Perchlorate (ClO₄^-) has both synthetic and natural sources. Stable isotope analysis of O (δ¹⁸O, δ¹⁷O) and stable (δ³⁷Cl) and radioactive (³⁶Cl) isotope analysis of Cl in ClO₄^- allows natural and synthetic ClO₄^- to be distinguished from each other and provides information on the potential origin(s) of natural ClO₄^-. The goal of this research effort was to develop an improved understanding of (1) the distribution and isotopic characteristics of natural ClO₄^- worldwide, (2) the mechanisms of natural ClO₄^- production, and (3) the contributing processes resulting in the ubiquitous distribution of this anion and its stable isotope characteristics in soils, groundwater, and vegetation. 

In order to achieve these goals, several different laboratory and field research tasks were undertaken. These objectives and tasks were designed to fill existing data gaps on the production, distribution and isotopic characteristics of natural ClO₄^-. The first tasks were designed to improve our understanding of ClO₄^- distribution, significant environmental ClO₄^- production mechanisms and the stable isotopic signature of the produced ClO₄^-. These tasks included the following: (1) determination of the isotopic signature of ClO₄^- produced by O₃ and ultra violet (UV) oxidation of Cl^- and relevant ClO_x species; (2) development of an improved understanding of the occurrence and isotopic signature of natural ClO₄^- and its relationship with NO₃^- and other anions in arid and semi-arid environments worldwide; and (3) evaluation of the potential for microbial production of ClO₄^-. A second set of tasks were designed to better understand plant accumulation of environmental ClO₄^-, whether its isotopic signature is altered during uptake and accumulation, and/or whether plants can generate ClO₄^- via an O₃-mediated mechanism.

The results from a range of different laboratory studies evaluating ClO₄^- formation mechanisms confirm that there are multiple potential pathways of ClO₄^- generation from both UV-photolysis and O₃-mediated oxidation of Cl^- and other ClO_x precursors.  Laboratory studies were successful in producing Cl and O isotopic variations in ClO₄^- that incorporate much of the reported stable isotope variation in natural ClO₄^-. Data indicate that final ClO₄^- isotopic composition is dependent on the precursor species oxidized. The reaction rates and intermediate species proposed to be involved in ClO₄^- formation require further study and additional experiments are required to resolve the reason for the enigmatic low δ³⁷Cl values of Atacama ClO₄^- (Chile), but significant progress was made in constraining pathways of ClO₄^- production in nature by application of stable isotope analysis.

Soil and groundwater sampling was conducted worldwide (including the continental US, South Africa, South America, China, Antarctica, and the Middle East) to evaluate concentrations of ClO₄^- and its relationship with common co-occurring anions such as Cl^- and NO₃^- among others. The data indicate that ClO₄^- is globally distributed in soil and groundwater in arid and semi-arid regions on Earth at concentrations ranging from 10^-1 to 10⁶ µg/kg. Generally, the ClO₄^- concentration in these regions increases with aridity index, but this also depends on the duration of arid conditions. In many arid and semi-arid areas, NO₃^- and ClO₄^- co-occur at consistent ratios (NO₃^-/ClO₄^-) that vary between ~10⁴ and ~10⁵. This is not the case for Cl^-/ClO₄^- ratios, which vary widely among locations. The NO₃^-/ClO₄^- ratios are largely preserved in hyper-arid areas that support little or no biological activity (e.g. plants or bacteria), but can be altered in areas with more active biological processes.

Stable isotope analysis of Cl and O and radioactive isotope analysis of ³⁶Cl in natural ClO₄^- confirmed and extended initial data suggesting that indigenous ClO₄^- sources in the southwestern U.S. show some isotopic variation by location and environment. ClO₄^-  concentration and isotope analysis was conducted in all five of the North American Great Lakes. The data showed average ClO₄^- concentrations ranging from 0.05 to 0.13 µg/L (varying by lake) with concentrations being nearly constant with depth. Interestingly, the overall ranges of stable isotopic compositions of Great Lakes ClO₄^- resemble those of indigenous natural ClO₄^- measured in groundwater of the western USA indicating a predominantly natural atmospheric source of ClO₄^- in all of the lakes.  Bomb-pulse ³⁶Cl is largely retained in Lake Superior because of the 191-yr water residence time in the lake.

A variety of plant species were also evaluated for their potential to accumulate and even generate ClO₄^- via O₃-mediated processes. Plants, particularly in arid environments, may contain abundant Cl- in their tissues; display a vast array of hydrated internal and external reaction surfaces; and catalyze a multitude of redox reactions that could be involved in biosynthesis of ClO₄^-. These factors, the ubiquitous distribution of plants, and the post-industrial increase in O3 exposure are consistent with the possibility that tropospheric O3 may induce biosynthesis of ClO₄^- from Cl- in plants. This was evaluated as a potentially novel source of ClO₄^- in the environment.

The impact of plant accumulation of ClO₄^- on Cl and O stable isotope values was also evaluated in both hydroponic laboratory studies and field crops grown in different parts of the U.S. In hydroponic studies with snap beans, no substantial differences were observed in the δ ³⁷Cl, δ¹⁸O, or Δ¹⁷O values of ClO₄^- between the growth solutions and leaf extracts. In contrast to ClO₄^-, δ¹⁵N of NO₃^- in plant tissue was fractionated substantially (~10-20‰).  The ε¹⁵N/ε¹⁸O ratios of 1.04 ‒ 1.07 support previous experimental studies showing similar ratios via assimilatory nitrate reductase. The data indicate that plants do not metabolize and assimilate ClO₄^- similarly to NO₃^-. Similar to hydroponically grown plants, field grown plants exposed to environmentally relevant ClO₄^- concentrations also did not appear to affect foliar ClO₄^- isotopic composition. 

Overall, the results of this project have provided important new information on natural ClO4- in the environment. Significant progress was made concerning potential mechanisms of its formation, isotopic characterization of natural ClO4- sources in groundwater, lakes, soils and plants, and its worldwide occurrence and accumulation in arid and semi-arid environments. The data support previous studies showing that natural and synthetic ClO4- can be differentiated by stable isotope methods, and suggest for the first time that the source(s) of ClO4- in food crops may be determined by isotopic analysis of ClO4- in plant tissue. (Project Completion - 2017)