Natural versus anthropogenic sources in the surface- and groundwater dissolved load of the Dommel river (Meuse basin): Constraints by boron and strontium isotopes and gadolinium anomaly

Abstract

The river Dommel, a tributary of the Meuse River, drains an area of intensive agriculture (livestock farming, maize and grassland over 50% of the basin), and a dense population of about 600,000 people representing 20% of the total area. The combined human activities in the Dommel catchment lead to a large amount of dissolved elements and compounds released in surface- and groundwaters. The aim of this study was to discriminate the natural (including infiltration of Meuse water) versus anthropogenic sources of the dissolved load, and to identify the various pollution sources such as agriculture, industrial activity, and wastewater treatment plants, using geochemical tools including major- and trace elements, Sr and B isotopes, and rare earth elements (REE). For that purpose, a same-day geochemical “Snapshot” picture of the entire basin was combined with monthly monitoring in strategic points. The major- and trace elements analyses allowed discriminating the main pollution sources affecting the basin, i.e. point versus diffuse sources. Strontium isotopes helped to identify each tributary and to calculate mixing proportions. Combining these calculations with the Sr-isotopic data obtained from the “Snapshot” sampling campaign during a low-flow period, shows that Meuse water infiltration represents 25% of the total Dommel discharge. Boron isotopes used for assessing the amount of water affected by anthropogenic input cannot discriminate between the two main anthropogenic inputs, i.e. urban wastewater and the zinc-smelter effluent, as they have similar δ11B values. Finally, the REE, and especially the use of Gd anomalies (Gd∗), demonstrated the generalized impact of urban wastewater on the streams of the Dommel Basin. The coupled use of different geochemical tracers (Sr and B isotopes together with Gd∗) in addition to the standard major-element analyses, led to discriminating the various anthropogenic components influencing the Dommel Basin water quality. With these tools it also became possible to assess the complex water circulation and exchanges between water compartments, including the major role of Meuse water through the Bocholt–Herenthals canal.