17_ 



discharge. Salinities were less than 28 ppt and temperatures exceeded 6°C within a few 

 meters of the surface. With increasing depth, salinities increased and temperatures 

 decreased, resulting in considerable density stratification over the depth range from 3 to 

 17 m. Below 25 m, variations in both temperature and salinity were minimal such that the 

 density stratification in the lower half of the water column was very weak, as had been 

 observed in February (Figure 3-la). 



By late spring (May 14), the upper water column had warmed further, resulting in a 

 20 m-thick layer having temperatures exceeding 6.6°C and salinities in the range from 

 roughly 29 to 31.5 PSU (Figure 3-lc). Beneath a moderate thermocline and pycnocline 

 extending from 20 to 30 m, all water properties were nearly constant throughout the lower 

 30 m of the water column, as had been observed in February and April. 



Comparison of water properties among the three vertical profiles acquired in spring 

 1996 is best accomplished through analysis of temperature/salinity (T/S) characteristics 

 (Figure 3-2). In this figure, T/S characteristics for each of the three profiles are presented to 

 illustrate the greater range of near-surface water properties during April and May compared 

 with the February profile. It is also interesting to note that bottom waters during February 

 were warmer than the near-surface waters, in contrast to the later profiles which showed 

 coldest waters at the bottom. Near-bottom salinities were always higher than near-surface 

 salinities, although the vertical salinity gradients were much greater in late spring due to river 

 runoff. Overall, near-bottom temperature, salinity, and density had not varied greatly during 

 the spring measurement period, which extended from late February to mid-May 1996; near- 

 surface water properties had, however, changed considerably due to river runoff and vertical 

 mixing to depths of 20 m. These results are generally consistent with the seasonal T/S 

 analyses presented by Colton et al. (1968) for the central Gulf of Maine and by Brown and 

 Beardsley (1978) for the coastal region southeast of Portland. 



With regard to the vertical density stratification at PDS, it is apparent that the entire 

 water column to a depth of 60 m is very weakly stratified throughout winter and early spring, 

 whereas the introduction of relatively fresh/warm waters at the surface during mid-spring 

 causes considerable stratification that may tend to decouple horizontal currents and other 

 transport processes within a basic two-layer system. Consequently, downward propagation 

 of storm-generated energy would be most effective during winter and late spring when the 

 water column is nearly void of density stratification. With the onset of spring and summer, 

 the bottom waters are less free to mix vertically with the upper layers, and the resulting 

 stratification acts as a partial barrier to the energy imparted by storms. Further discussion of 

 vertical mixing and storm effects on near-bottom currents and material transport is presented 

 in Section 3.3. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



