Occurrence and formation 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. ¹ Ericksen also reported a trace of perchlorate in the "blister" caliche of a nitrate deposit in southeastern California². However, certainly up to the very recent time period the natural occurrence of perchlorate was thought largely to be strictly confined to the Atacama dessert and its well studied nitrate deposits, with the exception of some now rejected reports³ and other unconfirmed findings.⁴ Interestingly, one of the first major hypotheses and now verified mechanism⁵ for the occurrence of perchlorate was atmospheric production and deposition over extremely long time periods. While it is certainly true that the Atacama is one of the driest places on Earth and has been for the last 6-15 million years⁵ other extremely arid areas exist, so it is reasonable that other arid areas should also posses some amount of perchlorate. Arid areas in this country, (e.g. the Death Valley region of the Mojave Desert in California) and other deserts throughout the world have been noted to contain natural surficial nitrate-rich salts and deep subsurface soils acting as repositories for nitrate and chloride. Stable isotope studies of these natural surficial nitrate rich salts by the United States Geological Survey (USGS)⁶ support the hypothesis that these long term nitrate accumulations must be of atmospheric origin. The combined oxygen and nitrogen isotope signatures of the nitrate in these deposits are significantly different from other natural and anthropogenic sources of nitrate. The processes that form and accumulate nitrate may offer clues to the formation of natural perchlorate. It is possible that the ultimate source of natural perchlorate could be aerosols formed in the atmosphere. Chlorine, along with other halogens, participates in numerous photochemical reactions in the atmosphere, and it is known that various oxidized forms of chlorine, including (ClO₄^-), are present in aerosols in trace amount ⁷. In fact, if recently reported precipitation data are applicable globally⁸ the question is not what areas contain natural perchlorate but rather what areas concentrate naturally deposited perchlorate to environmentally relevant concentrations. The issue of documenting perchlorate's natural occurrence has been certainly problematic given older analytical methodologies, the typically low levels of perchlorate occurrence (commonly a few tens of ppb or less), the inconsistent results of various investigators, and reservations about the existence of naturally-occurring perchlorate in areas other than the Atacama Desert. In addition, at least in the U.S. where interest in perchlorate is highest, the high profile industrial occurrences in the American Southwest (prime potential area for accumulation of perchlorate) have probably served to overshadow any anomalous detections potentially due to natural sources. This combined with the extreme difficulty in proving the origin of perchlorate found in the environment have led to a rather sparse data set on its potential occurrence. With the growing interest in its occurrence and production from natural mechanisms as well as the important isotopic work reported by Bao and GU,⁹ Sturchio et al.,¹⁰ and Michalski et al.⁵ to help determine its source, the overall importance as both an environmental pollutant and component of basic geochemical cycling may soon be at hand. This chapter is focused on documenting both the few known occurrences of perchlorate believed to be of natural origin as well as a discussion of the current theories on possible production mechanisms and the overall importance of naturally occurring perchlorate in the environment.