Combining 3H, 14C, and lumped parameter models to characterize modern groundwater residence time in a highly anthropized watershed

Paul Baudron, Florent Barbecot, Marina Gillon, Yves Travi, José Luis García Aróstegui, Francisco Gomariz Castillo, David Martinez-Vicente, Christian Leduc

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Isotope Tracing I

ABSTRACT: We applied Lumped Parameter Models in order to assess the residence time of modern groundwater based on the radiocarbon nuclear peak. The study area was an intensively irrigated semiarid area in southeastern Spain, the Campo de Cartagena. The approach was implemented in parallel for i) radiocarbon and for ii) tritium. The tritium input function had to be reconstructed due to the highly variable 3H content of irrigation water. An irrigation peak around year 2000 has had more impact on groundwater samples than the tritium bomb peak. As a first approximation, only piston flow model or dispersion model with very low dispersion parameter were able to explain the measured 3H and 14C activities. Nonetheless, exponential model was the most likely lumped parameter model according to hydrogeological context. Good results were finally obtained by a combined Exponential Flow and Piston Flow model in order to take into account the variation in the recharge rates since the development of drip irrigation. Apart from presenting an uncommon approach for radiocarbon modern water dating, this study highlights the need of considering not only the variation of the tracer but also the variability of recharge rates in lumped parameter models. ACKNOWLEDGEMENTS This work was developed within the scope of the Project fiModelación Hidrológica en Zonas Semi Aridasfl financed by the Regional Ministry of Universities, Business and Research (Region of Murcia, Spain). The authors acknowledge the Fundación Instituto Euromediterráneo del Agua (Murcia, Spain) for its fundamental financial support. Additional supports came through the fiCARTAG-EAUfl project financed by the French SICMED initiative and the 08225/PI/08 research project financed by fiPrograma de Generación del Conocimiento Científico de Excelenciafl of the Fundación Seneca, Región de Murcia (II PCTRM 2007-10).

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