Global patterns and environmental controls of perchlorate and nitrate co-occurrence in arid and semi-arid environments
Introduction
The oxyanion perchlorate (ClO4−) has received increasing attention due to its widespread occurrence on Earth and Mars, and yet its distribution and relation to other more understood atmospheric species are poorly defined. Terrestrial ClO4− is largely produced in the atmosphere and deposited in dry and wet deposition (Rajagopalan et al., 2009, Andraski et al., 2014). It is abiotically stable in most near-surface environments but can be irreversibly reduced biologically under anoxic conditions. In these respects ClO4− is similar to NO3−, although NO3− may be biologically reduced preferentially to ClO4− in mixed redox conditions. Major differences between the two species include a biological production mechanism for NO3− (nitrification) and the assimilation of NO3− by plants, from which N may be removed from the NO3− reservoir into stored organic matter or returned through subsequent nitrification. The terrestrial NO3− mass balance is complicated further by varying amounts of N2 fixation, which may result in net changes to the free NO3− reservoirs in soils and groundwaters. Given these attributes, NO3− and ClO4− should co-occur in arid environments. In the driest and coldest locations, where biological activity is minimal, the ratio of NO3−/ClO4− should be similar to that of total atmospheric deposition; whereas in less arid environments, NO3−/ClO4− ratios could be higher or lower than the deposition ratio, depending on the relative importance of net biologic NO3− (or N) addition or removal. Further, the isotopic composition of NO3− in relation to the NO3−/ClO4− ratio should be consistent with the net effects of mixing of atmospheric and biogenic NO3−, commensurate with the degree of assimilation and reprocessing of the NO3− atmospheric fraction. Such a conceptual model can be used to evaluate environmental conditions under which atmospherically deposited species accumulate and the net effects of soil processes on these species.
Until recently, ClO4− was considered to be present in the environment largely from military and commercial sources, but its occurrence in pre-industrial soils and groundwater in the Atacama Desert (Ericksen, 1981), Antarctic Dry Valleys (Kounaves et al., 2010, Jackson et al., 2012), Mojave Desert and Southern High Plains (Jackson et al., 2010), Middle Rio Grande Basin (Plummer et al., 2006), as well as on the surface of Mars (Hecht et al., 2009, Glavin et al., 2013), all demonstrate that ClO4− forms naturally. Isotope data including Δ17O and 36Cl/Cl values of natural ClO4− indicate that it is produced largely in the stratosphere from oxidative reactions of chloro-oxyanions by O3 oxidation and/or perhaps UV-mediated photo-oxidation (Bao and Gu, 2004, Sturchio et al., 2009, Jackson et al., 2010). ClO4− is deposited at the Earth’s surface by wet and dry atmospheric deposition. Modern ClO4− wet deposition rates measured in North America averaged 64 mg/ha-year (Rajagopalan et al., 2009) and total deposition rates measured in the Amargosa Desert (southwestern Nevada) over a 6-year period averaged 343 mg/ha-year (Andraski et al., 2014). Accumulations of ClO4− (93–1050 g/ha) have been observed in deep unsaturated-zone salt bulges throughout the southwestern United States (U.S.) that accumulated during the late Quaternary, roughly over the last 100,000–10,000 years based on Cl− deposition rates and inventories (Rao et al., 2007).
ClO4− is abiotically unreactive under typical terrestrial conditions but can be microbially (Archea and Bacteria) reduced under anoxic conditions as an electron acceptor (Coates and Achenbach, 2004, Liebensteiner et al., 2013). Reduction of ClO4− can be coupled to oxidation of various electron donors including organic matter, sulfide, and H2. In electron donor-limited environments, the presence of NO3− at greater concentrations has been shown to inhibit ClO4− reduction (Tan et al., 2004a, Farhan and Hatzinger, 2009). The capacity for ClO4− reduction appears to be common and has been demonstrated in a number of environments including Antarctic Dry Valley lakes (Jackson et al., 2012). Plants accumulate ClO4− primarily in transpiring tissue (e.g. leaves) (Jackson et al., 2005, Voogt and Jackson, 2010), and do not generally appear to transform it substantially (Tan et al., 2006, Seyfferth et al., 2008), although this may occur in some cases (Van Aken and Schnoor, 2002). Bioaccumulated ClO4− can be returned to the environment by senescence and leaching of plant tissue (Tan et al., 2004b, Tan et al., 2006, Andraski et al., 2014).
The largest and best known occurrence of terrestrial ClO4− is in the NO3− deposits of the Atacama Desert (Ericksen, 1981, Pérez-Fodich et al., 2014). Elevated ClO4− concentrations also have been reported in the clay hills near Death Valley (Jackson et al., 2010, Lybrand et al., 2013), subsurface salt accumulations in the southwestern U.S. (Rao et al., 2007), surface soils of the Dry Valleys of Antarctica (Kounaves et al., 2010), and surface soils in northeastern China (Ye et al., 2013) and the Amargosa Desert (Andraski et al., 2014). These occurrences have been interpreted to be of natural origin based on the distribution and age of the accumulations and /or the stable isotopic composition of the ClO4− and coexisting NO3−. ClO4− has also been reported in groundwater and surface water in many areas including China, India, South Korea, Antarctica, and the U.S. (Plummer et al., 2006, Rajagopalan et al., 2006, Quinones et al., 2007, Parker et al., 2008, Kannan et al., 2009, Ye et al., 2013). Some of these occurrences could be related to human activities including release of electrochemically produced ClO4− and distribution of NO3− fertilizers containing ClO4− from the Atacama Desert. Other occurrences are interpreted to be indigenous based on location, groundwater age, and/or stable isotopic composition (Plummer et al., 2006, Rajagopalan et al., 2006, Jackson et al., 2010, Jackson et al., 2012, Kounaves et al., 2010).
Here we present new data for natural ClO4− occurrences in selected globally distributed settings and an overview of the factors controlling ClO4− accumulation in the environment. We measured indigenous ClO4− concentrations in soil/caliche and groundwater samples from a variety of arid and semi-arid locations in Antarctica, Chile, China, southern Africa, United Arab Emirates (UAE), and the U.S. We also investigated the relationship between ClO4− and co-occurring Cl− and NO3− as well as the sources and sinks of NO3− based on isotope data (δ15N, δ18O, and Δ17O). Our objectives were to evaluate the occurrence and fate of ClO4− in arid environments and the relationship of ClO4− to the better studied atmospherically deposited species NO3− and Cl− as a means to understand the prevalent processes that affect the accumulation of these species over various time scales. We developed a conceptual model of ClO4− occurrence in relation to NO3− that incorporates the overall impact of biological processes to the co-occurrence of these important oxyanions. Our results firmly establish the widespread global occurrence of ClO4−, provide insights about environmental conditions controlling its distribution, and appear to indicate a new approach for evaluating arid-region biogeochemistry with particular relevance to NO3− processing and Cl− cycling. Our results also may contribute to understanding the prevalence of ClO4− on Mars and its implications to the co-occurrence of NO3− and possible extraterrestrial biologic impacts on these species (Stern et al., 2015).
Section snippets
Materials and methods
Soil, unsaturated subsoil, caliche-type salt deposits, and groundwater samples were collected for this study or obtained from archived samples of previous studies from sites in the U.S., Southern Africa (Namibia, South Africa, and Botswana), UAE, China, Chile, and Antarctica (Fig. 1). Sample sites are described below and summarized in Table 1, Table 2. All samples were analyzed for Cl−, NO3−, and ClO4− concentrations and a subset was evaluated for NO3− stable isotopic composition as described
Results and discussion
Results are summarized for all samples in Fig. 2, Fig. 3 and Table 1, Table 2 to highlight major global patterns and processes affecting the distribution of ClO4− and NO3−. Results for individual sites and sample types are then presented in more detail in Fig. 4, Fig. 5 to facilitate discussion of local patterns, controls, and exceptions to the global patterns.
Conclusions
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 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. These ratios are largely preserved in hyper-arid areas that support little or no biological
Acknowledgements
This work was supported by the Strategic Environmental Research and Development Program (SERDP Project ER-1435) of the U.S. Department of Defense; the U.S. Geological Survey Toxic Substances Hydrology Program, National Research Program, Groundwater Resources Program, and National Water Quality Assessment Program; the U.S. Antarctic Research Program[NSF]; and the Abu Dhabi Emirate National Drilling Company. CL acknowledges additional support from grants ICM P05-002 and PFB-23 to the IEB. We are
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2021, CatenaCitation Excerpt :Extensive research in the last decades has established that ClO4− is also formed naturally, probably in the atmosphere, involving reactions of common chlorine species (chloride) through a variety of chemical reaction mechanisms (Bao and Gu, 2004; Dasgupta et al., 2005; Sturchio et al., 2009; Jackson et al., 2010; Rao et al., 2010). Naturally formed ClO4− is readily removed from the atmosphere through deposition and enters into the surface environment such as glaciers and ice sheets (Furdui and Tomassini, 2010; Jiang et al., 2016; Cole-Dai et al., 2018), soil (Jackson et al., 2015a; 2015b), and water systems (Plummer et al., 2006; Rajagopalan et al., 2006). Nitrate deposits from the Chilean Atacama Desert is well-known as containing atmospherically produced ClO4−, based on the evidence of isotopic composition of ClO4− (36Cl/Cl ratios and oxygen-17 anomaly) (Bao and Gu, 2004; Sturchio et al., 2009).