HYDROLOGIC BUDGET OF A GROUNDWATER FLOW-THROUGH RESERVOIR ON SANDSTONE-PAVEMENT AT ALTONA FLAT ROCKS, NORTHEASTERN NEW YORK STATE

We report the results of a study to determine the hydrologic budget of a groundwater flow-through reservoir located on a sandstone- pavement at Altona Flat Rocks in northeastern New York State. Altona Flat Rock is one of a chain of unique ecosystems ranging through Clinton County (northeastern New York State) and into southern Quebec located on the Cambrian Potsdam Sandstone. These ecosystems were stripped of much of their soil and other unconsolidated material during catastrophic flooding during deglaciation. The reservoir is a remnant of a dam that was abandoned in the 1930’s The Potsdam Sandstone has a low permeability matrix. Much of the water flow is through bedding-plane fractures and vertical joints. Infiltration is very low. The small patches of soil and vegetation that do exist are quickly saturated to capacity. Consequently, there is a significant overland flow during precipitation events. Stream flow to and from the reservoir was monitored. Streams and reservoir stages were measured every 15-minutes. Rating curves were used to calculate actual discharge. We also monitored precipitation and other meteorological parameters. Evaporation rates were calculated. To test the water holding capacity of soils and vegetation mats, laboratory experiments were conducted on soil and vegetation samples. Groundwater flux was estimated from the hydrologic budget and checked using a groundwater flow model (MODFLOW) and the Dupuit-Forcheimer Equation. The hydrologic budget of the reservoir is strongly dependent on stage in the reservoir. The net groundwater fluxes fluctuate as a function of reservoir stage. As the stage of the reservoir increases, the groundwater inflow becomes less influential and seepage becomes greater. We found a critical water stage in the reservoir such the export of groundwater surpassed other hydrologic inputs, causing a negative reservoir budget. This is supported by historic anecdotes concerning the dam.