Project Highlights

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Identifying Pressures

Vartry Reservoir is a very important drinking water source in Ireland. Since 2013, spring diatom algae blooms have been occurring in the Vartry Reservoir. In this project, in addition to collecting and analysing historical water quality and ecology data, a 3-year monitoring programme (from 2016 to 2018) and a series of laboratory experiments were carried out to investigate the causes of diatom blooms and identify the pressures. The study monitored nutrient levels in both the feeding rivers and the reservoir. It was found that, although the silica and nutrient concentrations in the reservoir were sufficient for diatom growth, the concentration of soluble reactive phosphorus affected the magnitude of the diatom bloom. The study also found that the ratio of diatom to zooplankton in the Vartry Reservoir was high compared with other lakes in Ireland.

Informing Policy

In Ireland, drinking water mainly originates from surface water, with lakes being the main sources. Of the 224 lakes monitored by the Irish Environmental Protection Agency, 113 lakes, including the Vartry Reservoir, were in high or good ecological status during the period 2013 to 2018. The findings of this project have implications for managing these lakes. Although the impact on the water quality is not a main concern in the Vartry Reservoir, diatom blooms pose great challenges to the Vartry water treatment plant, resulting in a significant decrease in water production rates. The data collected in this project will benefit stakeholder decision making in terms of the management of the Vartry Reservoir.

Developing Solutions

Vartry Reservoir serves more than 220,000 customers and its supply area stretches from Roundwood, through north Wicklow up to south Dublin. The spring diatom algae bloom results in serious clogging of the downstream slow sand filters in the Vartry water treatment plant. This project provides solutions to (1) prevent the diatom algae bloom in the reservoir and (2) mitigate the impact of diatom algae bloom on the water treatment plant. The project found that, by increasing the zooplankton population to a threshold level, the diatom algae bloom could be inhibited. Coagulation and filtration could be considered as short-term measures to mitigate the impact of diatom blooms on the water treatment plant. Using aluminium sulfate and ferric chloride as coagulants can remove more than 90% of the diatoms. Denim and filter nets with a mesh size of 63 µm can also be used as filter materials to remove more than 77% of diatoms.