Effects of High pH on Arsenic Mobility in a Shallow Sandy Aquifer and on Aquifer Permeability along the Adjacent Shoreline, Commencement Bay Superfund Site, Tacoma, Washington

A groundwater arsenic plume, derived from arsenite wastes disposed at a chemical plant in Tacoma, WA, extends to the shore of the Hylebos Waterway. The plume is characterized by high-pH, high-silica concentrations generated by past disposal of high-pH brines on site. Aquifer K d values for arsenic decrease at least 10-fold as the pH increases from 8.5 to 11. Near the shore, aquifer sands are cemented, predominantly by opal quartz. Cementation reduces porosity to about 19%; however, very little pore space is interconnected. Along the shore face, a massive amorphous precipitate, high in Si and Mg, is found. SOLMINEQ calculations show that mixing high-pH, high-silica groundwater with seawater causes initial supersaturation of brucite [Mg(OH) 2 ] and magnesium hydroxysilicates. The cementation has likely considerably reduced the cumulative discharge of arsenic to the waterway.