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3. TEXT CHANGES <br />Depressed peak flows during spring and summer could <br />reduce shmb-scrub acreage, particularly on the outer fringes <br />of overflow areas and upper channel terraces. The shmb- <br />scrubbelt on the lower terraces would likely shift its position <br />relative to the stream, encroaching on the upper portion <br />of the facultative herbaceous zone. BLM would not expect <br />shrub-scmb acreage to change by more than ] 0 to I S percent, <br />with distal greasewood and sagebrush fringes bearing the <br />greatest impact. <br />Although a certain amount of streambed substrate <br />available for aquatic plant attachment (i.e., pondweed) may <br />become exposed under these Oow regimes, the overall affect <br />should be minor, since aquatics are typically found in deeper <br />pook where water depths would be least affected by flow <br />loss. Macroinvertebrate habitat would not expectedly <br />diminish except during drought influenced years when <br />constrictions in wetted channel width would likely occur. <br />4.9.6.2 Salinity <br />Committed mitigation provides for detection of <br />groundwater quality changes as well as required on-lease <br />containment of dissolved solids. if this mitigation strategy <br />k successfully implemented, dissolved solids contributions <br />to affected stream segments would be largely avoided. <br />In the event mitigation was not exercised or failed to <br />provide intended results, the fallowing discussion k provided. <br />BLM has been unable to calculate specific values for <br />expected salinity increases to Yellow Creek attributable to <br />mining. Increased dissolved solids Vaasport from the saline <br />zone is expressed as a range due to uncertainties concerning <br />natural versus post-mining dissolution rates (Table 43A, <br />Hydrology section). <br />Maximum expected TDS increases in Yellow Creek for <br />all but the 500,000 TPY Altemative, would not exceed <br />6.5 percent. Since Yellow Creek's wetlands currently tolerate <br />average annual TDS Fluctuations of 750 mg/1, we would <br />not expect the salinity tolerance of any wetland species to <br />be exceeded, even with sustained maximum 160 mg/I <br />increases. <br />Under the 500,000 TPY Alternative, the given TDS range <br />becomes less workable. Maximum 600 mg/1 increases would <br />increase salt concentrations by 25 percent. Although we <br />are not aware of the salinity thresholds for plants making <br />up Yellow Creek's wetland association, sustained 25 percent <br />increase (average 3,120 mg/I TDS) could effect vegetation <br />composition shifts or decrease vegetative productivity. <br />Plants are generally most susceptible to the effects of <br />salinity early in their life cycle, becoming increasingly tolerant <br />with age. Since maximum TDS concentrations coincide with <br />low Oow periods (July, August), phtnt development might <br />be expected to progress more or less normally during the <br />first half of the growing season (high flow periods). The <br />effects of increased salinity during the latter half of the <br />growing season would be thus somewhat moderated, <br />particularly since these plant associations would not then <br />be in direct contact with surface flows. BLM aka bas <br />empirical evidence that wetland communities composed of <br />sedges and tamarisk develop rapidly along well discharge <br />paths exhibiting constant TDS values of 5,000 mg/I. <br />Based on this analysis and with mitigation, we would <br />not expel salinity changes attributable to any altemadve <br />to significantly alter Yellow Creek's wetlands, particularly <br />those elements providing locally unique community stmcture <br />and composition (e.g., tamarisk, sedges, bulrush). <br />Page 4-30 <br />(11) <br />Column 1, paragraph 4, sentence l: "...the L-SE Zone, <br />lying ... "should read: "...the L-SE Bed, lying . . <br />(11) <br />Column 1, Section 4.18.1.4, Water Resources: Replace <br />entire section. <br />4.18.1.4 Water Resources <br />There would be the potential for an unavoidable impact <br />to the water quality of the base of the lower aquifer becattse <br />of the amount of increase in the surface area of the base <br />of the lower aquifer in contact with saline minerals as the <br />result of cavity collapse. This potential impact would persist <br />for the foreseeable future (less than 100 years) and would <br />be locally significant if the state standard for total dissolved <br />solids (TDS) concentration was exceeded and remedial <br />measures to lower the TDS level were unsuccessful. <br />Page 4-31 <br />(11) <br />Column 2, Section 4.18.2.2, Water Resources: Replace <br />entire section. <br />3-24 <br />