Arsenic distributions in porewaters and sediments of Puget Sound, Lake Washington, the Washington coast and Saanich Inlet, B.C.

Abstract

Solid phase As, Mn, Al, and excess 210Pb and dissolved total As, As+3 As+5, Mn+2, Fe+2, PO−34, NH+4, and H2S profiles in 12 cores from Puget Sound, Lake Washington, the Washington coast, and Saanich Inlet, B.C., are presented. Typically, strong correlations between solid phase As and Mn concentration maxima in surficial sediments are a result of intense As scavenging in the overlying waters by Mn or Ferich phases, or solid phase As dissolution at depth followed by upward diffusion and co-precipitation with Mn or Fe oxides at or near the sediment-water interface. Atypically, anthropogenic inputs cause high sedimentary As concentrations without accompanying high Mn levels. Sedimentary As fluxes are an order of magnitude greater in Puget Sound and Lake Washington than off the Washington coast and in Saanich Inlet. Porewater total As concentration profiles exhibit sub-surface maxima with magnitudes 10–60 times greater than in overlying bottom waters. Porewater maxima in marine environments are proportional to associated solid phase As concentrations. These maxima consistently occur within ± 1.0 cm of the bottom of the 210Pb-derived surface mixed layer, suggesting solid phase As release to porewater in response to a redox gradient across this mixing boundary. Dissolved As behavior beneath these maxima indicates As removal to solid phases in marine, but not lacustrine porewaters. Porewater As+3As+5 ratios are <1.0 for Washington coastal environments and usually between 1.0–4.0 for Puget Sound and Lake Washington. As+5 is below the detection limit in porewaters of Saanich Inlet when the overlying bottom water is anoxic, but As+3As+5 ratios as low as 2.8 exist in near surface porewater under oxic bottom water conditions.