to PDS. These data were used to assess water column structure and facilitate comparison 

 with the near-bottom water temperature data acquired by the moored instrument array. 



2.1.5 Sediment Grab Samples 



During the final instrument recovery cruise on May 14, 1996, bottom sediment 



samples were collected using a grab sampler and returned to SAIC's laboratory for visual 



assessment of sediment characteristics at the site of the moored instrumentation. Visual 



analysis showed that the sediments were comprised of fractions of small rocks, gravel, 



sand, silt, and clay. The predominant fractions were a sand, silt, and clay. The water 



content of this sediment was approximately 129%, which was determined using the 



formula: 



/weight of wet sediment sample \ „ , _ _ 

 water content = Ueight of dry sediment sample ) x 10 ° 



As described below, the sediment samples were used for post-cruise laboratory 

 calibration of the optical turbidity sensors mounted on the instrument array. 



2.2 Instrumentation and Data Acquisition Procedures 



2.2.1 Bottom-Mounted Instrument Array 



The instrument array was composed of an aluminum tripod frame, an electro- 

 magnetic current meter, two turbidity sensors and a water temperature sensor interfaced to 

 a datalogger, and an acoustic recovery system (Figure 2-2). The tripod was constructed of 

 2.5-in diameter Schedule 80 aluminum round stock, and 0.5-in aluminum flat stock welded 

 and bolted together to facilitate deployments from small vessels-of-opportunity. Because 

 of concern about the reliability of data from the electromagnetic current meter in close 

 proximity to the tripod hardware, no zinc anodes were attached to the tripod. As an added 

 precaution, Delryn material was placed between all tripod elements and stainless steel bolts 

 were used to join the tripod elements. 



2.2.2 Near-Bottom Current Velocity and Turbidity Sensors 



An InterOcean S-4DW internally recording, electro-magnetic current meter was 

 used to acquire current velocity data from a height of 46 cm above the seafloor (Figure 

 2-2). The S-4DW was mounted beneath the apex of the tripod to minimize flow turbulence 

 in close proximity to the legs of the tripod. With regard to data recording capabilities, the 

 S-4DW was equipped with 1 megabyte of memory and programmed to record vector- 

 averaged data in a "burst sampling" mode. During each burst, current vector data were 

 sampled at 0.5 Hz intervals over a 1 -minute period, yielding 30 current measurements per 

 Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



