Case Study:  LNAPL Remediation

Situation:  An old chemical plant had accumulated at least 3 feet of thick benzene and styrene floating on groundwater in thick gravels, which periodically produced slicks on the Ohio River. Norwest performed site assessment, risk assessment. and pilot testing.

Solution:  Initial mass reduction was accomplished by vapor extraction with a thermal oxidizer until vapor yield declined.  Norwest then engineered a sparge fence along the riverbank and created sparging clusters in hot spots. Periodic air injection via a large underwater mobile compressor broke up floating hydrocarbon that essentially lifted vapors into the vadose zone and promoted aerobic biodegradation above and below the water table. This “biosparging” mode required no recovered vapor or water treatment, and no fugitive vapors were detected. Monitored oxygen and carbon dioxide in the vadose zone correlated with hydrocarbon destruction. The floating product was completely removed after one year of operation and groundwater quality was improved to the point of meeting risk assessment criteria.

Result:  Norwest remediated a site that had been a candidate for NPL listing in one year’s active operation, with no groundwater treatment and vapor treatment limited to an initial vapor extraction phase.