Anthropogenic impact on solute fluxes in a headwater catchment in Western Germany.

Headwaters are a source of fresh water and material transported to the downstream sections of river channels. In most regions of Europe, the natural environment of headwaters has been significantly transformed by human activities, which is reflected in the quality of surface waters and the rate of solute fluxes. The aim of the study was to determine the anthropogenic impact on solute fluxes in a headwater catchment in Germany. The study area (Wüstebach experimental catchment) is located in the Eifel Mountains and is part of the TERENO (Terrestrial Environmental Observatories) network. The catchment is currently subject to the following anthropogenic pressures: the use of de-icing salts on the motorway running through the catchment and the logging of 22% of the forest area in 2013 by the National Park to promote the regeneration of near-natural beech forest. We used data from almost 13 years (June 2009-April 2022) of environmental monitoring in the Wüstebach catchment to calculate monthly and annual rates of solute fluxes. The concentrations of ions in stream water and precipitation were determined: Ca2+, Mg2+, Na+, K+, Al3+, Fetot, Mn2+, NH4+, NO3−, SO4−, and Cl−. Data from the Wüstebach catchment were compared with data from the control (untreated) catchment. The average annual rate of total dissolved solids in the Wüstebach catchment was 83 t/km2/a. Only the loads of four ions performed a significant difference after deforestation (SO4−, NH4+, Al3+, Fetot). However, total dissolved solids and loads of most of the ions did not show any significant differences after deforestation, which indicates a slight impact of logging on the chemical denudation. This resulted from the limited soil erosion that followed deforestation as the soil was well protected during logging works by covering harvester lanes with branches. However, there is a significant impact of road salting on the solute fluxes in the headwater catchment. Assuming that the control catchment is not affected by using de-icing salts, the road salting inputs in the Wüstebach catchment calculated based on the precipitation share for Na+, Ca2+, Mg2+, K+, and Cl− ions were, on average, 88%, 53%, 18%, 53%, and 90%, respectively, of the original ion loads. The input with road salting was so large that elevated concentrations of Na+ and Cl− were noted throughout the study period with slight seasonal fluctuations. This proves the subsurface build-up of salt due to the long-term application of de-icing salt. Without the input of de-icing salts, the average annual rate of chemical denudation in the catchment would be about 19 t/km2/a. Thus, on average, this ion storage effect in the soil has increased the total dissolved solids rate by 62%. In summary, it can be concluded that long-term intensive road salting in winter has a greater impact on the solute fluxes than the clear-cut of 22% of the catchment area. [ABSTRACT FROM AUTHOR]