Using Boron Isotopes to Enhance Nitrate Source Tracking – Tracing with Triple Isotopes (δ11B, δ15N, δ18O)
Natural denitrification and mixing processes may extensively alter the concentration of dissolved nitrate and its isotopic signature (δ15N and δ18O) in ground and surface waters. This can make the differentiation of urban and agricultural origins of nitrate very challenging. As δ11B is not affected by fractionation, the isotopic ratio of boron combined with oxygen and nitrogen isotopic ratios of nitrate prove to be a very powerful tool for tracing contaminant sources.
Nitrate water pollution is a major ecological concern that has been heightened by the introduction of anthropogenic sources of nitrogen into the cycle, e.g., fertilizers, sewage (wastewater), and animal manure. Elevated concentrations of nitrate in water pose a global human health problem through contaminated drinking water (Ward et al., 2018) and cause eutrophication of water systems which is detrimental to many organisms (Vitousek at al., 1997). The ability to identify and quantify the sources of nitrate in water plays an important role in managing and mitigating pollution. Measuring the δ18O and δ15N of nitrate in water has been demonstrated to be a reliable nitrogen source tracking tool (Kendall et al., 1997). Fertilizers, soil, and manure and sewage have characteristic nitrate isotopic signatures, however, the ranges overlap making it difficult to distinguish discrete sources (Figure 1) . Differentiating between manure and sewage is particularly difficult as the characteristic nitrate isotopic signatures for these sources are indistinguishable. Additionally, denitrification and nitriﬁcation of organic compounds cause isotopic fractionation further obscuring determination of the original nitrate source(s) (Bourke et al., 2019).
Figure 1. Representative ranges of δ18O and δ15N of various nitrate sources (Kendall et al, 2007; Hastings et al., 2013).
Boron to the Rescue
The aforementioned limitations can be overcome through the use of an additional complementary tracer such as boron. Boron is naturally derived from the weathering of rocks and is a common constituent of ground and surface waters. Boron is also commonly used in detergents, cosmetics, agricultural products and is present in animal manure. This results in a wide range of B isotope ratios and corresponding sources. Studies have shown that boron is substantially enriched in most nitrate contamination sources, and has the advantage of exhibiting distinct isotopic signatures for wastewater and animal manure (Figure 2) (Briand, et al. 2013; Eppich et al., 2013; Lasagna and DeLuca, 2017; Ransom et al., 2016; Widory et al., 2004) Further, boron is not affected by oxidation/reduction and biological reactions involving nitrogen compounds. Thus, the use of the isotopic ratio of boron (δ11B) combined with O- and N-isotopic ratios of nitrate (δ18ONO3, δ15NNO3) proves to be a very powerful tool for tracing contaminant sources, particularly effective at distinguishing sewage from animal waste (Figure 3).
Figure 2. Compilation of δ11B ranges for various earth system materials and environmental conditions.
Figure 3. Source signature of boron and nitrogen isotopic ratios in water (Briand et al., 2013).
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