Regional hydrology and simulation of flow of stratified-drift aquifers in the glaciated northeastern United States

Abstract

The glaciated Northeast, which encompasses New England, New York, and northern parts of New Jersey, Pennsylvania, and Ohio, contains more than 5,000 aquifers composed of sand and gravel deposited by or in glacial meltwater. These stratified-drift aquifers are largely independent of one another but have similar hydraulic properties, similar modes of recharge and discharge, and similar interchange of water with regional streams that flow across them. Collectively, they are the principal source of ground water in this region, supplying about I billion gallons per day in 1987. The first part of this paper summarizes knowledge of the hydrologic processes and properties that control the water resources of stratified-drift aquifers and the second part focuses on simulations of those aquifers. The paper includes regional maps of average precipitation, runoff, and evapotranspiration that are mutually consistent in that precipitation minus evapotranspiration equals runoff at all locations; each map reflects the spatial variation in both precipitation and runoff data. The low sireamnows typical of late summer throughout the region reflect spatial variation in precipitation, in wetland area, and, most important, in properties of stratified-drift aquifers, chiefly their areal extent but also their hydraulic conductivity, specific yield, and topography. Large variations in annual precipitation between successive years or nearby basins are not accompanied by large variations in evapotranspiration. Ground-water evapotranspiration in the region is thought to decrease as depth to the water table increases beyond the reach of plant roots. In stratified drift, however, many trees send roots to depths of 20 feet or more. Seasonal changes in the relation of ground-water stage to base flow, which have been used to estimate ground-water evapotranspiration, can also be explained by seasonal variations in recharge resulting from variations in evapotranspiration from the unsaturated zone. Seasonal cycles in evapotranspiration and in river stage are the principal causes of water-table fluctuations in stratified-drift aquifers, but a persistent departure from average precipitation will cause a tong-term net water-table rise or decline in stratified drift that is either remote from streams or somewhat fine-grained. The horizontal hydraulic conductivity of stratified drift ranges from several feet per day for very fine sand to several thousand feet per day for gravel. Samples of clean, well-sorted sand, analyzed in the laboratory for the relation between grain size and hydraulic conductivity shows with remarkable consistency an upper limit of hydraulic conductivity, which increases as a log-linear function of median grain size. The hydraulic conductivity ranges over an order of magnitude below the upper limit in each grain-size class as a result of decreased sorting or increased silt content. Transmissivity of stratified drift has been estimated at many sites in the glaciated Northeast from aquifer tests specific capacity tests or the summing of estimated hydraulic conductivity values assigned to lithologic descriptions in borehole logs; where two or three of these approaches have been applied at the same sites however, the results are only weakly correlated.