2005-10-19_REPORT - M2003016 - Laserfiche WebLink

Home Browse Search

Mr. earl Mount September 1 a-,L005 Page ~°roposed gt°igger~. "Che.Tuly 2003 monitoring and mitigation plan 6y R.'right b~rater'/~• ^^-ecrs, Inc. (W W E) proposed that an impact to groundwater wi it he considered to have occurred when post- mining da~r? show: 1) a lowering of the average (graving or non- ^'owing season) gr'ounc . ater level by more than 2 feet, or 2) a lowering of two or more consecutive readings by more Than 2 feet. compared to preanining data at the same time of year. The 2-~;~nt trigger was recommended based on the modeling conducted that showed a maximum change of 2 feet in groundwater levels Crom the operation. The monitoring data (Tables 1 and 2) show that groundwater did not fluctuate greatly over the monitoring period. The maximum ilucnration was L53 feet during the growing scasan, and l.S~ feet during the non-gro~~n~ season, both at MW-10. The largest annual fluctuation was 2.76 at MW-10. These findings, coupled vvith the tnode(ing results, indicate that the nigger of 2-feet proposed in the permitt!ug process is reasonable for average fnoundwater levels. ; lowever. 2-feet is too srnail and within the measured range of variation in groundwater levels for consecutive. readings. A trigger of 2-feet For two or more consecutive readings could cause it Co be reached due to existing changes in groundwater levels. Data collected indicate that this part of the trigger shotad be 3-feet. A period of hvo months is too short to indicate real changes in groundwater. Also, the original trigger only addressed the lowering of greuudwarer levels and naf mounding ef~P~ts, whic,'r could also occur. Therefore, it is proposed that the text describing the tugger for the -~ennii he changed as follows: "A potential impact to groundwater will have occurred wizen post-mining groundwater monitoring data show: 1) a change in the average (growing or non- growing season) groundwater by more than 2-feet, or 2) a change in three or more consecutive readings by more than 3-feet compared to data taken the same time of year prior to mining." MndeEing r•esal4s~ Modeling was completed using the U.S. Geological Survey (iJSGS) MODFLOW-2000 model to evaluate potential effects on groundwa4r from Ire proposed opera' on. The results of this modeling are contained in the March 2003 y>rNd c report. MODFLOW is afinite-difference model that was used to predicC stead;-acre conditions. The model was calibrated to published pre-mining water levels in the vicinity of the Heit Pit. The nxricl was then used to assess changes in groundwater levels with a lined pit in plat. The modeling predicted a maximum mounding of groundwater on the. west side of the pit of 2 feet, and a maximum drawdown or' "shadow" effect north of the pit of 3 feet. The model also s :oo._~d that groundwater levels returned to near pre-mining elevations, with no shadow elf ct, within approximately 1,000 feet north of the pit Additional derails of the modeling can be found in the March 2003 repotti. The MODFLOW model is capable cf transient analysis using a porticn, if not ail.. o^ the data obtained over die past eighteen months (Table t). However, Ute average piezometric surface rcpresent~:d by the inonitcring data (Figures 2A and 2B} conr~ares favorably with the pre-mining water levels generated by the steady-state model runs. This comparison shows similarity in the shape of Uic groundwater table, the average slope of the groundwater table across the Ra'. Pit (0.25