2005-06-24_REVISION - M2000016 (13)

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~1FARG E NORTH AMERICA AGGREGATES, CONCRETE !k ASPHALT example if more than one cell will be dewatered at any one time, this scenario should be modeled and reported. Wells beyond 600 feet that lie within the modeled area of affected ground water levels must also be located and identified. Thai maps and model must define or predict the cone of depression and/or shadow and mounding eff pct expected for any areas to be dewatered and dry mined or as a result of installation of clay liners or silt storage ponds. The cone of depression and assessment of other effects should define, at a minimum, the horizontal and vertical extent of expected impacts. If, during the assessment, the applicant determines that a drawdown or shadow effect, or mounding will occur offs ite that may impact a current ground water user, property owner (e.g., basement flooding, leach field inundation, decreased sub irrigation, affected sewer or water lines), and/or wetlands, the applicant must provide a map showing the location of all such structures that may be potentially affected and a table that lists the structure type and the name, address, and phone number of the structure owner. Please also commit to mitigation measures to be implemented and trigger points (both increases and decreases in water level, or with other evidence f impacts) that would put mitigation measures into effect. Enclosed with this letter are 22 maps that show the horizontal and vertical extent of the effects of the mining operation on the surrounding groundwater table. The maps were prepazed using the output data from the AMEC model; predicted groundwater levels during mining were compared with the baseline groundwater models to determine the decrease (or increase) in groundwater levels. Both the baseline and the mining phase models were transient models that were run over the life-span of the model. The AMEC model therefore took into account seasonal fluctuations in groundwater elevations. To determine the estimated drawdown atthe point of maximum disturbance for each phase, the baseline groundwater elevations were subtracted from the computed groundwater elevations at the ~~nd of mining of each phase. The end of each phase was considered the point of maximum distiarbance (to the surrounding groundwater table) because it is at these times when the dewatering operation will be completed to the maximum depth and has come into equilibrium witri the surrounding groundwater table. After the end of mining of each phase, dewatering ends, and,the groundwater table begins to return to its pre-mining status. Please refer to Table-1 in the AMEC report for the schedule of mining used to complete the mapping. Our analysis of the results of the AMEC model show that the maximum horizontal extent of the effects of the dewatering operation at any time during the life of the mine is no more than one mile from the cell that is being actively dewatered. The maximum vertical extent of the effects of the dewatering operation occur, as you would expect, nearest the cell that is being actively dewatered, and diminish rapidly away from the cell. Drawdown in the vicinity of the actively minc;d cell is not expected to be more than 5 to 10 feet outside of 200 feet from the active cell. After reclamation is complete and the groundwater flow has stabilized, the results of the model shove no more than 0.5 foot difference, plus or minus, in groundwater elevations anywhere in the vicinity of the site. These differences are negligible when considered with the natural seasonal fluctuation in the groundwater table. LAFARGE NORTH AMERICA INC. 'Lafarge Construction Materials 10170 Church Ranch Way, Suite 200, Westminster, Colorado 80021 Telephone: (303) 657-4000 Facsimile: (303) 657-4037