Environmental geochemistry of the San Antonio-El Triunfo mining area, southernmost Baja California Peninsula, Mexico

The San Antonio-El Triunfo (SA-ET) area has been exploited for gold, silver and lead for about 200 years. Approximately 800,000 tonnes of tailings are present in the area. The current research focuses on the evaluation of the tailings, their effect on groundwater, and adsorption of arsenic as natural control for groundwater pollution.;The tailings were classified as: oxidized tailings, low-grade ore, cyanide heap-leached material, and byproducts of processing plants. Tailings samples were tested in humidity cells to quantify acid-producing and acid-consuming processes, and the potential leaching of arsenic (As) and zinc (Zn). A rough, but conservative calculation indicates leaching of the tailings could contribute 0.4 mg/l As and 0.5 mg/l Zn to groundwater.;The SA-ET and La Paz-Los Planes areas were divided into eleven sub-areas based in the analysis of groundwater. The highest values of total dissolved solids (TDS) and arsenic are in the mining area (;Several experiments on total As (arsenate + arsenite) adsorption by aquifer material of the SA-ET area were conducted to test the feasibility of this material acting as a natural control for As concentrations in groundwater. The experimental % adsorbed vs. pH curves obtained matched the topology of total As adsorption onto iron oxihydroxide surfaces. The experimental adsorption isotherm at pH 7 indicates saturation of adsorbing sites at high solute concentrations. Surface titration of the aquifer material indicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (iron oxihydroxides = 7.9 to 8.2). Comparison between experimental and modeled results (MICROQL and MINTEQA2 codes) suggests As is being adsorbed mostly by oxihydroxide surfaces in the natural environment. A travel time of about 700 to 5,000 years was estimated for the arsenic plume from SA-ET to La Paz.