Contaminants:
cis-1,2-Dichloroethene, Vinyl chloride, Trichloroethene, Benzene-toluene-ethylbenzene-xylene (BTEX)

Reactive Media:
Fe⁰, O

Construction:
Funnel and Sequenced Gate

Point of Contact:
Mary Morkin
GeoSyntec Consultants
Tel: 925-943-3034
Ext: 203
Fax: 925-943-2366
Email: mmorkin@
geosyntec.com
1500 Newwell Avenue
Suite 800
Walnut Creek , CA 94596

The second part of a pilot-scale demonstration of an in situ sequenced permeable reactive barrier (SPRB) for the remediation of chlorinated solvents and petroleum hydrocarbons was conducted at Alameda Point (formerly U.S. Naval Air Station Alameda) in Alameda, CA.

The initial phase of this demonstration, which had been conducted at Canadian Forces Base Borden, Ontario, Canada, evaluated three technologies for their ability to treat perchloroethylene (PCE), carbon tetrachloride and toluene. The technologies were: (1) abiotic reductive dechlorination using zero valent iron followed by oxygen releasing compound (ORC™) to promote aerobic biodegradation; (2) natural attenuation; and (3) a permeable nutrient injection wall, using benzoate to promote anaerobic biodegradation, followed by an aerobic (oxygen) biosparge gate for aerobic biodegradation. The Alameda demonstration used zero-valent iron followed by oxygen biosparging in a funnel-and-gate system to remediate trichloroethylene (TCE); cis-1,2-dichloroethylene (cDCE); vinyl chloride (VC); and toluene, benzene, ethyl benzene, and xylene (BTEX). Total initial (upgradient) concentrations of chlorinated VOCs exceeded 100 mg/L, and toluene was found at levels of up to 10 mg/L. Historical air photos of the site indicate open disposal pits upgradient of the SPRB.

The shallow aquifer is composed of 22-24 ft of sandy artificial fill material that was hydraulically placed on bay silts and clays. Depth to ground water ranges from 4 to 7 ft below ground surface. The hydraulic conductivity of the overlying sandy fill material is 0.057 ft/day (~21 ft/year). The underlying bay silts and clays are 15-20 ft thick and act as a confining unit.

During construction of the funnel-and-gate system, the artificial fill sand was excavated to the top of the confining bay mud unit. To prevent settling, a concrete pad (nominally 2 ft thick) was placed at the bottom of the excavation; the gate was then constructed on this base. The gate is 10 ft wide and 15 ft long. As ground water passes through the gate it contacts the following media: about 18 in of coarse sand mixed with 5% zero valent iron, 5 ft of zero-valent iron, a 3-ft pea gravel transition zone, a 3-ft biosparge zone, and a 2-ft pea gravel zone. The 10-ft funnels were placed on either side of the gate, perpendicular to the direction of water flow.

Between February 1997 and May 1998, two pumping wells were used to operate the system under controlled conditions. For a period of about 70 days, the system operated at a flux rate of approximately 45 ft³/day to determine the maximum velocity it could process. At this velocity, breakthrough was observed in several downgradient monitoring points. Then, the system operated for about one year at a flux rate of approximately 12 ft³/day, more representative of conditions that would exist as a result of the funnel sections. Finally, the system was allowed to operate under natural gradient conditions.

The remedial objectives of the project generally were met, except with respect to cDCE and VC, with typical effluent concentrations of about 136 µg/L and 217 µg/L respectively. Retardation of the toluene or other hydrocarbons as a result of sorption to the granular iron precluded an assessment of petroleum hydrocarbon degradation. Breakthrough of cDCE and VC indicated that biodegradation (likely via aerobic oxidation) of these compounds was occurring in the biosparge zone. An estimated 66% of the VC and 30% of the cDCE was volatilized. Assessment of multilevel data showed excellent degradation (>91%) of the chlorinated organics using the granular iron at high influent concentrations (>100 mg/L total VOCs). At lower influent concentrations, almost complete degradation (>99%) was observed. The biosparge zone supported aerobic biodegradation of VC and cDCE, and by January 1998 remedial objectives were being met at the last set of sampling wells in the gate.

Results obtained to date suggest sparging rates in the biosparge zone should be minimized to reduce volatilization of contaminants from the water column. In addition, monitoring should continue so that long-term performance of the SPRB can be assessed.

The U.S. Navy has begun to operate the site. Current plans call for a hydraulic study to examine ground-water flow in the funnel-and-gate area and for monitoring to continue on a quarterly basis.