Two permeable iron reactive barriers
(a continuous trench and a funnel-and-gate
system) were installed in late November, 1997 at the U.S.
Department of Energy’s Y-12 National Security Complex in
overall goal of this research was to determine the effectiveness of the use of
zero-valent iron (Fe0) to retain or remove uranium and other
contaminants such as technetium and nitrate in groundwater.
The long-term performance issues were investigated by studying the
biogeochemical interactions between Fe0 and groundwater constituents
and the mineralogical and biological characteristics over an extended field
operation. Results from nearly 3 years of monitoring indicated that the Fe0
barrier was performing effectively in removing contaminant radionuclides such as
uranium and technetium. In addition, a number of groundwater constituents such
as bicarbonates, nitrate, and sulfate were found to react with the Fe0.
Both nitrate and sulfate were reduced within or in the influence zone of
the Fe0 with a low redox potential (i.e., low Eh).
An increased anaerobic microbial biomass was also observed within and
in the vicinity of the Fe0 barrier, and these microorganisms were at
least partially responsible for the reduction of nitrate and sulfate in
groundwater. Decreased concentrations of Ca2+ and bicarbonate in
groundwater occurred as a result of the formation of minerals such as aragonite
(CaCO3) and siderite (FeCO3), which coincided with the Fe0
corrosion and an increased groundwater pH. A
suite of mineral precipitates was identified in the Fe0 barrier
system, including amorphous iron oxyhydroxides, goethite, ferrous carbonates and
sulfides, aragonite, and green rusts. These
minerals were found to be responsible for the cementation and possibly clogging
of Fe0 filings observed in a number of core samples from the barrier.
Significant increases in cementation of the Fe0 occurred
between two coring events conducted at ~1 year apart and appeared to correspond
to the changes in an apparent decrease in hydraulic gradient and connectivity.
The present study concludes that, while Fe0 may be used as an
effective reactive medium for the retention or degradation of many redox-sensitive
contaminants, its long-term reactivity and performance could be severely
hindered by its reactions with other groundwater constituents; and groundwater
flow may be restricted because of the build up of mineral precipitates at the
soil/Fe0 interface. Depending
on the site biogeochemical conditions, the rate of Fe0 corrosion may
increase; therefore, the life span of the Fe0 barrier could be
shorter than predicted in previous studies (~15–30 years).