To understand spatial and temporal variability of AA usage in the two catchment areas, AA loads in wastewater were normalised to population size (PNDLs) to allow a representative comparison (appendix 1). When comparing overall distribution of AA and metabolite loads per capita between Bath and Keynsham, similar usage of AAs in the two communities can be observed (figure ). When investigating annual trends with monthly averages (figure ), higher population normalised loads of AAs could be observed for Bath in general all year round when compared to Keynsham. For example, monthly averages of SMX were found consistently higher in Bath in comparison to Keynsham throughout the sampling period by 60 ± 17% (with overall year averages 121 ± 43 vs 45 ± 36 mg/1000inh/day respectively). Unsurprisingly, aSMX followed a similar trend with population normalised loads as its parent compound, with ~60 % higher each month in Bath when compared to Keynsham. Similar results of consistent higher loads per capita in Bath were observed for MTZ and the two antiviral drugs 3TC and FTC (51 ± 22, 54 ± 20, and 53 ± 9 % higher each month in Bath vs Keynsham, respectively). This confirms the hypothesis that daily loads of AAs in wastewater are population size driven.