The groundwater in Zimapán, Mexico has arsenic concentrations that range from below detection limits to >1 mg l−1. Rural residents of the valley need a low-cost, low-tech remediation process to reduce the arsenic concentrations to <50 μg l−1, the Mexican drinking water standard.

Laboratory experiments show that the arsenic remediation potential of the Soyatal Formation, an ubiquitous clay-rich limestone, is superior to that of other rocks from the region. Experimentally contaminated water (ECW) was produced by reacting de-ionized water with tailings. The ECW (0.6 mg As l−1) was then reacted with various rocks from the Zimapán region. Although all rocks caused a decrease in the aqueous arsenic concentration, the arsenic concentration was below detection limits (<0.030 mg l−1) in any ECW that had been reacted with the Soyatal Formation. Other experiments established that a rock:water weight ratio of 1:10 can reduce the aqueous arsenic concentration in native water from 0.5 mg l−1 to <0.030 mg l−1.

The calcareous shale of the Soyatal Formation contains kaolinite and illite. Both minerals are known to adsorb arsenic. The adsorptive characteristics of the Soyatal Formation may provide the basis for an acceptable low-cost low-tech remediation system.

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