background concentrations. These natural background concentrations are 
generally at low, nontoxic levels; however, low concentrations 
associated with a large material volume result in a high mass loading. 
Riverine loadings reflect natural loadings as well as loadings from 
human activities occurring upstream. The relative contribution of 
natural versus human-induced loadings is unknown at this time. As for 
shoreline erosion, contaminant concentrations in riverine inflows 
generally occur at low, non-toxic concentrations. Moderate inputs of 
metals are exhibited by atmospheric and industrial sources with 
relatively low contributions from point discharges. 
For the selected organic compounds, the total inputs are substantially 
lower than for the metals. Polynuclear aromatic (PAH) compounds appear 
to be associated with riverine and atmospheric sources, while 
polychlorinated biphenyls (PCBs) appear to originate primarily from 
point sources. It should be noted however that the uncertainty in the 
reported numbers may be large enough that these differences are 
insignificant. Additionally, the result that high PAH loadings are 
associated with riverine and atmospheric inputs is due, in part, to the 
way that source concentrations were estimated and is probably not 
totally accurate given the lack of major known sources to most rivers 
and the fact that loadings calculated for atmospheric inputs in 
nonurban areas were also high. 
Areas of Puget Sound Receiving the Largest Input of Toxic Chemicals 
To illustrate the relative contribution of each source to the 
contaminant loading in each Puget Sound subregion, the total loading 
for six metals (arsenic, copper, lead, mercury, silver, and zinc) and 
two classes of synthetic organic compounds (combustion polynuclear 
aromatic hydrocarbons - CPAH and polychlorinated biphenyls-PCBs) 
corresponding to each source was computed as shown in Figures 2 and 3. 
It is apparent that in Whidbey Basin which receives greater material 
(water and suspended sediments) from river discharges than other 
basins, the riverine inputs dominate the remaining nonpoint source 
inputs and the point source contributions. Recall that the same 
concentrations are used to calculate riverine loadings to all basins, 
differences in total loading therefore reflect only the differences in 
river inflow volume. For the Main Basin of Puget Sound where most of 
the industrial, commercial and residential development occurs, the 
point source contributions are enhanced with respect to the nonpoint 
sources. The southern Sound and Hood Canal receive comparatively 
reduced contaminant loadings due to both the decrease in nonpoint 
source contributions and the low population density, commercialization 
20 
