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

Abstract

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 well-oxygenated (278 ± 128 μM), contained moderate levels of nitrate/nitrite (57 ± 26 μM) and ammonium (5.9 ± 13 μM), and had low dissolved organic carbon levels (DOC = 71 ± 21 μM). 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 μM) levels, low oxygen (20.3 ± 15.3 μM) and nitrate/nitrite (11 ± 8.9 μM) 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 μM) and chlorophyll (8.4 ± 4.1 μg l-1), and hosted a microphytoplankton community that was comprised primarily of dinoflagellates and was N-replete. This contrasted with neighboring, non-leachate-influenced areas which had low ammonium (0.6 ± 0.6 μM) and chlorophyll (1.7 ± 0.7 μg 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.