Dune water abstraction: flexible pond management requires tailoring

In the dune water utilities (DPWE) ‘Open filtration pond management’ project of the Sources Advisory Group, research was conducted into the possibilities of modifying pond management for the purpose of increasing production, without having any deleterious impact on the microbial safety and nature values. It was found that an increase in the pond levels can contribute to higher production, but that the effects on production, nature and safety can vary among different types of systems. In the system that was studied, small water level increases were not sufficient to entirely counter seasonal effects.

In the years ahead it is expected that there will be a growing demand for drinking water, which will have to be met (in part) by production using the existing infrastructure of the dune water utilities. One possible means of satisfying this growing demand for water, is a more intensive use of dune infiltration systems. This could for instance involve flexible pond levels, or higher pond levels as means of increasing the infiltration capacity.

Effect on water production

In this project a hydrological model was used to simulate water flow, temperature fluctuations and substance transport in a cross-section of a dune filtration system. The model was used to study the impact of seasons – with varying precipitation surpluses and temperatures – on production. Different variations in the pond levels were also simulated, both constant as well as fluctuating over the course of the year. The impact on the groundwater levels in the zone, where valuable dune valley vegetation can occur, was also investigated. As were the travel times: are these sufficiently long under different conditions to be microbially safe?

Groundwater levels

The results show that small increases in the pond level can contribute to higher production, but that this, in the type of system studied, is not sufficient for instance to entirely counter the seasonal effects. Sudden pond level changes can quickly impact both groundwater levels as well as production. It is important in the interpretation of the results to bear in mind that they are strongly dependent on the properties of the system, particularly the geometry. In shallower systems, fluctuations in groundwater levels can have a relatively far larger impact on the production than in deeper systems. The impact on dune valley vegetation also depends on these properties. Moreover, the sludge layer in the infiltration canal is also important: if it is or becomes poorly permeable, this can have a big impact on both the production as well as the groundwater levels in the dune valley vicinity.

Given that local conditions can have such a significant impact, a tailored approach is required when consideration is given to modifying the water levels in a dune filtration system.