Organic micropollutants in the Litani River: Occurrence, Fate, and Impacts in Relation to Microplastic Pollution

Abstract

Freshwater ecosystems worldwide are increasingly threatened by complex mixtures of emerging organic micropollutants and microplastics, notably in areas facing rapid urbanization with limited control on municipal and industrial wastewater discharge. Despite growing concerns, knowledge gaps persist regarding the fate of these organic micropollutants, their combined ecological risks, and microbial responses, particularly in rivers subjected to chronic, multi-source pollution. This work addresses some of these gaps through an integrated, multi-scale assessment of a highly impacted Mediterranean river system, the Litani River in Lebanon, used as a representative model of stressed freshwater environments globally.

The primary objectives of this work were to (i) develop and apply a robust high-resolution mass spectrometry-based suspect screening strategy for broad organic micropollutant characterization in the Litani River, (ii) quantify and evaluate spatial and seasonal variability of target micropollutant in the Litani River and their associated ecological risks in water and sediments, (iii) determine the microbial community structure and functional potential in response to long-term contaminant exposure, and (iv) critically assess the role of polyethylene microplastics in contaminant adsorption and biodegradation processes.

To achieve these objectives, a suspect screening workflow employing full-scan, data-dependent acquisition, and data-independent acquisition was implemented to screen approximately 850 candidate compounds, leading to the identification of 75 OMPs at level 2a confidence and confirmation of 20 compounds using authentic reference standards. Detection frequency analysis highlighted valsartan, climbazole, trimethoprim, N,N-diethyl-meta-toluamide (DEET), and cocaine as the most ubiquitous contaminants, reflecting consumption patterns and highlighting their relevance for quantitative monitoring and prioritization.

Subsequent quantitative analyses of target contaminants in the Litani River revealed pronounced spatial and seasonal variabilities in pharmaceutical and aromatic hydrocarbon concentrations in surface water and sediments. Several pharmaceuticals were detected at high concentrations reaching 23.6 µg/L for caffeine, 4.7 µg/L for ibuprofen, 4.1 µg/L for acetaminophen, and 2.2 µg/L for valsartan. Sediments showed widespread contamination by both pharmaceuticals and hydrocarbons. Spatial patterns revealed pollution hotspots associated with untreated municipal wastewater, hospital, and industrial discharges. Risk assessment identified telmisartan, ibuprofen, diclofenac, climbazole, caffeine, and selected hydrocarbons as priority contaminants posing substantial ecological risks.

At the biological scale, sediment-associated microbial communities were characterized using 16S rRNA gene sequencing, revealing diverse assemblages dominated by Pseudomonadota, Bacillota, and Actinomycetota- taxa commonly reported in sediments worldwide. Significant correlations between microbial community structure and specific contaminants were observed, while functional predictions indicated a predominance of chemoheterotrophic metabolism, reflecting microbial adaptation to sustained organic contaminant pressure and intrinsic potential for natural attenuation.

The role of microplastics in determining the fate of the contaminants, namely pharmaceuticals, was investigated through controlled laboratory experiments using pristine polyethylene microplastics and showed limited sorption of most pharmaceuticals. These results demonstrate that pharmaceutical microplastic interactions may be weaker than previously assumed and are not governed by hydrophobicity alone. Complementary water-sediment microcosm experiments further showed that pristine polyethylene microplastics did not significantly alter pharmaceutical biodegradation kinetics or microbial community dynamics over 63 days incubation period, indicating that intrinsic chemical properties and microbial resilience dominate contaminant fate under environmentally relevant conditions.

Overall, this work advances knowledge by integrating chemical analyses, microbial ecology, and microplastic interactions within a unified framework. Although this study focused on pristine microplastics of a single polymer type, the findings provide mechanistic insight and help establish clear directions for future research, including the investigation of aged microplastics and a broader range of polymer types. The framework and insights developed here are broadly transferable and support improved monitoring, risk assessment, and management of contaminated freshwater systems worldwide.