AN ALTERNATIVE TO EXCAVATION FOR REMEDIATION OF FUEL OIL SPILLS
Bioventing is the process of aerating soils to stimulate insitu biological activity and promote bioremediation. The process is applied to vadose zone (above the water table) soil. As of 1995 bioventing had been applied to over 150 petroleum hydrocarbon sites by the U.S. Environmental Protection Agency (EPA) and the U.S. Air Force. The process is one of the most cost effective methods available to remediate vadose zone soils.
The method consists of installing a number of perforated vent wells in the ground. The wells are connected to a blower which suppliers air to the soil. No excavation is required, therefore the expense of soil excavation, disposal and backfill are avoided. Foundation underpinning and floor slab reconstruction are not necessary where bioventing is used on spills that have occurred below buildings. In addition, the system components are relatively inexpensive. A blower for a small project can be purchased for under $2,000.
The method is not a panacea. Compared to excavation the time to remediate is long. It may take months or years to complete the cleanup, depending on the applicable remediation objectives which dictate the maximum amount of hydrocarbons that may remain (normally based on land and groundwater use). Such a long term approach is not feasible in many instances. Bioventing is a good alternative in situations where excavation is not feasible; for example at a spill where the home is located on permeable sandy soils, and the resulting contamination can extend to depths of more than 10 feet.
Factors Affecting Bioventing
The geology of the site affects the soil gas permeability. The soil must be sufficiently permeable to allow air to reach the contaminate. Therefore, sandy sites are candidates for bioventing but clay soil sites are not.
Bioventing has difficulty treating significant quantities of hydrocarbons that are located in or just below the water table.
Various environmental parameters affect the microbial activity that results in biodegradation of the hydrocarbon. The most important facto is the presence of oxygen as an electron receptor.
Measurement of the oxygen concentration in the soil at the site is usually carried out. Levels below 5% indicate that biodegradation is occurring and the site is suitable for bioremediation. Levels above 5% indicate that either the soils are adequately oxygenated and that the soil will naturally attenuate without any further action, or that some other factor inhibits bioremediation.
Soil moisture is required by micro organisms for their metabolic processes. Low soil moisture contents only limit bioventing in very dry desert environments.
Soil pH can limit bioventing, however, almost all soils are within the required range of 5-9.
Soil temperature significantly affects the rate of bioremediation which doubles for every 10°C rise in temperature. Biological activity can occur in the range of -10°C to over 50°C.
Nutrients are found in sufficient quantities in most soils to sustain microbial growth and the addition of nutrients has not often been found necessary. The nutrients of greatest concern are phosphorous and nitrogen and these can be added when necessary.
The levels of the lighter hydrocarbon compounds such as benzene, which are more mobile and toxic are the first to be remediated to low levels with bioventing. Typically this takes less than a year, wheras the heavier molecules comprising most of fuel oil take longer to remediate.
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