Chemical variations of tephra-fall deposit leachates for three eruptions from Popocatépetl volcano

Leachates from ash samples of the Popocatépetl eruptions of April 30, 1996, May 12, 1997, and October 17, 1998 settled at different distances from the crater were analyzed for anions (SO₄²⁻, Cl⁻, F⁻) and some metals. This study is aimed at determining the causes of the compositional variations of the leachates, to assist the assessment of water, soil and crop contamination due to ash deposits. Different behavior was observed in the ion concentrations with distance for the three eruptions. On April 30, 1996, SO₄²⁻ and F⁻ concentrations increased with distance, and Cl⁻ remained almost constant. On May 12, 1997, concentrations of the three anions decreased with distance. On October 17, 1998, F⁻, Cl⁻ and SO₄²⁻ increased more than three-fold with distance. Tephra size distributions were also different for the three eruptions. The observed trends of the leachates’ anion concentrations may have different causes: the type and intensity of the eruptions, the distribution of the tephra sizes, the degree of interaction of the tephras with volcanic gases, humidity, static charge, the original characteristics of the solid material, the transport time from the crater to the site of settling, and the relative angle between the wind direction and the sampling line. Enrichment factors and concentration trends for metals with distance suggest that Co, Ni, Cu and Pb in the leachates resulted mostly from volcanic gas adsorption.