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Nitrogen from a variety of sources can make its way into groundwater and waterways. A certain level of nitrogen is naturally present in the environment and nitrate in low quantities are necessary nutrients. However, the high levels of nitrate found in major anthropogenic contributors such as agricultural runoff, nitrogen-based fertilizers, animal manure and sewage pose a problem.
These various nitrogen sources have known isotopic signatures, thus measuring δ18O and δ15N of dissolved nitrate in water samples can help identify the source. Knowledge of the source is required to address the primary causes of contamination.
High concentrations of nutrients in the water table can cause drinking water to become toxic. High levels of nutrients in surface waters will cause overstimulation of algal growth, such as blooms. These blooms block sunlight and create anoxic zones (very low concentrations of dissolved oxygen) which create an uninhabitable environment for marine life, often referred to as eutrophication.
Eutrophication can have large negative impacts on marine and fishery industries.
Nonpoint source refers broadly to sources of contamination from many diffuse sources such as suburban developments, cities and fertilizers. Nonpoint source is usually driven by runoff events such as snow melts and rain.
Point source refers to a discrete, traceable source of pollution that is not driven by runoff.
Being able to identify sources of pollution is the first step in remediation.
High nitrate content in drinking water poses a concern for public health, particularly for infants as it can impair the blood’s ability to transport oxygen around the baby’s body – a condition known as methemoglobinemia or blue baby syndrome.
Algal blooms caused by high levels of nitrate can also have severe economic consequences resulting from losses in the tourism and commercial fishing sectors. The value of real estate through waterfront property value decline affects local economies and communities.
Nitrogen in water can originate from both natural and anthropogenic sources. Natural sources contribute minimally to high levels of nitrate in water compared with the agricultural sector, for example.
The overuse of nitrogen-based fertilizers, use of animal manure, and seepage from wastewater and sewage are major contributors. Nitrate is particularly mobile through both water and soil, thus excess nitrate from sewage, agricultural fertilizers or intensive farming easily makes its way into underground aquifers and surface waters.
Nutrient source analysis is often used by water managers to track the source(s) of nitrate after a contamination event.
The different sources of nitrate mentioned above have characteristic oxygen (δ18O) and nitrogen (δ15N) isotopic signatures. Therefore measuring δ18O and δ15N values for dissolved nitrate extracted from water can provide insight on the origin of the nitrate. Data on source is crucial for managing, tracking and mitigating further nitrate contamination of waterways.
Measuring the isotopic signature over time can also help identify when denitrification is taking place. During denitrification, nitrogen oxides like nitrate are reduced and eventually form nitrogen gas, causing isotopic fractionation. The occurrence of denitrification processes can therefore be identified in the oxygen and nitrogen isotope results.
Beta Analytic performs fast, ISO 17025-accredited oxygen and nitrogen isotope measurements using an isotope ratio mass spectrometer (IRMS). The interpretation of δ18O and δ15N results lies with the submitter as familiarity with local hydrological and environmental systems is required.
British Geological Survey. (2017). Nitrate Contamination in Groundwater. Available at: http://www.bgs.ac.uk/research/groundwater/quality/nitrate/home.html (accessed December 2017).
Eppich, G. R., Singleton, M. J., Roberts, S. K., Wimpenny, J. B., Derbueis, E., Moran, J. E., Esser, B. K. and Yin, Q. (2012). Source determination of anthropogenic NO3 in groundwater by analysis of d15N, d18O and d11B: A case study from San Diego County, California. Groundwater Resources Association of California.
EWG. (1996). Pouring it on: Nitrogen use and sources of nitrate contamination. Available at: https://www.ewg.org/research/pouring-it/ (accessed December 2017).
US EPA Fact Sheet – Preventing Eutrophication: Scientific Support for Dual Nutrient Criteria
Water Research Center. (2014). Nitrates and Nitrites in Drinking Water and Surfacewaters. Available at: https://www.water-research.net/index.php/nitrate (accessed December 2017).