Laserfiche WebLink
October 19, 2017 Page 5 <br />conditions in the Collom area, AAI asked Colowyo to collect monitoring well water elevations <br />from open and accessible wells. <br />2.2 Collom Site Visit <br />During the last week of May 2016, the AAI Project Team performed a 2 -day tour of the <br />existing surface pits at the Colowyo Mine and of the Collom Pit site. AAI visited the currently <br />operating South Taylor Pit, and the West Pit which is undergoing reclamation activities, to <br />observe strata conditions and assess them from a hydrogeologic perspective because similar <br />ground conditions are anticipated in the Collom Pit. The Project Team spent most of the time <br />touring the Collom Pit area, which included visits to the pilot dewatering test location, Little <br />Collom Gulch, Jubb Creek, and Collom Gulch and a few seeps/springs. The Project Team <br />traversed the length of Collom Gulch and the west fork of Jubb Creek, which represent the <br />western and eastern boundaries, respectively, of the Collom Pit. While inspecting these areas, <br />AAI performed stream -flow measurements, investigated likely subsurface infiltration sources, <br />and inspected a few coal burn zones, seeps/springs, and slump/collapse zones. AAI had <br />developed a conceptual hydrogeologic model of the Collom Pit based on its preliminary review <br />of the available hydrogeologic data, and the field visit helped to validate the conceptual model. <br />The findings of the site visit are summarized below: <br />1. The exposed pit slope strata in the South Taylor and West Pits indicated that the overall <br />grain size and joint network in the sandstone, siltstone, and mudstone layers are very <br />similar, essentially meaning overall hydraulic conductivity and storage coefficient values <br />for these rock types are likely to be very similar in the Collom Pit. <br />2. Any dewatering design needs to rely on strata whose horizontal hydraulic conductivities <br />are equivalent to those of a sandy clay (approximately 0.05 ft/day) or higher to be <br />effective. Flow within strata less permeable than this will tend to be vertical and <br />extremely slow, so that the net effect is not dewatering but depressurization. For the <br />Collom Pit, only the coal layers meet the aforementioned conductivity criterion. <br />3. Although groundwater infiltration into stream bottoms in the Project Area was observed <br />during WMC's 2005 study, the AAI team did not find evidence of the same during our <br />site visit. Rough streamflow measurements along the Collom Gulch and little Collom <br />Gulch indicated little or no loss to subsurface infiltration. <br />4. The overall conceptual hydrogeologic model formed by WMC during their 2005 study <br />still appears valid. <br />2.3 Dewatering Study Approach and Model Development <br />In order to develop and evaluate the effect of a dewatering program for the Collom Pit, <br />AAI utilized the three-dimensional (31)) finite -difference numerical modeling code, Visual <br />MODFLOW (version 2.8.2, Waterloo Hydrologic 1999) (McDonald and Harbaugh 1988). <br />Visual MODFLOW provides a fully integrated graphical modeling environment for groundwater <br />flow and contaminant transportation simulations with both pre-processor and post -processor <br />units. This program is a modified version of the MODFLOW program originally developed by <br />the USGS (McDonald and Harbaugh 1988) for hydrological studies. A 3D modeling program <br />allowed AAI to overcome the limitations of prior modeling efforts, through representative <br />Agapito Associates, Inc. <br />