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U <br />0 <br />• <br />CO-0000906 <br />New Elk Mine Water Quality Assessment <br />Group, and culminated in the most recent policy entile (hereafter the Narrative Standard policy) <br />Discharge Permits for the Protection of Irrigated Crops <br />Based on available information, neither the Middle Fork of the es forerriRiver nor the Left Fork gation water. However, <br />Apache Canyon Creek and Apache Canyon Creek are used as sour <br />two miles downstream from Outfall 001 in the Purgatoire River, and just below the conflu <br />tionence of <br />Apache Canyon Creek with the Purgatoire River, the Purgatoire River used for irrig water. <br />The evaluation of the suitability (i.e., quality) of irrigation water is complex and involves the detailed <br />understanding of the interactions of plant tolerances, soil types, and agricultural management <br />practices. Irrigation water has two properties - salinity and sodicity - that can have concurrent <br />impacts on the irrigated crop beneficial use. The Division has thus determined that two parameters, <br />specifically electrical conductivity (EC) and sodium, are the best parameters to regulate in discharge <br />permits to control levels of salts to minimize both the loss of irrigated crop yield and the sodium <br />hazard. <br />In order to establish limits for EC and sodium, the Division must: (1) determine the most sensitive <br />crop usually grown in the area downstream of the discharge and determine the corresponding E the <br />irrigation water (ECH,) threshold value for no reduction in yield below 100%; and (2) determine <br />SAR based on the EC value, and calculate the maximueedani of that can be discharged, with <br />the SAR. <br />consideration of existing water quality, to prevent the <br />Electrical Conductivity: The electrical conductivity (EC) is also known as specific conductance, <br />conductance, conductivity, or specific conductivity. Crops have varying sensitivity to electrical <br />conductivity. Studies have established the maximum conductivity in the water in the root zone that <br />will result in no reduction of crop yield. This value is referred to as the EC saturation extract or ECe. <br />However, the ECe is not the same as the EC of the irrigation water (EC,,,). The EC, is the maximum <br />conductivity in the irrigation water that will result in no reduction in crop yield. Division procedures <br />are to estimate EC,,, by dividing the ECe values by 1.5. <br />Table A-7: Maximum <br />Appendix A cWQA version s-a(FnaL)) Page 18 of 29 Last Revised August 26, 2008 <br />Common crop ECW thresholds are reproduced from the Narrative Standards policy, and are <br />summarized in Table A-7.