Linking fecal bacteria in rivers to landscape, geochemical, and hydrologic factors and sources at the basin scale

Abstract

Linking fecal indicator bacteria concentrations in large mixed-use watersheds back to diffuse human sources, such as septic systems, has met limited success. In this study, 64 rivers that drain 84% of Michigan’s Lower Peninsula were sampled under baseflow conditions forEscherichia coli,Bacteroides thetaiotaomicron(a human source-tracking marker), landscape characteristics, and geochemical and hydrologic variables.E. coliandB. thetaiotaomicronwere routinely detected in sampled rivers and anE. colireference level was defined (1.4 log10most probable number⋅100 mL−1). Using classification and regression tree analysis and demographic estimates of wastewater treatments per watershed, septic systems seem to be the primary driver of fecal bacteria levels. In particular, watersheds with more than 1,621 septic systems exhibited significantly higher concentrations ofB. thetaiotaomicron.This information is vital for evaluating water quality and health implications, determining the impacts of septic systems on watersheds, and improving management decisions for locating, constructing, and maintaining on-site wastewater treatment systems.