Environmental Engineering Reference
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have also been the result of the biopolymer used in the construction of the
PRB systems. The lowest DOC concentration measured within the PRB in
2010 was at TEPA-1C (6.4 mg/L), which coincided with the location where pH
also decreased sharply over the past 2 years. DOC concentrations at all other
locations monitored within the PRB in 2010 were greater than 10 mg/L and
generally greater than 20 mg/L.
8.2 South Carolina PRB
Results for the pilot PRB installed at the former phosphate fertilizer pro-
duction facility in South Carolina, (USA) have been previously described
(Ludwig et al., 2009). The PRB at this location consisted of a mixture of gran-
ite pea gravel (45%), composted yard waste (30% v/v), ZVI filings (20% v/v),
and limestone (5% v/v). The pilot PRB measured 7.9 m in length, 4.1 m in
depth, and 1.8 m in width and was installed in pyrite-containing fill sedi-
ments which were disposed of at the site over decades of operation. The pilot
PRB was installed near the edge of a tidal marsh located downgradient of
the site. The ZVI was added to help neutralize pH, ensure removal of arsenic
by promoting sorption of arsenic to ZVI surfaces, and to attempt to extend
the life of the PRB through sustained production of H
2
for utilization by sul-
fate reducers. Column studies have indicated that the presence of ZVI in
combination with organic carbon can enhance sulfate reduction rates by up
to 15% and extend the life of an organic carbon matrix by nearly twofold
(Guo and Blowes, 2009). Although the average pH of groundwater entering
the pilot PRB (pH 3.68) was slightly higher than at the Louisiana site, the
acidity of the groundwater entering the South Carolina pilot PRB was much
higher than that of the Louisiana site. Iron acidity was greater than 25-fold
while aluminum acidity was approximately twofold greater than at the
Louisiana site. Vertically averaged concentrations of As, Pb, Cd, Ni, and Zn
entering the South Carolina PRB (based on 48-month data) were 59.2, 0.744,
0.067, 0.481, and 215 mg/L, respectively. The PRB effectively removed heavy
metals, acidity, and arsenic from the acid rock drainage-impacted ground-
water, with concentrations of As, Pb, Cd, Ni, and Zn within the PRB (based
on 48-month data) averaging 30, <3, <1, <1, and 37 μg/L, respectively. Iron
concentrations were reduced from an average of 3384 mg/L to an average
of 3.15 mg/L within the PRB, while aluminum concentrations were reduced
from 520 mg/L to <17 μg/L within the PRB. Although dissolved iron concen-
trations entering the South Carolina PRB were high, average arsenic concen-
trations entering the same PRB were three orders of magnitude higher than
average arsenic concentrations entering the Louisiana PRBs, thus making it
conceivable that without ZVI, iron coprecipitation reactions alone would not
have been sufficient to remove all of the arsenic.
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