Hypolimnetic withdrawal as a lake restoration technique: determination of feasibility and continued benefits

The withdrawal of hypolimnetic water enriched with nutrients and reduced substances has been used as a lake restoration technique (hypolimnetic withdrawal, HW) for more than 60 years. By reducing internal phosphorus loading, HW treats the cause of much of the water-quality deterioration in eutrophic stratified lakes, including cyanobacteria blooms. To support future applications, a feasibility study is presented that determines the applicability of HW to a drinking water source lake for the City of Stockholm as an example. Necessary treatment of the HW water includes a technically advanced facility. The possible application of passive siphoning by gravity limits recurring energy costs. The most efficient performance is ensured by the monitoring of HW operational variables and water quality in the source and receiving water. A review of the scientific literature and the worldwide web confirms that new and continued HW applications improve water quality in stratified lakes with anoxic hypolimnia. A previously developed model for the prediction of epilimnetic TP decreases from the long-term TP export via HW was supported by results from several new applications. Benefits of flow regulation considering climate change variability have been reported, suggesting that HW could play a progressively important role in lake management.