Autonomous Underwater Vehicle Water-Quality Measurements in Back River, Savannah, Georgia, 2018-2019

The Savannah Harbor Expansion Project (SHEP) began in January 2015 with the purpose of deepening the Savannah Harbor federal shipping channel to allow access to larger shipping vessels (U.S. Army Corps of Engineers, 2018a). There are various features and mitigation efforts to the SHEP, one of which is the installation of a dissolved-oxygen (DO) injection system at two locations along the Savannah River to maintain pre-SHEP DO levels in the harbor (U.S. Army Corps of Engineers, 2018b). The injection systems are installed upstream near Georgia Power Plant McIntosh in Effingham County, Georgia and downstream near Hutchinson Island, Chatham County, Georgia. Designed to maintain DO levels in the harbor as they were before deepening, the system will pull water from the river into the Speece cones and inject it with oxygen before returning it to the river (Tetra Tech, 2013). Water-quality and bathymetric data were measured with 2 Autonomous Underwater Vehicles (AUV) during spatial surveys in the study area near the Hutchinson Island DO injector site in August 2018, February 2019, and April 2019. Water-quality and bathymetric data are measured by the AUV and logged at 1-second intervals. Water-quality parameters measured by the AUVs during the survey may include: Specific conductivity (microSiemens per centimeter at 25 degrees Celsius; YSI sensor 6560), water Temperature (degrees Celsius; YSI sensor 6560), Salinity (parts per thousand; computed from conductivity), Phycocyanin Fluorescence (estimated as Blue-green Algae cell density in cells per milliliter; YSI sensor 6131), Chlorophyll Fluorescence (estimated in micrograms per liter; YSI sensor 6025), Rhodamine Fluorescence (micrograms per liter; YSI sensor 6130), Dissolved Oxygen (milligrams per liter; YSI optical sensor 6150), pH (standard units; YSI sensor FR 6589), and Turbidity (formazin nephelometric units; YSI sensor 6136). Bathymetric data was measured with the on-board Doppler Velocity Log. During the August 2018 and April 2019 data collection, water-quality data was measured with a YSI 6600 V2 sonde mounted in the nose cone of the IVER2 EcoMapper AUVs. During the February 2019 data collection period, an IVER3 EcoMapper AUV with an EXO3 multiparameter sonde was used while one of the IVER2 AUVs was temporarily out of service. The sondes were calibrated before each data collection trip per USGS water-quality sampling protocols. Calibration checks were completed after each data collection trip. Data were measured at various periods of the tide cycle but generally targeted at or near slack tide conditions.

萨凡纳港扩建项目（Savannah Harbor Expansion Project, SHEP）于2015年1月启动，旨在疏浚萨凡纳港联邦航运航道，以保障更大吨位的船舶通行（美国陆军工程兵团，2018a）。该项目包含多项配套设施与生态减缓措施，其中一项为在萨凡纳河沿线两处位置安装溶解氧（dissolved-oxygen, DO）注入系统，以维持港口在项目实施前的溶解氧水平（美国陆军工程兵团，2018b）。该注入系统分别安装于佐治亚州埃芬汉姆县境内佐治亚电力麦金托什发电厂附近的上游区域，以及佐治亚州查塔姆县哈奇森岛附近的下游区域。本系统旨在维持港口疏浚前的溶解氧水平，其工作流程为从河中取水注入斯皮斯锥（Speece cones），向水中充入氧气后再将水体排回河道（特泰克公司，2013）。2018年8月、2019年2月及2019年4月，研究团队在哈奇森岛溶解氧注入点附近的研究区域开展空间勘测期间，使用2台自主水下航行器（Autonomous Underwater Vehicles, AUV）采集了水质与水深测量数据。自主水下航行器将采集水质与水深数据，并以1秒的间隔进行记录。本次勘测中自主水下航行器采集的水质参数包括：电导率（Specific conductivity，单位为25℃下的微西门子/厘米；YSI 6560传感器）、水温（Temperature，单位为摄氏度；YSI 6560传感器）、盐度（Salinity，以千分比为单位，由电导率计算得出）、藻蓝蛋白荧光（Phycocyanin Fluorescence，以每毫升细胞数估算的蓝藻细胞密度；YSI 6131传感器）、叶绿素荧光（Chlorophyll Fluorescence，单位为微克/升；YSI 6025传感器）、罗丹明荧光（Rhodamine Fluorescence，单位为微克/升；YSI 6130传感器）、溶解氧（Dissolved Oxygen，单位为毫克/升；YSI 6150光学传感器）、pH值（pH，标准单位；YSI FR 6589传感器）及浊度（Turbidity，单位为formazin nephelometric units；YSI 6136传感器）。水深数据通过机载多普勒测速日志（Doppler Velocity Log）采集。在2018年8月与2019年4月的数据采集阶段，使用的是安装在IVER2 EcoMapper自主水下航行器鼻锥内的YSI 6600 V2多参数水质仪。2019年2月的数据采集期间，其中1台IVER2自主水下航行器临时停运，因此改用搭载EXO3多参数水质仪的IVER3 EcoMapper自主水下航行器。所有水质仪均在每次数据采集任务前，按照美国地质调查局（United States Geological Survey, USGS）水质采样规程完成校准，并在每次任务结束后进行校准核查。本次数据采集覆盖潮汐周期的多个时段，但主要目标时段为平潮或平潮附近时段。