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.
The GeoSiphon concept isnt 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 SRSs 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,
scheme for siphoning groundwater
|As used here, head is the pressure on a liquid arising from a difference in elevations, expressed in feet (cubic feet of water in a column). One foot is about 0.43 pounds per square inch.|
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.
The GeoSiphon has broad applicability in a number of configurations. Testing at Savannah River Sites 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 technologys 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
For more information on the GeoSiphon, contact Mark Phifer at the Savannah River Technology Center at (803) 725-5222, or firstname.lastname@example.org.