SIMULATION OF SHALLOW GROUND-WATER DISCHARGE TO COASTAL AREAS OF MARYLAND'S INLAND BAYS

Natural resource managers in the Mid-Atlantic are concerned about the potential for eutrophication of estuarine waters from the delivery of excess nutrient loads from coastal watersheds. Ground water and surface water are significant sources of nutrients to coastal bays along the Atlantic Coast of Maryland. Knowledge of the relative importance of nutrient sources and the spatial distribution of ground-water nutrient loads is needed to target areas for monitoring and management.

The U.S. Geological Survey is conducting a hydrologic investigation to estimate the total flux of nutrients transported by ground water to the coastal bays. The study integrates several standard and new technologies for field data collection. The data are being used to model ground-water flow, nutrient transport, and the mixing of fresh and saline waters beneath the bays.

SUTRA, a variable-density, finite-element model, is used to simulate ground-water flow and investigate the spatial distribution of fresh ground-water discharge to the bays. Simulations will also determine zones of ground-water discharge and the position of the freshwater/saltwater interface. Parameters varied in the model include bay width, hydraulic conductivity of bottom sediments, and layers representing aquifers and confining beds.

Simulations indicate that coastal geomorphic features such as bay width are important controls on zones of fresh ground-water discharge to tidal waters and on the position of the freshwater/saltwater interface in the surficial aquifer. In simulations where the bay width was narrow (1 to 2 km) the interface was seaward of the barrier island. For a wider bay (8 km) the model results show discharge of fresh ground water is close to the landward margin of the bay. Simulations also showed vertical movement of saline waters from the bays downward through the underlying sediments.