2005-01-31_REVISION - M1977208 - Laserfiche WebLink

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Response to Adequacy Letters (dated November 22 and December 17, 2004), CEMEX, Inc., Lyons Quarry, Permit No. M-1977-208, TR-04 and TR-O5. Page 2 of 3 downward seepage of ponded water in C-Pit as vertical flow in the Dakota is upwazds. If continuous vertical fractures, which have not been observed, existed between C-Pi[ and the Dakota, groundwater in the Dakota would flow upwazds along these fractures and discharge to C-Pit as artesian flow. Although is not reasonable to assume that C-Pit seepage could flow to the Dakota aquifer, Cemex modeled a hypothetical release of C-Pit water on Dakota aquifer water quality. Model details and results were submitted with TR-04 as the report Evaluation of the Potential Impact of Ponded C-Pit Water on Groundwater Quality in the Dakota Sandstone. In modeling the potential water quality impact, CEMEX conservatively assumed [hat C-Pit seepage was released directly to the Dakota, ignoring transport through the bedrock units between C-Pit and the Dakota which could take several hundred yeazs at the estimated groundwater flow rate in the shales of 1 foot per year. Seepage was released at the point labeled "modeled contaminant source ielease at 0 years" on Attachment 1. The annual volume of seepage released to the Dakota was conservatively assumed to be about 7% of the total water volume in C-Pit (about 2.5 million gallons). Seepage was released to the Dakota for a period of 3 years, a total of about 7.5 million gallons. The concentrations of constituents released to the Dakota were the maximum constituent concentrations measured in C-Pit prior to November 2004. Constituent transport was also conservative as constituent sorption or decay was not allowed. The constituents were allowed to travel at the same rate as groundwater. The water quality impacts were modeled at a hypothetical receptor well in the Dakota located about 3,000 feet downgradient from the injection point directly in the Dakota as shown in Attachment 1. Model results predicted that a constituent plume would travel from the source release point to the hypothetical well in the Dakota with the maximum constituent concentration predicted to arrive at the well in about 30 years. Even considering the conservative assumotions used in the model, none of the constituents modeled (oH, chloride, sulfate. TDS, selenium or thallium) exceeded the Colorado eroundwater protection standards at the hypothetical receptor well. The impact of the conservative model design on the predicted results was summarized indicating that: • The estimated annual seepage volume of 7% from C-Pit is likely overestimated, thus the constituent mass released directly to the Dakota was also likely overestimated. • If the constituent mass released is overestimated, the constituent concentrations at the hypothetical receptor well may be significantly less than predicted. • Constituent transport across the intervening low permeability, heterogeneous strata between C-Pit and Dakota would naturally attenuate seepage constituent concentrations via dispersion, sorption, or geochemical reaction prior to their arrival in the Dakota.