Environ. Sci. Technol. 1995, 29,2157-2161
Speciation and Mobiiii of Akenic in a Dimictic b k e H E N R Y M . SPLIETHOFF, ROBERT P. MASON,*,+AND HAROLD F. HEMOND Ralph Parsons Laboratory, MIT, Cambridge, Massachusetts 02139
Introduction Recent studies (1-5) have demonstrated that the speciation and distribution of dissolved arsenic (As) in natural waters is often far from thermodynamic equilibrium, with arsenite [As(III)lpredominating in oxic surface waters and arsenate [As01 persisting in anoxic and even highly sulfidic environments ( 2 , 3 , 6 , 7 ) . Reduction of arsenate in surface waters by microorganisms (8-101, the differential scavenging of As(II1) and A s 0 by iron and manganese (hydr)oxides (11-141, and a relatively slow reduction of A s 0 in anoxic waters (3, 14, 15) are often quoted reasons. Our recently reported investigations in 1991/1992 (21, which focused on the dimictic Upper Mystic Lake which is within the historically contaminated [with As and other metals (16, 131 Aberjona Watershed, presented a picture of As cyclingand speciationthat generallyparalleled results found elsewhere (3-5). We report here additional data collected in 1993 that provide a striking example of how sensitive As speciation and concentrations can be to subtle changes in redox conditions. In 1992, there was little evidence of As(111) in the hypolimnetic waters, even after a prolonged period of low oxygen (no sulfide present) (2). In contrast, in October 1993when sulfidewas present in the lake bottom waters, As(II1) and total dissolved As concentrations were more than an order of magnitude higher than those found in October 1991 or 1992. These results raise important questions about the role of interannual variability and atypical seasonal cycles on the release and potential contamination ofthe water column with As. Not only were the 1993 As concentrations in the hypolimnetic waters of the Upper Mystic Lake greatly elevated with respect to the previous years [although still far lower than the solubility limits (18)and lower than those found in permanently stratified and anoxic waters (2,5)], but a study of the mixing regime of the lake in the fall of 1993 using microstructure profiling techniques (19) demonstrated that the hypolimnetic waters of the lake can be mixed into the surface layer by thermocline seiching induced by axial winds. This mixing enhances the potential for drinking water and food chain contamination in this urban lake, which is used extensivelyfor swimming,fishing, and other recreational activities.
Methods The Upper Mystic Lake is the first of two lakes receiving drainage from the Aberjona Watershed. The sediments of + Present address: Chesapeake Biological Laboratory, Solomons, MD 20688.
0013-936W95/0929-2157$09.00/0
0 1995 American Chemical Society
this 51-ha eutrophic dimictic lake contain alarge inventory of As and other trace metals as a result of historic contamination of the watershed (20). The lake stratifies in summer, with oxygen depletion of the bottom waters beginning in May and persisting into late fall (November). Mixing in November and December leads to an isothermal water column and oxygenation of the bottom waters (2). Arsenic speciation and concentration were measured using hydride generation techniques with detection by atomic absorption. Samples, stored cold, were analyzed for As speciationwithin 24 h of collection. Analytical details are described elsewhere (2). Dissolved oxygen concentrations and temperature were measured using a portable oxygen meter, and water samples were collected using a peristaltic pump into acid-washed bottles. Total (acidsoluble) and filtered (0.45 pm) iron and manganese concentrations were measured by ICP-ES. Particulate concentrations were estimated by difference. Sulfide analysis of samples, stored in BOD bottles, was completed within 6 h using the methylene-blue method (21).
Results The results obtained in 1992 were described in detail in Environ. Sci. Technol. byAurilio et al. (2). Concentrations of A s 0 increased in the hypolimnion throughout the fall of 1992 and in October, the maximum measured A s 0 concentration for that year (20 nM) occurred at the deepest depth sampled. In November, concentrations had decreased but were still elevated relative to the concentrations found in midsummer. Similar A s 0 concentrations (> 10 nM in the bottom waters) had been measured in October and November 1991. In 1993, the As(II1) and A s 0 concentration profiles for the spring and summer were similar to those found in 1992 (Figure 1). In addition, methylated As species were again found in the mixed layer, with concentrations increasing through the spring to a maximum of 3.9 nM dimethylarsenate in May before declining throughout the summer. The concentration and speciation found in the hypolimnion in the fall of 1993, however, were substantially different from those obtained either in 1991or 1992 (Figure 2). First, much higher concentrations of inorganic As were found in the hypolimnion, with the maximum concentration (380 nM) occurring in October. Second, the arsenic speciation in the bottom waters changed dramatically in 1993relative to the speciation in 1991 and 1992. In 1991/1992,As(II1) was always a small fraction (
