Defining freshwater outcrops in West Neck Bay, Shelter Island, New York using direct contact resistivity measurements and transient underflow measurements

Mapping out groundwater discharge zones can be a formidable task due to the highly variable nature of the discharge zone, temporal and spatial variability of seepage rates within the zone, and complications from tidal loading. An integrated approach incorporating direct contact resistivity logging and transient seepage rate measurements was undertaken to delineate the seepage zone and monitor discharge on a bay-wide scale. Conductivity values for the saturated sediments ranged from 280 S/cm in freshwater zones to 12,800 S/cm in zones with high salinity. The discharge zone at West Neck Bay, Shelter Island was observed to extend to 10-75 feet offshore. The groundwater seepage within the discharge zone was measured using a time transient seepage meter that was developed with ultrasonic technology. Seepage velocities in the study area ranged from 1.27 x 10 -3 cm/s to 3.94 x 10 -5 cm/s, equivalent to a mean value of 16 /m 2 /d. Integrating over the horizontal extent of the seepage zone, the total daily discharge was estimated to be 1.72 x 10 6 /day for the north-east section of West Neck Bay. This estimate of the total discharge due to underflow is comparable to the recharge in the contributing area, estimated to be 1.50 x 10 6 /day for this section of the bay.