Perchlorate Production in the Environment: Atmospheric and Soil Contributions
Natural Resources and Environmental Science
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Perchlorate (ClO4-) is a naturally occurring and human generated anion. Perchlorate can have human health impacts because it can act as a thyroid inhibitor. Because of health concerns, some states regulate perchlorate in drinking water, which shows the need of understanding sources of perchlorate in the environment. The major objectives of this project focus on better understanding perchlorate in the environment including perchlorate formation and movement through the environment in precipitation. The work in Chapter 1 explored mechanisms for formation of perchlorate. Previous research has shown that naturally occurring perchlorate is formed in the atmosphere; however, our results suggest that soil surfaces irradiated with sunlight may also allow for the formation of perchlorate. To evaluate if perchlorate would form when exposed to UV light in a controlled environment, titanium and silica gel were irradiated using UV lamps. Perchlorate was found to form on titanium dioxide (concentrations measured below limit of detection up to 934 µg/kg) and to a lesser extent on silica gel (concentrations measured below limit of detection up to 29.1 µg/kg). Soil samples irradiated with sunlight were also found to produce perchlorate (concentrations measured below limit of detection up to 159 µg/kg); however, production of perchlorate was variable, and did not occur in all soils. These results suggest that perchlorate contributions from soil cannot be ignored when evaluating sources of naturally occurring perchlorate to the environment. The work in Chapter 2 explored concentrations of perchlorate in rainwater across the continental United States. Previous study has indicated that perchlorate in precipitation is significantly (p < 0.001) correlated with concentrations of other ions found in precipitation (Rajagopalan et al, 2009). The primary objective of this chapter was to expand the analysis of environmental factors that contribute to perchlorate in precipitation by examining the impacts of surface ozone, solar radiation, and precipitation data to evaluate perchlorate in precipitation across the United States. We identified temperature, precipitation nitrate, and precipitation pH as variables that are important to the presence of perchlorate in precipitation through regression analysis (r2 = 0.57, p < 0.05). Interpolation surfaces were then created of each these variables in order to obtain values across the United States. These interpolation surfaces were then used in an overlay model with the coefficients determined from the regression analysis resulting in an overlay model estimating perchlorate concentrations across the United States. The perchlorate prediction overlay model was generally consistent with perchlorate concentrations measured in a previous study (Rajagopalan et al, 2009). This work will help to better understand the of movement of perchlorate within the environment.