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•'~ .. _ _ STATE OF COLORADO line E, uerton, EmeL~ve D;reR4r ~ [kaie~red ~ proe-.Jn~ ,nd imprwrrb u+e Mal;n ,;nd mvuenmrnc of :hr people ci Co~ernc~ "c uCC Ctiarrv Creek C-. 5. Comer, Cdorado 9C2<G1570 Labcnray and Zadi:uon Serviea CHnion B• 70 Lowry atvd. ~~~ • , ?hone 1]031 69.-0;CC Comer CO aC2]Ca92a ~' TDD Lic• I3o31 091.77(!7 13031 691•]C9C ~F~ t,AlO[id0 t11r LoeYed in C:Md:IC. C~IOr]ZO ~ rl,ou~ eoldr hmst-mw.cdphe.r~x.-.ru f'~~~' ~' ZQ~ v1dFl:vltOnmrnt ..:. ~ G8010~ March 8, 2000 Mr. Rich Ivfuza, Hydrologist Ecosystems Protection Program Office o Ecosystems Protection and Remediation L'.S. Env'soamental Protection Agency, Region 9 999 Eighteenth Street' St]ite 500 Denver, Colorado 80202.2466 RE: Recotameadations for Ground Water Monitoring in the San Lt2is Area, Cosri'•la County .. yVe have received your report;.dated January28, ?000, and the follow up clarltication lexer of. ~ .. ; Febtuary•17, 2000, on the recommendarion§ for ground water monitoring in the Sao Luis area.. We have also received tha tiraR questions and growers on this situation that you pr~epazed to explain the approaches and terms used is the report and have received copies of the let[ers from the stare Derision of Minerals end Geology (I]MCr) writteg is response to your report and carification letter. You will recaIl that, pursuant to a request from the San Luis Water gad S agitation District (District), we requested your assistance is determining the best location for a dtinkiag water monitoring well for the District. A representative of the District had contacted us exprssiag coneera about possible contamination frnm "a nearby gold mining operatiea" The District had decided that they wasted to tnoaitor the situation and asked for essistaace in determining the best location to drill a weft. The report and follow up clarification describe the assumptions used sad research ttndertakea to cog3plete ea analysis using the wellhead protection assessment (WHPA) model descn~bed ur Colorado's wellhead protection plain The model insults indicate teat it would take ag estimated 15 years for a contaminant to move through the Alamosa aquifer from the southwest corner of the Battle Mountain Resources, Inc. (BMRn mining site to the District's northern weI1. Linder Co[oradq's wellhead protection plea, technical assumptions and a modeling approach a:e established to generate very conservative results in order to identify potential risks to drinking water supplies, It, after implementing such coasetvatlve sssumptions, the predicted time of Tavel (TOT) to the drinking water supply well exc:eds five years then ao real threat has been identified and no further action is taken. The Division agrees with the EPA that the methods and assumptions used to arrive a< your caaclusions predict a significantly shorter time of travel than would more sopaisticated models that account for a number of additional factors. The Division believes that, where such models and data to calibrate them are available, we should take advantage of them to develop a more accutatt prediction of TOT.