Using Biodegradation as a Remediation Tool.

Throughout the world, environmental engineers and consultants apply natural biodegradation (i.e., natural attenuation or intrinsic biodegradation) as an accepted form of remediation for many contaminated-soil sites. In the United States, there are, however, several challenges in presenting this type of remediation to the U.S. Environmental Protection Agency (U.S. EPA) and other regulatory agencies. For example, how does one determine what favorable conditions are required for biodegradation? Which type of monitoring indicates that natural attenuation is actually taking place? U.S. EPA and the Air Force Center of Excellence have assisted by publishing documents that may soon become industry standards for demonstrating natural attenuation.

After completion of site characterization and contaminant identification the next step is to assess the presence of natural bacteria by testing a soil sample. A heterotrophic-plate-count (SM9215B) analysis indicates the number of bacteria able to grow, and a most-probable-number (SM9223B) coliform analysis assesses the presence of aerobes such as nitrate and sulfate reducers and methanogens. Also, the hydrocarbon-degrading-bacterial-plate-count (ASM) analysis can enumerate populations of bacteria that have the ability to degrade gasoline, diesel, jet fuel, and other hydrocarbons. Other tests can be performed to monitor the effects of attenuation, including analyses for nitrate (EPA 300.0), nitrite (EPA 300.0), sulfate (EPA 300.0), sulfide (SM4500-s), chloride (EPA 300.0), methane (RSK-175), ethene (RSK-175), alkalinity (SM2320B), and phenol (420.1 or EPA 625).

The analytical data then can be evaluated against historical results. Decreased levels of nitrate and increased levels of nitrite indicate microbial denitrification. Decreased levels of sulfate and increased levels of sulfide indicate that sulfate-reducing activities are occurring. If methane is present, or if its levels are increasing, hydrocarbon biodegradation may have taken place. If benzene has been identified as a contaminant of concern and increased levels of alkalinity are occurring over time, the benzene may be attenuating. To further address benzene attenuation, phenol levels also can be evaluated; an increase in phenol is a first step in benzene and hydrocarbon degradation. If chlorinated solvents have been identified as contaminants of concern, ethene and chloride levels may be evaluated, as these are by-products of chlorinated solvents and may indicate biodegradation.