Bioremediation of petroleum hydrocarbon-contaminated ground water: The perspectives of history and hydrology

Bioremediation, the use of microbial degradation processes to detoxify envi ronmental contamination, was first applied to petroleum hydrocarbon-contami nated ground water systems in the early 1970s, Since that time, these techn ologies have evolved in some ways that were clearly anticipated by early in vestigators, and in other ways that were not foreseen. The expectation that adding oxidants and nutrients to contaminated aquifers would enhance biode gradation, for example, has been born out by subsequent experience. Many of the technologies now in common use such as air sparging, hydrogen peroxide addition, nitrate addition, and bioslurping, are conceptually similar to t he first bioremediation systems put into operation. More unexpected, howeve r, were the considerable technical problems associated with delivering oxid ants and nutrients to heterogeneous ground water systems. Experience has sh own that the success of engineered bioremediation systems depends largely o n how effectively directions and rates of ground water now can be controlle d, and thus how efficiently oxidants and nutrients can be delivered to cont aminated aquifer sediments. The early expectation that injecting laboratory-selected or genetically eng ineered cultures of hydrocarbon-degrading bacteria into aquifers would be a useful bioremediation technology has not been born out by subsequent exper ience. Rather, it appears that petroleum hydrocarbon-degrading bacteria are ubiquitous in ground water systems and that bacterial addition is usually unnecessary. Perhaps the technology that was least anticipated by early inv estigators was the development of intrinsic bioremediation, Experience has shown that natural attenuation mechanisms - biodegradation, dilution, and s orption - limit the migration of contaminants to some degree in all ground water systems. Intrinsic bioremediation is the deliberate use of natural at tenuation processes to treat contaminated ground water to specified concent ration levels at predetermined points in the aquifer, In current practice, intrinsic bioremediation of petroleum hydrocarbons requires a systematic as sessment to show that ambient natural attenuation mechanisms are efficient enough to meet regulatory requirements and a monitoring program to verify t hat performance requirements are met in the future.