The role of arsenic-bearing rocks in groundwater pollution at Zimapán Valley, México |
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Authors: | M A Armienta G Villaseñor R Rodriguez L K Ongley H Mango |
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Institution: | (1) Instituto de Geofísica, UNAM, México D.F., Mexico. Circuito Exterior, C.U., México 04510 D.F., México e-mail victoria@tonatiuh.igeofcu.unam.mx, Fax: 525-5502486, MX;(2) Instituto de Geología, UNAM, México, MX;(3) Androscogin Valley Environmental Center, 86 Russell St, Lewiston, ME, USA, US;(4) Castleton State College, Vermont, USA, US |
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Abstract: | Interaction of groundwater with As-bearing rocks has been proposed as one of three main sources of arsenic at Zimapán valley,
México. The complexity of the geology and hydrogeology of the valley make it difficult to identify the natural causes of arsenic
poisoning. Samples from the different rock outcrops and water from wells tapping various rock formations were analyzed. The
rocks from mineralized areas contained higher concentrations of arsenic with respect to the same formations in non-mineralized
areas. The arsenic minerals arsenopyrite, scorodite, and tennantite were identified in some rock samples. Higher temperature
and lower Eh values were found for those wells containing more arsenic. The physicochemical characteristics of these naturally
polluted well waters could be produced by arsenopyrite oxidation. The geochemical model PHREEQCI was used to perform the inverse
modeling of two wells located along the same fault. Arsenopyrite oxidation and scorodite dissolution appear to be the geochemical
processes producing the natural pollution according to the model. The release and transport of arsenic mainly occur through
fractures within the cretaceous limestones where the most productive wells are drilled. The presence of arsenic should be
expected also in other formations near mineralized zones in the Zimapán Valley. Field determinations of Eh and T could be
used to detect potentially polluted wells.
Received: 29 April 1999 / Accepted: 18 July 2000 |
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Keywords: | Arsenic Arsenopyrite Groundwater México Zimapán |
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