Since July 2000, Lawrence Livermore National Laboratory has been using a passive, in situ treatment system known as GeoSiphon (Tech ID 2063) to remediate groundwater contaminated primarily with volatile organic compounds. A variant of a permeable reactive barrier, a GeoSiphon system takes advantage of natural gradient and pressure differences to induce contaminated groundwater to flow at an accelerated rate through a permeable reactive medium. At LLNL, the GeoSiphon has been designed to cause groundwater to flow aboveground into a series of drums filled with iron filings, where zero-valent iron reduces trichloroethylene to ethane, ethene, and chloride ions. Nitrate is reduced to ammonium and possibly nitrogen.

Schematic of the Iron Filings/GeoSiphon Treatment System.	Graphic provided by Lawrence Livermore National Laboratory.

The GeoSiphon concept isn’t new. In 1997 and 1998, two DOE organizations initiated demonstrations at the Savannah River Site that have contributed to the knowledge base for implementing this technology. The Office of Environmental Restoration (now known as the Office of Project Completion, EM-40) and the Office of Science and Technology (EM-50) have sponsored demonstrations of different GeoSiphon configurations at SRS’s TNX facility and in the D-Area, respectively, that have opened the way for a technology that enjoys distinct advantages over both pump and treat and competing permeable reactive technologies. GeoSiphon was developed by the Savannah River Technology Center and is part of the Subsurface Contaminants Focus Area inventory of technologies for treating groundwater or surface water contaminated with heavy metals, volatile organic compounds, or radionuclides.

A general scheme for siphoning groundwater

A GeoSiphon system is configured to take advantage of significant natural hydraulic head differences between two points. A siphon is used to produce a flow between the two points without mechanical pumps. The up-gradient initiation point is within a contaminated aquifer, and the down-gradient discharge point can be to the subsurface, the surface, or a body of water on the surface. The permeable treatment medium can be placed at the initiation point or the discharge point, be applied in situ or ex situ, and be either permanent or rechargeable. Among the reactive media that can be used are granular cast iron, activated carbon, ion exchange materials, limestone, zeolites, iron foam, bimetallics, peat, phosphate rock, dolomite, concrete, fly ash, blast furnace slag, sulfur, or pyrite.

A GeoSiphon system is sustainable and low maintenance, making it a promising alternative to the higher capital outlay and more expensive operating and maintenance costs involved in a pump-and-treat solution. Due to slow contaminant dissolution and/or migration, groundwater pump-and-treat systems often require extended treatment periods, ranging from tens to hundreds of years to reduce contaminant levels to regulatory standards. These extended periods of operation can require significant energy, resulting in high operating and maintenance costs. A GeoSiphon system also enjoys advantages over permeable reactive barriers, another passive, in situ method of removing contaminants from groundwater. While permeable reactive barriers rely on groundwater’s natural flow rate for transport of contaminants into reactive media, GeoSiphon systems are designed to enhance natural flow rates and can, therefore, potentially achieve faster cleanup results.

GeoSiphon at LLNL

The GeoSiphon system at LLNL’s Site 300 has been installed on the floor of a steep-walled canyon. The contaminated groundwater, which lies 40 to 60 feet below the canyon floor, contains 70 parts per billion trichloroethylene, 20 ppb nitrate, and 6.5 ppb perchlorate. The source area, which is 1,000 feet up canyon of the GeoSiphon treatment system, contains concentrations of these contaminants two to three orders of magnitude higher.

At LLNL, a siphon for transporting groundwater toward the treatment cell is a backup option. The LLNL GeoSiphon system employs three artesian wells for reaching water that rises to the surface by internal hydrostatic pressure. The artesian extraction wells have natural flow rates of 5 to 8 gallons per minute and are located at an elevation approximately 30 feet higher topographically and approximately 400 feet up canyon from the treatment system. In the event of artesian pressure drop, the wells can be siphoned and the groundwater will flow to the treatment system by force of gravity.

Four drums filled with zero-valent iron provide primary treatment of the groundwater. The GeoSiphon system at LLNL is enhanced by the addition of two granular activated carbon drums that provide secondary treatment, as well as aerators and sand beds for removing iron oxides. Accelerated bioremediation is also used as a final treatment to remove nitrate, with acetate provided as the carbon source.

TNX GeoSiphon

Technical expertise from SRS has contributed to the transfer of this technology to LLNL. EM-40 provided funding for the world’s first GeoSiphon, which was installed at the TNX facility in July 1997 to treat trichloroethylene and carbon tetrachloride–contaminated groundwater. The TNX GeoSiphon treatment cell is an 8-foot-diameter well, containing granular cast iron. A siphon passively induces the treated water to flow from the GeoSiphon cell to the point of discharge, an outfall ditch draining into the Savannah River. The TNX GeoSiphon is an example of a presiphon treatment cell configuration—groundwater is first treated then transported by siphon to the place of discharge.

As a zero-valent iron medium, granular cast iron is an effective reducing agent in enhancing a reaction known as reductive dechlorination. Although we do not completely understand the various steps by which zero-valent iron degrades trichloroethylene (a chlorinated volatile organic compound, or CVOC) into its final reaction products—ethane, ethane, and chloride ions, we do know that providing contaminated groundwater with sufficient contact time with zero-valent iron reduces CVOCs to intermediate and final degradation products.

During testing at the TNX GeoSiphon, approximately 49.7 tons of granular cast iron was used in the treatment cell, and flow rates were manipulated with pumping to determine the maximum acceptable treatment flow rate that allowed reduction of 200 micrograms per liter (mg/l) trichloroethylene to below drinking water standards (5 mg/l). The maximum acceptable treatment flow rate was found to be between 7.8 and 8.3 gallons per minute. During Phase II testing, the actual flow rate with an optimized siphon configuration was found to average 2.62 gallons per minute, resulting from a head differential of approximately 1.43 feet between the cell and the outfall ditch, the point of discharge. A new siphon line to increase the head differential and induce a treatment flow rate that more closely matches the maximum acceptable treatment flow rate will increase the efficiency of the system.

D-Area GeoSiphon

During FY98–99, EM-50 funded a full-scale pilot test of a GeoSiphon system at Savannah River’s D-Area Coal Pile Runoff Basin, a 12.5-acre basin that received acidic runoff contaminated with metals and sulfate from the 8.9-acre D-Area coal pile. Seepage from the basin resulted in groundwater contaminated with ferrous iron, aluminum, nickel, chromium, and other metals.

The treatment system was designed to be two-stage. Groundwater flowed below ground into a limestone-filled trench, the primary treatment cell, where aluminum and chromium were precipitated. Receiving the groundwater through a siphon line, the secondary treatment cell oxidized, precipitated, and collected the remaining metal contaminants, mostly iron and nickel. The natural head difference of less than 5 feet between the up-gradient limestone trench and the discharge point into the secondary treatment cell, induced groundwater to flow through the D-Area GeoSiphon system. With a siphon connecting two treatment cells, the D-Area GeoSiphon can be thought of as combining both presiphon and postsiphon treatment cell configurations.

The limestone trench raised the groundwater pH, which caused the precipitation of dissolved metals, primarily aluminum and chromium. The partially treated groundwater was then transferred by natural siphon to a downstream settling container. The supernatant from the settling container moved to a secondary treatment system, where iron and nickel were oxidized, precipitated, and settled. The treated water was then discharged to a man-made stream that flowed to a permitted surface water outfall.

The D-Area GeoSiphon is an example of a postsiphon treatment cell configuration.	Graphic provided by Savannah River Site

During testing of the D-Area GeoSiphon, several siphon configurations were evaluated as well as the capability of several oxidants and bases to oxidize iron and raise pH in the secondary stage. The most effective reactants for the secondary stage were found to be calcium peroxide and hydrogen peroxide/sodium carbonate.

Ready, set, siphon!

The GeoSiphon has broad applicability in a number of configurations. Testing at Savannah River Site’s TNX facility demonstrated its treatment of VOCs using a zero-valent iron medium. The configuration at the D-Area, with limestone and oxygen-generation compounds, shows that the technology aids in metals remediation and may also remediate the VOC-portion of contaminated groundwater simultaneously. Westinghouse Savannah River Co. has a patent pending for the GeoSiphon technology.

Mark Phifer, an engineer at the Savannah River Technology Center, has been instrumental in this technology’s deployments at Savannah River and Lawrence Livermore and is available to help other sites that are considering a GeoSiphon. For Phifer, the beauty of a GeoSiphon is its synergy with nature. “Rather than looking at the natural environment as something to overcome, the GeoSiphon concept looks for and works with existing natural energy potentials to achieve groundwater cleanup.”

For more information on the GeoSiphon, contact Mark Phifer at the Savannah River Technology Center at (803) 725-5222, or mark.phifer@srs.gov.