31 



these independent parameters, Figure 3-5 presents time series results from the entire 

 measurement program. Inspection of the pressure record (third tier from the bottom in 

 Figure 3-5) reveals data gaps on April 6 and 22 associated with servicing of the instrumen- 

 tation. Also evident is the shift in mean pressure associated with each redeployment of the 

 instrumentation, which is a result of deployment at a slightly different location and water 

 depth (see discussion of local bathymetry in Section 2.1.2). Aside from the pressure (depth) 

 offsets between these three pressure records, we see the periodic semi-diurnal fluctuations in 

 water level having a range of 3.5 m during the spring tide period. 



The water temperature record from February to May (fourth tier from the bottom in 

 Figure 3-5) exhibits considerable variability over the range from 3.6° to 5.5 °C. Semi- 

 diurnal temperature fluctuations on the order of a few tenths of degrees were evident during 

 the first three weeks of March, presumably due to tides (either vertical excursions within a 

 near-bottom thermocline, or horizontal displacement of water parcels within a region of 

 horizontal temperature gradients). Later in the deployment period, short-term fluctuations 

 were practically nonexistent, especially during the period from April 18 to May 4 which was 

 characterized by isothermal water of 4. 1 °C. The consistency of this temperature reading was 

 initially considered suspect, but as it spanned two deployment periods (at slightly different 

 locations) it is highly probable that the mooring region contained a considerable near-bottom 

 layer of isothermal water which was advected away after May 5. This thick layer of 4°C 

 water was evident from the CTD profile acquired on April 22 (see Section 3.1). Overall, the 

 near-bottom water temperatures measured by the CTD profiler on February 27, April 22, 

 and May 14 were consistent with the moored temperature record, although the CTD values 

 were consistently 0.4 °C lower than the thermistor on the S4 current meter. 



Hourly averaged near-bottom current speed during the February to May measurement 

 period (upper tier in Figure 3-5) ranged from approximately to 20 cms" 1 , with the majority 

 of the variability occurring at periods of approximately 12 hr in association with the semi- 

 diurnal tide. Significant aperiodic variability in current speed was observed on time scales 

 ranging from hours to a few days, possibly related to atmospheric forcing. Mean current 

 speeds for each of the three deployments were similar (near 7 cms" 1 ), but the third 

 deployment period (from April 23 to May 14) was characterized by significantly less speed 

 variability than the first two deployment periods, while the second deployment period (from 

 April 6 to 22) exhibited the largest speed variability. The noticeable changes in current 

 speed characteristics at both of the instrument servicing events (April 6 and 22) suggest that 

 redeployment of the instrumentation had an effect on the subsequent current measurements. 

 This is even more obvious upon inspection of the current direction data (second tier from the 

 top in Figure 3-5), which shows relatively consistent flow toward -60° (300 °T) during the 

 third deployment, dominant flow toward 140°T during the first deployment, and flow toward 

 -150° (210° T) during the second deployment. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



