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.

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