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was observed in the Nitrex, and TCE concentrations were found to decline
in the ZVI with an observed half-life of 5.7 h.
Concentrations of DOC in the Nitrex effluent were elevated, which could
potentially increase passivation of ZVI. The study suggested that the increase
in DOC in the effluent of the Nitrex resulted in an observed reduction in TCE
degradation rates in the ZVI. However, GHD (2007) found in a pilot-scale
denitrification PRB that total organic carbon (TOC) concentrations (and pre-
sumably DOC) actually decreased in the effluent due to increased biological
ac t iv it y.
Studies by Robertson et al. (2007, 2008) indicated that the use of highly
reactive carbon sources with longer-than-necessary retention times may
result in excess carbon stimulating reactions such as sulfate reduction, excess
DOC leaching, and ammonium production. This suggests that to minimize
concentrations of DOC in the effluent of the denitrification PRB, it is impor-
tant to not overdesign the required retention times and carefully choose a
less labile carbon source.
At the former waste control site (WCS), two source areas of chlorinated sol-
vents were identified as residual DNAPL in the unsaturated zone. One of the
sources has resulted in concentrations of TCE at 1000 μg/L in groundwater,
which needed to be reduced to concentrations of approximately 330 μg/L
prior to discharge at the downgradient Helena River. The installation of a
ZVI PRB was identified as the prime candidate technology; however, further
investigation identified that nitrate passivation could be a significant issue.
This chapter presents the field investigation, laboratory testing, and full-
scale installation of a sequenced PRB for the purpose of pretreating the
nitrate concentrations in order to remediate TCE using a ZVI PRB.
7.2 WCS Site Description
The WCS site is located 15 km northeast of Perth, Western Australia. The
site historically operated as a chemical/oil recycling and treatment facil-
ity until a destructive fire in February 2001. Following the fire, the owners
of the property went into receivership and the site has remained unoccu-
pied since then. The site is currently an “orphan site,” owned by the State
of Western Australia and managed by the Department of Environment and
Conservation of Western Australia. Since 2001, a series of investigations and
risk assessments have been undertaken that have identified two main source
areas of contamination.
During operation, the WCS had a history of accepting a wide variety of
chemicals. While some records were found indicating the potential type and
amount of chemicals on site at the time of the fire, it was impossible to be cer-
tain whether all potential contaminants had been identified. For this reason,
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