Remediation of Diesel and Fuel Oil Hydrocarbons in High Clay Content Soils: A Field Comparison of Amendment Performance Conducted at the Mare Island Naval Shipyard

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Courtesy of Weston Solutions, Inc

ABSTRACT

Past practices at the now closed Mare Island Naval Shipyard in Vallejo, CA caused diesel and fuel oil range organic (DFRO) contamination of the soil. The high clay content of the soils makes any remediation effort especially challenging. Chromatography of methylene chloride extracts of Mare Island soil confirmed that little weathering or degradation of petroleum hydrocarbons had occurred and very few hydrocarbon degrading bacteria were present in contaminated soil samples. A treatability study demonstrated that the addition of nitrogen, phosphorus, and rice hulls to contaminated soil stimulated the outgrowth of an active population of hydrocarbon degrading bacteria.

To test these findings, excavated soil (concentration of DFRO 560-1,670 ppm) amended with nitrogen and phosphorus was used to construct biopiles with and without rice hulls. The performance of these piles was compared to soil amended with rice hulls, nitrogen, and phosphorus that was mechanically mixed at regular intervals, a static pile with rice hulls but no nutrients, and a static unamended control pile. Contaminated soil was also treated with a microbial humic polymer nutrient mix and a time release oxygen compound supplemented with nitrogen and phosphorus.

Unexpectedly, the concentration of hydrocarbons increased in some of the soils after the initial sampling. It was determined that rice hulls and other non-petroleum organic compounds in the soil, artificially inflated the concentration of residual hydrocarbons. This problem was solved in the final sampling round by changing the extraction procedure.

After six months, the concentration of DFROs in soil treated with the humic polymer and the time release oxygen compound were less than 100 ppm which was the local water board limit for soil reuse. The concentration of DFROs in the static pile with no rice hulls was slightly above the water board limit. The concentration of DFROs in the rice hull amended soil was < 300 ppm. These results demonstrate that high clay content soils can be remediated to acceptable levels within a reasonable time.

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