PERMFILE126519 - Laserfiche WebLink

Home Browse Search

Exhibit Page 27-B • able G-4. Drawdown levels Distance Drawdown 50 10.41 100 6.96 150 5.82 200 5.12 400 3.61 600 2.79 800 2.25 1000 1.82 1200 1.47 b. These dimensions are shown on Figure G-2. Calculations show that the drawdown at the wells will not be excessive under the worst-case assumptions stated. Drawdown at the Robinson Well is estimated at 1.8 feet. c. Since dewatering discharges will enter the river upstream from the point in the aquifer in which the Robinson and Akin wells are placed, the drawdown will be less than that found in the above calculations. d. For more shallow aquifer depths (65 feet, 36 feet, and 25 feet), the drawdown cone is much smaller, and there is no drawdown more than 200 feet from the edge of the pit. While this is shown in the spreadsheet, the model actually does not apply to these shallow conditions, as the volume for recharge (from the river and downstream discharges) is significantly greater than the volume of the aquifer itself. e. The estimates are applicable primarily to the northern and southern sides of the pit, as the aquifer does not exist much further west than the Highway 145 embankment, where alluvial deposits end. The pit is between 200 and 500 feet of the Dolores River to the east, and the drawdown curve will be much steeper due to seepage from the river. This should greatly reduce the impact of the drawdown on cottonwoods to to the east. The impact on the trees to the south, approximately 500 feet south of the pit, will also be relatively small, with the drawdown at about three feet (not counting any recharge from the river). This is within the tolerance of cottonwoods, which can accept depths to water of from essentially zero to approximately 10 feet. Assuming a "normal" five foot depth to ground water, drawdown will lower the water table to about eight feet in this area, or less than the ten which would be expected to create poor conditions for sustainment of the grove. In addition, by limiting the drawdown to actual operational periods (as discussed below), the period for any impact, significant or not, would be greatly reduced. 6. Prevention and mitigation actions: As discussed above, the permit area is more than 200 feet away from the Dolores River main channel, and while it does wme within 200 feet of a wetland which may be either a branch channel or former channel, no disturbance of that area will take place. Storrs water pollution prevention and management actions, including erosion and sedimentation control, will be implemented as required by the Colorado Discharge Permit System storm water general permit for which coverage has been obtained for this site (prior to beginning mining). This meets the requirement for an NPDES permit and covers dewatering and process (wash) water as well as storm water, and application has been made for coverage. Since water pumped or allowed to drain by gravity for dewatering purposes is returned to the Dolores River, no net impact on river flow is anticipated. Overall design of the pit was done to plan for a minimum impact on surface and ground water, including phasing to require a minimum amount of dewatering, and to limit that dewatering to as short a season and as few years as possible while recovering the sand and gravel from the site. In addition, water pumped will be reintroduced into the river (and therefore available to the aquifer) as quickly as possible, thus reducing water loss and impacts downstream. 7. Project water requirements: Water use for extracting and processing aggregates from Line Camp will be low, amounting to 4.4 acre-feet per year under worst case conditions. This water will come from the Home Ditch. The Home Ditch provides water for irrigation of pasture and hay grass form 54.7 acres total, including 39.9 acres in the field in which the 19.1 acre Four States Aggregates, LLC 15 MAR 2001 Application for Permit: Line Camp Pit M-2001-001 FSA-LCP-D2-001