Monitoring the effect of capping sediments of a shallow Dutch lake with aluminium hydroxides and sand on internal phosphorus loading over ten years after treatment

In lake restoration, internal phosphorus (P) loading is often obstructing the improvement of water quality even years after decreasing external P loading. Here, we report on a field study in the Lake Bergse Voorplas (The Netherlands), in which treatment with Al hydroxides was combined with installing a sand layer on top of the sediment. Besides surface water monitoring, sediment and porewater samples were taken and analysed before, in the year after, and ten years after treatment. Porewater profiles indicate that the treatment was successful leading to a decrease in average total P and chlorophyll-a concentrations by about 30% and 50%, respectively. At sites with the highest Al dose, porewater P concentrations at the interface between capping layer and sediment decreased from values above 6 mg P/L to concentrations close to detection limit, one year after treatment. The sand layer ensured the physical integrity of the Al-rich layer, which was still effective in reducing the flux of legacy P from the covered sediment to the overlying water, 10 years after treatment. Adsorption experiments, however, indicate that the added Al hydroxide approached P saturation. Additionally, indications were obtained that about half of the Al hydroxides were converted into clay minerals being the most important factor affecting the long-term capacity of the added Al hydroxides to adsorb P. This study demonstrates that stabilizing the active Al-rich layer with a sand layer provides a promising approach to reduce internal P loading in shallow lakes over the time-scale of years.