1996-06-12_HYDROLOGY - M1977378 - Laserfiche WebLink

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COLORADO DEPARTMENT OFP IC HEALTH AND ENVIRONMENT, Water lity Control Division Rationale - Page 5, CO- 0027529 e. Antidegradation - Since the receiving water is Use Protected, an antidegradalion review is not required pursuant to section 3.1.8(1)(b) of The Basic Standards and Methodologies for Surface Water. f Salinity Regulations - In compliance with the "Regulations for Implementation of the Colorado Salinity Standards Through the CDPS Permit Program'; the permittee shall monitor for total dissolved solids on a quarterly basis. Samples shall be taken at the effluent discharge point(s). Net Limits - Before the last renewal of this permit, effluent limitations were based upon net limits, allowing credits for contaminants in certain measured ground water seeps into the American Tunnel below the SGC property line. In the last renewal, net limitations were removed because the Division concluded that the ground water inflow was not being returned to the same body of water from which it came. The permittee continues to feel that net limitations are appropriate under the regulations, particularly if, in the future, waters in the American Tunnel are returned to ground water. No intake credits have been allowed in the effluent limitations for this permit renewal. g• 4. Whole Effluent Toxicity (WET) Testing In compliance with the previous permit, the permittee has been performing chronic WET testing. Results over the last two years have all passed the chronic WET limitations, with the dilution at which a significant difference in toxicity between the dilution and the control being greater than 100% in most cases. This indicates that the likelihood of ever having a chronic WET failure is very small. As a result of the plugging of the American Tunnel, it is likely that the Instream Waste Concentration (IWC) would be less than what was used in the previous permit, even further reducing the possibility of a WET failure. Another factor to consider is that, as a result of the consent agreement between the permittee and the State, the permittee will treat Cement Creek streamflow which historically has had higher pollutant concentrations than the treated effluent. When these factors are processed in a mass - balance equation, the result is a reduced pollutant loading and concentration in the Animas River, which is the first downstream waterbody classified for aquatic life use. The Division expects that an overall result of the consent agreement will be the maintenance of both the numeric and narrative standards for the Animas River. As a result of the above considerations, the Division has determined that neither WET limitations nor WET monitoring for this permit are required after treatment of Cement Creek is begun. Until that time, chronic WET testing and limitations will be included in the permit, based upon the same IWC that was used in the previous permit, which was 10.3% . The results of the testing are to be reported on Division approved forms. The permittee will be required to conduct a statistical derivation on the data, looking for any statistically significant difference in toxicity between the control and the effluent concentrations. This set of calculations will look at the full range of toxicity (lethality, growth and reproduction). If a level of chronic toxicity occurs, such that there is a statistically significant difference in the lethality (at the 95% confidence level) between the control and any effluent concentration less than or equal to the Instream Waste Concentration (IWC), the permittee will be required to follow the automatic compliance schedule identified in Part 1. B. of the permit, if the observed toxicity is due to organism lethality. Only exceedance of the limitation specified in Part I.A. will trigger the requirement for conducting the automatic compliance schedule identified in Part I.B. of the permit. If the toxicity is due to differences in the growth of the fathead minnows or the reproduction of the Ceriodaphnia, no immediate action on the part of the permittee will be required. However, this incident, along with other WET data, will be evaluated by the Division and may form the basis for reopening the permit and including additional WET limits or other requirements.