We present the study of the geochemical processes associated with the
first successful remediation of a marine shore tailings deposit in a
coastal desert environment (Bahia de Ite, in the Atacama Desert of
Peru). The remediation approach implemented a wetland on top of the
oxidized tailings. The site is characterized by a high hydrauliz
gradient produced by agricultural irrigation on upstream gravel terraces
that pushed river water (similar to 500 mg/L SO(4)) toward the sea and
through the tailings deposit. The geochemical and isotopic
(delta(2)H(water) and delta(18)O(water), delta(34)S(sulfate) ,
delta(18)O(sulfate)) approach applied here revealed that evaporite
horizons (anhydrite and halite) in the gravel terraces are the source of
increased concentrations of SO(4), Cl, and Na up to similar to 1500 mg/L
in the springs at the base of the gravel terraces. Deeper groundwater
interacting with underlying marine sequences increased the
concentrations of SO(4), Cl, and Na up to 6000 mg/L and increased the
alkalinity up to 923 mg/L CaCO(3) eq. in the coastal aquifer. These
waters infiltrated into the tailings deposit at the shelf-tailings
interface. Nonremediated tailings had a low-pH oxidation zone (pH 1-4)
with significant accumulations of efflorescent salts (10-20 cm thick) at
the surface because of upward capillary transport of metal cations in
the arid climate. Remediated tailings were characterized by neutral pH
and reducing conditions (pH similar to 7, Eh similar to 100 mV). As a
result, most bivalent metals such as Cu, Zn, and Ni had very low
concentrations (around 0.01 mg/L or below detection limit) because of
reduction and sorption processes. In contrast, these reducing conditions
increased the mobility of iron from two sources in this system: (1) The
originally Fe(III)-rich oxidation zone, where Fe(II) was reduced during
the remediation process and formed an Fe(II) plume, and (2) reductive
dissolution of Fe(III) oxides present in the original shelf lithology
formed an Fe-Mn plume at 10-m depth. These two Fe-rich plumes were
pushed toward the shoreline where more oxidizing and higher pH
conditions triggered the precipitation of Fe(HI)hydroxide coatings on
silicates. These coatings acted as a filter for the arsenic, which
naturally infiltrated with the river water (similar to 500 mu g/L As
natural background) into the tailings deposit.