Silver Creek - Division of Water Quality - Utah.gov

Silver Creek Watershed TMDL Final Report
impacted areas and should reduce metal concentrations in this portion of the watershed. Areas in
the watershed that are currently being developed include the upper watershed area and the East
side of the meadow area (Figure 22). It is expected that all future development activities will
avoid contaminated areas and, as a result, it is expected that these areas will not contribute zinc
or cadmium load to Silver Creek.
The reach of Silver Creek between Atkinson and Wanship shows a decrease of approximately
one-third in zinc loadings. This is probably associated with precipitation/sedimentation, which
suggests that the zinc is still present and could be mobilized by high flow events or a change in
water chemistry. However, the historic data set that encompasses more than 10 years does not
indicate that accumulated metals are being released in disproportionate quantities. Assuming
that clean-up and remediation take place in the upper and central portion of the watershed,
additional remediation in the lowest reach of the stream would be a last priority, undertaken if
this reach appears problematic following watershed work upstream.
Cadmium
Some 52% of the cadmium observations exceeded the chronic water quality standard. Clean up
priorities for cadmium based on loading analysis should be targeted at the stream reach above the
Park City monitoring station and between Richardson and Atkinson.
4.5 Zinc and Cadmium Geochemistry Overview
Many heavy metals become more water soluble under acid conditions and can move downward
with water through the soil, and in some cases move to aquifers, surface streams, or lakes. If
these ions are found in high concentrations, their toxicity is dependent on water hardness, pH,
temperature and the presence of other dissolved substances.
The distribution and transport of zinc in water, sediment and soil are dependent upon the species
of zinc present and the characteristics of the environment. The solubility of zinc is primarily
determined by pH. At acidic pH values, zinc may be present in the aqueous phase in its ionic
form. Zinc may precipitate at pH values greater than 8.0. It may also form stable organic
complexes. The formation of such complexes can increase the mobility and/or solubility of zinc.
Cadmium is also naturally present in soil but in a predominantly insoluble and harmless form.
However, through pollution-formed pH-decreasing agents like acid-rain, the cadmium in the soil
may convert into a soluble form, become available to plants and enter the food chain.
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