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-2- <br />It is not practical for WDLC to monitor subsi- <br />dence over large areas or large numbers of panels. <br />As described in the permit application and subse- <br />quent adequacy response. Areas to be retreat mined <br />that are located where the overburden is more than <br />30 times seam height (approx. 210 feet) aze not <br />subject to detrimental surface or near surface <br />effects from subsidence which would cause depletion <br />of water resources. Zb provide a conservative <br />approach, WD~'~C has used 40 times seam height (approx. <br />280 feet) and rounded the number upwards to 300 feet. <br />Subsidence does occur at overburden depths greater than <br />300 feet; However, the surface effects that would cause <br />water depletion diminish to an insignificant level as <br />overburden increases in depth beyond 30 times seam height. <br />This rule of thumb (30 times seam height) has been proven <br />in actual mining conditions at underground opal mines in <br />the United States. Score coal mines in Colorado have <br />also used this approach with regazd to providing in- <br />formation on impact to water resources. <br />Based on the information contained herein and in the <br />permit application, WDCC will agree to observe on a basis <br />consistent with good engineering practice known water <br />resources that aze located in azeas to be retreat mined <br />that have less than 300 feet of overburden. The obser- <br />vations to determine subsidence effects (if any) that <br />could impact water resources will be done during the <br />period of retreat mining during which subsidence could <br />reasonably be expected to occur. The observations will be <br />carried out in these azeas only during the season of usage <br />by landowners and livestock. <br />ESchibit 2.8.1.U.1 shows areas in the lease block as well <br />as five year permit boundary which are less than <br />300 feet of overburden. ESchibit 3.4.4.B.1 shows pro- <br />posed mining in the lease block and five year permit area. <br />Most water usage (springs and stock ponds) is located in <br />azeas with more than 300 feet of overburden and hence will <br />not be affected or is located in azeas where subsidence <br />will not occur or mining will not occur. There aze a few <br />springs and stock ponds located where there is less than <br />300 feet of Dover and subsidence will occur. These will <br />be observed for impacts from mining as described herein. <br />