United States Environmental Protection Agency
Region VIII
POLLUTION REPORT

Subject: 

The Site is in a residential area called Aspen Park, which is located near Conifer, Colorado, in the foothills of the Rocky Mountains.    
        
In October of 1994 EPA found Carbon Tetrachloride (CCl4) at levels as high as 120,000 parts per billion (ppb) in residential wells.

On October 25, 1994, EPA initiated the Removal Action and began supplying bottled water to 33 residents.  A Treatability Study was conducted by EPA’s Emergency Response Team (ERT) to determine the best method of removing the CCl4, which ranged from 6 to 99,000 ppb.  Four air strippers and 29 carbon filters were installed on 33 homes.  A thorough investigation was conducted by EPA Contractors (TAT and REAC) in 1995 and 1996.  A source removal system was installed in March of 1997; it consisted of 4 pumping wells, 6 reinfiltration wells, a shallow tray air stripper system, and carbon polishing tanks.  The current system has 2 pumping wells feeding the air stripper and has treated over 20.2 million gallons of water as of June 30, 2010.  There are currently 29 residential carbon systems installed- 7 original systems were removed and 3 new systems were added, including 1 installed on a newly built house.  All 4 air stripper systems have been converted to carbon-only systems.

In 2010, the average carbon tet concentration of all monitoring and residential wells samples decreased to 20.8 ug/L from 31.7 ug/L in 2009.  After 15 years of treatment by individual home systems and 13 years of pumping and treating in the source area, the residential sampling results for carbon tet can be summarized as follows:

5    homes at non-detect;
9    homes between 0 and 5 ug/L;
5    homes between 5 and 11 ug/L;
3    homes between 11 and 20 ug/L;
4    homes between 21 and 49 ug/L;
1    homes between 51 and 100 ug/L; and
1    home greater than 100 ug/L, with the highest at 315 ug/L.

The carbon tet plume has diminished greatly in both size and concentration.  There are currently only 2 hot spots and the source area that show carbon tet concentrations > 5 ug/l.

Continue running system until the last well is below 100 ug/l.  Monitoring of the removal system and the affected homes should continue.

Carbon systems need monitoring, as they can slug contaminant at much higher concentrations into the homes once they saturate.  Homeowners may also disconnect the systems.  There is no one else willing to do this and pay for it, and there are no vendors willing to change out carbon in this area.

Yearly sampling for plume monitoring should continue to verify continued downward trends in contaminant concentration.