SIMONIS, ED, and JOHN BOWER, Groundwater Technology, West Sacramento, CA

ABSTRACT: Soil Vapor Extraction from Heterogeneous Lithology, with Thermal Oxidation Emission Control

Soil vapor extraction from a hydrocarbon-impacted vadose zone is a proven method for removing absorbed petroleum hydrocarbons and enhancing natural biodegradation. However, a heterogeneous vadose zone lithology complicates equivalent vapor extraction rates from each stratigraphic horizon.

Assessment of a leaking service station site in central California determined that two point sources (leaking tanks and a leaking product line) contributed to the subsurface problem. The heterogeneous site stratigraphy consists of four alluvial units, alternating silty clays and sands with variable pneumatic properties. The depth to the water table is 70 ft. Soil beneath the storage tanks is impacted to within 5 ft of the water table. Groundwater is not yet impacted.

To extract vapors from all four horizons at an equivalent rate, the screened interval of the extraction wells was staggered, with a greater proportion of screen towards the silty horizons. Three vadose monitoring wells, consisting of 1-ft-long monitoring points centered in each stratigraphic unit, were also installed. During initial pilot tests, all four units recorded similar pressures. Pulse tests extracted vapors with an average concentration 45,000 mg/L of total petroleum hydrocarbons for 5 days.

To minimize the total time for remediation and protect groundwater quality, a thermal oxidation treatment unit was chosen for emission control. The unit breaks down complex petroleum hydrocarbons to simple carbon and hydrogen oxides at 1400 degrees F on site. Initially, three vapor extraction wells were used at a total flow rate of 200 scfm. After 10 days of operation, extraction

from only one well was required, and sands cleaned up more rapidly than silty clays.

The system is successful in providing equivalent extraction and destruction of petroleum vapor from a lithologically heterogeneous vadose zone, as well as independent monitoring of each unit during remediation.

Cost analyses of various pumping and treatment remediation systems indicate that expenditures associated with thorough aquifer characterization represent less than 5% of the total remedial project cost. These expenditures represent a savings through the life of a project due to greater efficiency of a properly designed remediation system.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.