AN INTEGRATED HYDROGEOLOGIC AND GEOPHYSICAL STUDY OF WATER FLUXES IN A NATURE PRESERVE

A 1.4 km² watershed adjacent to the Binghamton University campus, southern New York, was the subject of a six-month investigation combining shallow geophysical methods with hydrologic measurements in order to identify the geologic controls on groundwater flow, and quantify surface and groundwater fluxes. The hills are covered with deciduous forests surrounding a pond and wetland. Beaver dams separate the surface water bodies into four ponds having hydraulic connection. One outflow controls all surface water discharging from the basin. Geophysical surveys (seismic refraction, gravity, electrical resistivity) constrained bedrock and water table elevations, and results were calibrated to a well drilled into bedrock. Hydrologic data (precipitation, evaporation, runoff, storage changes, groundwater flow) were collected throughout the basin from May through October 2000 for the generation of a weekly hydrologic budget. Groundwater gradients were measured using shallow wells (<1m deep) located within 15 meters from shorelines. Piezometers were embedded in pond bottom sediments in order to quantify seepage rates. Sediment cores were taken from the ponds and surrounding hillslopes to estimate grain size and permeability.

Geophysical surveys have shown the area to be shale bedrock overlain by deposits of glacial till of varying thickness (35-50m). A glaciolacustrine clay facies is thought to perch the ponds above the surficial aquifer. The watershed is bounded to the north by a ribbed moraine and to the south by steep valley walls where bedrock outcrops in several places. Groundwater gradients indicate that surface water recharge occurs via groundwater flow from hillslopes, however sediment permeabilities (~10^-6 cm/s) cause the recharge rate to be low. High springtime water levels are caused by runoff from melting snow, however low levels recorded during dry summer months are due to a regional depression of the water table. Therefore, water fluxes during the dry season are strongly governed by groundwater levels and flow rates, whereas fluctuations observed during the spring wet season are more likely influenced by surface hydrologic processes.