Determining the drivers of harmful algal blooms and their impact on public water supply resilience during droughts

Cantwell, Helen 2021. Determining the drivers of harmful algal blooms and their impact on public water supply resilience during droughts. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Climate change poses a significant risk to water supply resilience. Reduced summer rainfall combined with elevated temperatures are likely to threaten both water quantity and water quality. With respect to water quality, shallow warmer reservoirs can increase risk of biological causes of poor quality, for example due to harmful algal blooms, cyanobacteria toxins and taste and odour metabolites. This was investigated at Llandegfedd, the largest potable water reservoir and a critical water supply system in Wales. The overarching aim addressed in this thesis was to determine if the reservoir would be vulnerable to poor water quality events in the future. To this end, the project comprised four main areas of research: 1. Historical data; 2. Water nutrient dynamics; 3. Sediment nutrient dynamics; 4. Effects of extreme weather events upon water quality. Analysis of water samples indicated phosphorus limitation and the reservoir overall was classified as oligotrophic; unusual for a lowland reservoir. Mass balance analysis demonstrated low internal loading potential even in summer months, which was supported by sediment phosphorus measurements. Allochthonous sources of phosphorus were responsible for the low levels of bioavailable P in the water column. In contrast, the shallowest site at the northern end of the reservoir, adjacent to the main pumped inflow, was eutrophic and at highest risk of water quality deterioration, cyanobacterial growth and geosmin production. Analysis of extreme weather events indicated this risk to be compounded by pulse rainfall events. The reservoir relied upon an allochthonous source of phosphorous, with the refilling process also changing nutrient dynamics through increasing phosphorous concentrations. An additional finding of this study was that analysis of nutrients pre and post water abstraction tower failure around 1999/2000 indicated that the current water abstraction method is inappropriate for monitoring water quality risk at this site, demonstrating the need for enhanced monitoring. This study has shown that Llandegfedd Reservoir remains oligotrophic despite potential for internal P loading, suggesting possible future issues with water quality at the site. This is compounded by extreme weather events and patterns of drawdown and refilling. This is an important warning to the wider Water Industry that oligotrophic water supply reservoirs still require significant monitoring to assess water supply resilience in the face of climate change.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Funders:	NERC
Date of First Compliant Deposit:	10 November 2021
Last Modified:	05 Aug 2022 01:37
URI:	https://orca.cardiff.ac.uk/id/eprint/145414

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