Existing quality

1. "Existing quality" is the 85th percentile of dissolved metals concentrations from representative water quality measurements.

2. This is defined in Colorado Rule 31 and applied in evaluating streams for classification as Outstanding Waters.

3. Additional guidance for comparing metals concentrations with criteria is given in Colorado's 303(d) Listing Methodology.

4. The chronic metals criteria shown above are hardness-adjusted, using mean hardness measurements for each stream.

5. That applies for all the metals except selenium, which does not require adjustment.

6. Existing quality is the range between the 15th and the 85th percentiles for pH.

7. It is the 15th percentile for dissolved oxygen and the geometric mean for E coli.

8. As evident from the two tables, none of the measurement values depicting existing quality exceeded the standards.​

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References

1. Code of Colorado Regulations, Water Quality Control Commission, 5 CCR 1002-31, Regulation No. 31, "The Basic Standards and Methodologies for Surface Water" (Rule 31), pages 4 and 17.

2. Colorado Department of Public Health, Water Quality Control Division, "303(d) Listing Methodology," May 2020, page 26.