and industrial activity. Due to dynamic water transport through 
relatively narrow channels, shoreline erosion provides the highest 
contribution to the total metal loading in the Straight of Juan de Fuca. 
For the synthetic organic compounds, the CPAHs are the largest 
contributors to organic contaminant loading for all subregions except 
for the Main Basin where municipal inputs appear to contribute equal 
loadings of PAHs and PCBs. Again, for the northern Puget Sound 
regions, riverine inputs exceed by far any other nonpoint and and point 
source input. However, for the Main Basin, municipal discharges appear 
to be responsible for the majority of the organic contaminant loading 
(relative to nonpoint sources; recall that loadings from industrial 
sources are not well-known at this time). It is also interesting to 
note that the atmospheric contributions to the CPAH loading is 
noticeable in areas where fossil fuel consumption is increased from 
industrial and residential activity. 
Quantifying Nonpoint Source Contributions 
As pointed out earlier, nonpoint sources are diverse and difficult to 
quantify; loading estimates to date have been based upon limited data. 
Available means by which nonpoint source contributions may be more 
accurately quantified are discussed below. 
Nonpoint source inputs can be estimated in a number of ways with 
increasing levels of complexity. The simplest method is applicable in 
subregions where major nonpoint source loadings is via riverine 
inflow. In this instance, it is possible to monitor concentrations of 
contaminants in the rivers or other drainage systems and measure their 
flows. Multiplying the two together will yield the contaminant mass 
flux (loading) from this source into the Puget Sound region under 
consideration. This type of approach was used to calculate the 
nonpoint source loadings presented earlier. 
A more detailed method is to catalog contaminant sources within a given 
area and quantify concentrations and area runoff. An example of this 
method is the Water Quality Assessment (WQA) procedure developed by EPA 
as a screening tool for local authorities to assess nonpoint source 
loadings. The method is based upon the results of previous studies 
which have found correlations of contaminant loadings with various 
measurements of land use, precipitation, and other factors. The 
accuracy of estimates obtained using this procedure varies depending on 
the amount of detail with which the basin is described. 
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