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Getty Task 2 Pnge 30 March 1, 1983 <br />i~ <br />be lowered enough to cause the aquifer to become unconfined, the water removed <br />i <br />will come fzom confined storage, and recovery of the aquifer should be <br />relatively rapid following completion of mining. No significant impacts to <br />this aquifer are anticipated, therefore. <br />The effects of mining on the Twentymile aquifer are more difficult to <br />estimate. Assuming that significant interconnection between the Twentymile <br />' and overburden aquifers does not ezist, the model has estimated that the total <br />volume of water removed from the aquifer is 6900 acre feet. Using drill logs <br />~ and wells completed is the Tweatymile Sandstone, a conservative estimate of <br />the average saturated thickness of this aquifer is 200 feet. Using a specific <br />yield of 0.05 and neglecting confined storage, approzimately 6400 acre feet <br />per square mile of water is stored is this aquifer. Since the areal eztent of <br />the aquifer is several square miles, a very small amount of the water stored <br />will be removed. In sddition, the recharge to the aquifer is probably is <br />excess of 100 gpm, so it is unlikely that the operation of the mine will have <br />' a significant impact on this aquifer. It should be doted that removal of <br />water from the Twentymile aquifer will result in a decrease of piezometric <br />i! head that may cause some wells that currently flow at the surface to cease to <br />flow. The wells that have been established in this aquifer near the proposed <br />mine should provide an indication of the amonat of drawdowa ezperienced is the <br />Tweatymile aquifer as a result of mining. It shoald be noted that the above <br />analysis is based oa the assumption that fracturing does not extend completely <br />~ through the marine shale separating the aquifers. A substantial amonat of <br />! fracturing could lead to more impact oa the aquifer and increased inflows. <br />The probability of this occnrence is deemed small given the large thickness of <br />the shale unit. <br /> <br />