Environmental Engineering Reference
In-Depth Information
250
200
150
Groundwater flow
100
PRB
50
0
TEPA-2A
TEPA-2B
TEPA-2C
TEPA-2D
TEPA-2E
TEPA-2F
TEPA-2G TEPA-2H
2006
2007
2008
2009
2010
FIGURE 8.4
Arsenic concentration trends along TEPA-2 from 2006 to 2010.
may have been a factor in the high arsenic concentrations observed. A low
iron concentration would be expected to limit arsenic removal by limiting
coprecipitation reactions with iron sulfides, iron carbonates, and/or iron
(oxy)hydroxides.
8.1.2.8 Sulfate/Sulfide
Sulfate concentrations entering the PRB ranged from 1649 mg/L at TEPA-1 to
6696 mg/L at TEPA-6, while concentrations entering the PRB in 2010 ranged
from 1835 mg/L at TEPA-1 to 6435 mg/L at TEPA-6 (Table 8.1). Within the
PRB, sulfate concentrations were significantly reduced, consistent with active
sulfate reduction although in 2010, sulfate removal along TEPA-2 decreased
significantly. Sinks for sulfate entering the PRB include formation of metal
sulfides, elemental sulfur, precipitation as gypsum, and formation of organi-
cally bound sulfur (Ludwig et al., 2009). Given the generally high iron concen-
trations entering the PRB, mackinawite (FeS) was possibly the largest sulfate
sink. Chemical equilibrium calculations indicated saturation conditions with
respect to gypsum along TEPA-2. However, undersaturated conditions with
respect to gypsum were indicated along the other three transects suggest-
ing gypsum precipitation was not a significant sulfate sink. The 2010 data
suggested only 15.3% of the sulfate was removed along TEPA-2, although
sulfide concentrations within the PRB along TEPA-2 were consistently the
highest, including a sulfide concentration of 23.9 mg/L in 2010. The sulfide
data indicated that despite the apparently more limited sulfate removal
occurring along TEPA-2, sulfate-reducing activity still remained sufficiently
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