Characterizing the drivers of organic matter fluctuations in surface water lakes: the case of the Gileppe water reservoir in Wallonia (Belgium)

Many surface water lakes in northern Europe have seen a rise in their organic matter content. When the water is used for human consumption, this has an indirect effect on human health, by increasing the risk of pathogens appearance and of biofilm formation after water treatment. Besides, organic matter negatively affects the color, odor and taste of water, which displeases the consumers. In order to ensure a good drinking water quality at the tap, its treatment should thus be adapted to the organic matter content.

In this study we (i) characterize the trend and seasonality patterns of organic matter in the Gileppe Lake over the last 20 years and (ii) unravel mechanisms causing the observed fluctuations using time series statistical modelling. The water reservoir created by the Gilleppe dam in the North of Wallonia has an available volume of 3.100.000 m³. It is used to supply a hydro-electric power plant, and to provide drinking water to the city of Verviers and its surroundings. The water producer « Société Wallonne des Eaux (SWDE) » extracts an annual volume of around 14 million m³ to that end. The SWDE has been measuring the quality of the extracted water since 1991, at an increasing frequency. These measurements include parameters related to organic matter content such as total organic carbon (TOC), color and chemical oxygen demand.

The TOC concentrations in the Gileppe lake indicate there has been a rise in organic matter in the Gileppe lake since the 90’s, as the concentration was 3,7 mg/l in October 1997, and increased to 10,4 m/l in October 2019. The TOC also has a seasonal variability, with the highest concentration peaks being reached during the autumn.

We characterize the evolution of the potential drivers of the increasing trend and the seasonality: climate (precipitation, T°), land use (mainly forest cover area and type) and anthropogenic pressure (presence of septic tanks, wastewater release, agricultural runoff). We then investigate if, according to the literature, the evolution of these variables could explain of the observed organic matter trends and seasonality.