Evaluating the Trend and Extreme Values of Faecal Indicator Organisms in a Raw Water Source: A Potential Approach for Watershed Management and Optimizing Water Treatment Practice

This study demonstrates the use of microbial load time series, through trend and extreme event analysis, to evaluate the effectiveness of watershed management strategies and to understand the probabilistic behaviour of extreme events. Heterotrophic plate count (HPC), Clostridium perfringens, intestinal enterococci, Escherichia coli, and coliform bacteria, were monitored from 1999 to 2012 at Nedre Romerike Vannverk (NRV) drinking water treatment plant, which takes its source water from Glomma River, Norway. Mann-Kendall test, Seasonal Mann-Kendall test, and Sen’s Slope Estimator were used for trend analysis over years and also seasonal trends were examined through linear regression. Mann-Kendall test results show a decreasing trend for all indicator microorganisms except Escherichia coli. Seasonal trend analysis results also indicate that Clostridium perfringens during autumn and intestinal enterococci during spring have a significantly decreasing trend. An increasing trend was observed for all pathogens during the summer season. Trend analysis results offer insights and crucial perspective for policy makers and planners to evaluate the existing watershed management strategies. Moreover, extreme microbial load events in the raw water was analysed using the POT method to estimate return levels of extreme indicator microbial load corresponding to selected return periods. It is of importance to calculate the return period of extreme microbial load events for the purpose of designing optimal pathogen barriers and performing risk analysis.