The subject site is located in downtown Denver, Colorado and was formerly operated as a bulk petroleum storage facility. Bulk storage of gasoline, diesel and fuel oil was documented at the site as early as the 1920’s with storage capacities as great as 100,000 gallons. Historic petroleum hydrocarbon contamination was discovered at the site during removal of 3 underground storage tanks. Orphan underground storage tanks were also discovered during initial abatement activities. Approximately 2,000 cubic yards of petroleum hydrocarbon contaminated soil was excavated from the site. This soil was incorporated into asphalt paving at off-site locations.
Significant soil and groundwater contamination were observed during assessment activities performed at the site. Total petroleum hydrocarbon (TPH) concentrations as high as 12,140 mg/Kg and total benzene-toluene-ethylbenzene-xylenes (BTEX) concentrations as high as 159,900 μg/Kg were observed in soil at the site and extended from the ground surface to a depth of approximately 15 feet where groundwater was present. TPH concentrations in groundwater were as high as 392 mg/L and BTEX concentrations were as high as 18,000 μg/L. The groundwater plume was approximately 500 feet long by 200 feet wide. Much of this soil and groundwater contamination was under significant structures and roadways which complicated remedial actions.
The site is underlain by high-energy alluvial deposits associated with the South Platte River. These high-energy deposits extended to depths of approximately 10 to 16 feet and are underlain by bedrock. The saturated thickness ranges from approximately 3 to 5 feet. Insitu bioremediation was determined to be the most cost-effective remedial approach for this site. The selected remedy consisted of the installation of oxygen release compound (ORC) to enhance the natural biodegradation of hydrocarbons which was already occurring at the site. Approximately 3,000 pounds of ORC was injected via direct-push rods in August 1997 and approximately 390 pounds of ORC was injected in September 1999. This approach produced ORC® lenses which were approximately 40 feet in diameter. Soil contamination is being remediated via a soil-vapor extraction (SVE) system. BTEX concentrations observed in the 12 groundwater samples collected from the site during the August 2000 sampling event did not exceed the Colorado Groundwater Standards with the exception of the ethylbenzene concentration in one (1) groundwater sample. The benzene concentration observed in the most contaminated well had been reduced by approximately 99% (from 316 μg/L to 2.7 μg/L) through the use of ORC and SVE. Postremediation monitoring was initiated in November 2000 and slight increases in BTEX concentrations were observed in some impacted areas of the site. Additional applications of ORC and additional operation of the SVE system may be warranted.