In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 9

occur in the form of As 2 S 3 or by coprecipitation with iron sulfides. If the concentration of sulfates
in the groundwater is too low, this could be increased by dissolution of gypsum. Several researchers
have studied this type of potential barrier, including Gubert et al . (2004) and Köber et al . (2003;
2005).

1.4 APPLICATIONS OF PRBS

1.4.1 Application of Montana
In June 2005, an experimental barrier was installed near the town of Helena, Montana (USA,
Wilkin et al ., 2006), with a length of 9 m, a thickness of 7.6 m and a variable width between 1.8
and 2.4 m. The barrier was located near an old lead smelter and was designed to treat underground
water with moderate concentrations of As(III) and As(V). Groundwater at the site had become
contaminated with arsenic due to leaching from the contaminated process ponds located over the
shallow groundwater. The arsenic plume was approximately 450 feet wide and extended 2100 feet
down gradient from the process ponds. The barrier was built in 3 days, using digging equipment
modified to allow the construction of deep trenches that were filled with a pulp of a biopolymer.
The reactive medium was completely composed of granular ZVI. Concentrations of As upstream
the barrier exceeded 25 g L − 1 . From the eighty samples taken downstream the barrier, eleven
exceeded 0.50 mg L − 1 , 62 had concentrations below 0.05 mg As L − 1 and 24 were below the
acceptable limit of 10 µ gL − 1 . After 2 years of operation, the concentrations downstream the
barrier were considerably lower.

1.4.2 Application to the treatment of groundwater contaminated
by acid drainage from pyrite mines
One of the biggest problems in the mining sector is the treatment and disposal of solid waste and
liquid effluents generated in the processing stages, especially when the facilities are abandoned.
The best example is mining dedicated to the processing of sulfides, where the adequate manage-
ment of acidic effluents generated in the process is the main objective to reduce the environmental
impact of the activity. Generally, mixtures of solid and liquid waste are accumulated in controlled
damps as disposal treatment. During these periods, soluble sulfide species can be oxidized by
dissolved oxygen in water according to the following reaction (Blodau, 2006):

H + Metallic sulfide + water + oxygen → soluble metal + sulfate + (1.13)

Therefore, the water of these ponds has variable amounts of ions (Fe, Zn, Pb, Cu), nonmetallic
ions [As(V), As(III)], a high acidity (pH between 1 and 2) and high sulfate contents. Among
the constituents of acid mine water, As has been recognized as one of the pollutants with the
greatest impact on aquatic ecosystems because of its persistence, toxicity and bioaccumulation,
and toxicological effects associated (Cundy et al ., 2008; Morgada et al ., 2009; Tyrovola and
Nikolaidis, 2009; Zouboulis and Katsoyiannis, 2005). In the absence of specific regulations, the
same regulation criterion of drinking water (10 gAsL − 1 ) has been selected for the remediation µ

process with PRBs in groundwaters.
While the case of surface run-off and drainage treatment is relatively affordable, in the case
of groundwater, pump-and-treat technologies with ex-situ treatment and return of the treated
water to the aquifer are hardly applicable due to its high cost and reduced capacity to achieve the
required quality standards (Spira et al ., 2006). It is therefore proposed the use of PRBs, replacing
the aquifer material by reactive materials to treat the pollution plume generated (Sacre, 1997;
Waybrant et al ., 1998). Groundwater moves through the barrier treatment by natural flow or,
when it is necessary to pump, wells can be installed in a way that the water contaminant passes
through the reactive barrier (Burghardt et al ., 2007; Lackovic et al ., 1999).