Processed continuous resistivity profile (CRP) data below the sediment water interface from Great South Bay on Long Island, New York, collected by the U.S. Geological Survey from Sept. 22 to Sept. 25, 2008 (ALLGSB_RESBSED_SEPT08.SHP)

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 https://cmgds.marine.usgs.gov/fan_info.php?fan=2008-007-FA and https://cmgds.marine.usgs.gov/fan_info.php?fan=2008-037-FA.本研究针对毗邻美国纽约州火岛国家海岸（Fire Island National Seashore）与长岛（Long Island）的海底含水层（submarine aquifers）展开调查，旨在评估海底地下水排泄（submarine groundwater discharge, SGD）作为向大南湾（Great South Bay）输运氮素的潜在面源（nonpoint source）的重要性。本次调查采集了超过200千米的连续电阻率剖面（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（fresh-saline groundwater interface）。此外，研究团队在两处区域开展了地下水采样工作：其一为大南湾北海岸（尤其帕乔格湾Patchogue Bay），该区域具有典型的开发型长岛海岸特征；其二为大南湾南侧长50千米的障壁岛（barrier island）——火岛及其毗邻区域。其他野外工作还包括沉积物取芯（sediment coring）、固定式电阻率剖面测量（stationary electrical resistivity profiling）以及原位孔隙水电导率（in-situ pore water conductivity）调查。大南湾沿岸的陆上与近岸浅层水文地层（hydrostratigraphy），尤其是海底隔水层（confining units）的赋存状态与特征，似乎对海底地下水流与排泄带的规模及化学特征起到主控作用。沉积物取芯结果显示，隔水层通常由被淹没且埋藏的泥炭层构成，这类泥炭层大概率形成于盐沼（salt marsh）环境。基于连续电阻率剖面数据，低盐度地下水在大南湾大部分南北岸区域向海延伸10至100米，尤其分布在潮汐河口外侧以及火岛北侧毗邻现代盐沼的浅滩下方。大南湾北岸多数岸线与近岸区域受到人类活动改造，包括盐沼填埋、岸堤（bulkheads）与码头修建以及航道（navigation channels）疏浚，这类活动通过截断隔水层以及增加填埋区域近岸的补给量，大幅改变了该海湾岸线的自然水文地质条件。深入理解此类海湾开发型与未开发型岸线的海底地下水排泄特征，有望助力改善河口湾（estuary）营养盐过富化（nutrient overenrichment）的模拟模型与修复策略。如需了解本项目涉及的调查工作详情，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2008-007-FA 与 https://cmgds.marine.usgs.gov/fan_info.php?fan=2008-037-FA。