GENERAL36945

• Permit No: M1988112
• IBM Index Class Name: General Documents
• Doc Date: 1/28/2000
• Doc Name: RESPONSE TO EPA RECOMMENDATIONS FOR GROUNDWATER MONITORING IN THE SAN LUIS AREA REPORT SAN LUIS PROJ
• From: DMG
• To: USEPA
• Media Type: D
• Archive: No

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STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1713 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FA%: (3031 8728106
January 28, 2000
Mr Rich Muza
United States Environmental Protection Agency
Region 8
999 18th Street -Suite 500
Denver CO 80202-2466
DIVISION OF
MINERALS
GEOLOGY
RECLAMATION
MINING•SAFETY
Bill Owens
Governor
Greg E. Walther
ExecW ive Dueclor
M¢hael B. Long
Division Director
Re: Response to EPA's Recornmendations for Groundwater Monitoring in the Sort Luis Area
Report, San Luis Project, M-88-112
Deaz Mr. Muza:
Thank you for providing the DMG with a copy of EPA's January 18, 2000 report regarding "the
potential for impacts to the town's public water supply wells from potential contaminant sowces
as prioritized by the locals." The DMG's review is based upon our knowledge of the site.
Apparently, Ms. Kathleen Reilly, WQCD, asked that the EPA review the groundwater flow
system in the vicinity of the Town of San Luis. The Division offers our help byway of
commenting on the EPA report and recommend you contact Mr. James Dillie, DMG, if
additional data are necessary to revise the reports conclusions or to discuss the reports findings.
The reports description of the regional hydrogeology and local groundwater usages implies that
the Rito Seco alluvial aquifer is a separate groundwater pathway that passes unde- the town of
San Luis; after which it joins with the Culebra Creek drainage system. However, ~a7e did not find
evidence in the report to substantiate the contention that this sepazate groundwater pathway does,
in fact, exist. The report uses this theory to project the time it will take for a contaminant plume
to reach the town's public water supply well on the north side of the community.
On page 3 of the report, under recommendations, it is stated that '"There is a distance of
approximately 2.8 miles from the southwestern corner of the permit boundary to the town's
closest water supply well." This is an accurate statement. However, the data in the permit file
indicates that an alluvial aquifer does not exist in this portion of the permit azea. 7'he shallowest
aquifer is the Santa Fe formation and the current groundwater level is approximau^ly 160 feet
from surface. The average measured hydraulic conductivity in this portion of the Santa Fe
formation is 1.04 feedday, the hydraulic gradient (dh/dl) is .OS fdit and the average porosity is
.25, so the seepage velocity is 0.208 fdday or 75.92 ft/yr. If a contaminant plume existed in this
portion of the permit area, and a preferential pathway allowed the groundwater to flow toward
the town's water supply well, it might, theoretically, take 195 years to reach the water supply
well. However, according to DMG files, a contaminant plume has not been detected in the Santa