In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 10

Table 1.2. Composition of groundwater from wells
near the Aznalcóllar dam used in the experiments with
columns for evaluation of reactive mixtures of lime,
organic matter and iron.

Composition Water I Water II

Ca(II) [mg L − 1 ]

360 360

Fe(II) [mg L − 1 ]

10 10

Zn(II) [mg L − 1 ]

20 20

Cd(II) [mg L − 1 ] - 2

Cu(II) [mg L − 1 ] - 20

Al(III) [mg L − 1 ] 10 10

As(V) [mg L − 1 ] 2 2

SO 2 4 [mg L − 1 ] 960 960

HCO 3 [mg L − 1 ] 24 24

pH 3.0 3.0

1.4.3 The Aznalcóllar pollution case
On 25 April 1998, a rupture occurred in the tailings dam at the Aznalcóllar mine (SW Spain,
Bolzicco et al ., 2001), and the Guadiamar river received a 6 hm 3 of sludge and acidic water. This
event caused a major ecological catastrophe, and life in the river disappeared completely, reaching
the acidic waters the entrance limits of the Doñana National Park, one of the most important
wetlands in Europe. Furthermore, in aquifers near the reservoir, detectable levels of metal species,
arsenic, sulfate and acidity were measured. For this problem, the best option for remediation was
the use of a PRB, based on previous experience with a similar situation in the Nickel Rim mine
(Dudbury, Canada), where sulfate (2000-5000 mg L − 1 ), iron (250-1300 mg L − 1 ) and slightly
acidic pH (5-6) have been detected (Joshi and Chaudhuri, 1996). Placing a barrier based on
calcite, organic matter and gravels, allowed the reduction of sulfate to values between 200 and
3600 mg L − 1 and iron to values between 1 and 40 mg L − 1 , maintaining the control of acidity near
7. According to these results, the PBR was considered a viable, efficient and economical solution
(USEPA, 1998).
Considering the analysis of groundwater from wells near the Aznalcóllar pond, two different
types of model waters were evaluated at laboratory scale as preliminary evaluation (Waters I and
II), whose composition is shown in Table 1.2 . As can be seen, waters were rich in calcium and
sulfate, with an acidic pH.
Given the situation of the Guadiamar river and taking into account the composition of ground-
water and the composition of the sludge dispersed, it was necessary to act on the following aspects:
(i) to regulate the acidity of water, (ii) to reduce sulfates, (iii) to reduce the levels of heavy metals,
especially zinc, and (iv) to be aware of the possible mobilization of As and the effect of the barrier
for its abatement. In this case, As removal was not a priority because its concentrations were close
to the limits set by the WHO (Gavaskar et al ., 2005).
The materials used in laboratory scale tests are described in Table 1.3 . The used calcite was
natural and had a particle size of 2 mm. Two types of vegetal compost were evaluated: one from
plant debris from the area close to Aznalcóllar and another one obtained from organic matter of
solid wastes and sewage sludge. The source of sulfate-reducing bacteria was obtained from river
sediments of an anaerobic zone. Shavings of metallic iron used previously in reactive barriers for
decontamination of organochlorinated compounds were used; shavings from machining processes
of metallic pieces of cast steel were also tested.
The efficiency for As removal of the reactive mixtures containing organic matter was very high
in both columns since the beginning of the experiments. The reactive material containing Fe(s)