Particulate Phosphorus Characterization along the River Continuum Indicates the Importance of Fine Sediment Management to Drinking Water Treatability

Abstract:

The accumulation of deposited cohesive sediment in drinking reservoirs can increase both phosphorus release to the water column and the risk of algal blooms and subsequent toxin releases or taste and odor events. The form and mobility of key limiting nutrients such as phosphorus in these systems are necessarily impacted by the biotic adjustments driven by landscape activities and disturbances along the river continuum. In this investigation, approximately ten years of historical water quality and flow data collected were evaluated along a downstream gradient from the headwaters to a downstream drinking water reservoir in the Elbow River watershed serving the City of Calgary in Alberta, Canada. Fine sediment size distribution, geochemical composition, and associated phosphorus forms and mobility were also subsequently evaluated to demonstrate its relevance as an in-stream (i.e., internal) source of bioavailable phosphorus. The effects of fine sediment-associated phosphorus availability on the proliferation of potentially toxin forming cyanobacteria was evaluated in the laboratory using microcosm approaches. The biomass associated with cyanobacterial blooms can increase water turbidity, challenge coagulation/flocculation/clarification processes, clog filtration and membrane processes, and increase the chemical oxidant demand required for disinfection, thereby increasing the potential for disinfection by-product formation. Both toxins (such as microcystin) and taste and odor compounds can be released by cyanobacterial cells prior to and during drinking water treatment. Their presence can challenge conventional water treatment processes or lead to customer complaints. Accordingly, microbial DNA was extracted from water and sediment samples, and the V4 region of the 16S rRNA gene was sequenced to describe the composition of the microbial communities present. To increase the resolution of taxonomy of cyanobacterial 16S rRNA gene sequences, the search tool BLAST was used to identify evolutionary relatedness between the samples, and characterized cyanobacteria in the NCBI database. This work demonstrates the critical importance of considering the role fine sediment and associated nutrients in the management of water quality and treatability in the Elbow River system.