Evaluation of fecal indicator bacteria concentration in different water sources, storage treatments, and holding times

Abstract

Accurate water quality monitoring of microbial indicators hinges on understanding the impact of holding times and treatments on water sample results. This study examined interacting factors influencing holding times for fecal indicator bacteria (FIB) in water samples held for a range of times (0–216 h) under three storage treatments (i.e., Controlled Cold [∼4°C], Controlled Ambient [∼21°C], and Changing Conditions [∼4°C–21°C]). Four separate trials were conducted during 2016 using two distinct water supplies: river water ( n  = 2) and post‐irrigation canal water ( n  = 2). Large carboys of water ( n  = 3/trial, ∼53L) were evenly divided into 500 mL aliquots for storage under different treatments ( n  = 105/treatment). Samples were processed in triplicate using standard membrane filtration for Escherichia coli and fecal coliforms. Linear regression with autocorrelation and Dunnett's test were used to analyze the concentration of FIB across holding times from baseline ( T  = 0). Canal water samples maintained FIB concentrations similar to T  = 0 for at least 72 h under Controlled Cold; at least 48 h under Changing Conditions, and at least 30 h under Controlled Ambient treatments. By contrast, river water samples maintained concentrations similar to T  = 0 at least 180 h without statistically significant changes under Controlled Cold, at least 72 h under Changing Conditions, and at least 36 h under Controlled Ambient treatments. Regardless of the treatments, river water samples were more stable than canal water samples, likely due to starting conditions. Holding times for water quality monitoring continues to be a multifaceted challenge and may warrant further exploration or more flexibility around regulatory requirements.