For example, 19 percent of chromium loadings to the Bay 
comes from industrial sources; an additional 19 percent is enter 
ing the Bay in waste water, a loading approximately equal to the 
loading from rivers. For some metals, in particular cadmium, 
the amount of anthropogenic input greatly exceeds natural river 
loadings. 
In addition to these averaged loadings for the entire Chesa¬ 
peake Bay, single estuaries may be heavily impacted. As an exam 
pie, copper enters the Patuxent River estuary primarily from 
four different sources? all four sources are approximately equal 
in magnitude (Table 3). The first source is natural weathering 
of rocks and soils; the second source is copper contained in 
sewage effluents; the third source is copper contained in 
cooling water effluent from a conventional power plant; the 
fourth source is copper leaching from bottom paints on 
recreational vessels. For this one sub-estuary, therefore, the 
inputs of copper from anthropogenic sources far exceed natural 
loadings. 
Association with Sediments 
Most of the metals that enter the Chesapeake Bay are 
associated with sediments. Many toxic metals have a high 
affinity for particles, as has been discussed earlier. 
Therefore, metals entering the Chesapeake Bay can end up in 
sediments, and not be transported to the ocean. 
Several metals are now found in sediments in concentrations 
that greatly exceed natural levels, in particular, cadmium, 
cobalt, lead, and zinc. If we determine the amount of metal in 
excess of natural levels, metal enrichment in the Chesapeake Bay 
is not significantly different from many other east coast 
estuaries that have been subjected to man's influence, such as 
the Delaware Bay, the Hudson River estuary, and Narragansett Bay 
(Table 4). 
Metal levels in sediments are in general higher in the 
Northern Bay and decrease seaward (Figure 1). This general 
decline is caused primarily by physical processes discussed 
earlier and the predominance of fine materials (clays and silts) 
in the Northern Bay. The metals are primarily associated with 
the fine fraction, and therefore are found in the Northern Bay. 
Metal distributions also follow general water circulation 
patterns, with higher concentrations along the western shore. 
In addition, some of the largest source of metals are in the 
Northern Bay. 
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