Using rare earth elements as geochemical tracers to distinguish groundwater from different Alpine aquifers

Rare earth elements(REE)were determined in 39 groundwater samples collected at 14 sites under low- and high-flow conditions. Water samples derived from aquifers hosted in crystalline, molasse, flysch, carbonate and evaporite rocks located in Western Switzerland. The concentration of REE in groundwater circulating in different rocks showed large variations: lowest concentrations(SumREE <10 ng/L)occurred in groundwater from evaporite aquifers; highest concentrations(SumREE up to 516 ng/L)were observed in carbonate aquifers, although REE in these waters do vary under different hydrological conditions; groundwater from other aquifers had SumREE from 10 to 100 ng/L. Distinct REE signatures were observed in waters draining specific rocks. The REE patterns in groundwater from crystalline, molasse and flysch aquifers showed heavy-REE enrichment at different degrees. Groundwaters circulating in crystalline rocks were distinguished by negative anomalies in Ce and Eu, whereas those from carbonate aquifers were nearly flat with SumREE and the magnitude of negative anomaly in Ce is likely to be controlled by iron concentrations. The REE-Post-Archean Australian Shales(PAAS)normalized patterns appear useful to recognize the aquifer type and suggest the possibility to use the REE as geochemical tracers.