Silver Creek - Division of Water Quality - Utah.gov

Silver Creek Watershed TMDL Final Report
2.0 WATER QUALITY STANDARDS
The Silver Creek watershed is listed on the State of Utah’s 303(d) list as impaired for zinc and
cadmium. Beneficial use 3A, protected for cold-water fish and other cold-water species, is
identified as impaired. Water quality data and analysis are discussed in Sections 3 and 4. Data
for the following constituents were gathered to quantify and evaluate this impairment:
• Total and Dissolved Cadmium
• Total and Dissolved Zinc
• Total Dissolved Solids
• Total Suspended Solids
• pH
Data for total dissolved solids and total suspended solids were gathered because metals such as
zinc and cadmium are often present within these solids. Values for all of these constituents are
sufficient to provide a good understanding of existing water quality impairments present in this
area. Data for the constituents were gathered from the Utah Division of Water Quality (DWQ)
and from the Environmental Protection Agency (EPA). Data beginning in January 1990 through
2002 were obtained for monitoring stations located on or near Silver Creek.
2.1 Water Quality Targets and Endpoints
Water quality standards for zinc and cadmium were obtained from the State of Utah, Rule R317-
2-14. This rule states that the standards for zinc and cadmium are dependent on the hardness of
the water. Hardness is used as a surrogate for a number of water quality characteristics which
affect the toxicity of metals. Increasing hardness has the effect of decreasing the toxicity of
metals. Water quality criteria to protect aquatic life may be calculated at different concentrations
of hardness measured in milligrams per liter (mg/l) as calcium carbonate (CaCO 3 ). (EPA,
National Recommended Water Quality Criteria 2002, www.epa.gov/waterscience/pc/revcom.pdf.)
Table 4 shows the equations provided by the State of Utah to calculate these standards.
Table 4: Water Quality Standard Calculations
Chronic
Zinc e (0.8473(ln(hardness))+0.884)
Cadmium e (0.7409(ln(hardness))-4.719)
The average hardness measured in the Silver Creek watershed was found to be 484 mg/l. The
equations shown in Table 4 that were used to calculate hardness adjusted water quality standards
for zinc and cadmium are only considered valid up to a hardness of 400 mg/l. The use of a
maximum hardness of 400 mg/l for calculating hardness adjusted water quality standards for
metals is in accordance with Utah’s Water Quality Standards R317-2-14. Utah’s use of a
maximum hardness of 400 was specifically recommended by Region 8 EPA in a letter to the
Division dated Dec. 20, 2001. The National Recommended Water Quality Criteria: 2002 (EPA
822-R-02-047 Nov. 2002) also specifically address this issue. At high hardness values such as
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