Environ. Sci. Technol. zyxwvuts 1989, zyxwvu 23, 1425-1428 Neijassel, 0. M. et al., Eds.; Elsevier Science: Amsterdam, 1987; Vol. 3, pp 515-519. zyxwvutsr (5) Henson, J. M. et al. zyxwvutsrq FEMS Microbiol. Ecol. zyxwvuts 1988, 53, 193-201. (6) Fliermans, C. B. et al. Appl. Environ. Microbiol. 1988,54, 1709-14. (7) Levenspiel, 0. Chemical Reaction Engineering, 2nd ed.; Wiley: New York, 1972; Chapter 9. (8) Garland, S. B. et al. The Use of Methanotropic Bacteria for the Treatment of Groundwater Contaminated with TTichloroetheneat the Department of Energy Kansas City Plant; ORNL/TM-llOM zyx Oak Ridge National Laboratory: Oak Ridge, TN, in press. Received for review January 17,1989. Revised manuscript re- ceived June 12,1989. Accepted July 24,1989. This work was supported by the Kansas City Plant, Office of Defense Programs, zy U.S. Department zyxwv of Energy, under Contract DE-ACO5- 840R21400. NOTES Hydrogen Peroxide Concentration in a Northern Lake: Photochemical Formation and Diel Variability William J. Cooper*,+ and David R. S. Lead Drinking Water Research Center, Florida International University, Miami, Florida 33 199, and National Water Research Institute, P.O. Box 5050, Burlington, Ontario, Canada L7R 4A6 Diel changes in H202concentration in Jacks Lake, Ontario, suggested that photochemical processes were re- sponsible for its formation. The concentrations of H202 reached 200-400 nM by late afternoon on a sunny day and declined to below 10 nM during the night. The depletion of H202 observed in near-shore lake sampling sites was faster than the dark decay rate of H20z. The dark decay rate of H20z obeyed first-order kinetics and was much faster than those previously observed in marine environ- ments. Rain was shown to have H202 concentrations up to 34 pM and may contribute to the surface water H20z concentration. Introduction Sunlight-induced photochemical processes in natural waters have implications in redox cycling, pollutant transport, and possibly biological activity (1, 2). Of the possible reactive photochemical intermediates that can be formed, H202 is one of the more stable species (3). While numerous studies have been published regarding the spatial and temporal variability of H202 in oceanic (4-10) and estuarine (11, 12) environments, relatively little is known about freshwater systems (13-20). The in situ photochemical formation of H202 is thought to result from the disproportionation of superoxide, Of (17, 18, 21-26). As such, 02- may also be important in aqueous processes and H202may be used as a way of estimating the formation rates of 02- in water (23). Several reports of diel and seasonal changes in metal speciation in freshwater have appeared (27-30). It is possible that these changes result from reactions that are initiated photochemically and may involve Hz02, as ob- served in oceanic environments (7,10,31). Because of the influence of pH and chloride ion on iron and copper speciation it is difficult to extend marine systems studies to freshwater. However, it is possible that reactions of this 'Florida International University. 8 National Water Research Institute. nature are at least in part responsible for the observation of reduced forms of metals in oxygenated water. Although H202 is thermodynamically a good oxidant, it is usually kinetically limited, at natural water pH and in the absence of catalysts, when reacting with organic compounds (32,33). Superoxide, on the other hand, may react directly with some pollutants and may be important in determining their fate in natural waters (3). Two such examples are benzidine and benzo[a]pyrene, which have second-order rate constants with 02- in water of >2.5 X lo7 and <1 X lo7 L mol-ls-l, respectively (34). The biological effects of H202 at concentrations found in natural waters have not been considered in any detail (35). Preliminary evidence suggests that H202 may inhibit some microbial processes (36), but more detailed investi- gations are necessary. Superoxide has been studied ex- tensively and is well-known to have adverse effects at the cellular level (37). However, no studies have been reported on its effects in natural waters. For these reasons, it appears that studies leading to a quantitative understanding of the formation and decom- position of H202 (and thus indirectly Of) in natural waters are important. This study was undertaken to examine the diel changes in H202 in a northern temperate oligotrophic lake, Jacks Lake, Ontario, Canada. The study extended over two time periods and is the first report of extended diel changes in H202 concentration in freshwater. Methods The H202 was analyzed by the scopoletin horseradish peroxidase fluorescent decay method (11, 20, 38). The lower limit of detection was 10 nM, while the upper limit was 250 nM. Sample dilution with distilled water was required for higher concentrations. A Turner Designs Model 10 fluorometer equipped for excitation and emission wavelengths of 365 and 490 nm, respectively, was used. Replicate analyses on the same sample were within *5% throughout the range of H202 concentrations reported. To minimize analytical errors, separate standard curves were 0013-936X/89/0923-1425$01.50/0 0 1989 American Chemical Society Environ. Sci. Technol., Vol. 23, No. 11, 1989 1425