2009-03-06_PERMIT FILE - C1981012A (6)

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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale - Page 14, Permit No. CO-0000906 iii. Bicarbonate and adjusted SAR (SAR-adj) - Studies by the Colorado State University Cooperative Extension and • by other entities have found that high proportions of sodium to calcium and magnesium in total dissolved solids negatively impacts plant growth and may contribute to reduced yields in farmland overtime. Additionally, high bicarbonate concentrations also adversely affect plant growth because bicarbonate combined with calcium and magnesium will precipitate out, leaving a higher concentration of sodium in the water. For this reason, monitoring and reporting of the bicarbonate ion concentration, in mg11, is required in this permit. Note the bicarbonate ion must be measured pursuant to a modified alkalinity test (Alkalinity can be measured via procedures approved pursuant to 40 CFR Part 136), that involves the titration of alkalinity in an acidic environment, thereby resulting solely in a measure of bicarbonate alkalinity. Although the SAR-adj of the downstream irrigation water is of interest to the Division, the Division does not intend to impose in-stream monitoring for calculations of SAR-adj at this time. However, effluent monitoring for calcium, sodium, magnesium, bicarbonate and conductivity is required under this permit to enable the Division to later calculate an effluent SAR-adj for use in correlating the effluent discharge to in-stream SAR-adj values that will be available based on in-stream sampling conducted by other entities. d. Pollutants Limited by Water OualiN Standards - The water quality assessment in Appendix A contains the evaluation ofpollutants limited by water quality standards. The mass balance equation shown in Section IV of Appendix A was used for most pollutants to calculate the maximum allowable effluent concentration, *, that could be discharged without causing the water quality standard or narrative standard to be violated. A detailed discussion of the calculations for the maximum allowable concentrations for metals and boron is provided in Section IV of the water quality assessment contained in Appendix A. The maximum allowable effluent pollutant concentrations determined as part of these calculations represent the calculated effluent limits that would be protective of water quality. These are also known as the water quality-based effluent limits (WQBELs). Both acute and chronic WQBELs may be calculated based on acute and chronic standards, and these may be applied as daily maximum (acute) or 30-day average (chronic) limits The Permits Section evaluated the calculated WQBELs and has made a determination as to whether there is a reasonable potential for the facility discharge to cause or contribute to an exceedance of a stream standard. If there is a reasonable potential for the discharge to contribute to an exceedance, effluent limits are included in the permit. The following paragraphs include a discussion of the reasonable potential analysis for ammonia, metals and boron. i. Total Ammonia as N - Ammonia concentrations oj'concern have been found in the discharges from some mining operations at concentrations of concern due to the use of ammonium based explosives. Using ammonia models, in-stream pH and temperature recovery and ammonia removal are generally simulated and effluent limitations for ammonia can be generated. Data for input into the models often requires the completion of a study ofstream and effluent pH and temperature. Because the process is resource intensive and would only be necessary if ammonia effluent concentrations are discharged at levels of concern, the Division is forgoing ammonia modeling until such time as data indicate that ammonia modeling is necessary; however, as part of this permit, the Division is establishing ammonia monitoring and reporting requirements for use in later analyses of the need for ammonia limits development. ii. Metals and Boron - Using the assimilative capacities contained in the WQA, an analysis must be performed to determine whether to include the calculated assimilative capacities as WQBELs in the permit. This reasonable potential analysis is based on the Determination of the Requirement to Include Water Quality Standards-Based Limits in CDPS Permits Based on Reasonable Potential, dated December 2002. This guidance document utilizes both quantitative and qualitative approaches to establish reasonable potential depending on the amount of available data. Recent effluent data (e.g., collected within the last five years) were not available for any pollutants for use in conducting a Reasonable Potential analysis. Thus, consistent with Division procedures, monitoring will be required to collect samples to support a Reasonable Potential analysis and subsequent decisions for a numeric limit. Monthly samples shall be collected at Outfalls MONI and MON9 and analyzed with all results being reported on a DMR. After the collection of a minimum of 12 months of samples, the permittee may request that a reasonable potential analysis be performed for the pollutants for Outfalls MON] and MON9. Depending on the results of the reasonable potential analysis, the permit may be reopened to add limitations, or remove monitoring requirements for the appropriate pollutant. Monitoring will continue at Outfalls MONI and MON9 until the permit expires, or until the permit has been amended.