35 



chromium levels and the Portland area in general is characterized by low 

 to moderate chromium levels (Fig. 14). Stations in Portland Harbor 

 proper and in mid and upper Casco Bay generally have higher than average 

 values. 



Sediment levels of copper in Casco Bay range from 2.40 to 44.5 ppm 

 with an overall mean of 15.5 (Table 4). Only five stations exhibit 

 copper levels of over 20 ppm. These are the three stations in Portland 

 Harbor and stations 52 and 53 (Fig. 15). The gradient of decreasing 

 copper levels down the Fore River is suggestive of an upstream source. 

 Once again, lowest copper levels are found in the sandy main shipping 

 channel into the Bay and at the offshore stations. Upper Bay stations 

 generally are close to the mean in copper concentration. 



Lead concentrations in the sediments range from 9.0 to 61.4 ppm 

 with an average of 26.8 (Table 4). Its distribution is similar to that 

 of copper, i.e. high concentrations in Portland Harbor with a decreasing 

 gradient down the Fore River, low concentrations in the tidal channel 

 and at most offshore sites, and low to moderate values throughout the 

 remainder of the Bay (Fig. 16). 



Unlike the other metals, nickel does not show a strong pattern in 

 its distribution (Fig. 17). Concentrations range from 4.53 to 32.0 ppm 

 with a mean of 17.6 ppm. The extreme high value occurs at station 17 in 

 upper Casco Bay. Low values are found principally in channel areas and 

 just offshore, but even this pattern is not as clearly developed as for 

 the other metals. 



Zinc concentrations in Casco Bay sediments average 65.4 ppm and 

 range from 20.5 to 100.5 ppm (Table 4). Highest values occur in 



