SOURCES AND OCCURRENCE OF NON-ANTHROPOGENIC PERCHLORATE

The issue of naturally occurring perchlorate is one that has environmental, agricultural, and economic implications. Natural perchlorate was first identified in Chilean nitrates over 100 years ago and was extensively studied by George Edward Ericksen and colleagues (see, e.g., 1981, USGS Professional Paper 1188). Until recently significant deposits of naturally occurring perchlorate were thought to be confined to the Atacama Desert and associated with its well-studied nitrate deposits. One of the earliest hypotheses, now verified, for the occurrence of perchlorate in the Atacama deposits is atmospheric production and deposition over extremely long time periods. Chlorine, along with other halogens, participates in numerous photochemical reactions in the atmosphere. It is now known that various oxidized forms of chlorine, including perchlorate, are present in aerosols in trace amounts. In fact, if recently reported precipitation data are applicable globally, the question is not what areas receive natural perchlorate but rather what areas concentrate naturally deposited perchlorate to environmentally significant levels. A small but growing number of data sets from semi-arid and arid regions in the southwestern United States suggest fairly widespread occurrence of perchlorate that is partly or fully due to natural processes. The study of naturally occurring perchlorate has been limited until recently by analytical resolution. Recent strides in analytical methodologies, namely ion chromatography and mass spectroscopy (IC & IC/MS) with pre-concentration for resolving to sub-microgram-per-liter levels, have greatly increased the ability to detect and differentiate sources of perchlorate. Ongoing studies of production and accumulation by natural mechanisms are leading to a better overall understanding of perchlorate, both as an environmental pollutant and as a component of basic geochemical and hydrologic cycles. This presentation summarizes the historical discovery of naturally occurring perchlorate, proposed mechanisms of production and accumulation, and new data on recent discoveries.