Impacts of human activities on recharge in a multilayered semiarid aquifer (Campo de Cartagena, SE Spain)

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English. The development of intense agriculture in semiarid areas modifies intensity and spatial distribution of groundwater recharge by summing irrigation return flow to limited rainfall infiltration. Environmental tracers provide key information, but their interpretation is complicated by more complex groundwater flow patterns. In multilayered aquifers, the real origin of the groundwater samples is hard to assess because of local mixing processes occurring inside long-screened boreholes. We use environmental tracers (C-14, C-13, H-2, O-18, H-3) to investigate the long-term evolution of recharge in the five-layer Campo de Cartagena aquifer in South-Eastern Spain, in addition to high-resolution temperature loggings to identify the depth of origin of groundwater. Despite the complex background, this methodology allowed a reliable interpretation of the geochemistry and provided a better understanding of the groundwater flow patterns. The tritium method did not give good quantitative results because of the high variability of the recharge signal but remained an excellent indicator of recent recharge. Nonetheless, both pre-anthropization and post-anthropization recharge regime could be identified and quantified by radiocarbon. Before the development of agriculture, recharge varied from 17mm.year(-1) at the mountain ranges to 6mm.year(-1) in the plain, whereas the mean annual rainfall is about 300mm. In response to the increase of agricultural activity, recharge fluxes to the plain were amplified and nowadays reach up to 210mm.year(-1) in irrigated areas. These values are strengthened by global water budget and local unsaturated zone studies. Copyright (c) 2013 John Wiley & Sons, Ltd.