Impacts of sewage treatment works versus diffuse pollution on bacterial water quality at rural wild swimming sites

Abstract

We investigated the factors shaping bacterial water quality at rural wild swimming sites located in catchments that are dominated by peatland, woodland, grassland, and arable land, with <3% (sub)urban land cover. We quantified faecal indicator bacteria and environmental DNA (eDNA) derived markers, including antimicrobial resistance traits (AMR), in the influent and effluent of a village sewage treatment works (STW) and across five river sampling locations. If formally designated and monitored, the wild swimming sites in this study would be unlikely to meet bathing standards, primarily due to elevated Escherichia coli concentrations. E. coli concentrations were positively correlated with other culture-based faecal pollution indicators ( faecal coliforms , faecal streptococci , intestinal enterococci ; Spearman ρ ≥ 0.88, p < 0.05), and with 24-h antecedent rainfall (Spearman ρ = 0.66, p < 0.05). Human gut-associated microbial markers (HF183, crAss64, and/or crAss56) were detected in all samples, indicating a widespread human contribution to faecal pollution. In a catchment with less than 1% (sub)urban land cover, treated STW effluent contributed an estimated 30 ± 7% of the riverine E. coli load. Assuming no attenuation during river transport and using the highest measured discharge loads, we estimated that treated sewage contributed <0.2% of river flow, <1% of bacterial loads, <4% of resistome traits, and <44% of general faecal pollution indicators. Diffuse sources thus dominated faecal pollution, with agriculture representing the most plausible source based on catchment land cover. Microbial water quality was insufficient for bathing at wild swimming sites that met nutrient criteria under the Water Framework Directive, demonstrating that nutrient status alone cannot be used to infer recreational safety. Microbial water quality should be included in regulatory monitoring programmes for all surface waters used for recreation.