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• GW-N4wascollectedsinceAugust,1979andcollectionofwaterleveldataforWellsGW-N14,GW- <br />N17,GW-N17P1,andGW-N20beganinJuly 1986. GW-N4R,GW-N29andGW-N30wereinstalled <br />in September, 1987 and collection of water level information began immediately after they were <br />completed and developed. Map 2.04.7-3 shows the Dakota coal potentiometric surface. Monthly <br />water level data including hydrographs for New Horizon 2 monitor well sites are contained in <br />Appendix D of Attachment 2.04.7-1. An additional well, GW-N37, was drilled in May, 1993 in the <br />north-east corner of the permit area. <br />Ground water in the Dakota coal is under artesian conditions. Mean artesian heads in the lower <br />Dakota coal range from 18 feet at Monitor Well GW-N 14 and 42 feet at Monitor Well GW-N29. The <br />average artesian head for the lower Dakota coal is 34 feet. Water levels in the Dakota coal display <br />seasonal variability that coincides with the irrigation season and operation of the ditch irrigation <br />system. The maximum seasonal water level fluctuation observed in Monitor Well GW-N4 is about <br />3.0 feet due to its location in the valley bottom. The New Horizon 2 wells display seasonal variability <br />from 2.8 feet at Monitor Well GW-N 17P1 to 14.6 feet at Monitor Well GW-N20. Shallowing occurs <br />from April to September and there appears to be little lag between when the irrigation season starts <br />• and theshallowingofthestaticwaterlevelsoccurs. There are no discerniblewaterlevelresponses <br />to precipitation, norcan any effectfrom mining be observed at any ofthe Dakota coal monitorwells <br />at the New Horizon 1 mining area. <br />Underburden Water Levels. Fourteen monitor wells have been installed to monitor water levels <br />in the underburden aquifer system. Six of those monitorwells(GW-N1,GW-N1PI,GW-N3,GW- <br />N5, GW-N6 and GW-N11) were located in and proximate to the New Horizon 1 mine area. The <br />remaining six monitorwells (GW-N13, GW-N16, GW-N16P1, GW-N19, GW-N33, GW-N34, GW- <br />N43andGW-N46)werelocated proximate to the NewHorizon2 mining area. Maps 2.04.7-1 and <br />2.04.7-4 show the underburden monitor well site locations and the potentiometric surface, <br />respectively. Monthly water level data including hydrographs for New Horizon 2 Mine sites maybe <br />found in Appendix D of Attachment 2.04.7-1. In may, 1993, an additional well (GW-N36) was <br />installed in the northeast corner of the permit area. <br />Ground water in the underburden aquifer is under artesian conditions. Mean artesian heads in the <br />underburden aquifer range from 9.7 feet at Monitor Well GW-N 11 to 86.7 feet at Monitor Wefl GW- <br />r~ <br />(REVISED 6/28/01) 2.04.7-4 <br />