Li and δ7Li as proxies for weathering and anthropogenic activities: Application to the Dommel River (meuse basin)

Abstract

Lithium (Li) contents and isotopes were studied in the Dommel catchment, a small riverine system in northern Belgium and the southern part of the Netherlands discharging into the Meuse River. The study covered surface- and groundwaters in and on sand and gravel in the catchment; it aimed at evaluating the potential of Li isotopes as effective tracers of anthropogenic activities in addition to tracing water/substratum interaction processes within a sandy environment. Dissolved lithium concentrations in the Dommel catchment span one order of magnitude, ranging from 1.55 to 39.20 μg/L with a mean concentration of 6.58 μg/L, higher than the worldwide riverine average of 1.9 μg/L. Dissolved δ7Li varies from +5.4‰ to +27.8‰. Part of the catchment is impacted by smelter effluents with Li concentrations in the range 91–526 μg/L (mean value 288.36 μg/L) and a δ7Li of around +25.6‰ before dilution along the flowpaths of the catchment basin. To interpret the dataset in terms of using Li isotopes as a proxy for anthropogenic activity, we first applied an atmospheric-input correction to waters for both Li concentration and δ7Li, as rain contributes an important fraction of the dissolved elements in Dommel waters (8–100% of Li in waters is of atmospheric origin; 100% means no interaction with rocks, no anthropogenic disturbance). Then, we determined and quantified the anthropogenic influence using δ7Li and mixing equations for the impacted parts of the catchment. Finally, we added constraints on the water/substratum interaction in this mostly sandy-feldspar soil catchment through the δ7Li and Li/Na ratios, highlighting the role of water-residence time in the system on weathering intensity.