Urban River Environments: Monitoring Antibiotics using Passive Samplers (POCIS) and Fugacity Modelling Techniques
Authors:   , Harrower Jamie 
Published: July 2024

Humans have become reliant on antibiotics in modern day life to treat many common bacterial infections, medical conditions, and also for use in major surgical procedures. However, after consumption, a fraction of these drugs is excreted by the patient, and such residues are not fully removed by WWTP. In recent decades, concerns have been raised over the presence of antibiotics in natural environments, where they may act as potential drivers for antimicrobial resistance. Advancements in analytical capabilities such as the application of Liquid Chromatography Mass Spectrometry (LCMS) have allowed the detection of antibiotics in complex matrices such as surface waters and wastewater treatment effluents at very low levels (ngL-1 – ugL-1). Monitoring antibiotics in field studies is challenging due to the intensive sampling procedures required, where spot sampling is the most commonly used method. Spot sampling however is unable to detect spikes in concentrations or take account of diurnal variation, which can lead to over- or underestimation of the total load. Application of Polar Organic Chemical Integrative Samplers (POCIS) is an alternative, in which devices containing a specific sorbent material sandwiched between a permeable membrane, housed in metal cages, are deployed in the aquatic environment. This allows contaminants in flowing systems such as rivers to sequestrate onto the sorbent, where following extraction and analysis using LCMS a Time Weighted Average (TWA) concentration can be determined. Application of fugacity modelling techniques can estimate the fate of antibiotics between different environmental compartments in a river system. This, together with monitoring data, can potentially provide a powerful tool to determining the concentrations of antibiotics, and hence the risk they pose to the environment. Building fugacity models requires specific knowledge of the antibiotic physiochemical properties such as partition coefficient (KOW), organic carbon coefficient (KOC), water solubility and dissociation constants (pKa), and also environment parameters, such as the dimensions of the environment model, including the volumes of each compartment, temperature and pH. The research investigated the occurrence and chemical behaviour of several classes of antibiotics (fluoroquinolones, tetracyclines, macrolides and sulphonamides) in a small river system (Breich Water, West Lothian), and determine whether a fugacity model could be applied to estimate the fate of antibiotics in the river system and be comparable with field data. The objectives were: 1) To develop a fast analytical method using UV-Vis spectroscopy to measure KOW of antibiotics in an octanol/river water system and use these values in the fugacity model; 2) To monitor antibiotics in the Breich Water using POCIS devices and evaluate the spatial distribution and potential sources along the Breich Water 3) To develop a fugacity fate model to compare this with the field monitoring data. The overall aim was to compare a fugacity model with field monitoring data, and determine whether the model could accurately predict concentrations in a river environment. The results confirmed that WWTP effluent discharge is a major source of antibiotics, which are potentially capable of travelling large distances from the source (WWTP) (the investigated stretch was approximately 10km long). Some compounds were also detected upstream from WWTP, indicating other sources, including clarithromycin, doxycycline, erythromycin, paracetamol and carbamazepine. Statistical analysis revealed that there was a significant difference in concentration of contaminants between different sampling locations, within each season. It was also demonstrated that POCIS devices were capable of detecting antibiotics in small burn, although significant biofouling was observed on the POCIS disks in both summer and winter seasons. Building a level 1 fugacity model demonstrated that the majority of antibiotics (99%) would end up in the sediment phase once discharged into the river environment. The level 2 model showed that most of the antibiotics within the system are removed through advection processes, as opposed to degradation, which partially explains why concentration of antibiotics at sites further downstream were comparable to concentration closer to the point source (WWTP). Overall, a level 1 fugacity models seems inadequate to accurately predict concentrations of contaminants in a dynamic river system, however it does give a basic understanding of which phase compounds such as antibiotics are likely to partition into.