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

• Published in: Environmental science & technology 1996-05, Vol.30 (5), p.1645-1651
• Main Authors: Mariner, Paul E, Holzmer, Fred J, Jackson, Richard E, Meinardus, Hans W, Wolf, Frederick G
• Format: Article
• Language: English
• Subjects: Aquifers; Arsenic; Chemistry; Contamination; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Groundwater; Hydrogeology; Hydrology. Hydrogeology; Marine; Oceans; Pollution, environment geology; Shorelines
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es9506420

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.