2009-10-16_PERMIT FILE - M2009082 (10)

Home Browse Search

what is shown on the USGS base mapping, the Jones Ditch has been revised such that flow re-enters the Cache La Poudre River at Weld County Road (WCR) 25. A field inspection of the ditch (near WCR 25) and a comparison of ditch bottom and groundwater elevation data indicate that the ditch is approximately 5 feet above the groundwater elevation in the and will be loosing water to the shallow alluvial aquifer. Ditch bed conductance and inflow values were estimated from the approximate bottom width of the inlet (10 feet), the assumed bed thickness (0.5 foot), and a bed hydraulic conductivity (2x 10-5 cm/sec). A hydraulic conductivity of 2x 10-5 cm/sec was selected to represent the ditch bed because this would be reflective of a predominantly silty material or the low end of a clayey sand (McWhorter and Sunada, 1977; Davis & DeWeist, 1966), which likely provides a reasonable representation of the nature of the ditch bottom. Poudre River Leakance Shallow aquifer leakance to the Poudre River was modeled using the MODFLOW drain package applied along its flow segment in the model domain (Figure 2). A field inspection of the Poudre River along mine site was conducted at two locations where the river intersects 95th Avenue and where it intersects Colorado Highway 83. Site inspection and groundwater data (wells MW-7 though MW-12) indicate that the river stage is several feet below the adjacent groundwater elevations and the river gains water from the shallow alluvial aquifer. The drain package was used to simulate aquifer losses to the Poudre River because of model mathematical instability using the river boundary condition during simulated post- mine reclamation activities. The drain boundary is justified for this purpose as the Poudre River appears to always gain water from the aquifer and simulated groundwater elevations are above the drain elevations even during mine dewatering activities. Drain conductance values were initially set to approximately the same values as those estimated for river conductance and adjusted to approximate the flow out of the model that was obtained using the river package. Additional variations in the conductance were made during the groundwater elevation calibration process. The river (and subsequently drain) conductance values were based on an approximate river bottom width of 50 feet, an assumed bed thickness of 1.0 foot, and a bed hydraulic conductivity 1x10-5 to 2x10-5 centimeters/second (reflective of a predominantly silty material). EnviroGroup Limited