FEASIBILITY OF IMPLEMENTING GEOCHEMICAL BARRIERS FOR AS REMOVAL IN A HISTORIC MINING SITE OF MEXICO

Mexico is a country rich in non-renewable natural resources, including minerals. Zimapan, is a Pb-Ag-Zn mining site with abundant As-minerals. Unfortunately, mining exploitation has also produced tailings impoundments, which have been altered over time; among these, Compañía Minera Zimapán (CMZ) and San Miguel Nuevo (SMN). These tailings contain soluble As concentrations up to 34 mg/L the former and 10 mg/L the latter. High Fe soluble concentrations were also measured; up to 645 (~80 % Fe(III)) and 303 mg/L (~20 % Fe(III)) respectively. These elements are transported through runoff towards soils, nearby shallow wells and the non-perennial Toliman River.

Limestone formations outcropping at Zimapan could be used in geochemical barriers for treatment of tailing leachates. The potential advantage of this procedure is that a passive system does not require constant supervision and the material is found natural and abundantly in the zone. This study shows the results of the treatment of representative leachates coming from CMZ and SMN tailings in percolation columns, simulating geochemical barriers with indigenous limestone as reactive material.

An experiment with two columns fed with CMZ leachates was run during 8 and 14 weeks respectively, until clogging, which was associated to high Fe(III) concentrations. Other two columns fed with SMN leachates worked 20 weeks without clogging signs. Results showed that As concentrations had a significant decrease from both leachates, even until below the analytical detection limit (0.001 mg/L); only a maximum value of 0.040 mg/L was reached in one column. Iron decreased below analytical detection limit (0.3 mg/L) for both cases. Besides, pH was neutralized; the inflow values of 2.5 and 3.42 for CMZ and SMN respectively, increased to outflow values between 6 and 8. Geochemical processes were assed through DRX, SEM-EDS, SEM-WDS determinations in the columns and estimation of saturation indexes in leachates with Visual MINTEQ®.