IDENTIFYING THE SOURCES OF NITRATE IN SURFACE AND GROUND WATERS USING STABLE ISOTOPES: A GLOBAL OVERVIEW

Abstract

Nitrate pollution of freshwater systems is a pressing issue that affects nearly every region of the globe. Excessive levels of nitrate in surface and groundwater environments can cause severe impacts to human and ecosystem health. To effectively mitigate the adverse effects of nitrate pollution, the sources of nitrate to systems must be identified. For identification of nitrate sources, the dual isotope technique (δ15N-NO3 and δ18O-NO3) has been widely used in current research. This thesis aimed to provide a global synthesis of nitrate pollution in surface and groundwater to understand better how nitrate pollution varies spatially around the world with different climates, and land uses. It also assessed the current state of practice of water quality management of nitrate, its relation to nitrate sources in surface and groundwater and the economic complexity of nations. A global database of nitrate in surface and groundwater was produced collating data from available literature, statistical analysis was then performed to assess changes of nitrate concentrations and isotopic compositions with various variables. Next, a stable isotope mixing model was used to determine the sources of nitrate in surface and groundwaters around the world. Finally, the state of practice of water quality management of nitrate and its relation to identified nitrate sources and economic complexity was assessed. The analysis found that nitrate concentrations and isotopic compositions vary significantly between several groups of climate and land use. The modelling results showed that manure + sewage was the most commonly identified source of nitrate around the world in both surface and groundwater across land use and climate types. The importance of selecting site-specific data in stable isotope mixing models was also highlighted. A relationship was found between economic complexity and levels of wastewater treatment, with low development corelating with low levels of wastewater treatment. It also found that pollution of groundwaters by manure + sewage is prevalent regardless of the economic power of a nation. This research highlights that much more still needs to be done to mitigate nitrate pollution of freshwater around the world, not only in developing nations but also in developed nations that depend on groundwater for drinking water supply.