VERTICAL AND HORIZONTAL HETEROGENEITY OF GROUNDWATER NITROGEN CONCENTRATIONS AT THE SEEPAGE FACE OF AN ESTUARY

Nitrogen is a key element influencing the ecology of coastal waters. In areas with sandy soils, water and nitrogen are transported to receiving waters via groundwater rather than by runoff. Extensive work has been done to understand the sources of nitrogen loads to estuaries, but there is little knowledge of the degree of groundwater mixing near the seepage face or of the nitrogen transformations and losses that occur within that transition zone. We mapped the fine-scale structure of the seepage face by sampling at several depths along a series of transects perpendicular to the shore from 4.5 meters upland to 1.5 meters beyond the shoreline of Edgartown Great Pond (Martha's Vineyard, Massachusetts). We measured salinity, ammonium, nitrate, dissolved organic nitrogen, and boron. The aquifer showed clear salinity gradients both seaward and with increasing depth, revealing a narrow zone of freshwater seepage and a zone of fresh and saltwater mixing seaward. Highest concentrations of ammonium were found farther offshore and at increasing depths, suggesting that in-situ decomposition of organic matter was the source. Nitrate concentrations were highest landward and decreased offshore. Concentrations relative to boron (a passive tracer for wastewater effluent) allowed evaluation of decreases in nitrate concentration due to dilution and losses by denitrification. In general, concentrations of solutes in groundwater were variable normal to the shoreline, and rates of solute delivery declined seaward. Understanding this heterogeneity is important for establishing a meaningful sampling protocol, and allows interpretation of the likely sources, as well as estimates of losses.