3-D MAPPING OF GLACIAL AQUIFER SYSTEMS IN NEW ENGLAND: CASE STUDIES IN THE BIG RIVER BASIN, CENTRAL RHODE ISLAND AND IN SOUTH HADLEY, MASSACHUSETTS

In New England, glacial stratified deposits are the principal aquifers for public water supply. Ground-water studies have traditionally characterized these glacial aquifers as unpredictably heterogeneous and “chaotic”. Geologic models developed during the past half-century of systematic surficial mapping at 1:24,000-scale in southern New England have produced a methodology that allows us to predict the distribution of sedimentary textures within these highly complex glacial meltwater deposits. In recent years, collaborative, synergistic studies have utilized the knowledge gained in 3-D mapping efforts to address the many ongoing issues regarding water supply and ground-water contamination.

The Big River basin in central Rhode Island is a predominantly north-flowing drainage system in which a series of glacial lakes developed during the retreat of the late-Wisconsinan ice sheet. A northerly younging and lowering sequence of ice-marginal deltas, subaqueous fans, and lake-bottom sediments records systematic ice retreat; understanding of ice-margin positions supplemented with subsurface well-log data allowed mapping of the thickness and distribution of coarse-, medium-, and fine-grained units within the so-called “stratified drift aquifer”. The 3-D stack-unit map was used to identify six optimal locations for wells in which to conduct aquifer testing. Wells were pumped at 325 to 920 gal/min and each was shown to be capable of yielding 1-2 million gal/day. In a subsequent study, a ground-water flow model constructed using the 3-D materials map simulated the effects on streamflow of various pumping scenarios in which ground-water withdrawal rates ranged from 2 to 11 million gal/day within the basin.

The area surrounding the Dry Brook public-supply well in South Hadley, MA is underlain by coarse-grained ice-marginal lacustrine fan and deltaic sediments and lake-bottom varved silts and clays of glacial Lake Hitchcock. A 3-D materials map with subsurface contours drawn on coarse- and fine-grained units was used to construct a ground-water flow model that delineates areas contributing water to the well. Precise geologic mapping of the lateral and vertical extent of fine-grained confining beds within the aquifer was shown to be an important factor for accurate delineation of the contributing area.