Occurrence and fate of emerging contaminants in rural endorheic basins: insights from the Gallocanta Lake Basin (Spain)

Endorheic basins with terminal lakes are particularly vulnerable to chemical pollution due to their closed hydrological nature, which promotes the retention and accumulation of contaminants. This study investigates the occurrence, sources, and environmental fate of contaminants of emerging concern (CECs) in the rural endorheic basin of Gallocanta Lake (Spain), one of the largest saline lakes in Europe and a protected Ramsar site. A spatially comprehensive sampling campaign was conducted in September–October 2024 across surface waters, groundwater, treated and untreated wastewater, and terminal water bodies. Non-target screening and semiquantative approaches based on liquid chromatography coupled to high resolution mass spectrometry (LC–HRMS) were applied to characterize complex contaminant mixtures across environmental compartments.

Multivariate analyses revealed clear chemical fingerprints associated with diverse sources, including effluents from rural wastewater treatment plants (WWTPs), untreated wastewater discharges, agricultural runoff, and natural stream waters. Primary-treated WWTP effluents showed variable and often limited removal of CECs, with selective attenuation of some pharmaceuticals but persistence or even enrichment of others, highlighting the limitations of basic treatment in rural settings. Terminal lakes exhibited the highest cumulative contaminant loads, reflecting their role as integrators of upstream pressures.

Particular attention was drawn on the insect repellent N,N-diethyl-meta-toluamide (DEET), due to its exceptional persistence and enrichment in Gallocanta Lake, reaching concentrations (>5,000 ng/L) far exceeding those in inflows (<1,000 ng/L). This pattern, resulting from the combination of compound stability and strong evapoconcentration under semi-arid conditions, will be discussed in detail, illustrating how moderate inputs can lead to disproportionately high accumulation in endorheic lakes.

Overall, the results demonstrate that endorheic basins act as hotspots for the accumulation of persistent CECs and emphasize the need to consider hydrological closure, in-lake processes, and rural wastewater management when assessing contamination risks and designing mitigation strategies.