T187 Advances in Understanding at the Groundwater–Surface Water Interface and Challenges for the Future (Posters): A Reflection on Tom Winter’s Legacy

Tuesday, 11 October 2011
9:00 AM - 6:00 PM
Hall C

THE HYDROGEOLOGY OF A HIGHLY UNSTABLE STREAMBANK AFFECTING NEW YORK CITY'S DRINKING WATER SUPPLY

Paper number: T187-13
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

In an effort to reduce chronic turbid conditions, the New York City Department of Environmental Protection (NYC DEP) has an ongoing plan to implement a stream restoration project on an actively failing bank of the Stony Clove Creek in the Catskill Mountains of upstate New York. The Stony Clove is a tributary of Esopus creek, which feeds the Ashokan Reservoir; one of New York City’s largest drinking water supplies. It is also the largest source of suspended sediments in the watershed. Monitoring wells were installed upslope of a large landslide during the planning stages of the bank stabilization project; however, the groundwater-surface-water interactions were neither explored nor taken into account in the design.

Water samples from each well, surface channel, and the Stony Clove were analyzed using an ion chromatography system, while slug and pump tests were performed on the wells to quantify aquifer properties. The Hvorslev Slug-Test Method was used to determine the hydraulic conductivity and transmissvity of the waterbearing units. The depth to water was higher than the depth to the well screen in all wells, demonstrating the confining effects of the clays which increase pore pressure and the potential for slope failure. Hydrogeochemical signatures indicate that the surface channels are not directly connected to the deeper groundwater system which seeps from the sandy layers of interbedded massive clays and sands and enters the Stony Clove Creek as baseflow. Hydraulic conductivity values as low as 0.0031 m/day and as high as 0.029 m/day were observed between the wells, as is typical for aquifers dominated by glacial clays. The lack of connectivity between the channels and the seeps located within the failure zone suggests that armoring the bank and diverting surface-waters, as is intended, will only offer a temporary fix.