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Waters sampled at this site during the irrigation seasons have a calcium magnesium <br />• bicarbonate sulfate type. During the irrigation season, milliequival ents of calcium are <br />only slightly higher than magnesium, while bicarbonate levels approach su 1'f ate. The <br />relative distribution of each anion at NPDES 006 during periods of irrigation result in a <br />bicarbonate dominated water type. <br />Based on the limited number of samples collected at NPDES 006, mean pH varied from 8.15 <br />during the irrigation seasons to 8.26 between periods of irrigation. For both seasons, <br />the range in pH was virtually identical (7.5 to 8.6). <br />TDS averages 995 mg/1 and ranges from 500 mg/1 up to 1,784 mg/1 during the irrigation <br />season. Between periods of irrigation, TDS averages 2,385 mg/1 and varies between 1,888 <br />and 3,090 mg/7. <br />Both mean TSS and FeT increase slightly for the seasons between periods of iir rigation. <br />Mean TSS and FeT are 14.9 mg/1 and .17 mg/l, respectively, for the months without <br />irrigation, and average 8.5 m/gl and .15 mg/1 during the irrigation seasons. Total iron <br />(FeD) averages slightly lower (.04 mg/1 versus .OS mg/1) during periods of no irrigation. <br />• MnT typically averages a higher concentration (.36 mg/1) when irrigation is not occurring <br />than during the irrigation period (.OS mg/1). <br />Effluent limitations (see Appendix 7-6) have been exceeded at any NPDES site only once <br />since 1980. This single excursion took place during the second quarter of 1984, when TSS <br />(total suspended solids) averaged 40 mg/1 over the 30-day period at NPDES Site 001, The <br />standard for the 30-day average of TSS concentration is 35 mg/1. CMLRD has communicated <br />to PCC that they have on file all NPDES discharge monitoring reports (DMR's) submitted to <br />the Colorado Department of Health prior to the first quarter of 1986. Subsequent DMR's <br />have been submitted to the CMLRD as part of Annual Hydrologic Reports (1986 and 1987 AHR's <br />for the Nucla Mine). <br />West Lateral Irrigation Ditch - Flow and Water Quality. The West Lateral irrigation ditch <br />is part of the Colorado Co-operative Company's main ditch system that originates on the <br />San Miguel River just upstream of Cottonwood Canyon (about 12 miles upstream of Naturita). <br />Construction of the main ditch commenced in 1896, and water began flowing to the First <br />• Park area (see Exhibit 7-1) via the ditch in the spring of 1904 (Mercer, 1967). The main <br />ditch, as well as the main laterals, is basically an unlined open-channel excavated in <br />7-163 Revised 04/11/88 <br />