1986-12-31_REPORT - C1981008 (3)

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that of the West Lateral . Mean TDS concentration for the period of record is 973 mg/l ranging from 488 to 1732 mg/l . 2. Site SW-N6 Site SW-N6 is located on the West Return Ditch immediately downstream of County Road 124 as shown on Exhibit 1. This site is also downstream of mining activity which has taken place north of this east-west road. Flow and quality data have routinely been collected from this site since August of 1980; these data are presented in Appendix B and statistically summarized in Table 15. A hydrograph constructed from monthly instantaneous flow measurements is shown in Figure 9. A trilinear plot of the major ions is depicted in Figure 2. As previously discussed, flow at this site is reflective of the seasonal use of irrigation water. For the period of record depicted in Figure 9, flow generally ranges between 0.0 and 1.25 cfs. This reach of the West Return Ditch is routinely dry from November to April . The statistically summarized water quality data from Table 15 used to construct Figure 2 indicate that this water is a Ca-Mg-SO4 type; mean TDS concentration for the period of record is 815 mg/1 , ranging from 405 to 1550 mg/l . Because of the limited upstream data collected to date at site SW-N7, it is difficult to assess any changes in flow or quality to the West Return Ditch water as it crosses mine spoils immediately north of County Road 124. However, from the data illustrated in Figure 5, it appears that the flow and quality of the water sampled at the two sites during the summer of this year (1986) are very similar showing little to no detectable change. 3. Site NPDES 001 NPDES discharge point 001 (Permit No. CO-0000213; see Appendix 7-6) is located on the West Return Ditch at the southernmost edge of mining disturbance as shown on Exhibit 1. Flow and quality data have routinely been collected from this site since 1975. Data 44