Forest Dieback in Drinking Water Protection Areas: A Hidden Threat to Drinking Water Quality

The forest dieback during the unprecedented drought years of 2018 to 2020 poses a severe but largely overlooked threat to drinking water quality, according to a new study by an interdisciplinary research team from the University of Freiburg, Germany. Published in the journal Earth’s Future, the researchers found that average nitrate concentrations in groundwater more than doubled in exemplary German drinking water protection areas affected by significant forest loss.

“In Germany, 43 percent of water protection areas are covered by forests, therefore the health of forests is crucial for maintaining water quality,” explains Dr. Carolin Winter, first author of the study and hydrologist at the Faculty of Environment and Natural Resources at the University of Freiburg. The researchers were able to show that within three years after the beginning of the drought period in 2018, approximately five percent of forest areas in water protection zones had died off. The impact on water quality varied significantly across different sites, with researchers warning of potential delayed effects that may emerge in the coming years.

“In Germany, 43 percent of water protection areas are covered by forests, therefore the health of forests is crucial for maintaining water quality.”

Dr. Carolin Winter

Faculty of Environment and Natural Resources at the University of Freiburg

The importance of forests for protecting drinking water

For centuries, forests have been recognized as natural safeguards for drinking water quality. Most drinking water in Germany comes from groundwater extracted within designated protection areas that have restrictions to minimize contamination risks. One aim of these rules is to prevent nitrate from entering the groundwater and making it unusable for drinking water supply. Normally, forests exclude common nitrate sources typically found in agricultural, urban, or industrial areas. Furthermore, forests actively retain nitrate, preventing it from leaching into groundwater. However, high rates of forest dieback may compromise this crucial protective role and potentially transform forests into sources of contamination.

Rapid forest loss in critical protection zones

By combining existing forest data and a novel dataset of all drinking water protection areas in Germany, the scientists determined that 43 percent of drinking water protection areas in Germany are forested. Approximately 5 percent of these forests were lost within only three years following the onset of drought in 2018. “This is a substantial loss within such a short period, considering the crucial role of drinking water protection areas and that the typical rotation length for most tree species in Germany ranges from about 60-160 years,” notes senior author Dr. Florian Schnabel, forest scientist at the University of Freiburg’s Faculty of Environment and Natural Resources. These losses occurred primarily in spruce-dominated forests, though other species such as beech also experienced unusually high mortality rates.

Doubling of nitrate concentrations where forest dieback was severe

The researchers analyzed groundwater nitrate concentration data from heavily forested drinking water protection areas that suffered substantial forest loss exceeding 25 percent. They found that, on average, nitrate concentrations in these areas more than doubled from 5 mg/L to 11 mg/L between the pre-drought period (2008–2017) and post-drought years (2021–2022). In contrast, areas with minimal forest loss (less than 3 percent) showed no increase in nitrate levels.

However, not all sites with high forest loss exhibited increased nitrate concentrations, and both the timing and magnitude of increases varied considerably between affected areas. “Different forest types could be one reason for these varied effects,” explains Winter. “Another factor could be the time lag between forest loss and increased nitrate concentrations in groundwater, which can sometimes take several years to decades. This means that similar increases might become evident in additional areas in the years ahead.” The authors conclude that more research is necessary to develop a comprehensive understanding of the mechanisms controlling how forest dieback impacts water quality. This knowledge will be crucial for effectively protecting drinking water resources in the long term.