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During both seasons (irrigation and non-irrigation), mean pH values were virtually <br />identical (8.2 units). The range in pH during irrigation (7.5 to 9.1) was slightly larger <br />than the range determined for non irrigation (7.68 to 8.80). <br />As previously discussed at other surface water sites (SW-N1 and SW-N3), TDS during the <br />non-irrigation period averaged almost twice the concentration averaged during the <br />irrigation season (1,379 mg/1 versus 729 mg/1). TDS ranged from 405 mg/1 to 1,550 mg/1 <br />during periods of irrigation, and varied from 990 mg/1 up to a high of 2,380 mg/1 when <br />irrigation was not occurring. <br />Both TSS and FeT averaged slightly higher during the non-irrigation season. During the <br />irrigation period, T55 and FeT averaged 10.4 mg/1 and .22 mg/1, respectively, while during <br />the remainder of the year, T55 and FeT averaged just 13.1 mg/1 and .24 mg/1, FeD averages <br />were identical for both seasons (.03 mg/1). Average MnT during the non-irrigation season <br />was .18 mg/1, three times the average value of MnT during periods of irrigation (.O6 <br />m9/1). <br /> Site 6W-N7 lies above the northern boundary of the Nucla Mine on the West Return Ditch <br />• (see Exhibit 7-1). Monitoring of water quality began at this site in March of 1987 (see <br /> Appendix 7-7 and Tab 16, Hydrologic Monitoring Program). Table 7-47 presen ts a <br /> statistical summary of historic rvater quality data obtained at Site SW-N7. Figure 7-28 <br /> presents a Schoeller diagram for Site SW-N7 of mean milliequivalent values for the major <br /> ions for both seasons. <br />As at SW-N6, the water type at SW-N7 is a calcium magnesium sulfate bicarbonate for both <br />seasons. Figure 7-28 shows the typical increase in mean milliequivalent values for the <br />major ions during periods of no irrigation. <br />Average pH values are only slightly lower at Site SW-N7 during the non-irrigation season. <br />pH values range from 7.15 to 7.94 during periods of no irrigation, compared to a range of <br />7.64 to 8.24 when irrigation is occurring. <br />TDS ranges between 1,270 mg/1 up to 1,732 mg/1 with a mean of 1,439 mg/1 during the <br />non-irrigation period. When irrigation is occurring, TDS averages just 678 mg/1, which <br />• probably reflects the proximity of SW-N7 to the source of irrigation water just upstream <br />of this site (see Exhibit 7-1 ). <br />7-139 Revised 04/11/88 <br />