126
Permeable Reactive Barrier
groundwater levels within the denitrification PRB have been identified on
a number of occasions as being a few centimeters higher than those upgradient, suggesting the possibility of flow bypass. However, the composition
of groundwater samples collected from wells screened in the Leederville
formation (D series monitoring wells) as well as at the ends of the PRBs
(MWG113, MWG114, MWG115, MWG116, MAG117, and MWG118) of the system do not suggest that flow bypass is occurring.
The survey elevation of the denitrification wells was confirmed ensuring that water levels were calculated correctly. Field observations have
suggested that groundwater in the denitrification and ZVI PRBs were initially slightly more viscous than background groundwater. This phenomenon was noticeably more pronounced within the denitrification PRB. The
dynamic viscosity of a fluid is inversely proportional to the hydraulic conductivity of a media. Based on this fundamental understanding, a marginal increase in viscosity of a fluid could directly result in an increase of
a few centimeters in water levels such as that noted within the denitrification PRB.
The increase in viscosity could be attributable to either bacteria, residual
guar gum present in the groundwater, or a combination of both. Research by
Rafai et al. (2010) has indicated that certain rod-shaped bacteria, which pull
fluid toward themselves with their flagella and drag it along with them from
behind, may increase fluid viscosity. This research has shown that increases
in both living and dead cells will increase the viscosity; however, living cells
result in a greater increase in viscosity. Rafai et al. (2010) found that as little
as a 15% increase in concentration of living cells resulted in a doubling of the
viscosity.
The TOC results indicate that despite the addition of a slurry breaking
agent, residual guar gum has persisted longer than anticipated within both
PRBs. TOC concentrations measured during the quarterly monitoring rounds
for the B series wells along the plume centerline are presented in Figure 7.11.
Initially, concentrations of TOC were found to be significantly elevated at
monitoring locations within both the denitrification and ZVI PRBs. Elevated
TOC was anticipated in the denitrification PRB due to the addition of a carbon source to stimulate denitrification; however, initial field measurements
were found to be 30 times greater than those measured in the laboratory
column experiments.
Field observations noted during the quarterly monitoring rounds also
identified that in addition to the increased viscosity, the groundwater at site
had a strong organic odor, and when exposed to oxygen resulted in the formation of an unknown white precipitate.
It is thought that the initial high concentrations of TOC were associated
with the guar gum and its decrease due to subsequent breakdown to shorterchain, more labile carbon forms. With time, the concentrations of TOC in
monitoring wells in the PRBs have decreased, as have the reports of odors,
Permeable Reactive Barrier
groundwater levels within the denitrification PRB have been identified on
a number of occasions as being a few centimeters higher than those upgradient, suggesting the possibility of flow bypass. However, the composition
of groundwater samples collected from wells screened in the Leederville
formation (D series monitoring wells) as well as at the ends of the PRBs
(MWG113, MWG114, MWG115, MWG116, MAG117, and MWG118) of the system do not suggest that flow bypass is occurring.
The survey elevation of the denitrification wells was confirmed ensuring that water levels were calculated correctly. Field observations have
suggested that groundwater in the denitrification and ZVI PRBs were initially slightly more viscous than background groundwater. This phenomenon was noticeably more pronounced within the denitrification PRB. The
dynamic viscosity of a fluid is inversely proportional to the hydraulic conductivity of a media. Based on this fundamental understanding, a marginal increase in viscosity of a fluid could directly result in an increase of
a few centimeters in water levels such as that noted within the denitrification PRB.
The increase in viscosity could be attributable to either bacteria, residual
guar gum present in the groundwater, or a combination of both. Research by
Rafai et al. (2010) has indicated that certain rod-shaped bacteria, which pull
fluid toward themselves with their flagella and drag it along with them from
behind, may increase fluid viscosity. This research has shown that increases
in both living and dead cells will increase the viscosity; however, living cells
result in a greater increase in viscosity. Rafai et al. (2010) found that as little
as a 15% increase in concentration of living cells resulted in a doubling of the
viscosity.
The TOC results indicate that despite the addition of a slurry breaking
agent, residual guar gum has persisted longer than anticipated within both
PRBs. TOC concentrations measured during the quarterly monitoring rounds
for the B series wells along the plume centerline are presented in Figure 7.11.
Initially, concentrations of TOC were found to be significantly elevated at
monitoring locations within both the denitrification and ZVI PRBs. Elevated
TOC was anticipated in the denitrification PRB due to the addition of a carbon source to stimulate denitrification; however, initial field measurements
were found to be 30 times greater than those measured in the laboratory
column experiments.
Field observations noted during the quarterly monitoring rounds also
identified that in addition to the increased viscosity, the groundwater at site
had a strong organic odor, and when exposed to oxygen resulted in the formation of an unknown white precipitate.
It is thought that the initial high concentrations of TOC were associated
with the guar gum and its decrease due to subsequent breakdown to shorterchain, more labile carbon forms. With time, the concentrations of TOC in
monitoring wells in the PRBs have decreased, as have the reports of odors,