Assessing Groundwater Residence Time in a Highly Anthropized Unconfined Aquifer Using Bomb Peak 14C and Reconstructed Irrigation Water 3H

Abstract

Radiocarbon decay is rarely used to assess the residence time of modern groundwater due to the low resolution of its long half-life in comparison to the expected range of ages. Nonetheless, the modern 14 C peak induced by the nuclear bomb tests traces efficiently the impacts of recent human activities on groundwater recharge, as well as for tritium. A simple lumped parameter model (LPM) was implemented in order to assess the interest of 14 C and 3 H nuclear peaks in a highly anthropized aquifer system of southeastern Spain under intense agricultural development. It required i) to assess a correction factor for modern 14 C activities and ii) to reconstruct the 3 H recharge input function, affected by irrigation. In such a complex hydrogeological context, an exponential model did not provide satisfying results for all samples. A better solution was reached by taking into account the qualitative recent variation of the recharge rates into a combined exponential flow and piston flow model. Apart from presenting an uncommon approach for 14 C dating of modern groundwater, this study highlights the need of considering not only the variation of the tracer but also the variability of recharge rates in LPMs. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.