A review of the arsenic cycle in natural waters

Abstract

A review of the occurrence and cycling of arsenic in fresh waters is presented. The fate of arsenic in natural waters has received little attention in past years, in spite of the fact that arsenic is toxic and probably carcinogenic through exposure by drinking water. The chemistry of arsenic in aqueous systems is reviewed. Thermodynamic information is summarized in an Eh pH diagram for a system including sulfur. Mechanisms for removal of arsenic from the solution phase to the sediments are discussed. The possible microbially-mediated reactions of arsenic, including oxidation of arsenite, methylation of arsenic species, and reduction of arsenate, are discussed with reference to the locale of the reaction in the water column or in the sediments and to the toxicological significance of the reaction products and the rates of reaction. A cycle of reactions for arsenic in a stratified lake is proposed and evidence is summarized relating to the occurrence and importance of particular reactions. The potential pollutional hazard of arsenic is from ingestion of drinking water with high concentrations of arsenic, rather than consuming arsenic containing aquatic organisms. Although arsenic is greatly concentrated in aquatic organisms, it is evidently not progressively concentrated along a food chain. In addition, arsenic when consumed as an organically-bound species in flesh evidently has low toxicity. The global cycle of arsenic is discussed. While volcanic activity is the original source of much of the arsenic in sedimentary rocks, in recent times weathering of arsenic has been approximately in balance with deposition of arsenic in sediments. Human activities, including the use of arsenic, the burning of fossil fuels, increased erosion of land and the mining and processing of sulfide minerals, have increased the amount of arsenic entering the oceans by at least a factor of 3. This increase will have no effect on the concentration in the oceans for many hundreds of years. However, these cultural contributions are the source of high localized concentrations in many fresh waters. Careful surveillance and increased knowledge of the fate or arsenic in the aquatic environment are needed to insure that there will be no public health hazard.