ACI Bioremediation™

Project Documentation

Project Description:

A 500 gallon waste oil Underground Storage Tank (UST), a 4000 gallon premium UST, a 6000 gallon unleaded UST and an 8000 gallon regular gasoline UST were removed from Westmoor Mobile in April. Upon removal and inspection of the tanks, the 6000 gallon unleaded and the 4000 gallon premium tanks were found to have holes up to 2" in diameter in the bottom. Inspection of the surrounding soils revealed extensive contamination, and the hole was over-excavated. Over-excavation continued until there was no more room on site for additional soil. Sample results indicated that contamination was still present to a depth of 41 feet, 20 feet below the bottom of the hole. Due to the extent of the contamination and the minimal work space available, bioremediation was recommended. Bioremediation commenced in July on the excavated and in-situ contaminated soils.
      

Westmoor Mobile, Daly City, California
Site Location of Underground Storage Tanks

Technology Description:

The objective of the biodegradation method is to provide an environment for the microorganisms in which they can thrive and use the petroleum hydrocarbons as a food source. The soil was initially checked to determine if it was capable of supporting the microorganisms.

      The soil pH was adequate, but nutrients and moisture were added as necessary. The bioremediation process was divided into two portions; the excavated soil piles and the in-situ materials. The in-situ bioremediation was carried out under the approval of both the San Mateo County Department of Environmental Health and the Region 2, Regional Water Quality Control Board.
      First, water was injected and sprayed onto the excavated soil to bring the moisture content up to 30-35%. The soil was never saturated to the point that free liquid would drain from the pile and potentially wash the hydrocarbons into the surrounding area.
      The microorganisms were added, as required, along with nutrients to enhance their rate of growth. Following each application of microbes and nutrients, the soil was covered with 6 mil black plastic to increase temperature to promote growth of the microorganisms. It is important to note the soil was not tilled or turned during the treatment process. A total of 494 cubic yards of soil was treated.
      In-situ, treating the soil in place, bioremediation was used for the contamination at depths to 40 feet below grade and 18 feet laterally at a depth of 15 feet. Microbes and nutrients were added as needed through a series of injection holes on approximately two-foot spacings.
      Over 70 injection holes were hydro-punched vertically into the bottom of the excavation, vertically and at an angle from the surface around the perimeter of the excavation, and horizontally into the sides. An estimated 1650 cubic yards of soil was treated in-situ.

Status:

On October 30 and 31, 1991 a site investigation was conducted to determine if the bioremediation had progressed far enough to allow the excavation to be backfilled. Seven samples were taken of the in-situ material. The analysis found no detectable contamination in the tested soil. Samples taken of the excavated piles revealed piles 2 - 5, and 8 were non-detect. Pile 1, 6, and 7 had minor residual contamination. The hole was back filled with the non-detect soil. The soil from piles 1, 7, and 8 was moved into smaller piles and further remediated to be used for on-site landscaping.