Impacts of anthropogenically influenced groundwater seepage on water chemistry and phytoplankton dynamics within a coastal marine system

Although landfills are a common method of solid-waste disposal, reports describing the impact of landfill leachate discharge on coastal surface-water quality are rare. In order to establish the impact of anthropogenically influenced groundwater seepage on a specific coastal marine ecosystem, a field campaign was conducted within North Sea Harbor (NSH), New York, USA, an embayment located northwest of, and down-gradient from, an unlined, municipal landfill. Most groundwater entering this system had modest seepage rates (0.020 to 1.18 cm h -1 ), was welloxygenated (278 128 M), contained moderate levels of nitrate/nitrite (57 26 M) and ammonium (5.9 13 M), and had low dissolved organic carbon levels (DOC = 71 21 M). In contrast, groundwater entering the SE portion of NSH was indicative of a landfill leachate plume, with high ammonium (>1 mM), and DOC (520 270 M) levels, low oxygen (20.3 15.3 M) and nitrate/nitrite (11 8.9 M) concentrations, and elevated seepage rates (8.33 4.54 cm h -1 ). Quantification of inorganic nitrogen fluxes revealed that seepage of this ammonium-rich groundwater accounted for ~80% of the inorganic N supply to the embayment receiving the discharge. The water column adjacent to this landfill-influenced groundwater seepage exhibited high levels of ammonium (61.7 69.3 M) and chlorophyll (8.4 4.1 g l -1 ), and hosted a microphytoplankton community that was comprised primarily of dinoflagellates and was N-replete. This contrasted with neighboring, nonleachate-influenced areas which had low ammonium (0.6 0.6 M) and chlorophyll (1.7 0.7 g l -1 ) levels and an N-limited microphytoplankton community dominated by diatoms. In summary, these results demonstrate that discharge of groundwater containing landfill leachate may be a significant, long-term eutrophication process in coastal environments.