Arsenic speciation in seawater and interstitial waters: The influence of biological‐chemical interactions on the chemistry of a trace element1

Four species of arsenic were found in samples of seawater and interstitial water from the northeast Pacific and the continental borderland off southern California: arsenate, arsenite, methylarsonate and dimethylarsinate. All were present in the photic zone, with a good correlation between photosynthetic activity and the concentration of the methylated arsenicals. There is less arsenite in the surface waters than in the deep waters, suggesting biological uptake and transport. A sharp increase of arsenate at the base of the photic zone indicates rapid regeneration of this species. In the deep waters arsenate is dominant, but arsenite is still present in excess of equilibrium concentrations. No arsenic biomethylation was found in the interstitial waters of oxic and anoxic sediments, but arsenite was present there. In Santa Barbara Basin arsenite accounts for 20% of the arsenic in the interstitial water, the highest proportion found. Total arsenic is lower in the interstitial than in the overlying waters, suggesting removal of arsenic into the solid phases by adsorption or coprecipitation. Biological‐chemical interactions and the kinetics of redox reactions are important in influencing the speciation of arsenic.