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.

The Problem

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).

δ18O and δ15N of nitrate sources

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).

δ11B for earth system materials

Figure 2. Compilation of δ11B ranges for various earth system materials and environmental conditions.


Boron and nitrogen in water

Figure 3. Source signature of boron and nitrogen isotopic ratios in water (Briand et al., 2013).

References

Bourke, S.A., et al. Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments. 2019. Hydrology and Earth System Sciences. 23, pp. 1355–1373

Briand, C.; Plagnes, V.; Sebilo, M.; Louvat, P.; Chesnot, T.; Schneider, M.; Ribstein, P. and Marchet, P., 2013: Combination of Nitrate (N, O) and Boron Isotopic Ratios with Microbiological Indicators for the Determination of Nitrate Sources in Karstic Groundwater. Environ. Chem., vol. 10, pp. 365–369. https://www.publish.csiro.au/en/EN13036. 

Eppich, G.R., Singleton,M.J., Wimpenny, J.B., Yin, Q.-Y., and Esser, B.K., 2013. California GAMA Special Study: Stable Isotopic Composition ofBoron in Groundwater –San Diego County Domestic Well Data, LLNL-TR-533174, 22 p.

Hastings, et al. (2013). “Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts”, Elements, 9(5), 339-344.

Kendall, C., Elliott, E.M., and Wankel, S.D., 2007. Tracing anthropogenic inputs of nitrogen to ecosystems, Chapter 12, In: R.H. Michener and K. Lajtha (Eds.), Stable Isotopes in Ecology and Environmental Science,  2nd edition, Blackwell Publishing, p. 375-449. https://doi.org/10.1002/9780470691854.ch12 

Lasagna, M. and DeLuca, D. A., 2017: Evaluation of sources and fate of nitrates in the western Po plain groundwater (Italy) using nitrogen and boron isotopes. Environmental Science and Pollution Research International, vol. 26, no. 3, pp. 2089-2104. DOI: 10.1007/s11356-017-0792-6 

Ransom, K. M., et al. (2016), Bayesian Nitrate Source Apportionment to Individual Groundwater Wells in the Central Valley by Use of Elemental and Isotopic Tracers, Water Resour. Res., 52, 5577–5597, doi:10.1002/ 2015WR018523.

Vitousek, P. J., et al. Human Alteration of the Global Nitrogen Cycle: Sources and Consequences. 1997. Ecological Applications. 7(3). pp. 737-750

Ward, M. H., et al. Drinking Water Nitrate and Human Health: An Updated Review. 2018. International Journal of Environmental Research and Public Health. 15(7): 1557.

Widory, D.; Petelet-Giraud, E.; Negrel, P. and  Ladouche, B., 2004: Tracking the Sources of Nitrate in Groundwater Using Coupled Nitrogen and Boron Isotopes: A Synthesis. E n v i r o n . S c i . T e c h n o l ., vol. 39, pp. 539-548