148
Permeable Reactive Barrier
Owing to pre-existing high levels of contamination on the downgradient
side of the PRB at the South Carolina site, the discharge plume from the PRB
could not be effectively monitored. However, as in the case of the Louisiana
PRBs, the high acidity groundwater entering the PRB was effectively converted to a net-acid consuming water, with alkalinities as high as 650 mg/L
as CaCO 3 measured within the PRB (based on 30-month data). Increases
in δ 34 S values, order of magnitude increases in sulfate reducing bacteria
counts, and order of magnitude decreases in sulfate concentrations within
the PRB provided strong evidence of microbially mediated sulfate reduction
occurring.
The much higher total dissolved solids (TDS) concentrations (up to
35,000 mg/L) entering the South Carolina PRB relative to the Louisiana PRBs,
coupled with the ZVI-induced higher average pH within the South Carolina
PRB (up to pH 9.28), resulted in much larger scale precipitation of secondary
minerals in the South Carolina PRB. This was reflected in decreased permeability observed within the South Carolina PRB over the course of just 1.5
years. The very high TDS concentrations and associated adverse impacts on
PRB hydraulic conductivity resulted in a decision to preclude further consideration of such a PRB for full-scale use at the site. However, organic carbon
combined with ZVI may offer significant benefits at other sites where TDS
levels are not so unusually high.
8.3 Conclusions
The findings of the Louisiana PRB study indicated that a cow manure-limestone-based PRB system effectively removed heavy metals, arsenic, and acidity from acidic sulfate-containing waters; although a significant dissolved
iron content entering the PRB may have been a prerequisite for removal of
arsenic. The results also indicated that the cow manure-based PRBs showed
persistence and performed well over a period of 7 years. The PRBs also produced an alkalinity discharge that helped to neutralize acid-impacted aquifer sediments downgradient of the PRBs. Less positive findings included the
observed mobilization of arsenic downgradient of the PRB and observed
groundwater mounding along the upgradient face of the PRB. Lower average hydraulic conductivity properties within the PRB were responsible for
the buildup of a hydraulic head along the upgradient face of the PRB and
resulted in some bypassing of groundwater around the ends of the PRB. This
significant shortcoming in the design of the PRB indicated that any refurbishment efforts should include formulation of a higher permeability treatment matrix. An alternative but less preferable option would be to extend the
length of the PRB to capture any potential bypass flow, as has been done by
others (Jarvis et al., 2006).
Permeable Reactive Barrier
Owing to pre-existing high levels of contamination on the downgradient
side of the PRB at the South Carolina site, the discharge plume from the PRB
could not be effectively monitored. However, as in the case of the Louisiana
PRBs, the high acidity groundwater entering the PRB was effectively converted to a net-acid consuming water, with alkalinities as high as 650 mg/L
as CaCO 3 measured within the PRB (based on 30-month data). Increases
in δ 34 S values, order of magnitude increases in sulfate reducing bacteria
counts, and order of magnitude decreases in sulfate concentrations within
the PRB provided strong evidence of microbially mediated sulfate reduction
occurring.
The much higher total dissolved solids (TDS) concentrations (up to
35,000 mg/L) entering the South Carolina PRB relative to the Louisiana PRBs,
coupled with the ZVI-induced higher average pH within the South Carolina
PRB (up to pH 9.28), resulted in much larger scale precipitation of secondary
minerals in the South Carolina PRB. This was reflected in decreased permeability observed within the South Carolina PRB over the course of just 1.5
years. The very high TDS concentrations and associated adverse impacts on
PRB hydraulic conductivity resulted in a decision to preclude further consideration of such a PRB for full-scale use at the site. However, organic carbon
combined with ZVI may offer significant benefits at other sites where TDS
levels are not so unusually high.
8.3 Conclusions
The findings of the Louisiana PRB study indicated that a cow manure-limestone-based PRB system effectively removed heavy metals, arsenic, and acidity from acidic sulfate-containing waters; although a significant dissolved
iron content entering the PRB may have been a prerequisite for removal of
arsenic. The results also indicated that the cow manure-based PRBs showed
persistence and performed well over a period of 7 years. The PRBs also produced an alkalinity discharge that helped to neutralize acid-impacted aquifer sediments downgradient of the PRBs. Less positive findings included the
observed mobilization of arsenic downgradient of the PRB and observed
groundwater mounding along the upgradient face of the PRB. Lower average hydraulic conductivity properties within the PRB were responsible for
the buildup of a hydraulic head along the upgradient face of the PRB and
resulted in some bypassing of groundwater around the ends of the PRB. This
significant shortcoming in the design of the PRB indicated that any refurbishment efforts should include formulation of a higher permeability treatment matrix. An alternative but less preferable option would be to extend the
length of the PRB to capture any potential bypass flow, as has been done by
others (Jarvis et al., 2006).