NATURAL PERCHLORATE IN SEDIMENTS, SOILS, AND DUSTS

Perchlorate contamination of water and soils became a matter of public interest in the mid-1980's. Most of this contamination was tied to anthropogenic point sources such as rocket fuel manufacturing plants and testing sites. Advances in the sensitivity of perchlorate detection techniques in the late 1990's showed that contamination at ppb levels was widespread, but not all the perchlorate could be traced to anthropogenic sources. Natural perchlorate salts were first identified in Chilean nitrate-bearing caliches more than 100 years ago. For most of the intervening time, that occurrence has been considered unique. However, recent work indicates that at least some perchlorate is of probable natural origin and that accumulations of perchlorate are most likely to occur in geologic materials under geologic and climatic conditions similar to, but not as pronounced, as those that existed during the formation of the Chilean deposits. This study has identified perchlorate in a variety of sediments, soils, and dusts from arid and semi-arid environments in the United States and elsewhere using a tandem IC-MS-MS analytical process.

When natural perchlorate is present, it typically occurs in amounts ranging from 1 to a few tens of ppb, although significantly higher concentrations have also been detected. Most of our recent samples were collected where no possible anthropogenic activity can be identified. George Ericksen had suggested that perchlorate in the Chilean nitrate deposits may have resulted from the reaction of chlorine with ozone. More recent work by Texas Tech researchers and others has shown that perchlorate can be formed under atmospheric conditions and that it can be detected in precipitation. Natural perchlorate appears to be concentrated in soils and sediments under climatic and geologic conditions similar to natural nitrate. These conditions include an arid to semi-arid climate and the presence of semi-permeable to impermeable soil and sediment layers. Other factors that may play a part in natural concentration of perchlorate include the length of time permissive conditions existed, closed basin hydrology, accumulation in plants, volcanic activity, presence/absence of organic matter, unusual climatic conditions such as drought, and overall sediment composition.