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- ~, <br />Mr. Larry Schults <br />March 8, 1999 <br />Page 2 <br />A. The fractured rock would result in greatly <br />increased mine-water inflows, thus increasing <br />costs, and <br />B. Mine water is a source of water supply for oil <br />shale recovery, but the inflow here at future <br />mines could be unusable because of salts, and <br />thus create an expensive brine-disposal <br />problem. <br />The monitoring for subsidence/caving, as described, wil <br />not provide any early warning of the upward progression <br />from the cavity. Because lateral dissolution rates in <br />each cavity will be variable, and probably will be faster <br />in rich Nahcolite zones, and because no direct method of <br />determining cavity geometry seems available, the <br />protective support pillars between cavities are at risk. <br />The rapid dissolution of a rich Nahcolite zone into or <br />through a pillar will greatly compromise its strength. <br />Monitoring: <br />A. Groundwater: It is essential that properly <br />constructed wells be placed so as to monitor <br />water levels/water quality near the <br />dissolution surface, at sites near the <br />production well. Water levels are the most <br />critical parameter to measure frequently, <br />significant changes should trigger water- <br />quality monitoring. Figure 2-11 in the EIS <br />provides no detail on at what depth <br />groundwater will be monitored. The site is in <br />a highly fractured zone (USGS PP. 1196, Figure <br />3), and outflow of brine to Piceance Creek is <br />likely to occur. <br />B. Subsidence/caving monitoring: Monitoring of <br />deep caving is not addressed, but is essential <br />if the problem is to be evaluated in a timely <br />manner. Monitoring wells are needed, again, <br />near the production well. (It may be possible <br />to utilize subsidence measurements in <br />production wells and in groundwater monitoring <br />wells.) Monitoring of pillars by wells will <br />be necessary if critical data are to be <br />collected on dissolution of Nahcolite into the <br />pillar. A "dry" monitoring hole into the <br />pillar center would provide data on deep <br />penetration of dissolution. <br />Thank you for the opportunity to comment on this EIS. <br />Singly, -/~ <br />v ~%fA ~ ~i%~e~ <br />Glen A. Miller <br />GAM/an <br />