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<br /> <br />Letter to Rich Muza <br />U.S. EPA Region 3 <br />Page 2 <br />DMG has principal permitting authority for the (BMRI) gold mining site. Their January 2Sth response <br />to your report contends that the Alamosa aquifer would not be the only expected ground water <br />pathway, which is an assumption that your analysis is based upon. DMG has detenu toed, through a <br />more detailed site-specific analysis, that the aquifer in the area where you indicate that the flow path <br />would begin is approximately 160 feet below the surface where there is a relatively impermeable layer <br />of the Santa Fe formation o~eEl the Alamosa aquifer. <br />U~.7DQR L4tN6 <br />According to the DMG calculations, if a contaminant plume existed in this area of the mine site, and a <br />preferential pathway was present [o allow the ground water to flow toward [he District's water supply <br />well, it would take l95 years to reach the well. We understand that your clarification letter of February <br />17, 2000, recognizes this fact but that you did not adjust your calculations to account for the different <br />hydro-geological properties of the Santa Fe formation. In addition, DMG's analysis would predict a <br />TOT via the Rito Seco alluvial and Alamosa aquifers in excess of 60 years. This is in contrast to the <br />conclusion in your January 28th report that flow via this pathway could result in a TOT of less than 15 <br />years. <br />The discrepancy between your calculations and DMG's has caused us to re-examine the two analytical <br />methods. We noted significant differences in the levels of sophistication inherent in the models used <br />to determine the respective agencies' conclusions regarding the vulnerability of the well. Specifically, <br />the WHPA model, the use of which is recommended in the state's wellhead protection pian, is limited <br />in its ability to account for a number of dite~rent aquifer conditions; a point you have made in your <br />clarification letter. On the other hand, the model used by DMG is a conservative model which is able <br />to factor in considerably more site-specific information. <br />In spite of their different approaches, both EPA and DMG have concluded that the District's source <br />water is not now endangered, nor is it reasonably likely to be endangered, by contamination associated <br />with the mining operation. The fact that the remedial measures undertaken by BMRI in consultation <br />with this Division and DMG have resulted in improvements in the quality of the ground water at the <br />mining site substantially reduces the possibility of contamination from impacted ground water flowing <br />off the mine property. Ongoing monitoring of the situation by DMG and this Divisi ~n should ensure <br />that any potential for off-site contamination from the mine is minimized. <br />As you know, the San Luis Water and Sanitation District, as a public water system, s scheduled to <br />undergo a source water assessment in the next three years to meet the requirements ~~f the Safe <br />Drinking Water Act. In the course of this undertaking, we will recommend evaluati ~n of [he <br />delineated wellhead protection area using the data [hat have been developed by DMA.; and the EPA in <br />this current evaluation. It is possible [hat the contaminant inventory will identify additional potential <br />sources of contamination that will be examined and ranked according to the threats ~~osed to the source <br />water as defined in the state Source Water Assessment and Protection (SWAP) Plan. <br />