Installation Date:
07/01/1997

Contaminants:
Trichloroethene, cis-1,2-Dichloroethene, Carbon tetrachloride, Nitrate

Reactive Media:
Fe⁰

Construction:
GeoSiphon Cell

Point of Contact:
Mark Phifer
Westinghouse SRC/SRS
Tel: 803-725-5222
Fax: 803-725-7673
Email: mark.phifer@
srs.gov
Building 773-42A
Aiken , SC 29808

The GeoSiphon Cell (patent pending) was installed in the TNX flood plain at the Savannah River Site (SRS) by auger and caisson methods in July 1997. The cell was installed to demonstrate treatment of ground water contaminated with chlorinated volatile organic compounds (CVOCs). Ground-water contamination has been detected in the TNX water table aquifer, but not in the semi-confined or deep aquifers underlying the site. Predominant contaminants, and average concentrations of each, detected in the TNX flood plain are trichloroethylene (TCE) at 200-250 µg/L, cis-1,2-dichloroethylene (cDCE) at 20-50 µg/L, carbon tetrachloride (CT) at 15-45 µg/L, and nitrate (NO₃) at 10-70 mg/L.

The TNX Area is a semi-works facility for the Savannah River Technology Center, which is located 0.25 mile from the Savannah River near Aiken, SC. The facility was used for pilot-scale testing and evaluation of various chemical processes associated with the SRS. The water table elevation averages 100 ft above mean sea level under the TNX site, while the Savannah River elevation averages 85 ft. In the flood plain where contamination was detected, the water table aquifer is approximately 35-40 ft thick. It consists of interbedded sand, silty sand, and relatively thin clay layers. Based on testing and modeling analysis, the aquifer may be characterized as having a horizontal hydraulic conductivity of 65 ft/day, vertical hydraulic conductivity of 30 ft/day, effective porosity of 0.15, pore velocity of 3 ft/day, and a horizontal gradient of 0.007.

The TNX GeoSiphon Cell is a large-diameter (8-ft) well containing granular zero-valent iron (Fe⁰) as a treatment media (in place of gravel pack). The cell passively induces flow by use of a siphon from the cell to the Savannah River. The flow is induced by the natural hydraulic head difference between the cell and the river. The passively-induced flow draws contaminated ground water through the treatment cell, where the Fe⁰ reduces the CVOCs to ethane, ethene, methane, and chloride ions. Treated water is discharged subsequently to the Savannah River.

During Phase I testing of this technology, which was completed in December 1997, flow through the TNX GeoSiphon Cell was induced by pumping and the treated water was discharged to the existing TNX National Pollutant Discharge Elimination System outfall. Testing indicated that TCE degradation is the limiting compound to treatment below the Primary Drinking Water Standard Maximum Contaminant Levels within the TNX GeoSiphon Cell. Data indicated that approximately 8 gal/hr of ground water contaminated with 200-250 µg/L of TCE could be treated, while maintaining the average discharge TCE concentration below 5 µg/L. Field first order rate constants produced from the steady state TCE data increased with flow rate from 0.347 to 0.917/hr. Phase I system costs are estimated at $119,115, including $26,400 for iron, $27,411 for other construction materials, and $65,344 for mobilization, labor, rentals, and related installation expenses. Approximately 49.7 tons of 0.25-2.0 mm (particle size) granular cast iron was used in the installation of the first TNX GeoSiphon Cell (TGSC-1).

During Phase II, flow through the TNX GeoSiphon Cell was induced by siphon and the treated water was discharged to an existing outfall ditch that flows into the Savannah River. To allow continuous operation, the siphon line configuration was optimized to include an upward rise from the cell to the outfall ditch, an air chamber at the crest adjacent to the outfall ditch, and a steep drop into the outfall ditch with line termination in a sump. The head differential available to drive the system (approximately 1.4 ft) produced a continuous flow rate of 2.5-2.7 gal/minute. Approximately 1.2 ft of head was utilized to drive flow through the cell itself, and approximately 0.2 ft of head was utilized to drive flow through the siphon line. Based on these results, a new siphon line will be installed between the cell and a target location, thus producing a 5-ft head differential capable of inducing an estimated 9.5 gal/min through the GeoSiphon Cell.

Phase III of this demonstration project will involve installation and operation of a full-scale GeoSiphon Cell system for treatment of the entire TNX contaminated ground-water plume.

Lessons Learned

The GeoSiphon Cell was selected for use at the TNX Area because it offers passive, in situ treatment (no power requirements) at lower operating and maintenance costs than pump-and-treat technology. In contrast to funnel and gate or continuous permeable wall technologies, the GeoSiphon Cell could be constructed using an existing foundation and well drilling techniques. In addition, there is potential for accelerating cleanup through the use of induced flow rates greater than natural flow. With a maximum siphon lift of 25 ft, application of the GeoSiphon Cell technology was found to be limited to areas of shallow ground water such as that existing at the TNX Area.

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