A rapid increase in the concentration of hydrogen peroxide was observed when samples of natural surface and ground water from various locations in the United States were exposed to sunlight. The hydrogen peroxide is photochemically generated from organic constituents present in the water; humic materials are believed to be the primary agent producing the peroxide. Studies with superoxide dismutase suggest that the superoxide anion is the precursor of the peroxide.

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Photochemical reactions may affect the photic zone, the surface sunlit layer of freshwaters, oceans, and estuarine regimes where fresh and saltwaters begin to interact. Hydrosphere photochemistry influences the world's oceans (about 70% of the Earth's surface), and the illuminated portion of the hydrosphere is especially active and diverse. 'Ransport, material exchanges, and biology (including aquatic primary pro-3 S A Environ. Sci.Technol.* The authors, ivlto also served as ihe orpni:f>!,s ~~~~i i t m i r i w ~! / t h c ,VAT0 Advanced Research Insriruw. ore (clockwise from u p p r 1c.h): Hod Zika. Riduird Zepp. Jucques Joussor-Duhien, and Oliver C. Z~!firiou.

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