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.