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Perchlorate (ClO4-) is an emerging contaminant whose presence and origin is of considerable environmental interest. This article discusses known sources of perchlorate and available forensic techniques to distinguish between naturally occurring and anthropogenic origins. Forensic techniques presented include stable isotopic analysis (Cl-37/Cl-35, Sr-87/Sr-86, O-16/O-17), surrogates analysis ( nitrates, sodium, chlorides, phosphate, nitroglycerins, metals, strontium), historical information assessment ( aerial photography, propellant use and chemistry, firing range usage), geologic analysis ( limestone, caliche, playa crusts, hanksite [Na22K(SO4)(9) (CO3)(2)Cl], potash, phosphate ore, etc.), x-ray diffraction (XRD), and scanning electron microscopy (SEM).

In this article, the detection of a source of naturally occurring perchlorate in soil and groundwater associated with the Mission Valley Formation in San Diego California is presented. Modification of existing agricultural soil sampling techniques USDA 60-6(26) and USDA 60-6(27a) were used for sample preparation along with EPA Standard Method 314 for extract analysis. X-ray diffraction (XRD), polarized light microscopy (PLM), scanning electron microscopy (SEM), surrogates analysis, geologic evaluation, and aerial photography were used in this investigation. The results of this analysis indicated a naturally occurring source of perchlorate is associated with the Mission Valley Formation. No statistical relationship between major ions (sulfate, total nitrate, chloride) and perchlorate was identified.