No abstract

Arsenic concentrations have been determined in mesquite ( Prosopis laevigata ) and huizache ( Acacia farnesiana ) growing on As-rich soils in the semi-arid mining area of Zimapán, México. The elevated As contents in the substrates (up to 32 000 mg/kg) were reflected in the dry weight concentrations of twigs (up to 82.7 mg/kg in mesquite and 225 mg/kg in huizache), and leaves (up to 78.2 mg/kg in mesquite and 67.0 mg/kg in huizache). Arsenic concentrations in twigs and leaves collected in less impacted soils (containing up to 110 mg/kg of As) reached a maximum of 20.8 mg/kg in mesquite twigs and 27.8 mg/kg in mesquite leaves. Differences of several orders of magnitude in surface substrate As concentrations were not reflected in plant As contents. A low proportion of available As in substrates, as well as deep plant roots which may reach less contaminated sediments, explains this behaviour. The pod As concentrations from tailings-affected sites were not significantly different from background samples. Nearly all mesquites and huizaches from the high-As substrates were above standards for foodstuffs. The As concentration in vegetation growing on soils affected by tailings and smelter/slags points to a potential contamination of the food chain, mainly through goats, the most common mammals in the area. Mesquites and huizaches are not hyperaccumulators but are As-tolerant plants and can therefore be considered as an option for a remediation programme to stabilize eroding tailings.

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.