Hydrologic Evaluation of Groundwater Discharge, Nauset Marsh, Cape Cod National Seashore, Massachusetts

Urish, Daniel; Qanbar, Entifadh

The principal objective of this 1992-1994 study was to evaluate the quantity and quality of coastal groundwater discharge into the Nauset Marsh System. Much of the 1927 hectare watershed of the Nauset Marsh System is developed for residential or commercial purposes. All of the area is serviced by on-site individual sewage disposal systems. Of particular concern is the potential for eutrophication in the confined, poorly flushed components of the Nauset Marsh System due to discharge of nutrient enriched groundwater. Five locations were selected for detailed field measurements of groundwater discharge characteristics and water quality. These locations represent a variety of regional conditions, namely variations in residential population, geologic characteristics and shoreline vegetation. Each site was subjected to groundwater piezometric measurements and water quality sampling to determine patterns of flow and variations in salinity and nitrogen levels. Discharge of groundwater occurs during low tide in a highly irregular pattern along the shore. Permeability tests and grain size analysis show that greater flow occurs in zones of clean sands, with little water passing through sediments where abundant vegetation tends to trap less permeable silt. Thermal infrared photography taken at low tide showed that groundwater discharge varies greatly along the shore margin in much the same pattern as that identified in the smaller scale site studies. At each site monitor wells were placed for water level measurements and water quality sampling. The usual pattern of beach groundwater in the inter tidal beach zone is that of an upper salty layer caused by salt water infiltration at high tide, a fresh water layer of inland origin moving to discharge at the coast, and an underlying salt water layer connected to the bay through permeable sediments. In more prolific spring areas, the maximum groundwater discharge rate at low tide was calculated to be 3.0 cubic meters/day/square meter of beach face. The water discharge is at a maximum velocity of 9.0 meters/day. As a general observation, field measurements at the sites showed a trend of increasing nitrogen with decreasing salinity indicating a primary fresh water source for the nitrogen. The least squares mean fit of all date showed an average groundwater nitrogen content for all sites of 1.92 mg/l, but varied from 0.1 mg/l for Salt Pond Bay to 3.9 mg/l for Weeset. The maximum average nitrogen content for freshwater, based on the maximum nitrogen concentration envelope, was 3.7 mg/l, with a range from 0.1 mg/l for Salt Pond Bay to 8.0 mgl/l for Weeset. The total estimated annual nitrogen contribution to Nauset Marsh from on-site sewage disposal was 10,146 kg/yr at an average concentration of 1.65 mg/l in the discharging groundwater, without allowance for denitrificaiton. This estimate does not include other nitrogen sources of potential importance such as lawn fertilizer.