Arsenic content and groundwater geochemistry of the San Antonio-El Triunfo, Carrizal and Los Planes aquifers in southernmost Baja California, Mexico |
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| Authors: | A Carrillo-Chávez J I Drever M Martínez |
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| Institution: | (1) Instituto de Geología, UNICIT-UNAM, Campus Juriquilla, A.P. 15, Juriquila, Qro., 76230, México e-mail: ambiente@conin.unicit.unam.mx Tel: +52-42-381108 Fax: +52-42-381101, MX;(2) Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 87201-3006, USA, US;(3) Depto. de Geología, UABCS, La Paz, BCS, 23080, México, MX |
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| Abstract: | The San Antonio-El Triunfo mining district, located at a mountainous region 45 km south-east of La Paz, Baja California,
has been worked since the late 1700s. Mine waste material produced during 200 years of mineral extraction area poses a risk
of local groundwater pollution and eventually, regional pollution to the Carrizal (west basin) and the Los Planes (east basin)
aquifers. There are different types of deposits in the mining area. These are dominated by epithermal veins, in which arsenopyrite
is an important component. Carrillo and Drever (1998a) concluded that, even though the amount of mine waste is relatively
small in comparison to the large scale area, significant As in groundwater derived from the mine waste piles is found locally
in the groundwater. This paper shows the results of geochemical analyses of groundwater samples from the San Antonio-El Triunfo
area and the Carrizal and Los Planes aquifers during several years of monitoring (1993–1997). The highest values of total
dissolved solids (TDS) and As are in the mineralized area where the mining operations occurred (∼1500 ppm TDS and 0.41 ppm
As). The lowest concentrations of TDS and As are, in general, away from the mineralized area (∼500 ppm TDS and 0.01 ppm As).
Sulfate and bicarbonate (alkalinity) are, in general, high near the mineralized area and low away from it. The arsenic concentrations
vary seasonally, especially after the heavy summer thunderstorms. Geochemical modeling (MINTEQA2 and NETPATH) and analysis
of the regional geochemical evolution of the groundwater from the mining area towards the aquifer of Los Planes shows that
the most likely hydrochemical processes include: dilution, precipitation of calcite, and adsorption of As onto surfaces of
iron oxyhydroxides (ferrihydrite). These processes act as natural controls to the extent and amount of As pollution in the
Carrizal and Los Planes aquifers.
Received: 4 May 1999 · Accepted: 22 February 2000 |
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| Keywords: | Arsenic in groundwater Baja California Peninsula Geochemical modeling Mine waste material |
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