A phytoplankton risk matrix: combining health, treatment, and aesthetic considerations in drinking water supplies

Worldwide, the risk profile of drinking water supplies is uniquely shaped by differing geology, climate, and human-based developmental practices. The World Health Organisation’s Water Safety Plans (WSPs) are designed to improve the quality of water supplies, particularly regional supplies. One advantage of WSPs is that they are water supply-specific, thus reflecting local conditions; however, WSPs are also limited by a sole focus on health risks, with a little reference to treatment or aesthetic risks such as filter clogging and taste and odour complaints. Appropriately managing these non-health risks is important in effective infrastructure management and in developing consumer trust; however, currently, there is a lack of practical techniques to enable water managers to identify and respond to the threefold-risk profile (health, treatment, and aesthetic) that exists in all regional water supplies. To address this, phytoplankton concentration and bio-volume data were analysed from freshwater samples collected over 2 years, from an ephemeral subtropical catchment in Central Queensland, Australia. This study investigated the temporal and spatial patterns of phytoplankton, as influenced by flow events. Based on field data, a semi-quantitative risk matrix methodology was developed for use as a supplement to the existing WSPs and allows a more comprehensive assessment of phytoplankton-based hazards related to health, treatment (e.g., clogging) and aesthetic risks for individual water supplies. For example, the primary risk identified for the studied catchment was the presence of Cyanobacteria in the pre-chlorinated supply chain immediately prior to entering the treatment plant, specifically Limnothrix redekei. Modelling of the data within these risk matrices demonstrates that the approach is useful in identifying overall catchment risks as well being applied for seasonal and event-based risk assessment of regional drinking water supplies.