HYDROGEOLOGIC CONTROLS OF WETLANDS IN THE POTSDAM SANDSTONE: ADIRONDACKS AND LAKE CHAMPLAIN BASIN, NEW YORK STATE

As part of a NSF-sponsored two-year project to study the effects of the 1998 ice storm on the Altona-Flat Rocks, we studied the groundwater flow system through the fractured Potsdam Sandstone. The Altona-Flat Rocks is one of a series of exposed bedrock surfaces of Potsdam Sandstone that was cleared of overlying soil and other surface material during post-glacial catastrophic flooding. The site is instrumented with two recording meteorological stations, stream gauging stations, reservoir stage recorder and a network of 16 groundwater-monitoring wells. Groundwater flows predominately through horizontal and vertical fracture networks in the bedrock. Down-hole geophysical tests show that these fractures are extensive, intercepting several different groundwater-monitoring wells. Due to the orientation of the fractures and the regional dip, there are many areas where the fractures are exposed at the surface. We have observed that these fracture systems provide a considerable source of surface water. The source of the hydraulic head that drives these flow systems through the fractures is uncertain. Where the fractures are exposed, there is often an associated wetland. These wetlands receive most of their hydrologic budgets from the discharging groundwater. The duration of the groundwater discharge at these wetlands is affected by topography. Fractures in higher elevations tend to become dry soon after the spring melt. While fractures in lower elevations tend to discharge groundwater for a longer duration. From chemistry data of basic cations and anions, the groundwater has very low concentrations of dissolved solids. Thus, even in wetlands that receive a significant amount of groundwater, the wetlands tend to be nutrient-poor and characteristic of bogs or poor-fens.