u_ 



burst; bursts were acquired at 10-minute intervals over the entire deployment period. In 

 addition, a single pressure measurement was acquired during each burst in order to assess 

 water level (tides). 



Hourly measurements of near-bottom turbidity were acquired using two optical 

 turbidity sensors manufactured by Seapoint Sensors, Inc. Analog data from the turbidity 

 sensors, located at heights of 33 and 81 cm above the seafloor (Figure 2-2), were recorded 

 by a programmable Model R2 data logger manufactured by Dryden Instrumentation Inc. 

 In addition, near-bottom water temperature data were acquired and recorded by the logger 

 at hourly intervals. 



For the overall measurement program, 100% of the current velocity, bottom 

 pressure, turbidity, and water temperature data were recovered over the 78-day 

 deployment period, except during two brief (less than 12-hour) servicing events. 



2.2.3 Post-Survey Laboratory Calibration of Turbidity Sensors 



Following the field measurement program, the relative turbidity sensors were 

 calibrated using standard laboratory procedures as recommended by the manufacturer 

 (Seapoint Sensors, Inc.) Sediment samples from the deployment site (composed 

 predominantly of silts and clays) were homogenized and wet sieved through 63-micron 

 mesh to remove gravel and sand, as the primary objective of the field program was to 

 assess whether currents were sufficient to resuspend fine-grained sediment residing on the 

 seafloor. The supernatant was dried and then weighed to determine the mass of the 

 sediment sample. A 1-gram subsample of fine sediment was then hydrated and serially 

 diluted with filtered seawater from Narragansett Bay to achieve a suite of representative 

 concentrations of suspended solids. 



One of the turbidity sensors was interfaced to the data logger used during the field 

 program and immersed in the slurry of suspended solids to acquire relative turbidity 

 measurements (in Nephelometry Turbidity Units [NTUs]) for comparison with the known 

 suspended solids concentration. Intercomparison data were acquired over a broad range of 

 suspended solids concentrations; results in the range from to 32 mg 1" are presented in 

 Figure 2-3. As indicated in the figure, a linear calibration was evident over the range of 

 suspended solids concentrations measured in the field (see Section 3). This relationship 

 was used to convert the field measurements of relative turbidity into units of mg f 

 suspended solids concentration. 



It is important to note, however, that the laboratory calibration of the turbidity 

 sensor is prone to some uncertainty at very low suspended solids concentrations (e.g., less 

 Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



