Variability of Escherichia coli Concentrations in Rivers during Base‐Flow Conditions in New Zealand

Escherichia coli concentrations in rivers are known to vary considerably. Much research has focused on storm events and the relationships between fecal microbe concentrations and flows. However, there is still considerable variability in microbial concentrations during base‐flow conditions, and little research has been conducted to understand this short‐term variability in rivers. We investigated the variability of E. coli concentrations in base flows at the time scales of minutes, hours, and days and compared this to variability from laboratory replication of the measurement methods. This was conducted in three different‐sized rivers in both summer and winter seasons. Estimates of variability were analyzed using the coefficient of variation (CV). The variability at the minute time scale was 17%, compared with the laboratory replication variability of 15%. The CV then increased to approximately 32 and 60% at the hourly and daily time scales, respectively. There is strong evidence that both time scale (p < 0.001) and river (p < 0.001) significantly affect the variation in E. coli concentrations. The E. coli concentrations were higher in summer than winter, with a marked effect in the smallest stream, where at one site the concentrations were >2000 most probable number (mpn) 100 mL−1 in all summer samples. This variability of concentrations should be considered when interpreting the results from a one‐off grab sample used to compare against water quality standards or for calibrating models. Core Ideas E. coli concentrations are highly variable during base‐flow conditions. The error of analytical techniques needs to be considered for interpreting grab sample results. The variability of E. coli concentrations increases with increasing time scale.