Elucidating groundwater anthropogenic contamination sources in an agricultural area impacted by urban stressors — A multi-isotope approach combined with emerging organic compounds

Isotopic tracers have been employed to investigate anthropogenic impacts on groundwater quality. Nevertheless, their effectiveness is limited by the overlap of isotopic fingerprints of some anthropogenic sources and by the influence of subsurface processes altering the initial isotopic signals of sources. To overcome such challenges, this research integrates a multi-tracer approach combining environmental (δ¹⁵N_NO3, δ¹⁸O_NO3, and δ¹¹B) and hydrologic (δ¹⁸O_H2O and δ²H_H2O) isotope tracers with 126 emerging organic compounds (EOCs) to acquire an enhanced understanding of the anthropogenic contamination within a groundwater system. This approach is applied to a Mediterranean aquifer underlying an economically important agricultural area affected by anthropogenic stressors. Nitrate, which is used as an indicator of anthropogenic contamination, has concentrations up to 180 mg/L in groundwater. By coupling δ¹⁵N_NO3 to δ¹¹B, nitrate mainly originates from manure used on agricultural lands without evidence of inputs from inorganic fertilizers or human wastes. Nonetheless, 68 of the 126 EOCs are detected with concentrations >10 ng/L in water samples, including EOC of human origin. This unveils an anthropogenic influence through groundwater-surface water interaction, which is supported by observation from δ¹⁸O_H2O and δ²H_H2O values. Exceptionally, the EOC bisphenol A was detected in all water samples with high concentration of up to 123,000 ng/L in groundwater, indicating the influence of plastic-derived contamination. This is suspected to originate from plastic pipes used in the irrigation activities, from untreated urban wastewaters discharged in the rivers crossing the study area, from possible contaminated leachate derived from a neighboring landfill, or from livestock manure, but these inferences require additional research. As the environmental isotope tracers used in this study are not able to identify some of EOC sources, this study highlights therefore the integration of EOCs with common environmental isotope tracers as an effective approach for identifying different anthropogenic sources affecting groundwater quality.