Variability of E. coli concentrations in rivers during base-flow conditions in New Zealand

E. coli concentrations in rivers are known to vary considerably. Much research has focused on storm events and the relationships between faecal microbe concentrations and flows. However, there is still considerable variability in microbial concentrations during base flow conditions and there has been little research conducted to understand this short-term variability in rivers. We investigated the variability of E. coli concentrations at the time scales of lab-replication, minutes, hours and days in 3 different sized rivers in both summer and winter seasons. Estimates of variability were analysed using the coefficient of variation (CV). Analysis of replicate samples in the lab showed that the variability of the lab replicates was 15% with a small increase to 17% at the minute scale. The CV then increased to approximately 32 and 60% at the hourly and daily time scales. There is strong evidence that both time-scale (p<0.001) and river (p<0.001) significantly affect the E. coli concentration variation. Actual concentrations were higher in summer than winter with a very marked effect in the small tributary where at one site the concentrations were >2000 E. coli 100mL-1 in all summer samples. This variability of concentrations should be considered when interpreting the results from a one-off grab sample against water quality standards or for calibrating models.