Monsoon-triggered surges of antibiotic resistance genes in urban Indian rivers impacted by untreated wastewater.

In many low- and middle-income countries, intense seasonal rainfall can strongly reshape pollutant dynamics in urban rivers receiving untreated sewage and solid waste, with potential consequences for antibiotic resistance dissemination. Here, we examined monsoon-driven changes in the relative abundance and distribution of antibiotic resistance genes (ARGs) in two heavily polluted sub-tributaries of the Ganga River, the Bindal and Rispana, flowing through Dehradun, India. Water and sediment samples were collected across three hydrological periods in 2019, summer (pre-monsoon), monsoon, and winter (post-monsoon), and analysed for faecal indicator Escherichia coli (yccT), the class 1 integron-integrase gene (intI1) as a proxy for anthropogenic pollution, four wastewater-associated model ARGs (sul1, sul2, ermF, tetW) conferring resistance to sulphonamides, macrolides, and tetracyclines. In addition, concentrations of ten heavy metals were quantified. E. coli was consistently detected in water samples across seasons in both rivers (1.0-3.6 log10 copies mL-1), indicating persistent faecal contamination. Monsoon rainfall was associated with a consistent increase in faecal/anthropogenic indicators and ARGs, reaching 1 to 3 orders of magnitude higher relative abundance compared to pre-monsoon conditions. Post-monsoon, ARG levels in water generally declined toward pre-monsoon levels, whereas sediment-associated ARG signals remained elevated post-compared to pre-monsoon, indicating persistence in depositional reservoirs. Although heavy metal concentrations frequently reached ranges previously linked to co-selection, they showed no significant correlations with ARG abundances, identifying faecal pollution as the dominant driver of seasonal resistance dynamics. No consistent increases in ARGs or faecal markers were detected downstream of wastewater treatment plants, supporting widespread non-point source inputs. Overall, monsoon events act as major amplifiers of antibiotic resistance in severely impacted urban rivers by mobilising faecal contamination and counteracting dilution. Mitigation should prioritise reducing unmanaged sewage and solid waste inputs, particularly before and during monsoon periods.