Precipitation and discharge changes drive increases in Escherichia coli concentrations in an urban stream

Determining the driving factors of E. coli dynamics and predicting future E. coli changes in urban aquatic systems are important for regulating water quality. In this study, data from 6985 measurements of E. coli from 1999 to 2019 in an urban waterway Pleasant Run in Indianapolis, Indiana (USA) were statistically analyzed by Mann-Kendall and multiple linear regression to assess the long-term trends in E. coli concentrations and to project E. coli concentrations under fu-ture climate change scenarios. E. coli concentrations monotonically increased over the last two decades, with the value increasing from 111 Most Probable Number (MPN)/100 mL in 1999 to 911 MPN/100 mL in 2019. E. coli concentra-tions have exceeded the Indiana standard of 235 MPN/100 mL since 1998. E. coli showed peak concentration in sum -mer and higher concentration in sites with combined sewer overflows (CSOs) relative to those without. Precipitation had both direct and indirect impacts on E. coli concentrations meditated by stream discharge. Multiple linear regres-sion results showed that annual precipitation and discharge accounted for 60 % of E. coli concentration variability. Based on the observed precipitation-discharge-E. coli concentration relationship, the projected results showed that, in the highest emission representative concentration pathways (RCP) 8.5 climate scenario, E. coli concentrations in the 2020s, 2050s, and 2080s will be 1350 +/- 563 MPN/100 mL, 1386 +/- 528 MPN/100 mL, and 1443 +/- 479 MPN/100 mL, respectively. This study illustrates that climate change can impact E. coli concentrations by altering tem-perature, precipitation patterns, and stream flow in an urban stream and predicts an undesired future situation under a high CO2 emission scenario.