Improving understanding of nitrate sources in connected river and groundwater systems through linking nitrate isotopes and contaminants of emerging concern

Increased nutrient levels in aquatic systems can trigger issues such as eutrophication, water quality degradation, and algal blooms that have negative environmental and economic impacts. Nitrate contamination makes water unconsumable hence, reducing access to drinking water - a key factor of well-being as recognized in the UN Sustainable Development Goals. Nitrate source identification remains challenging using hydrochemical measurements, but the analysis of stable isotopes in nitrate (δ¹⁵N and δ¹⁸O) opened the possibility to track sources and processes. The efficiency of this isotopic approach lay in simple and precise field and analytical methods, with low cost and easy sample preparation. Despite the potential and usefulness, nitrate isotopes on their own cannot differentiate closely related sources of nitrate contamination with overlapping isotopic signatures, such as sewage (human sources) and manure (agricultural sources), as well as treated versus raw sewage inputs from the catchment. One solution is to combine isotopic techniques with analysis of compounds of emerging concern (CECs). Some CECs are ideal chemical markers of faecal contamination (sewage or manure) as they are usually linked to a specific source. They are ubiquitous in that source and are persistent and present at detectable concentrations in contaminated environmental samples. Their high solubility in water and low volatility facilitates their use as tracers for components originating in sewage and manure. Our study is focused on the innovative approach of combining stable isotopes with CECs in surface and groundwater to improve nitrogen source tracking and source delineation, and more precise quantification of groundwater/surface water interaction. As a proof of concept, we have combined stable isotopes of nitrate with CECs in surface water (large European river) and groundwater (shallow aquifer) case studies. Preliminary results provide a unique and versatile framework for expanding the use of isotopic techniques to tracing nitrate pollution sources and assessing water quality in the catchments worldwide.