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<br />4 <br />p. 3-12 What is meant by, "Flow in the lower aquifer is towards a sink" and <br />the "characteristics of any flow out of this sink are unknown"? As <br />Figure 3-2 an~i Page 3-9 note, there is flow out of the lower aquifer <br />with discharges to Piceance and Yellow Creek. This appears <br />consistent with EPA's understanding of the regional hydrogeology. <br />We suggest that these references to a ground water "sink" be deleted <br />from the final) EIS since they appear to conflict with the other <br />conclusions about the discharge of flow from the Basin. <br />p. 3-13 The human health standard for fluoride concentration in drinking <br />water established by EPA under the Safe Drinking Water Act has been <br />recently changed to 4.0 mg/liter. <br />p. 4-4 The EIS correctly notes that as a result of the prediction for <br />exceeding the 24-hour particulate concentration of the Class II PSD <br />increments, ttie applicant would either have to: 1) use a different, <br />less conservative, model, 2) reduce the emission estimates <br />appropriately, or 3) provide additional controls to assure <br />compliance with the Class II increment standards. Due to the <br />predicted violation of PSD Class II increments, EPA concludes that <br />the alternative of 500,000 TPY is unacceptable until this issue is <br />resolved. However, note that since the Nation's current output of <br />sodium bicarbonate is less than that produced under this <br />alternative, we suggest that this alternative be dropped from <br />further analysis. If this alternative is to be retained in the <br />analysis, the applicant's opinion of market expansion that would <br />make this a plausible alternative should be included in the final <br />EIS. <br />p. 4-11 The conclusion by Weston (1985) that subsidence could cause a mixing <br />of the upper and lower aquifers requires much additional attention. <br />The EIS could ;present a more thorough evaluation of the quantity of <br />salt that could enter the groundwater system from the solution <br />cavities each ,year. Diffusion rates (including temperature effects) <br />groundwater flow rates, saturated salt content of the brine-filled <br />cavities, and •the effect of cavity collapse and subsidence fractures <br />on groundwater flow rates, could be evaluated to predict the amount <br />of salt moved •From the cavities into the groundwater system and the <br />eventual, if any, impact on surface water quality. EPA suggests <br />that consideration be given to the development of a hydrologic model <br />to evualate such an eventuality using the data obtained from the <br />pilot project. <br />p. 4-12 EPA suggests that consideration be given to separate wells, or <br />isolated multiple completion wells, to monitor the Uinta aquifer, <br />the upper Mahogany aquifer, the lower Mahogany aquifer, and the zone <br />above the dissolution surface to better detect brine leaks. <br />