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

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• Agricultural Use Protection -Section 31.11(1)(a)(iv) of The Basic Standards and Methodoh¢ies for Surface Waters (Regulation No. 31) includes the narrative standard that State surface waters shall be free of substances that are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life. The water quality assessment in Appendix A contains the evaluation of pollutants limited by narrative standards. The mass balance equation shown in Section IV of Appendix A was used for electrical conductivity and sodium to calculate the maximum allowable effluent concentration, M.,, that could be discharged without causing the narrative standard to be violated. A detailed discussion of the calculations for the maximum allowable concentrations for electrical conductivity and sodium is provided in Section V of the water quality assessment contained in Appendix A. In accordance with the discussion that follows, the Division is establishing monitoring and limitations for several parameters in accordance with the Division's Implementing Narrative Standards in Discharge Permits for the Protection of Irrigated Crops policy (hereafter the Narrative Standards policy). Electrical Conductivity (EC) - The calculated chronic limit for EC in deciSiemens per meter (dS/m), as set out in Appendix A, is established as a 30-day average limit, effective immediately. EC is also known as specific conductance, conductance, conductivity, or specific conductivity. Because EC can be measured in various units, which has led to confusion in accurate reporting of results, the Division is providing the following conversions to dS/m: Multiply pmhos/cm _ 1000 to convert to dS/m Multiply pS/cm = 1000 to convert to dS/m Multiply mmhos/cm x I to convert to dS/m ii. Sodium and SAR - SAR, which stands for Sodium Adsorption Ratio, is a representation of the relative proportion of sodium cations to other cations in total dissolved solids. SAR is a parameter used for determining suitability of water for irrigation purposes. However, Division procedures are to impose sodium limits in lieu of SAR limits because of implications of SAR being a ratio of three different parameters, and therefore the discharge may meet the SAR limit, however, after blending with the receiving stream, the limit may not be met. If sodium is limited, we can be certain that the SAR of the receiving stream will meet the limitation. The calculated chronic limit for sodium, as set out in Appendix A is established as a 30-day average limit, effective immediately. Although no limit for the SAR will be imposed at this time, calculating and reporting the SAR will be specified in the permit. Additionally, because SAR is a calculated parameter based on a formula (see the paragraphs that follow) incorporating sodium, calcium and magnesium analytical results, monitoring requirements for calcium and magnesium (along with limits and monitoring for sodium) are also established in this permit. The SAR reported as part of permit requirements must be calculated using the following formula: Na+ SAR = F': + Mg++ +i The e values for sodium (Na+), calcium (Ca++), and magnesium (Mg++) in this equation are expressed in units of milli-equivalents per liter (meq/1). Most monitoring data will typically be reported in terms of mg/1, which must be converted. The conversions are: Concentration in mg /I meq/1 = Equivalent weight in mg / meq r? ?J COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale - Page 13, Permit No. CO-0000906 Where the equivalent weights are determined based or: the atomic weight of the element divided by the ion's charge: Na' = 23.0 mg/meq (atomic weight of 23, charge of 1) Ca" = 20.0 mg/meq (atomic weight of 40.078, charge of 2) Mg" = 12.15 mg/meq (atomic weight of 24.3, charge of 2)