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high enough to effectively remove incoming heavy metals and arsenic from
the groundwater. Interestingly, contrary to the other three transects, calcium
concentrations within the PRB along TEPA-2 increased steadily since 2007,
despite pH remaining relatively constant. This suggests that the limestone
in the PRB along TEPA-2 may have made an increasing contribution to pH
buffering relative to the bicarbonate generated from microbially mediated
sulfate reduction.
As might be expected, the PRB discharges a sulfide plume. The highest
average sulfide concentration measured in H wells near the creek downgra-
dient of the PRBs since 2004 was 1.2 mg/L at TEPA-6. The highest single mea-
surement of sulfide at an H well location was 4.0 mg/L at TEPA-6H in 2007.
8.1.2.9 Dissolved Organic Carbon
DOC concentrations increased significantly within the PRB relative to upgra-
dient locations as expected (Table 8.1). Since installation, the average DOC
concentrations upgradient of the PRB ranged from 4.04 mg/L at TEPA-1 to
16.0 mg/L at TEPA-6. Within the PRB, average DOC concentrations since
installation of the PRB ranged from 19.0 mg/L at TEPA-1 to 69.9 mg/L at
TEPA-6. However, DOC concentrations within the PRB declined signifi-
cantly with time (Figure 8.5), with concentrations in 2010 ranging from
9.28 mg/L at TEPA-1 to 33.0 mg/L at TEPA-6. Decreasing DOC concentra-
tions were consistent with that observed for other organic carbon-based PRB
systems and were likely a reflection of decreasing labile organic substrate
content in the PRB with time. The initially high DOC concentrations may
160
140
120
100
80
60
40
20
0
2004
2005
2006
2007
2008
2009
2010
TEPA-1C/D
TEPA-2C/D
TEPA-5C/D
TEPA-6C/D
FIGURE 8.5
DOC trend over time within PRB along four monitored transects.
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