Paleogeochemical deposits in northern Chile are a rich source of naturally occurring sodium nitrate (Chile saltpeter). These ores are mined to isolate NaNO3 (16-0-0) for use as fertilizer. Coincidentally, these very same deposits are a natural source of perchlorate anion (ClO4-). At sufficiently high concentrations, perchlorate interferes with iodide uptake in the thyroid gland and has been used medicinally for this purpose. In 1997, perchlorate contamination was discovered in a number of US water supplies, including Lake Mead and the Colorado River. Subsequently, the Environmental Protection Agency added this species to the Contaminant Candidate List for drinking water and will begin assessing occurrence via the Unregulated Contaminants Monitoring Rule in 2001. Effective risk assessment requires characterizing possible sources, including fertilizer. Samples were analyzed by ion chromatography and confirmed by complexation electrospray ionization mass spectrometry. Within a lot, distribution of perchlorate is nearly homogeneous, presumably due to the manufacturing process. Two different lots we analyzed differed by 15%, containing an average of either 1.5 or 1.8 mg g-1. Inadequate sample size can lead to incorrect estimations; 100-g samples gave sufficiently consistent and reproducible results. At present, information on natural attenuation, plant uptake, use/application, and dilution is too limited to evaluate the significance of these findings, and further research is needed in these areas.

Adsorption and release of perchlorate in a variety of soils, minerals, and other media were studied when the solid media were exposed to low and high aqueous solutions of perchlorate salts. Low level ClO4- exposure was investigated by subjecting triplicate 5.0 g portions of a solid medium (38 different soils, minerals, or dusts) to 25 mL of an aqueous ammonium perchlorate (NH4ClO4) solution containing 670 ng mL(-1) (6.8 microM) perchlorate. This corresponds to a perchlorate-to-soil ratio of 3.4 microg g(-1) (34 nmol g(-1)). At this level of exposure, more than 90% of the perchlorate was recovered in the aqueous phase, as determined by ion chromatography. In some cases, more than 99% of the perchlorate remained in the aqueous phase. In some cases, the apparent loss of aqueous perchlorate was not clearly distinguishable from the variation due to experimental error. The forced perchlorate anion exchange capacities (PAECs) were studied by soaking triplicate 5.0 g portions of the solid media in 250 mL of 0.20 M sodium perchlorate (NaClO4) followed by repeated deionized water rinses (overnight soaks with mixing) until perchlorate concentrations fell below 20 ng mL(-1) in the rinse solutions. The dried residua were leached with 15.0 mL of 0.10 M sodium hydroxide. The leachates were analyzed by ion chromatography and the perchlorate concentrations thus found were subsequently used to calculate the PAECs. The measurable PAECs of the insoluble and settleable residua ranged from 4 to 150 nmol g(-1) (micromol kg(-1)), with most in the 20-50 nmol g(-1) range. In some soils or minerals, no sorption was detectable. The mineral bentonite was problematic, however. Overall, the findings support the widely accepted idea that perchlorate does not appreciably sorb to soils and that its mobility and fate are largely influenced by hydrologic and biologic factors. They also generally support the idea that intrasoil perchlorate content is depositional rather than sorptive. On the other hand, sorption (anion replacement) of perchlorate appears to occur in some soils. Therefore, the measurement of perchlorate in soils requires accounting for ion exchange phenomena; leaching with water alone may give inaccurate results. If perchlorate anion exchange is confirmed to be negligible, then leaching procedures may be simplified accordingly.

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Variable availability of the purified dihydrochloride salt of 3,3'-dimethoxybenzidine (DMB; ortho-dianisidine) led us to investigate the effects of reagent purity on the analytical results obtained when this reagent is used in the photometric determination of the disinfection byproduct bromate. After analyte ions are separated by ion chromatography, a solution of DMB (post-column reagent) is added to the eluate and the DMB is oxidized, thereby producing a chromophore detected by its absorbance. Although some commercial products of undefined grade performed well, others did not. Variability was also observed between lots of purified material. Sensitivity at low concentrations (< 5 micrograms L-1 BrO3-) varied by a factor of up to 10. In some cases, the lower limit of detection for photometric detection was greater than that obtained using conductivity detection, as high as 5-7 micrograms L-1 BrO3-. An impurity or several impurities are suspected to be responsible for deviations from linearity at low analyte concentrations. This investigation underscores the need for ensuring reagent purity in environmental analyses. Ideally, chemical manufacturers will meet the needs of analytical chemists who test potable water and begin producing a high grade material in sufficient quantities to meet monitoring requirements. The establishment of third-party standards for a spectrophotometric grade of DMB.2HCl would be helpful in ensuring that a variety of manufacturers could supply products of uniformly high quality that would be suitable for the measurement of bromate in public drinking water supplies.