Laserfiche WebLink
Mr. Douglas M. lkenberry, P.E. <br />Page 2 <br />October 27, 2012 <br />1. The materials being placed are effectively isolated from the environment by a very <br />conservative disposal cell design. This means that releases (if any) are likely to <br />be slow and of small quantity. <br />2. Because the materials being placed are from only a few limited sources, they are <br />very chemically consistent. Thus, the chemistry of facility releases (if any) should <br />also be consistent over time. <br />3. The materials being placed are relatively inert (both chemically and biologically). <br />This means that leaching should be minimal and that releases (if any) should not <br />be highly concentrated. Sparingly concentrated releases pose less threat to the <br />environment and to downstream receptors. <br />GROUNDWATER HYDROLOGY & USE <br />Doty & Associates, LLC <br />Following discussions in the groundwater monitoring plan (Doty, 1998), the uppermost <br />aquifer consists of clayey backfill, undisturbed bedrock (primarily silty claystone), and the <br />Ennis Draw alluvium northeast of the site. These materials form a single interconnected, <br />unconfined hydrologic system. The clayey backfill and undisturbed bedrock are the least <br />permeable portion of the system and these materials have roughly equal saturated <br />hydraulic conductivities of approximately 3x10" centimeters per second (cm /s) or lower. <br />The alluvium, on the other hand, is more permeable, having a conductivity of 1x10 cm /s <br />or higher. <br />Prior to mining, flow in the overburden was to the northeast toward the alluvium. Flow <br />in the alluvium is to the northwest following the alignment of Ennis Draw. When the pit <br />was open to expose the coal at an average depth of approximately 90 feet, groundwater <br />flow was inward toward the excavation. After the pit was backfilled with overburden, flow <br />remained inward in response to the existing drawdown at the pit perimeter and the <br />moisture demand of the less than saturated overburden backfill. Because of the relatively <br />low hydraulic conductivity of the surrounding materials, the relatively large quantity of <br />overburden backfill and the low infiltration recharge due to the arid climate, inward flow <br />(resaturation of the backfill) is still occurring. An analysis presented in the groundwater <br />monitoring plan (Doty, 1998) showed that this long -term recovery of the groundwater <br />system was consistent with hydrologic conditions. <br />The most recent evaluation of the rate and direction of groundwater flow (using December <br />2011 data) indicates that the flow system continues to be dominated by resaturation of <br />the backfill. These data imply that the maximum groundwater flow velocity is toward the <br />area being resaturated and it is estimated to be approximately 1 foot per year. The <br />gradient away from the facility toward Ennis Draw is approximately 0.005 feet per foot and <br />the flow velocity is on the order of 0.5 feet per year. <br />Also as discussed in the groundwater monitoring plan, there is very limited use of <br />groundwater in the site vicinity. A recent review of the State Engineer's website indicates <br />