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,, Letter to tick M~:za U.S. EPA Ire; or. 3 Page ? DMG has pr'rcipa: pertnit::n; authority for the ($4L1tI) fold miZirg site. Thei: January 38th !esponse to your ~epor. car.:ends that ::tc Alamosa aquifer would nct be t;.e aniy expected ground water pathway, white is sn assuazption that your analysis is based upon. DY[G has d_lerzined, through a mare detailed site-speci:ic aaaly=_is, that the aquifer in the area where you indicate that the flow path would beein is approximately 160 feet below the surface where thcrc is a relatively ic!permeable layer of the Santa Fa fot:ttatiattruverlymg the A?autosa aquifer. According to she DViG calculations, if a contaminant plume existed in this a: es of the mine site, and a prefermtial pathway was present to allow the ground ware: to Eow toward the District's water supply well, it would take 195 years to reach the well. We understand that yotu cltu.Scatian letter of February 17, 2000, recognizes this fact but ~1tat you did not adjust your calculators to accotmt for the different hydro-geological properties of the Santa Fe formation. Ia addition, DMG's analysis would predict a TOT via the lti:o Seco alluvial and Alamosa aquifer in excess of 60 years. Tcis is in contrast to the conclusion in your January ?8th report that tlaw via this pathway could result i_z a TOT of less than 15 years. The discrepancy between your calculations and DMG's has caused us to re-examine the two analytical methods. We toted significant diffaeaces in the levels of sophistication inherent is the models used to determine the r:apective agencies' conclusions regarding the vulnerability of the well, Specifically, the WF}PA model, the use of which s recommended in the state's wellhead protection plea, is limited is its ability to account for a number of difeeeat aquifer conditions; a point you have made in your clarification letter. OA the other hand, the model used by DING is a conservative made! which is able to factor in wnsiderably more site•specific information. In• spite of their different approaches, both BPA and DMG have concluded that the District's source water is not now endangered, nor is it reasonably likely to be endangered, by contamination associated with the mining operation. The fact that the remedial measures undertaken by B:4Ili.i is ecnsultatioa with this Division and DMG have resulted is itapmvements is the quality of the grow :d water at the mining sin substantially reduces the possibility of contamination from itttpacted ground water Rowing off the mine ptropery. Ongoing monitoring of the situation by DMG and this Division should mstue that any potential for Off=51LG COALaIniI3aL1aII from the tlliIlG L7 mi»imi~~, As you know, the San Luis Water and Sanitation Ilistrict, a9 a public water system, is scheduled to undergo a sour:: water assessment is the next three years to meet the requirements of the Safe Drirtkittg Waer Act In the course of this undertalong, we will recommend evaluation of the delineated we.lhesd protection area using the data that ltave'~eea developed by bMG and the EPA in this current evaluation. It is possible that the contaminant inventory will identify additional potential sources of eoacataiaation that will be examined and ranited accordiao to the threats posed to the source wazer as defined in [tt: state Source Water Assessment gad Protection (SWAP) Plan.