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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/l versus 729 mg/1). TDS ranged from 405 mg/1 to 1 ,550 mg/l <br /> during periods of irrigation, and varied from 990 mg/l up to a high of 2,380 mg/l when <br /> irrigation was not occurring. <br /> Both TSS and Fe averaged slightly higher during ,the non-irrigation season. During the <br /> irrigation period, TSS and Fe averaged 10.4 mg/l and .22 mg/l, respectively, while during <br /> the remainder of the year, TSS and Fe averaged just 13.1 mg/l and .24 mg/1 . Fe averages <br /> were identical for both seasons (.03 mg/1). Average MnT during the non-irrigation season <br /> was .18 mg/l, three times the average value of MnT during periods of irrigation (.06 <br /> mg/1 ). <br /> Site SW-N7 lies above the northern boundary of the Nucl a 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 15, Hydrologic Monitoring Program). Table 7-47 presents a <br /> statistical summary of historic water 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/l with a mean of 1,439 mg/l during the <br /> non-irrigation period. When irrigation is occurring, TDS averages just 678 mg/l , 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 />