This project will provide new insight into how aeration-induced shifts in (oxygen) O2 and mixing influence manganese (Mn) oxidation and transport. Field campaigns are currently ongoing to characterise spatial and temporal variations in Mn on reservoir-wide and seasonal scales, respectively. Specific objectives include: investigating in-situ changes in the distributions of O2 and Mn (both dissolved (reduced) Mn2+ and solid (oxidised) Mn4+) in the sediment and water-column as a function of aeration operation in two reservoirs, Blagdon Lake (Bristol Water) and Durleigh Reservoir (Wessex Water); determining the potential drivers of Mn cycling within aerated reservoirs; and developing a 3D numerical model for hydrodynamics and water quality that characterises how destratifying aeration systems influence reservoir-wide mixing and Mn concentrations.

Project outline

This research investigates how engineered aeration of drinking-water supply reservoirs influences the transport and removal of Mn, which is a significant problem for UK water utilities due to aesthetic issues, customer complaints, and resultant fines. The primary research questions are: 1) how do aeration systems influence the distribution of Mn, O2 and mixing, the primary geochemical and physical controls on Mn cycling, within UK reservoirs, and 2) can aeration be optimised to reduce problematic levels of Mn in the water entering the treatment plants? These questions are being answered based on comprehensive physical and geochemical in situ measurements in two aerated drinking-water-supply reservoirs coupled with the development of a Mn-focused numerical model characterising the influence of aeration in reservoirs.

This project is using two drinking-water supply reservoirs as ‘natural laboratories’ in which the influence of aeration system type, operation scheme, reservoir bathymetry, and sediment composition on Mn distributions is being assessed. Geochemical, hydrodynamic, and engineered aeration operation data obtained via this study, in pairing with historical data, will be ultimately be used to 1) assess the controls on Mn cycling, and 2) develop a numerical model that describes aeration-induced changes in mixing and Mn geochemistry within these reservoirs.

Field sites: Blagdon Lake and Durleigh Reservoir (Fig. 1), managed by the project partners Bristol and Wessex Waters, respectively, were selected as field sites for this project to 1) assess effects of high sedimentary levels of Mn present in the southwestern UK, 2) facilitate comparison between different bathymetry, land use, regional geology, and aeration strategies (surface aerator vs. sediment-based bubble plume systems) and 3) allow for systematic field sampling in light of logistic constraints. These reservoirs are representative per geology and depth of many of the UK water-supply reservoirs aerated for Mn issues.

Measurements have been performed ongoing since summer 2015 in both reservoirs to capture the influence of aeration during summer stratification. Mn is largely a seasonal issue due to temperature-driven stratification and resultant hypolimnetic anoxia during summer. Water column geochemistry is characterised by vertical water-column measurements via 1) water samples obtained using a Niskin bottle and analysed for Mn via inductively coupled plasma spectrometer (ICP) analysis and 2) continuous profiling using a YSI EXO1 water-quality sonde equipped with O2, pressure, temperature, conductivity, algae and pH/redox sensors. Bulk sediment samples are obtained at each sample site using a Uwitec sediment corer and analysed for Mn and total organic carbon (TOC; indicative of precipitated algal biomass) via acid-digestion and ICP analysis.

Science

Currently, little is known on the influence of aeration strategies (e.g., the type of aeration system used, duration of aeration periods, and placement in the reservoir) on Mn transport and resulting water quality. To address this research gap, this project uses an interdisciplinary, field-based approach to characterize how Mn and O2 levels vary in the sediment and water column on seasonal and reservoir-wide scales as a function of aeration. PGR student Mahan Amani worked on this project and is in the final stages of writing up her thesis; she has presented results at several conferences, including Physical Processes of Natural Waters workshops 2015-2017. PGR student Jack Waterhouse is in the first year of his doctoral studies within the WISE CDT program.

Impact

Particularly in lakes and reservoirs, which are recognized as being highly sensitive environmental systems, the incorporation of Mn biogeochemical cycling effects into mitigation and treatment strategies is crucial to preserving water quality and ecosystem health. Furthermore, dissatisfaction with aesthetic (odour, taste, and colour) problems associated with high Mn levels result in up to 50% of UK customer complaints regarding drinking water supplies. Removal of metals during water treatment is feasible; however, treatment of source water with elevated Mn levels can be difficult and costly due to the complexity of Mn biogeochemical reactions. In addition to water quality issues, it is now emerging that pipeline blockages by Mn-based cohesive material are a cause of drinking-water supply and food security problems on an international scale.