Isotopes Help Trace the Origin of Urban Water Pollution in Mauritius

Nitrate is a nitrogen compound and an essential nutrient for plants. In excessive quantities, nitrate poses a concern for public health, since it can impair the blood’s ability to transport oxygen around the body. It can also cause algal blooms in lakes and rivers, reducing biodiversity and the ability of aquatic ecosystems to support valuable services, such as tourism and commercial fishing.

In 2016, scientists began working with the IAEA to use isotopic techniques to assess the origin of nitrate pollution after authorities had detected nitrate contamination around Port Louis. Contaminants were found in streams and rivers, threatening protected areas, such as the Rivulet Terre Rouge Estuary Bird Sanctuary. Toxic algal blooms in the ocean caused episodes of fish kill, raising complaints among local fishermen.

The sources for such pollution could have been manifold: inappropriate wastewater disposal systems from households, faulty septic tanks, industrial discharges, animal breeding and agricultural activities. Knowing who or what is responsible for nitrate pollution can help policymakers take informed action to protect rivers and groundwater.

Deciphering the origins of nitrates in waterways can be difficult. “Conventional chemistry will tell you how much nitrogen pollution there is in a river, but not where this pollution comes from,” said IAEA isotope hydrologist Ioannis Matiatos. “Analysing the isotopes of nitrate can give this kind of information.”

With technical assistance, scientists from the National Environmental Laboratory collected chemical and isotopic data in fourteen river stations and fifteen water boreholes around the city, and identified mechanisms influencing water quality around the area. They sampled and analysed nitrogen compounds in Port Louis’ waterways with support of the IAEA’s technical cooperation programme in the form of expert visits, training and equipment. The method they used involved analysing the unique ‘fingerprints’ of nitrate in water molecules by looking at the molecules’ isotopic composition (read more about this on page 5).