Nutrient Source Tracking – Nitrate in Water

Nitrate is a naturally occurring ion which contains 3 oxygen atoms surrounding a central nitrogen atom. Nitrate is formed naturally by bacterial nitrogen fixation and is therefore found in soils, effluents and surface waters. Nitrate is also produced artificially on a commercial scale, used as a preservative in cured meats and, due to its high solubility and biodegradability, as a carrier for other nutrients in fertilizers.

Beta Analytic’s Nitrate Testing Services

ISO 17025-accredited Beta Analytic provides nitrate concentration and isotopic measurements, which can be used to decipher the source(s), environmental alterations and fate of nitrate in the environment.

Samples are analyzed by the chemical reduction of nitrate to nitrous oxide followed by continuous flow (CF) Isotope Ratio Mass Spectrometry (IRMS) (Casciotti et al., 2002; Foreman et al., 2016; and Altabet and Wassenaar, 2017). Isotope ratio data are reported as delta (δ) values in units of parts per thousand (per mill) (‰) (Coplen, 2011).

Nitrogen isotope ratios are reported relative to N2 in air (Mariotti, 1983) and oxygen isotope ratios are reported relative to VSMOW reference water and normalized on a scale such that δ18OSLAP = -55.5‰ (Coplen, 1994; IAEA, 2017).

The results are also presented graphically on a plot which includes representative areas of the isotopic composition (δ18O and δ15N) of various nitrate sources (Kendall et al, 2007; Hastings et al., 2013).

nitrogen isotopes plot

Source

A nitrate ion naturally contains different isotopes of oxygen and nitrogen, the most common being oxygen-16 and nitrogen-14 and the rare being oxygen-18 and nitrogen-15. As nitrate is formed and utilized naturally or artificially, the mass of the nitrate ion varies.

These differences are expressed as delta values and are compared to internationally accepted standards.
For example: 

Alterations

A nitrate ion in nature will likely be altered, for example by denitrification, a process by which bacteria utilizes NO3 . During denitrification, microbes preferentially target the nitrogen-14 and oxygen-16 bearing nitrate ions, leaving the residual nitrate enriched with the oxygen-18 and nitrogen-15 isotopes. Nitrification has the opposite effect.

Impact

Source, oxygen availability, pH and land use all play a part in the isotopic values of nitrate and should be considered when analyzing isotopic data. The isotopes of nitrate are a useful tool in understanding where nitrate is originating from and what processes the ion had undergone before reaching your sample vial.

Beta Analytic performs fast, ISO 17025-accredited oxygen and nitrogen isotope measurements using an isotope ratio mass spectrometer (IRMS).

Why is an excess of nitrate a concern?

The overuse of nitrate-based fertilizers, natural and artificial, in this past century has resulted in an abundant amount of nitrate in top soils, which is then transported into the water table via infiltration. High concentrations of nitrate in drinking water can be toxic (10mg/L or higher EPA, 2018), starving blood of oxygen via methemoglobinemia.

Furthermore, high levels of nitrate in surface waters will cause an overstimulation of algal growth. These algal blooms block sunlight and create anoxic zones (very low concentrations of dissolved oxygen) which create an uninhabitable environment for marine life. Eutrophication can have large negative impacts on marine and fishery industries.


References:

Altabet, M.A. and L. I. Wassenaar. New Chemical methods for the precise determination of nitrate isotopic composition Presented at Chemical Oceanography at the 86th annual Gordon Research Conference; 2017 July, 23-28; Colby-Sawyer College, New London, NH.

Aravena, R., Evans, M. L., & Cherry, J. A. (1993). Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems. Groundwater, 31(2), 180-186.

Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Anal. Chem, 74(19), 4905-4912.

Coplen, T. B., 1994. Reporting of Stable Hydrogen, Carbon, and Oxygen Isotopic Abundances, Pure and Applied Chemistry, v. 66, p. 273-276.

Coplen, T. B., 2011, Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results. Rapid Communications in Mass Spectrometry, v. 25, 2538–2560. 

Florida Department of Health Fact Sheet – Chemicals in Private Drinking Water Wells

Foreman, R. K., Segura-Noguera, M., & Karl, D. M. (2016). Validation of Ti (III) as a reducing agent in the chemiluminescent determination of nitrate and nitrite in seawater. Marine Chemistry, 186, 83- 89.

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

Hosono, T., Tokunaga, T., Kagabu, M., Nakata, H., Orishikida, T., Lin, I. T., & Shimada, J. (2013). The use of δ15N and δ18O tracers with an understanding of groundwater flow dynamics for evaluating the origins and attenuation mechanisms of nitrate pollution. Water Research, 47(8), 2661-2675.

International Atomic Energy Agency (IAEA), Reference Sheet for International Measurement Standards, 2017

Isotope Tracers in Catchment Hydrology (1998), C. Kendall and J. J. McDonnell (Eds.). Elsevier Science B.V., Amsterdam. pp. 519-576.

Kendall, C., Elliott, E.M., and Wankel, S.D. (2007), “Tracing anthropogenic inputs of nitrogen to ecosystems.” In Stable Isotopes in Ecology and Environmental Science, 2nd ed., Ch. 12.

Li, S. L., Liu, C. Q., Li, J., Xue, Z., Guan, J., Lang, Y., … & Li, L. (2013). Evaluation of nitrate source in surface water of southwestern China based on stable isotopes. Environmental Earth Sciences, 68(1), 219-228.

Mariotti, A., 1983, Atmospheric nitrogen is a reliable standard for natural 15N abundance measurements: Nature, v. 303, p. 685-687.

US EPA Fact Sheet – Preventing Eutrophication: Scientific Support for Dual Nutrient Criteria

US EPA National Primary Drinking Water Regulations