ARCHIVED REPORTS XR0004798 - Laserfiche WebLink

Home Browse Search

r! y r r 1 ' F V l 4 28 February'1997 , AGE-NC Project No 95-0144 ' Page 12 of-20 5 4 NATURAL ATTENUATION r Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural processes to degrade' hydrocarbons Biodegradation is the most important natural attenuation mechanism No additional bacteria or nutrients are added to the soil Essential nutnents required for biodegradation are usually naturally present in the subsurface However, where high concentrations ' of hydrocarbons are present, nutrients may be consumed,before hydrocarbon remediation is complete Oxygen depletion in{'the subsurface is another characteristic of passive biodegradation The core of a contaminant plume,typically exists under anaerobic conditions and only the margins are aerobic Therefore, even though anaerobic biodegradation is much slower than aerobic' i biodegradation, anaerobic processes may dominate the degradation of contaminants Natural attenuation is normally used where low concentrations of hydrocarbons are present or after completion of another method of remediation , r ' The,na#ural attenuation process is time consuming, it will take many years for the hydrocarbon plume to reach levels comparable to those of,active remediation methods Costs for natural attenuation are definitely lower than the active remediation options However,,the periodic costs could accumulate over time and potentially be considerable depending on-the duration and type of monitoring requirements 1 r i Fate and Transport modeling may be used to demonstrate that hydrocarbons in soil do not pose a ' threat to ground water Where ground water'has been impacted or has a high potential to become impacted,modeling may be used to evaluate thepossibility of hydrocarbons to impact nearby ground water receptors L 6 0. GROUND WATER REMEDIATION ALTERNATIVES ; 61 IN-SITU AIR SPARGING In-situ air sparging (IAS) utilizes air injection to volatilize dissolved hydrocarbons and strip hydrocarbon vapor,into the vadose zone, where it can be withdrawn by a vapor extraction system Air sparging is commonly conducted concurrently with soil vapor extraction 61 1 PRINCIPLES IAS induces the transfer of hydrocarbons from ground water to soil gas in the vadose zone by 1 ' creating a strong concentration gradient between the two regimes Commonly, an extraction well is surrounded by specifically designed air injection(sparging) wells that are screened only within the t I rt