Drivers of Contaminants of Emerging Concern (CECs) concentration in aquifers and rivers of the Besòs basin (Catalunya, Spain)

Contaminants of emerging concern (CECs) are increasingly detected in European waters, yet they are rarely studied jointly across wastewater, surface water, and groundwater at basin scale, limiting understanding of groundwater vulnerability. CECs comprise a heterogeneous group of mainly anthropogenic compounds, including pharmaceuticals, personal care products, food additives, pesticides, veterinary and industrial chemicals, and their transformation products. Here, we investigated the occurrence of 120 CECs in wastewater treatment plant (WWTP) effluents (n = 10), rivers (n = 27), and groundwater (GW; 42 wells and 6 springs) across the Besòs catchment (NE Spain). We combined non-parametric correlation analysis, principal component analysis (PCA) of major ions and CECs, and grouping strategies to assess shared sources and attenuation mechanisms across basin compartments.
  Domestic and industrial CECs reached higher concentrations than pesticides across all compartments, and compounds ubiquitous in rivers and WWTPs were also detected basin-wide in GW. PCA of major ions showed that most river sites downstream of WWTPs plot between WWTPs and GW along the first two principal components (60–80% of ionic variance per sub-basin), with higher K and PO₄, whereas GW is characterized by higher Ca+Mg relative to Na+K and higher NO₃.
  In most river sites and in 62% of groundwater, total CEC pollution was largely explained by a small group of 16 compounds with high detection frequency in rivers and peak concentrations exceeding 200 ng L⁻¹ in at least one river sample. In these sites, attenuation was primarily inferred to result from mixing between unpolluted waters and WWTP-impacted flows, with additional non-conservative processes. This interpretation is supported by systematic decreases in individual CEC concentrations from rivers to groundwater while maintaining CEC:sucralose ratios comparable to, but lower than, those measured in WWTP effluents. Consistent with PCA results, the ubiquitous occurrence of these compounds in groundwater was not accompanied by major ion shifts in 50–70% of sites per sub-basin, indicating dilutionof small volumes of polluted water with larger volumes of resident groundwater.
  In contrast, 38% of groundwater sites showed total CEC pollution dominated by a larger group of 53 compounds with lower ubiquity and peak concentrations in surface water, including 23 CECs below detection limits in WWTPs and most river sites. Many of these compounds exhibited higher CEC:sucralose ratios in groundwater than in surface waters, suggesting inputs from subsurface sources rather than recent inflow from losing river reaches. At these sites, weaker relationships between CEC concentrations and hydrophilicity (log Dₒw) indicate reduced sorption control and a greater influence of desorption or legacy release from organic matter. PCA of selected CECs further showed co-variation among compounds sharing similar river ubiquity and peak concentration, highlighting common attenuation and release controls at sub-basin scale.
  Overall, mixing, sorption, and degradation emerge as key processes controlling CEC persistence and protecting groundwater quality at basin scale under chronic wastewater pressure.

Acknowledgements: Financial support from MCIU/AEI/10.13039/501100011033, the European Union and NextGenerationEU/PRTR through grants CEX2018-000794-S, PCI2024-153452 (WATER4MED project, PRIMA) and CNS2023-144051. Doctoral fellowship from ”la Caixa” Foundation (ID 100010434), code B006133.