Geochemistry of highly acidic mine water following disposal into a natural lake with carbonate bedrock


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Geochemistry of highly acidic mine water following disposal into a natural lake with carbonate bedrock
Applied Geochemistry
Wisskirchen C., Dold B., Friese K., Spangenberg J.E., Morgenstern P., Glaesser W.
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Acid mine drainage (AMD) from the Zn-Pb(-Ag-Bi-Cu) deposit of Cerro de
Pasco (Central Peru) and waste water from a Cu-extraction plant has been
discharged since 1981 into Lake Yanamate, a natural lake with carbonate
bedrock. The lake has developed a highly acidic pH of similar to 1. Mean
lake water chemistry was characterized by 16,775 mg/L acidity as
CaCO(3), 4330 mg/L Fe and 29,250 mg/L SO(4). Mean trace element
concentrations were 86.8 mg/L Cu, 493 mg/L Zn, 2.9 mg/L Pb and 48 mg/L
As, which did not differ greatly from the discharged AMD. Most elements
showed increasing concentrations from the surface to the lake bottom at
a maximal depth of 41 m (e.g. from 3581 to 5433 mg/L Fe and 25,609 to
35,959 mg/L SO(4)). The variations in the H and 0 isotope compositions
and the element concentrations within the upper 10 m of the water column
suggest mixing with recently discharged AMD, shallow groundwater and
precipitation waters. Below 15 m a stagnant zone had developed. Gypsum
(saturation index, SI similar to 0.25) and anglesite (SI similar to 0.1)
were in equilibrium with lake water. Jarosite was oversaturated (SI
similar to 1.7) in the upper part of the water column, resulting in
downward settling and re-dissolution in the lower part of the water
column (SI similar to -0.7). Accordingly, jarosite was only found in
sediments from less than 7 m water depth. At the lake bottom, a layer of
gel-like material (similar to 90 wt.% water) of pH similar to 1 with a
total organic C content of up to 4.40 wet wt.% originated from the
kerosene discharge of the Cu-extraction plant and had contaminant
element concentrations similar to the lake water. Below the organic
layer followed a layer of gypsum with pH 1.5, which overlaid the
dissolving carbonate sediments of pH 5.3-7. In these two layers the
contaminant elements were enriched compared to lake water in the
sequence As < Pb approximate to Cu < Cd < Zn = Mn with increasing depth.
This sequence of enrichment was explained by the following processes:
(i) adsorption of As on Fe-hydroxides coating plant roots at low pH (up
to 3326 mg/kg As), (ii) adsorption at increasing pH near the
gypsum/calcite boundary (up to 1812 mg/kg Pb, 2531 mg/kg Cu. and 36
mg/kg Cd), and (iii) precipitation of carbonates (up to 5177 mg/kg Zn
and 810 mg/kg Mn: all data corrected to a wet base). The infiltration
rate was approximately equal to the discharge rate, thus gypsum and
hydroxide precipitation had not resulted in complete clogging of the
lake bedrocks. (C) 2010 Elsevier Ltd. All rights reserved.
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26/09/2012 14:11
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20/08/2019 12:34
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