ARSENIC MOBILITY FROM HIGH-SILICA ASH FLOW TUFFS

Volcanic rocks typically have only low to moderate arsenic concentrations although elevated levels of arsenic in groundwaters have been associated with pyroclastic and volcaniclastic rocks and sediments in many parts of the world. In this study we analyzed 49 samples of ash-flow tuffs, and 11 samples of tuffaceous sediments representing a wide spectrum of chemical weathering and devitrification. Total and partial digestions, and water extractions of samples were used to determine the total, environmentally available, and readily leachable fractions of arsenic present in all tuff samples.

The 49 tuff samples have a mean arsenic content of 7.5 mg kg^-1, a median arsenic content of 5.2 mg kg^-1, and a maximum arsenic concentration of 81 mg kg^-1. The mean and median values are higher than the average crustal abundance of 1.7 mg kg^-1 (Wedepohl, 1995), and consistent with previously reported values for volcanic glasses and felsic volcanic rocks (Onishi and Sandell, 1955; Wedepohl, 1995), although the maximum arsenic content is higher than previously reported in the literature. In addition, the arsenic concentrations of tuffs were found to be highly heterogeneous, both between and within individual units, and in some cases, individual outcrops. Results of whole rock and leachate analyses indicate no significant difference in the total arsenic content of tuffs as a result of devitrification or weathering, but both devitrified and weathered tuffs contain higher levels of environmentally available and readily leachable arsenic than unweathered glassy tuffs. The results of this study indicate that devitrification and weathering processes determine the host phases, degree of adsorption, and overall mobility of arsenic from ash-flow tuffs.