Identification of riverbank filtration sites at watershed scale: A geochemical and isotopic framework

Sampling methods may drive short-term groundwater nitrate variability in an irrigated watershed connected to a coastal lagoon (Campo de Cartagena-Mar Menor, SE Spain)

This study highlights concerns regarding the reliability of groundwater nitrate data used in official surveys, such as within the EU-mandated Water Framework Directive (WFD). The focus is on the Campo de Cartagena - Mar Menor hydrosystem in Spain, a region known for its intensively irrigated watershed and eutrophicated lagoon, where monitoring the evolution of nitrate contamination in surface and groundwater is crucial but challenging due to the risk of inconsistent characterization leading to erratic remediation measures. The study employed an experimental approach in private wells that belong to a longstanding official nitrate survey network marked by irregular sampling practices. Importantly, these wells lacked comprehensive design documentation and were frequently used by farmers. The study aimed to evaluate the representativity of dissolved nitrate measurements in such an emblematic case, while investigating the source of the water using geochemical and isotope tracers. This assessment considered the effects of different sampling techniques (bailer or pumping) and sampling parameters (depth and time), acknowledging actual practices. The research highlights several key findings. Firstly, the bailer sampling method proved to account for a substantial portion of the observed variation in nitrate content. Secondly, in some cases, pumping introduced contributions from different water horizons, complicating the interpretation of nitrate data. Thirdly, alterations in the sampling protocol had a notable impact on the resulting nitrate measurements. Furthermore, the study emphasized a critical issue: the lack of analytical uncertainty estimation in official surveys introduces significant bias in result interpretation, with discrepancies exceeding 100 mg/L in four of the six wells analyzed. This underscores the pressing need for improved sampling protocols, dedicated borehole infrastructure and precise data interpretation. Given the potential unreliability of some official groundwater nitrate data shared under EU or other regulations, with corresponding economic and environmental impacts, the study recommends meticulous verification before transmitting data.