Control of Submarine Groundwater Flow and Chemistry by Onshore and Offshore Buried Peat Along a Developed Long Island Shoreline
Abstract
Submarine groundwater discharge (SGD) along the north shore of Long Island’s Great South Bay estuary was investigated to characterize and quantify nutrient delivery to the bay from groundwater. Recent field activities included onshore and offshore piezometer and well sampling, sediment coring, stationary resistivity profiling, and surveys of in situ porewater conductivity. Groundwater and surface water were sampled for analysis of nutrients, water quality parameters (conductivity, temperature, pH, ORP, dissolved oxygen), and age tracers. Results from detailed study at Patchogue Bay sites showed shallow plumes of low-salinity groundwater (salinity <3.5) located within 0.5 m of the estuary floor that decreased in thickness along one transect from approximately 4.5 m near shore to <1 m at 90 m offshore. Sediment cores collected near shore recovered buried submarine peat layers up to 49 cm thick that isolated low-salinity (salinity = 2-4) porewater below from brackish porewater above. In situ measurements of porewater conductivity and temperature in the upper 0.7 m of the sediment at one site verified the presence and extent of the peat layer there, as well as the isolation of terrestrially-derived groundwater beneath the layer extending offshore about 100 meters. Stationary electrical resistivity measurements also revealed the presence of high-resistivity (low-salinity) groundwater beneath the peat layer and more conductive porewater above the peat layer. Peat was found adjacent to existing or filled tidal creeks but was absent in cores collected offshore of the ends of peninsulas between tidal creeks. Much of the natural shoreline of the bay was developed in the 1950s and 1960s after construction of bulkheads and filling of tidal marshes with dredged or dumped fill material. Recharge of groundwater through buried peats and organic-rich soils in developed filled areas results in adjacent submarine groundwater plumes that are anoxic and ammonium-rich (average = 255 micromoles of ammonium per liter in offshore samples; max = 593 [n = 41]). Tritium data collected previously in this area indicate that groundwater recharged within the last few decades discharges relatively close to shore, but that the age of groundwater increases with depth and distance offshore. Recent samples from offshore profile wells contained detectable concentrations of MTBE and volatile organic compounds in the deeper low-salinity intervals, consistent with recharge of this groundwater further inland than younger and shallower submarine groundwater that did not contain detectable concentrations of these compounds. Land use patterns in the upland area of the watershed support this trend. Better understanding of the distributions of age and chemical composition along developed shorelines such as this could lead to improved models and mitigation strategies for estuarine nutrient over-enrichment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H42A..01B
- Keywords:
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- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 1830 HYDROLOGY / Groundwater/surface water interaction;
- 3021 MARINE GEOLOGY AND GEOPHYSICS / Marine hydrogeology;
- 4217 OCEANOGRAPHY: GENERAL / Coastal processes