Drought effects on the occurrence of emerging contaminants in an alluvial aquifer: Implications for groundwater resources management.

Human activities, such as agriculture and reclaimed water discharges, are the main sources of pharmaceuticals (PhACs) and emerging contaminants (ECs) in groundwater. Their occurrence is expected to be linked to the hydrogeological dynamics of the aquifer, especially during severe drought periods when recharge from human sources dominates; therefore, it is paramount to relate groundwater flow dynamics and pollutant occurrence to properly control and manage water resources quality.

This study evaluates the presence of ECs, including PhACs and endocrine disrupting compounds (EDCs) along the Onyar River alluvial aquifer (NE Catalonia, Spain; 295 km²) in two sampling campaigns depicting different hydrological scenarios. The first survey took place in June-July 2021 including the analysis of 45 PhACs in 18 groundwater samples, and the second one in March 2023, after almost two years of a severe drought, focused on 12 groundwater and 10 stream water samples for the determination of 45 PhACs, plus 9 transformation products (TPs) and metabolites, and 32 EDCs. Hydraulic head and hydrochemical data were also collected.

Five PhACs were detected in groundwater (acetaminophen, carbamazepine, hydrochlorothiazide, ibuprofen, and venlafaxine) at 0.6 to 57 ng/L in 2021. Sulfamethoxazole was the only antibiotic found. The second campaign (2023) identified eight PhACs in river samples, including trimethoprim, clindamycin, sulfamethoxazole, sulfapyridine, flubendazole, carbamazepine, citalopram, and venlafaxine, ranging from 5.9 to 81.8 ng/L, whereas in groundwater only six PhACs: sulfamethoxazole, sulfamethazine, sulfadiazine, carbamazepine, venlafaxine, and hydrochlorothiazide were identified at 0.5 to 20 ng/L. TPs and metabolites, such as carbamazepine-10,11-epoxy and carbamazepine-2-hydroxy were only found in river samples, while metoprolol acid was present in both river and groundwater samples. Several EDCs were present in both river and groundwater samples, including tolyltriazole, benzotriazole-1H, bisphenol A, caffeine, methylparabens, TCEP, TBEP, TCPP and estrone at concentrations from 0.3 to 142.2 ng/L.

Such distinct results are influenced by hydrological factors. As a general interpretation supported by head and chemical data, stream water induced aquifer recharge due to groundwater withdrawal is more intensive during drought periods when the water table is lower, as in 2023. This enhances the transport of ECs introduced by reclaimed water inputs towards the aquifer. Conversely, in periods with a higher water table: June-July 2021, the hydrological setup reverses and pollutants introduced by stream recharge, yet relevant, do not reach wells located far away from the drainage network because of larger groundwater recharge and reduced withdrawal irrigation rates. Therefore, a temporal variability of ECs concentration in groundwater is not a matter of uncertainty, but a consequence of observable and predictable changing hydrological conditions. Ignoring hydrological variability in the interpretation of PhACs and ECs will result in erroneous actions about preventing pollution migration and understanding their actual hazard to human and environmental health.

Funding: project EC-FATE, call “MINECO-AEI, PID2022-139911OB-C42” and the Ramon y Cajal contract (RYC2020-030324-I).