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described will be several years before the TDS levels begin attenuating. Based on analyses of alluvial <br />aquifer data below the 004 pond at Seneca II, the alluvial TDS levels have peaked and begun declining <br />• even though the spoil discharge TDS levels are still increasing. <br />The AVF study conducted by Peabody Coal Company determined that there are two subirrigated grass <br />hay fields (Section 9, T5N, R88 W, Exhibit 16-31 that could potentially be affected by higher TDS <br />levels within Dry Creek due to mining activities. The following equation, provided in the CMLRD <br />guidance document " A Description of the Material Damage Assessment Process Pertaining to Alluvial <br />Valley Floors, Surface Water, Ground Water and Subsidence at Coal Mines", can be used to estimate <br />the effects of increased dissolved solids levels on crop species: <br />Y = 100-BIECW-AWI, where: <br />Y = Relative Yield, <br />B = Percent yield decrease per unit salinity increase, <br />ECW = Predicted conductivity of irrigation water (mmhos/cm), and <br />AW = Salinity threshold of irrigation water (mmhos/cm). <br />The subirrigated hay fields are composed of alfalfa (20%1, orchardgrass f30%I, and smooth brume <br />• (50%I• TDS related impacts to these iwo hay fields were reassessed in 2004 to address potential <br />additional impacts from the Seneca 17-W South area. These revised predictions may be found in this <br />tab on pages 729 and 130 (Ground Water Seneca II-W South ArealPotential Impacts of Replaced Soil <br />on Surface Water DualitylSubirrigated Cropsl• <br />The overall significance of the impact to the Dry Creek alluvial aquifer will be minimal, The alluvia! <br />ground water has a slow movement rate, and the undisturbed alluvial aquifer is marginal to unsuitable <br />IOlson's stock Well IS-45) exceeded the Hg limit for stock water and the irrigation limits for Cd, <br />conductivity, Fe, Mn, Mo, and Zn) for stock water use and the subirrigation of crops. In addition, the <br />aquifer will not yield sufficient water to make pumpage of alluvial water for irrigation purposes <br />feasible. <br />The TDS increases in the 009 tributary and Sage Creek alluvium will have negligible significance. The <br />009 tributary alluvium is dry and thus, had no premining water use. The limited amount of spoil <br />discharge should only saturate the 009 alluvium to a depth of <br />• <br />TR-50 47 Revised 11/04 <br />