Processed continuous resistivity profiling data collected in the Indian River Bay, Delaware, on April 15, 2010, on U.S. Geological Survey Field Activity 2010-006-FA

A geophysical survey to delineate the fresh-saline groundwater interface and associated sub-bottom sedimentary structures beneath Indian River Bay, Delaware, was carried out in April 2010. This included surveying at higher spatial resolution in the vicinity of a study site at Holts Landing, where intensive onshore and offshore studies were subsequently completed. The total length of continuous resistivity profiling (CRP) survey lines was 145 kilometers (km), with 36 km of chirp seismic lines surveyed around the perimeter of the bay. Medium-resolution CRP surveying was performed using a 50-meter streamer in a bay-wide grid. Results of the surveying and data inversion showed the presence of many buried paleochannels beneath Indian River Bay that generally extended perpendicular from the shoreline in areas of modern tributaries, tidal creeks, and marshes. An especially wide and deep paleochannel system was imaged in the southeastern part of the bay near White Creek. Many paleochannels also had high-resistivity anomalies corresponding to low-salinity groundwater plumes associated with them, likely due to the presence of fine-grained estuarine mud and peats in the channel fills that act as submarine confining units. Where present, these units allow plumes of low-salinity groundwater that was recharged onshore to move beyond the shoreline, creating a complex fresh-saline groundwater interface in the subsurface. The properties of this interface are important considerations in construction of accurate coastal groundwater flow models. These models are required to help predict how nutrient-rich groundwater, recharged in agricultural watersheds such as this one, makes its way into coastal bays and impacts surface water quality and estuarine ecosystems. For more information on the survey conducted for this project, see https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA.2010年4月，美国特拉华州印第安河湾（Indian River Bay）海域开展了一项地球物理勘测，旨在圈定该海湾下方的淡水-咸水地下水界面及其相关的海底沉积构造。 本次勘测包含在霍尔特登陆点（Holts Landing）研究区域周边开展的高空间分辨率测量工作，该区域后续完成了密集的陆上与近海研究。 本次勘测的连续电阻率剖面法（continuous resistivity profiling, CRP）测线总长度达145千米，另在海湾周缘布设了36千米的声呐地震（chirp seismic）测线；研究采用50米拖缆（streamer）在全海湾网格内开展了中分辨率CRP测量工作。 勘测结果与数据反演结果显示，印第安河湾下方埋藏有大量古河道（paleochannels），在现代支流、潮汐水道与滨海沼泽分布区域，这些古河道大体垂直于海岸线延伸。在海湾东南部靠近怀特溪（White Creek）的区域，成像发现了一处规模格外宽阔深邃的古河道系统。 多数古河道还伴随有高电阻率异常，对应着与其相关的低盐地下水羽流（low-salinity groundwater plumes）；这一现象大概率源于河道充填物中赋存的细粒河口泥质与泥炭，此类物质可作为海底隔水层（submarine confining units）发挥作用。若此类隔水层存在，陆上补给的低盐地下水羽流便可越过海岸线运移，进而在地下形成复杂的淡水-咸水地下水界面。 该界面的特征是构建精准海岸地下水流模型（coastal groundwater flow models）的重要考量因素。此类模型可用于预测在类似的农业流域（agricultural watersheds）中补给的富营养地下水如何运移进入近岸海湾，并对地表水水质与河口生态系统（estuarine ecosystems）造成影响。 如需了解本次项目勘测的更多详情，请访问：https://cmgds.marine.usgs.gov/fan_info.php?fan=2010-006-FA。