Fate of artificial sweeteners in urban aquifers: Evaluation of natural attenuation using mixing models.

Groundwater represents a critical source of freshwater, yet shallow aquifers are increasingly threatened by anthropogenic contamination from urbanized river recharge. Due to their source domestic origin, high mobility in water, resistance to conventional wastewater treatment, widespread consumption, and constant discharge loads, artificial sweeteners (ASs) may serve as good candidate tracers for wastewater inputs in aquatic systems. The occurrence, distribution, and natural attenuation of four ASs, namely acesulfame (ACE), cyclamate (CYC), saccharin (SAC), and sucralose (SUC), were evaluated across the river-groundwater interface of the Besòs River Delta aquifer (NE Spain) under contrasting hydrological regimes. The Besòs River was identified as the primary contamination source, with groundwater AS concentrations exhibiting a progressive downgradient decrease along the flow path. Mixing analysis demonstrated a distinct pattern from dry-period recharge regime dominated by low-flow end-members (93% combined contribution) to a wet-period regime with a substantially higher contribution of rainfall-driven recharge (35% W1 end-member). Natural removal efficiencies (Rabs) and first-order attenuation kinetics revealed distinct seasonal and compound-specific behaviors. Wet conditions enhanced natural attenuation, yielding higher mean Rabs, lower spatial variability, and usually, shorter half-lives across the aquifer. SAC and CYC were the most naturally attenuated, achieving near-complete attenuation under wet conditions (average 99.5% and 98.0%). ACE showed high but transient attenuation (82.7% dry vs. 96.5% wet). In contrast, SUC shows marked persistence and potential aquifer apparent accumulation during low-flow periods (15.6% dry vs. 67.0% wet), maintaining the highest persistence across both seasons (apparent half-lives of 16.0-21.3 d). Depth-dependent contrasts were also observed during dry periods, with deeper groundwater exhibiting higher attenuation. The results demonstrate that hydrological variability is a key control on the natural attenuation of ASs and identify SUC as a conservative tracer of wastewater-derived recharge. This integrated approach improves the assessment of contaminant fate in urban aquifers and supports groundwater quality management.