Influence of net groundwater discharge on the chemical composition of a coastal environment: Flanders Bay, Long Island, New York

Seasonal (October 1997 and May 1998) concentrations of dissolved (<0.45 <mu >m) trace metals (Ag, Al, Cd, Cu, Fe, Mn), inorganic nutrients (NO3, PO4, S i), DOC and DON were measured at seven wells during periods of low and high groundwater flow, in the aquifer around Flanders Bay in Eastern Long Islan d, New York. Similar measurements were made in surface waters of Flanders B ay, a shallow coastal embayment with restricted water flushing and river in put. Dissolved constituents in the groundwater were classified according to their behavior under different flow conditions as follows: (1) peak during high flow (DOC, pH, Si, NOB, Al and Cu); (2) peak during low flow (salinit y, DON, Ag, Cd, Mn); and (3) concentrations independent of flow conditions (PO4 and Fe). The primarily urban and agricultural land use on the North Fo rk of Long Island was reflected in higher concentrations of nutrients, Cu a nd Cd in groundwater, compared to samples from the South Fork which is most ly open parkland. Principal component analysis indicated that groundwater s eepage could influence the chemical composition of Flanders Bay with respec t to the major geochemical carriers (e.g. Fe and Mn). However, mass balance estimates for Cu indicated that, during low flow conditions, net groundwat er Cu input was about 10% of the total input. In contrast, during high flow , net groundwater flow could account for up to 58% of all Cu inputs. Nevert heless, a large imbalance, which accounted for up to 70% of the outflux dur ing low aquifer recharge, suggested that the Cu budget of the Bay was not a dequately described by the inputs considered (river, net groundwater flow, atmospheric deposition, and tidal exchange). Important missing components o f the Cu mass balance in Flanders Bay may include groundwater circulation d riven by tides and waves as well as diffusive benthic fluxes.