Deciphering the presence of wastewater in a medium-sized Mediterranean catchment using a multitracer approach

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The impact of wastewater disposal on the water resource in a medium-sized Mediterranean watershed, the Hérault River basin, has been studied using a multitracer approach including NO 3 - , Cl - , B concentrations and the Gd anomaly. The Hérault watershed supplies drinking water to approximately 300,000 inhabitants and receives the effluents of 96 sewage treatment plants. The geological context is heterogeneous with 4 main lithologies: a Palaeozoic basement in the north, a karstified Mesozoic cover in the middle, an evaporite layer in the western part of the basin and alluvial deposits in the southern plain. The concentration of major ( Ca 2 + , Cl - , NO 3 - ) and trace elements (B, Sr and rare earth elements) has been monitored in streams, aquifers and sewage treatment plant effluents. Because of the particularly heterogeneous and rich geological context of the Hérault basin, the challenge of using NO 3 - , Cl - and B as wastewater tracers was to distinguish their natural and anthropogenic origins. Indeed, none of these wastewater borne elements can be considered as a highly reliable tracer by itself because they are ubiquitous in water where they can be supplied by both natural (evaporite dissolution, rainfall) and anthropogenic sources (wastewater or agriculture). However, the respective contributions of different sources of candidate tracers, NO 3 - , Cl - and B could be roughly assessed using Sr/Ca, B/Sr and Cl −/B ratios which, combined with the information provided by each tracer, allowed the ascertainment of the presence of wastewater in two tributaries of the Hérault River and in a few wells tapped for drinking water in the alluvial plain.