracturing characteristics of the major bedrock geologic units in the state, as depicted on the Connecticut State Geologic Map [preliminary map]. An abstract has already been published on this concept.


Aleinikoff, J.N., and Walsh, G.J., 2002, SHRIMP U-Pb geochronology of zircon and sphene from the New Milford quadrangle, eastern Hudson Highlands, Connecticut: Geological Society of America Abstracts with Programs, v. 34, no. 1, p. 7.

Aleinikoff, J.N., Wintsch, R.P, Scott, R.B., Walsh, G.J., and Yacob, E.Y., 2003, New SHRIMP U-Pb ages for Late Proterozoic and Paleozoic igneous rocks in the Avalon and Bronson Hill terranes of south-central Connecticut: confirmations and surprises: Geological Society of America Abstracts with Programs, v. 35, no. 3, p.24.

Stone, J.R., Walsh, G.J., and Thomas, M.A., 2002, Characterization of bedrock aquifers in Connecticut: (1) geologic mapping and analysis: Geological Society of America Abstracts with Programs, v. 34, no. 1, p. 23.

Walsh, G.J., Aleinikoff, J.N., and Fanning, C.M., 2004, U-Pb geochronology and evolution of Mesoproterozoic basement rocks, western Connecticut, in Tollo, R.P., Corriveau, L., McLelland, J.M., and Bartholomew, M.J., editors, Proterozoic Tectonic Evolution of the Grenville Orogen in North America: Boulder, Colorado, Geological Society of America Memoir No.197, p. 729-753.

Walsh, G.J., 2001, Bedrock geology of the New Milford quadrangle, Connecticut: Geological Society of America Abstracts with Programs, v. 33, no. 1, p. 80.

Online Publications

Bedrock Geologic Map of the New Milford Quadrangle

Walsh, G.J., 2003, Bedrock geologic map of the New Milford quadrangle, Litchfield and Fairfield Counties Connecticut: U.S. Geological Survey Open-File Report 03-487, 49 p., scale 1:24,000.

Preliminary Bedrock Geologic Map of the Old Lyme Quadrangle

Walsh, G. J., Scott, R. B., Aleinikoff, J. N., and Armstrong, T. R., 2006, Preliminary bedrock geologic map of the Old Lyme quadrangle, New London and Middlesex Counties, Connecticut: U.S. Geological Survey Open File Report 2006-1296, scale 1:24,000.

Related links:

USGS Connecticut District
Connecticut Geological and Natural History Survey


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