CHARACTERIZATION OF BEDROCK AQUIFERS IN CONNECTICUT: (1)GEOLOGIC MAPPING AND ANALYSIS

In the glaciated terrain of New England, bedrock aquifer characterization is complex because the character of both bedrock and overlying glacial materials varies greatly over short distances. Surficial materials range locally from thin to absent to several hundred feet thick. Textures range from high permeability sand and gravel to low permeability glacial lake clay and till. Bedrock in much of Connecticut is highly metamorphosed and structurally complex. Traditional bedrock geologic maps provide limited information about the physical properties and fracture characteristics of the bedrock. New 1:24,000-scale geologic mapping, with emphasis on the geometry of fracture networks, was undertaken in the New Milford and Old Lyme quadrangles to assess the water-bearing potential of bedrock aquifers and to maximize the ability to conceptualize and model ground-water flow. The new mapping also delineates surficial units where these materials are sufficiently thick to affect the water-bearing characteristics of the bedrock.

A conceptual model for the statewide characterization of bedrock classifies the formational units of Rodgers (1985) into "layered" and "nonlayered" groups. Layered rocks include well-foliated gneisses and schists of the crystalline uplands and sedimentary rocks of the Hartford Basin. Nonlayered rocks include basalt and diabase of the Hartford Basin, poorly foliated metaigneous rocks, and massive marble. In layered rocks, parting along compositional layers or foliation surfaces may significantly control fracture geometry. Where layer-parallel fractures dip steeply, subhorizontal unroofing joints may produce hydraulically significant low-angle fractures. Where layer-parallel fractures dip gently, strain release during unroofing has taken place along layer-parallel fractures. In nonlayered rocks, significant fractures may include subhorizontal unroofing joints and steeply dipping joints, including cooling joints.

Models for characterizing fracture geometry in various types of bedrock must be tested by wellfield-scale investigations that include aquifer testing and ground-water-flow modeling. Results of a study in Old Lyme indicate that preferential fracturing along foliation planes may produce significant directional anisotropy in the bedrock aquifer.