Baldwin Pit Groundwater Inflow Assessment
Wenck completed a hydrogeologic and groundwater inflow assessment for a proposed gravel mine near Marysville, California. The proposed WURM mine is located along the Yuba River where numerous other pits had been completed, but those pits were typically mined “wet.”
To complete the evaluation, Wenck coordinated the drilling and aquifer testing of the alluvial aquifer. Three, 4-inch diameter test wells and several 2-inch diameter observation wells were constructed. Following well completion, Wenck conducted stepped and constant rate tests on three of the test wells to determine hydrogeologic parameters for the alluvial aquifer and assess parameter variability across the site.
Based on the hydrogeologic data acquired, Wenck developed estimates of groundwater inflow to the proposed WURM pit under two development scenarios and two different depths, assuming equilibrium conditions for both scenarios. The first scenario entailed dewatering the entire mine pit area. The second scenario entailed dewatering three, 200-foot wide mine blocks on either end of the mine pit. These scenarios were evaluated with dewatering to depths of 40 feet and 70 feet from ground surface. This two-pronged approach allowed Wenck to estimate the maximum amount of groundwater that could inflow the pit under the worst case (whole mine) and potentially more manageable conditions (mine blocks).
The assessment concluded that relatively large inflows of groundwater were expected during mining on the WURM property. Groundwater inflows under Scenario 1 were estimated to be between 11,000 to 22,000 gallons per minute (gpm) if the entire site was dewatered to depths of 40 and 70 feet, respectively. When only mine blocks were considered, groundwater inflows ranged from 6,000 to 11,000 gpm on the West End to depths of 40 and 70 feet, respectively, and from 2,000 to 6,000 gpm on the East End to depths of 40 and 70 feet. Subsequent mining of the property revealed that dewatering pumping rates were in line with modeled estimates.
Major Project Features:
- Hydrogeologic Assessment
- Aquifer Testing and Analysis
- Pit Dewatering Assessment
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