144
Permeable Reactive Barrier
2010 ranged from 189 μg/L along TEPA-1 to 395 μg/L along TEPA-6. Within
the PRB, Ni was generally treated to <5 μg/L with the exception of TEPA-1C
and TEPA-2D, where concentrations of 47.1 and 7.46 μg/L were measured,
respectively. The highest concentration of 47.1 μg/L measured at TEPA-1C
coincided with the low pH of 4.64 also measured at this location in 2010. The
increased presence of Ni relative to Pb and Cd under the declining pH conditions at TEPA-1C is consistent with the higher solubility of NiS relative to PbS
and CdS. Ni in the downgradient half of the PRB at TEPA-1D was measured
at 1.63 μg/L in 2010, indicating the PRB along TEPA-1 still remains effective in treating Ni. The overall Ni removal efficiency of 87.6% along TEPA-1
was significantly less than the greater than 98% removal efficiency observed
along the other three transects in 2010. Ni concentrations were also higher
than Pb and Cd concentrations downgradient of the PRB along all transects,
with concentrations as high as 24.9 and 47.8 μg/L, respectively measured at
TEPA-2H and TEPA-6E. At all other downgradient transect locations, Ni concentrations were less than 8 μg/L.
8.1.2.7 Arsenic
Average yearly arsenic (As) concentrations entering the PRB ranged from
6.33 μg/L at TEPA-1 to 139 μg/L at TEPA-5 (Table 8.2). Arsenate was the predominant form of arsenic entering the PRB while arsenate, arsenite, and thioarsenic forms were observed downgradient of the PRB, without any one form
dominating. The highest arsenic concentration entering the PRB in April 2010
was 58.7 μg/L at TEPA-5. Within the PRB, arsenic was treated to less than
10 μg/L along all four transects. However, contrary to the heavy metals, arsenic concentrations rebounded downgradient of the PRB as shown in Figure
8.4, eventually exceeding concentrations upgradient of the PRB. This is presumed to be due to reductive dissolution of arsenic-containing iron minerals
in the downgradient aquifer sediments. Arsenic concentrations at TEPA-2H
near the creek were measured at 68.1 μg/L in 2010 relative to the average arsenic concentration within the PRB of only 6.2 μg/L. The value of 68.1 μg/L,
however, represents a decrease from previous sampling rounds in 2009 and
2008 when arsenic concentrations at this same location were measured at
102 and 123 μg/L, respectively and in 2006 when the arsenic concentration at
this location was measured at 230 μg/L. The data suggest that although arsenic was being mobilized from aquifer sediments downgradient of the PRB,
the amounts being mobilized were gradually decreasing over time.
Arsenic was also monitored along a fifth transect across the full-scale PRB
where concentrations within the PRB averaged 820 μg/L since 2006. This is
despite sulfide concentrations within the PRB at this location having averaged
11.5 mg/L during the same period. Thioarsenic species accounted for over
25% of the dissolved-phase arsenic present within the PRB along this fifth
transect. Iron concentrations within the PRB at this location, however, averaged only 1.5 mg/L since 2006, which suggests that low iron concentrations
Permeable Reactive Barrier
2010 ranged from 189 μg/L along TEPA-1 to 395 μg/L along TEPA-6. Within
the PRB, Ni was generally treated to <5 μg/L with the exception of TEPA-1C
and TEPA-2D, where concentrations of 47.1 and 7.46 μg/L were measured,
respectively. The highest concentration of 47.1 μg/L measured at TEPA-1C
coincided with the low pH of 4.64 also measured at this location in 2010. The
increased presence of Ni relative to Pb and Cd under the declining pH conditions at TEPA-1C is consistent with the higher solubility of NiS relative to PbS
and CdS. Ni in the downgradient half of the PRB at TEPA-1D was measured
at 1.63 μg/L in 2010, indicating the PRB along TEPA-1 still remains effective in treating Ni. The overall Ni removal efficiency of 87.6% along TEPA-1
was significantly less than the greater than 98% removal efficiency observed
along the other three transects in 2010. Ni concentrations were also higher
than Pb and Cd concentrations downgradient of the PRB along all transects,
with concentrations as high as 24.9 and 47.8 μg/L, respectively measured at
TEPA-2H and TEPA-6E. At all other downgradient transect locations, Ni concentrations were less than 8 μg/L.
8.1.2.7 Arsenic
Average yearly arsenic (As) concentrations entering the PRB ranged from
6.33 μg/L at TEPA-1 to 139 μg/L at TEPA-5 (Table 8.2). Arsenate was the predominant form of arsenic entering the PRB while arsenate, arsenite, and thioarsenic forms were observed downgradient of the PRB, without any one form
dominating. The highest arsenic concentration entering the PRB in April 2010
was 58.7 μg/L at TEPA-5. Within the PRB, arsenic was treated to less than
10 μg/L along all four transects. However, contrary to the heavy metals, arsenic concentrations rebounded downgradient of the PRB as shown in Figure
8.4, eventually exceeding concentrations upgradient of the PRB. This is presumed to be due to reductive dissolution of arsenic-containing iron minerals
in the downgradient aquifer sediments. Arsenic concentrations at TEPA-2H
near the creek were measured at 68.1 μg/L in 2010 relative to the average arsenic concentration within the PRB of only 6.2 μg/L. The value of 68.1 μg/L,
however, represents a decrease from previous sampling rounds in 2009 and
2008 when arsenic concentrations at this same location were measured at
102 and 123 μg/L, respectively and in 2006 when the arsenic concentration at
this location was measured at 230 μg/L. The data suggest that although arsenic was being mobilized from aquifer sediments downgradient of the PRB,
the amounts being mobilized were gradually decreasing over time.
Arsenic was also monitored along a fifth transect across the full-scale PRB
where concentrations within the PRB averaged 820 μg/L since 2006. This is
despite sulfide concentrations within the PRB at this location having averaged
11.5 mg/L during the same period. Thioarsenic species accounted for over
25% of the dissolved-phase arsenic present within the PRB along this fifth
transect. Iron concentrations within the PRB at this location, however, averaged only 1.5 mg/L since 2006, which suggests that low iron concentrations