GROUNDWATER REGIME OF A TIDALLY INFLUENCED COASTAL POND

Hydraulic gradients between the water level of Little Pond, a small, t idally influenced coastal pond located on Cape Cod, Massachusetts, and 41 mini-piezometers located along the shores and in the sand layer un derlying soft bottom sediments indicate that the flow of groundwater t o the Pond is mainly concentrated along the western shore, within a ne arshore sandy strip. Similar results are obtained from the distributio n of salinity in shallow bottom sediments. The point measurements of g roundwater head are translated to long-term values by approximating ti dal movements to a sinusoid and using a simple mathematical model. Sal inity depth profiles in the shore and bottom sandy sediments indicate at least occasional outflow of salt water to the ground, and suggest t hat a density-driven convection may play an important role in determin ing the direction of flow and the distribution of solutes. Salinity de pth profiles in the soft sediments compared with the results of an adv ection-diffusion model indicate a locally strong upward flow of fresh groundwater. The model is also used to estimate long-term average velo cities of groundwater flow within soft bottom sediments. This flow mak es up only about 1% of total groundwater inflow, but may be important for the exchange of solutes across the sediment-water interface. In 19 89, groundwater contributed about 60% of total freshwater input to the Pond.