Rapid Redox Transformations of Arsenic and Characterization of the Internal Seiches in the Upper Mystic Lake, Medford, Massachusetts

The rate constants for in situ arsenic redox transformations in the surface waters of a eutrophic lake were estimated from observed horizontal heterogeneity in the arsenic species concentrations in the epilimnion of the Upper Mystic Lake. The smallest rate constants necessary to explain the observed horizontal heterogeneities were 10 times higher than rate constants estimated from monthly profiles of the arsenic species in the lake. These high in situ rate constants confirm the high rate constants measured by Spliethoff and Hemond (1995) in laboratory experiments. Particulate arsenic and particulate iron were very strongly correlated in samples collected from the hypolimnion of the lake (r=0.94,n=116). In addition, the apparent partition constant between dissolved arsenate and arsenate sorbed to iron oxides was inversely proportional to the particulate iron concentration, an example of the "particle concentration effect" (Gschwend and Wu, 1985). These observations suggest that arsenate is sorbed to iron oxides, and that a fraction of the "dissolved" arsenate is really arsenate sorbed to iron oxide colloids. If arsenate sorption to the iron oxides in the hypolimnion is actually at equilibrium, a simple surface complexation model calculation (Dzombak and Morel, 1990) shows that the ratio of the dissolved arsenate and arsenite concentrations nearly matches the ratio predicted from thermodynamics for the pH and pE of the hypolimnion. Therefore, the arsenic species in the hypolimnion may be at thermodynamic equilibrium. The first horizontal modes of the first and second vertical mode internal seiches (VH1 and V2H1) were identified as the dominant observed seiches in the Upper Mystic Lake. The V2H1 seiche is excited by wind stress forcing close to its natural frequency. Internal seiches alone cannot explain an observed mixing event which suggests that boundary mixing in the shallow regions of the lake may also be important. Thesis Supervisor: Harold F. Hemond Title: Professor of Civil and Environmental Engineering