RISK REDUCTION AT A LEAKING STORAGE TANK SITE BY INSITU INJECTION OF A CARBON BASED COMPOUND

A gasoline release occurred at a storage tank site in southwest Missouri. The site was formerly operated as a gasoline service station from the 1950’s to the 1990’s. The duration and amount of release(s) is unknown. Investigations including the sampling of soil and groundwater indicated the presence of a long narrow plume in the soil and groundwater. The contaminated area extended from the site, across a highway to at least 600 feet down gradient. The sediments beneath the site consist of clay, sandy clay and chert that extend from the surface to the top of the bedrock at depths ranging from 12 to 30 feet below grade. A risk assessment following the guidelines of the Missouri Department of Natural Resources (MDNR), concluded that a risk from fire and explosion existed at the site and required corrective actions. A carbon based compound, BOS 200 was selected based on its demonstrated effectiveness with light non aqueous phase liquid (LNAPL). Soil samples were collected on a high density basis to better determine the location and concentration of petroleum hydrocarbon mass in the subsurface. This data was used to finalize an injection plan. Over 40,000 pounds of a carbon based compound and over 50,000 pounds of gypsum at approximately 300 locations. The compound and gypsum were mixed with water and injected into the subsurface at predetermined intervals and concentrations utilizing a specialty pump and standard direct push injection points. Injection pressures ranged from 100 to 1000 pounds per square inch. Prior to injection Lower explosion Limit (LEL) readings were collected at various locations to establish background readings. Subsequent to the start of the injection, groundwater samples were collected to determine dissolved contaminants of concern (COC) concentrations and select dissolved inorganic constituents. In addition LEL measurements were collected. Monitoring and sampling conducted during and subsequent to injection showed dramatic decreases of dissolved COC in many wells adjacent to the injection areas. Longer term monitoring results showed overall decreases of dissolved COCs in groundwater at most locations. LEL readings and thus fire and explosion risk was reduced in most locations. Longer term monitoring is needed to document continued reduction of dissolved COC by biological degradation.