Processed continuous resistivity profile data collected by the U.S. Geological Survey in Great South Bay on Long Island, New York, on May 19, 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 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与长岛的海底含水层开展调查，旨在评估海底地下水排泄（submarine groundwater discharge, SGD）作为氮素输入大南湾的潜在非点源的重要性。研究团队采集了总长超200千米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，研究人员在两处区域开展地下水采样工作：一是大南湾北岸，尤其是代表长岛开发岸线特征的帕乔格湾沿岸；二是位于大南湾南侧、长50千米的障壁岛火岛及其毗邻区域。其余野外作业还包括沉积物取芯、固定式电阻率剖面测量以及原位孔隙水电导率调查。大南湾岸线的陆上与近海浅层水文地层特征，尤其是海底隔水单元的赋存状态与性质，似乎对海底地下水流与排泄带的规模及化学特征起到首要控制作用。沉积物取芯结果显示，此类隔水单元多由淹没埋藏的泥炭层构成，这类泥炭大概率形成于盐沼沉积环境。基于连续电阻率剖面法数据，低矿化度地下水沿大南湾多数南北岸向海延伸10至100米，尤其在潮汐河口外侧以及火岛北侧毗邻现代盐沼的浅滩下方区域分布更为广泛。大南湾北岸多数岸线及近岸区域受人类活动改造，包括盐沼填埋、护岸墙与码头修建以及航道疏浚，这些活动通过破坏隔水单元、增加填埋区域近岸的地下水补给量，大幅改变了该湾岸线的自然水文地质条件。深入明晰此类海湾开发与未开发岸线的海底地下水排泄特征，有望助力优化河口湾营养盐富营养化的模拟模型与修复策略。如需了解本项目所涉调查的更多详情，请访问以下链接：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。