ARSENIC CONTAMINATION IN THE SOILS AND SEDIMENTS OF ZIMAPÁN, MEXICO

The occurrence and related processing of arsenic rich ores in the Zimapán Valley has had a significant impact on the soils and sediments in the basin. Arsenic concentrations measured by INAA and/or acid digestion and ICP in more than 150 soils and sediment samples collected from 1995-9 range from 6 to 30 000 mg As/kg in the soils, 57 to 6 725 mg/kg in river sediments and 650 to 32 000 mg/kg in the tailings near the river. Average background arsenic concentrations (about 20 mg/kg) in the Zimapán valley are generally higher than US and Canadian clean-up guidelines. Soil and sediment arsenic concentrations appear to correlate with Cu, Pb, and Zn. Fe is not statistically correlated with As. Although samples with more than 8 wt% Fe all had more than 2 000 mg As/kg, samples with about 5 wt% Fe had arsenic concentrations ranging from 6 to 10 000 mg As/kg.

Soil arsenic concentrations exceed 400 mg/kg only within 1000 m of a known arsenic source, a mine or tailings heap. Residents of the areas around slag heaps grow fruits and vegetables in soils with as much as 13 000 mg As/kg. River sediment arsenic concentrations decreased downstream of an arsenic source.

Hypotheses to explain the occurrence of arsenic contaminated soils and sediments include: soil contamination by wind blown arsenic-rich particulates from tailings and smelter fumes, and naturally arsenic-rich soils developed as a result of weathering of non-ore rocks with slightly higher than average arsenic concentrations.