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• ditches are flowing and flood irrigation is occurring. Water level responses to <br />precipitation events are indistinguishable, as the effects of recharge from the ditch <br />irrigation system have masked any evidence of recharge that could be attributable to <br />precipitation. Mining at the Nucla Mine area has had no discernible affects on the water <br />levels of the overburden monitor wells. <br />Dakota Coal Water Levels. Eight monitor welts have been installed to monitor water Levels <br />in the Dakota coal aquifer. Two of the monitor wells (CW-N4 and GW-N4R) are located at <br />the Nucla Mine and the remaining six monitor wells (GW-N14, GW-N17, GW-N17P1, GW-N20, <br />GW-N29 and GW-N30) are located at Nucla East (Exhibit 7-1 ). Water level data for Well <br />GW-N4 has been collected since August, 1979 and collection of water level data for Wells <br />GW-N14, GW-N 17, GW-N17P1, and CW-N 20 began in July 1986. The remaining three wells <br />(CW-N4R, CW-N29 and GW-N30) were recently installed (September, 1987) and collection of <br />water level information began immediately after they were completed and developed. <br />Exhibit 7-3 shows the Dakota coal potentiometric surface. Monthly water level data <br />including hyd rog ra phs for all monitor well sites are contained in Appendix 7-1. <br />Ground water in the Dakota coal is under artesian conditions with a general downward <br />• vertical head, but there are areas with upward vertically recharging head pressures. Mean <br />artesian heads in the loner Dakota coal range from 18 feet at Monitor Well GW-N14 and 42 <br />feet at Monitor Well GW-N 29. The average artesian head for the lower Dakota coal is 34 <br />feet. Water levels in the Dakota coal display seasonal variability that coincides with <br />the irrigation season and operation of the ditch irrigation system. The maximum seasonal <br />water ievel fluctuation observed in Monitor Well GW-N4 is about 3.0 feet due to its <br />location in the valley bottom. The Nucla East wells display seasonal variability from 2.8 <br />feet at Monitor Well GW-N17P1 to 14.6 feet at Monitor Well GW-N20. Shallowing occurs from <br />April to September and there appears to be little lag between when the irrigation season <br />starts and the sh allowi ng of the static water levels occurs. There are no discernible <br />water level responses to precipitation, nor can any affect from mining be observed at any <br />of the Dakota coal monitor wells at the Nucla or Nucla East mining area. <br />Underburden Water Levels. Twelve monitor wells have been installed to monitor water <br />levels in the underbu rden aquifer system. Six of those monitor wells (GW-N1, CW-N1P1, <br />GW-N3, GW-NS, GW-N6 and GW-N 11) are located in and proximate to the Nucla Mine site. The <br />remaining six monitor wells (GW-N13, GW-N16, GW-N16P1, GW-N19, GW-N33 and GW-N34) are <br />• located proximate to the Nucla East mining area. Exhi hi is 7-ll and 7-4 show the <br />7-8 Revised 04/11/88 <br />