2014-06-17_REVISION - M2000041 - Laserfiche WebLink

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• Dr.Angela Bellantoni RE:Evans#2 Pit Hydrogeologic Evaluation Review May 27, 2014 northern aquifer boundary removes the opportunity for adjacent aquifer flow into the dewatering radius of influence, likely increasing dewatering impacts to the north, while recharge from the river would be expected to decrease the radius of influence south and southwest of the site. These and other aquifer inconsistencies, such as mine cut-off walls and settling ponds, could not be quantitatively reconciled using the analytical solution, but will undoubtedly affect the extent of the dewatering radius of influence. General approximations for the radius of influence in these areas were presented in the report to help identify the aerial extent of proposed groundwater monitoring. Parameters used in the Marinelli and Niccoli analytical solution that show the most sensitivity for affecting the dewatering radius of influence results are the saturated thickness of the alluvial aquifer penetrated by the pit, the aquifer hydraulic conductivity, and the alluvium recharge. Assumptions made in estimating these parameters were questioned in the review comments. The estimated pre-mining average alluvial groundwater elevation (also used to estimate the saturated thickness of the alluvial aquifer penetrated by the pit) presented in the report is consistent with the limited regional information available from published reports. Unfortunately, more detailed and area specific baseline groundwater studies were not found to be readily available. Borelog data from a 2006 subsurface investigation on and adjacent to the Evans#2 Pit site included the groundwater contact elevations during drilling. The approximated groundwater elevations recorded during drilling were potentially experiencing affects from active Evans #2 Pit dewatering, adjacent irrigation well pumping, and seasonal/annual fluctuations, but these records were the only source of site specific baseline groundwater information available. The site specific borelog data was used in conjunction with published report information to provide approximate baseline groundwater contours for backfilling operations during site reclamation.. The approximate pre-mining groundwater contours may not provide adequate information to evaluate dewatering impacts at specific well or borehole locations, especially considering seasonal and annual groundwater elevation fluctuations. Temporal groundwater monitoring information at identified locations of interest is recommended to evaluate and quantify specific dewatering impacts. Similar to estimating pre-mining alluvial groundwater elevations, borehole data from 2006 was used to approximate the alluvial aquifer hydraulic conductivity. Information from published reports suggested that the hydraulic conductivity could vary significantly within the Arkansas River alluvium, stating ranges from 70 to 1,200 feet/day. Gradation results from the 2006 alluvium borehole samples also showed significant variability in grain size and size distribution from hole to hole and with depth. Since particle size and distribution was used to estimate alluvial aquifer hydraulic conductivity, it is consistent that the published variability in hydraulic conductivity was also observed on and near the site. Although many boreholes were drilled during the 2006 subsurface investigation, gradations were only performed on two of the borehole samples collected on or adjacent to the site. Material particle size and size distribution are commonly used to estimate groundwater permeability or hydraulic conductivity. Several methods are available for calculating hydraulic conductivity, including the Hazen, Beyer, Prugh, and Kozeny-Carman. Each method includes empirically derived formulas with limitations on applicability, generally related to sample particle size and particle distribution range,that affect method accuracy. 4- �: e.-A 1 1 f i Page 2 of 3 SOLUTIONS