Soil and Groundwater Bioremediation: UNOCAL Old Seattle Marketing Fuel Terminal

Problem

A former marketing fuel terminal in the Western United States had been contaminated by losses incurred during the handing of petroleum products during 65 years of operation. Over 60,000 cubic yards of soil were contaminated with petroleum hydrocarbons at a mean concentration of 2,660 parts per million (ppm). Groundwater analyses identified benzene as the compound of concern. Ethylbenzene, toluene, and xylenes were present at relatively low levels. A Remedial Action Plan for the site was required prior to full-scale remediation.

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Solution

Pelorus As the Senior Scientist for this project Dr. Mahaffey was responsible for the development of a Remedial Action Plan for the fuel terminal site and to remediate the site. His staff conducted a laboratory treatability evaluation to assess treatment options and cleanup levels achievable from those options. Options studied included no action, excavation and offsite disposal, offsite treatment, and onsite treatment focusing on bioremediation.

The recommended treatment program included a solid-phase biological soil treatment and an in situ biological soil and groundwater treatment. These options were chosen because of the terminal destruction of the contaminants and significant cost savings over offsite disposal.

Four areas of contamination were treated as follows: onsite solid-phase biotreatment of soil in Areas A and B, followed by disposal of treated soil at an approved landfill; onsite solid-phase biotreatment followed by in situ biotreatment of contamination below the water table in Area B, and in situ biotreatment and vapor extraction in Areas C and D.

After demolition of existing structure onsite, the majority of contaminated Area A soil (approximately 20,000) cubic yards excavated and transported to Area B for solid-phase biological treatment. Additional soil was treated in Area A. Solid-phase treatment involved the excavation and processing of the contaminated soil with a carefully controlled combination of oxygen, water and nutrients. This treatment promoted the rapid growth of naturally occurring bacteria which feed on and completely break down the hydrocarbon contaminants. When treatment was complete, the Area B soil (approximately 25,000 cubic yards) was treated in the same manner.

The in situ remediation system was designed to capture groundwater from a down gradient infiltration trench, treat contaminated water in a submerged fixed film bioreactor, and return the treated, oxygenated water via an upgradient trench. A total of approximately 40 million gallons of water was treated and reinjected through the two trench systems.

Analysis of groundwater identified benzene as the compound of concern. Ethylbenzene, toluene, and xylenes are also present at lower levels. Free-phase product was identified in the extracted groundwater.

The surface remediation unit consisted of an oil-water separator (OWS) and a bioreactor. The OWS included a 7,000 gallon separator tank and a dissolved air flotation (DAF) unit used to help separate the oil-water phases. Oil was skimmed from the OWS, stored and then recycled. Approximately 3,000 gallons of free product were recycled and sent to nine local recycling plants. The bioreactor consisted of a chambered 12,000 gallon tank where the water was amended with nutrients and oxygen. The amended water was reinjected into the subsurface environment. The in situ system was designed to stimulate petroleum degrading microorganisms in the bioreactor tank and in the subsurface environment.

Bioremediation of the soils lowered the hydrocarbon contaminant level to below the agreed upon cleanup level of 200 ppm. Water cleanup met the cleanup criteria for the contaminants as follows: total hydrocarbons to below 15 ppm and benzene to below 40 ppb.

This successful project was the largest bioremediation activity in the State of Washington during the period of 1989 to 1992.

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