HYDROLOGIC EFFECTS OF SHUTDOWNS OF THE RONDOUT-WEST BRANCH WATER TUNNEL ON THE GROUNDWATER-FLOW SYSTEM IN WAWARSING, NEW YORK

Flooding of residential basements and bacterial contamination of private-supply wells are recurring problems in the Rondout Valley near the Town of Wawarsing, Ulster County, New York. Leakage from the Rondout-West Branch Water Tunnel and above-normal precipitation has been suspected of causing elevated groundwater levels and basement flooding. A network of 41 wells (23 unconsolidated aquifer and 18 bedrock wells) and 2 surface-water sites was used to monitor the hydrologic effects of water tunnel shutdowns in a 12-square-mile study area. The area is underlain by a sequence of northeast-striking sedimentary bedrock that contains dissolution features, fractures, and faults. Inflows of less than 1 to more than 9,000 gallons per minute from the fractured bedrock were documented during construction of the water tunnel, which is located more than 700 feet beneath the study area. Glacial deposits infill the valley above the bedrock sequence and consist of clay, silt, sand, and gravel. The groundwater-flow system in the valley consists of both fractured-rock and unconsolidated aquifers. Potentiometric-surface maps indicate that groundwater in the bedrock flows from the surrounding hills on the east and west sides of the valley toward the center of the valley. Water levels in the bedrock and unconsolidated aquifers fluctuated about 5 feet seasonally.

Some bedrock wells had tunnel-leakage influence changes in water levels that ranged from 1.5 to 12 ft, and delay times ranging from 0.5 to 60 hours, at distances up to 7,000 ft from the tunnel. Tunnel-influence response of the bedrock aquifer is consistent with the presence of discrete high-transmissivity networks along fractured limestone bedding strike that have undergone dissolution.

Some unconsolidated aquifer wells indicated water-level changes due to tunnel leakage from the bedrock. Water levels changed in these wells as much as 2.5 ft within 18 hours of tunnel shutdowns, but water-level changes in other unconsolidated-aquifer wells were smaller or nonexistent.