Processed continuous resistivity profile data collected by the U.S. Geological Survey in Great South Bay on Long Island, New York, on Sept. 23, 2008

An investigation of submarine aquifers adjacent to the Fire Island National Seashore and Long Island, New York, was conducted to assess the importance of submarine groundwater discharge (SGD) as a potential nonpoint source of nitrogen delivery to Great South Bay. More than 200 kilometers (km) of continuous resistivity profiling (CRP) data were collected to image the fresh-saline groundwater interface in sediments beneath the bay. In addition, groundwater sampling was performed at sites (1) along the northern shore of Great South Bay, particularly in Patchogue Bay, that were representative of the developed Long Island shoreline, and (2) at sites on and adjacent to Fire Island, a 50-km-long barrier island on the southern side of Great South Bay. Other field activities included sediment coring, stationary electrical resistivity profiling, and surveys of in-situ pore water conductivity. The onshore and offshore shallow hydrostratigraphy of the Great South Bay shorelines, particularly the presence and nature of submarine confining units, appears to exert primary control on the dimensions and chemistry of the submarine groundwater flow and discharge zones. Sediment coring has shown that the confining units commonly consist of drowned and buried peat layers likely deposited in salt marshes. Based on CRP data, low-salinity groundwater extends from 10 to 100 meters (m) offshore along much of the northern and southern shores of Great South Bay, especially off the mouths of tidal creeks, and beneath shallow flats to the north of Fire Island adjacent to modern salt marshes. Human modifications of much of the shoreline and nearshore areas along the northern shore of the bay, including filling of salt marshes, construction of bulkheads and piers, and dredging of navigation channels, has substantially altered the natural hydrogeology of the bay's shorelines by truncating confining units and increasing recharge near the shore in filled areas. Better understanding of the nature of SGD along developed and undeveloped shorelines of embayments such as this could lead to improved models and mitigation strategies for nutrient overenrichment of estuaries. For more information on the surveys involved in this project, see http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2008-007-FA and http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2008-037-FA.

本研究针对美国纽约州长岛毗邻火岛国家海岸的海底含水层（submarine aquifers）开展调查，旨在评估海底地下水排泄（submarine groundwater discharge, SGD）作为氮素向大南湾输送的潜在面源的重要性。研究采集了超过200公里的连续电阻率剖面法（continuous resistivity profiling, CRP）数据，以成像大南湾下方沉积物中的淡水-咸水地下水界面。此外，研究团队在两处区域开展地下水采样：其一沿大南湾北岸，尤其是帕乔格湾，该区域为具有代表性的开发型长岛海岸线；其二位于大南湾南侧长50公里的障壁岛火岛及其毗邻区域。其余野外作业包括沉积物岩心取样、固定式电阻率剖面测量以及原位孔隙水电导率调查。大南湾沿岸的陆上与近岸浅层水文地层结构，尤其是海底隔水层的赋存状态与性质，似乎对海底地下水流与排泄带的规模及化学特征起到主要控制作用。沉积物岩心取样结果显示，隔水层通常由淹没埋藏的泥炭层构成，这类泥炭大概率形成于盐沼环境。基于连续电阻率剖面法数据，低盐度地下水沿大南湾多数北岸与南岸区域向海延伸10至100米，尤其在潮汐水道口外侧以及火岛北侧毗邻现代盐沼的浅滩下方。大南湾北岸多数沿岸及近岸区域受到人类活动改造，包括盐沼填埋、护岸与码头修建以及航道疏浚，这些活动通过破坏隔水层以及提升填埋区域近岸的地下水补给量，极大改变了该海湾沿岸的自然水文地质条件。相较于此类海湾的开发与未开发岸线，更好地理解海底地下水排泄的特征，有助于优化河口湾营养物过量富集的模拟模型与缓解策略。如需了解本项目所涉调查的更多信息，请访问：http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2008-007-FA 与 http://woodshole.er.usgs.gov/operations/ia/public_ds_info.php?fa=2008-037-FA。