Organic-Based Permeable Reactive Barriers for the Treatment of Heavy Metals, Arsenic, and Acidity - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 142

CaCO 3 equivalents along TEPA-1 to a high of 2489 mg/L as CaCO 3 equiva-
lents along TEPA-6. Table 8.1 indicates almost 100% removal of Al (presum-
ably almost entirely as Al(OH) 3 ) along transects TEPA-2, 5, and 6 and nearly
90% removal along TEPA-1. Consistent with the decline in pH at TEPA-1C,
aluminum concentrations increased to 9.62 mg/L at this location in 2010.
This concentration, although still relatively low, was approximately 20 times
the concentration observed within the PRB along any of the other three tran-
sects in 2010.
Iron-based acidity accounted for approximately 15% of the total mineral
acidity entering the PRB and ranged in 2010 from 111 mg/L as CaCO 3 equiv-
alents at TEPA-1 to 377 mg/L as CaCO 3 equivalents at TEPA-6. Major sinks
for iron entering the PRB are expected to be ferrous sulfide (mackinawite)
and ferrous carbonate (siderite) precipitation. Iron removal efficiency in 2010
ranged from 72.1% along TEPA-1 to 91.8% along TEPA-6. The removal of alu-
minum and iron-based acidity and corresponding precipitation of secondary
minerals such as aluminum hydroxides, mackinawite, and siderite may have
led to a gradual passivation of limestone surfaces. It was unlikely, however,
that secondary mineral formation adversely impacted the hydraulic conduc-
tivity over time, given the relatively large sized limestone gravel used in the
PRB mixtures and the gradual consumption of the organic substrate that
occurs with time.
8.1.2.5 Alkalinity
The conversion of the impacted groundwater from an acid-producing
potential to an acid-consuming potential is an important objective of the
organic-based PRB systems. Alkalinity data indicated both pilot PRBs and
the full-scale PRB were successful in converting the high acidity groundwa-
ter entering the PRB to an acid-consuming potential (Table 8.1). Alkalinities
within the PRB based on April 2010 data ranged from 155 mg/L as CaCO 3
along TEPA-2 to 1100 mg/L as CaCO 3 along TEPA-6. Net alkalinities (total
alkalinity minus [Fe + Al + Mn + pH] acidity) ranged from 49 mg/L as
CaCO 3 along TEPA-2 to 1010 mg/L as CaCO 3 along TEPA-6. A gradual decline
in alkalinity values within the PRB with time was observed along all four
transects (Figure 8.3), which possibly indicates reduced sulfate reduction
rates and/or increased passivation of limestone surfaces. Net alkalinity was
maintained in groundwater downgradient of the PRB along each of the four
transects although gradual reductions were observed with distance from the
PRB. At H well locations near the creek, net alkalinity values in 2010 ranged
from 229 mg/L as CaCO 3 at TEPA-2H to 524 mg/L as CaCO 3 a t T E PA- 6 H .
8.1.2.6 Lead (Pb), Cadmium (Cd), and Nickel (Ni)
Metal sulfides are expected to precipitate out in accordance with their solubil-
ity products in the preferred order CuS > PbS > CdS> ZnS > NiS > FeS > MnS