ARSENIC MOBILIZATION FROM SILICIC VOLCANIC ROCKS IN THE SOUTHERN WILLAMETTE VALLEY

Volcanic tuffs and tuffaceous sediments are frequently associated with high As groundwater concentrations though the process of As mobilization from these rocks is not fully understood. Although high silica rocks and volcanic glasses may be slightly enriched in As relative to other volcanic rocks, concentrations are still typically < 8 ppm indicating that As release must be facilitated by conditions particular to these rocks. Alkaline desorption, anionic competition, reactive glass dissolution, and reductive dissolution of iron oxides are proposed processes of As release from volcanic rocks. Geogenic As contamination of groundwater in the southern Willamette Valley in western Oregon has been well-documented since the early 1960s, and previous studies have identified the Little Butte Volcanics Series and Fisher and Eugene Formations as the source of As contamination. This study examines 19 samples from 10 units of ash flow tuffs and tuffaceous sediments within the Fisher Formation and Little Butte Volcanics Series, representing a range of weathering and devitrification, to determine conditions of mobilization and mineralogical constraints that control As release into solution. Preliminary leachate studies were conducted over a range of pHs from 7 to 11 and variable phosphate concentrations. Resulting aqueous As concentrations range from non-detect to 900 μg/L, with the majority exceeding 10 μg/L. Initial results show that the addition of phosphate produces the highest concentrations of As, indicating that surficial desorption more strongly controls As mobility, although dissolution processes may still be a factor. Despite being geochemically similar, As mobility varies significantly even within the same ash flow tuff, possibly as a result of post-depositional alteration, e.g. devitrification.