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Anthropization of a semiarid Mediterranean multi-layer aquifer system (Campo de Cartagena, SE Spain) : hydrodynamic, geochemical and isotopic approaches

Abstract : The Campo de Cartagena area in the Murcia region (SE Spain) is an emblematic case of the hydrological and environmental changes caused by the intensive use of groundwater for agriculture in semiarid Mediterranean areas. Agricultural development supported by the multi-layer aquifer system has led to the overexploitation of the deep layers, while irrigation return flow and the subsequent increased recharge rates increased water table levels in the unconfined layer. In addition, a large number of boreholes of the area are screened in several aquifers and allow an artificial connection between different groundwater masses. Moreover, as a consequence of the water table increase in the shallow aquifer, a permanent flow appeared in the last kilometres of the surface watershed. Together with the uncontrolled release of brines from private groundwater desalination, it induced a permanent surface flow of water to the main outlet of the system, the Mar Menor lagoon. In this context, understanding the complex evolution of the whole system and how the water balance is affected is a hard task.The first research focus aimed at collecting and reviewing all kind of existing data in order to reconstruct the evolution along one century of the multi-layer aquifer system. It highlighted an inversion of the vertical hydraulic gradient between aquifers and the decrease of water table levels up to 500 m for the deepest layer. Facing the difficult identification of the origin of groundwater samples, a method based on the Random Forest (RF) machine learning technique was developed. Despite the difficulty of an unequal dataset, accuracy over 90% was reached and 107 groundwater samples of unknown origin could be classified. Results were much better than using Linear Discriminant Analysis (LDA) or Decision Trees (CART).The second research axis was motivated by the difficulty to update the water balance of the multi-layer aquifer where irrigation return flow represents an additional source of recharge, added to the limited rainfall infiltration. Environmental tracers (14C, 13C, 2H, 18O, 3H) were combined to high-resolution temperature loggings to investigate the long-term evolution of recharge in the Campo de Cartagena aquifer system and discriminate local mixing processes (infra-boreholes) from regional mixing processes (between aquifers). Both pre-anthropization and post-anthropization recharge regime could be identified and quantified. Before the development of agriculture, recharge varied from 17 mm.a-1 in the mountain ranges to 6 mm.a-1 in the plain. In response to the increase of agricultural activity, recharge fluxes were amplified up to 210 mm.a-1 in irrigated areas.The third research axis consisted in quantifying submarine groundwater discharge (SGD). In order to decipher the influence of the different water sources on the Mar Menor, a radon (222Rn) and radium (223Ra, 224Ra) survey was combined with the hydrodynamic modeling of the lagoon. The areas of influence of a plume of radionuclides from the river were identified, the main areas of SGD were located and a location for a submarine emissary was proposed. Mass balances in winter and summer seasons provided total yearly SGD fluxes of water of 7.2-15.9 108 m3.a-1 (222Rn), 21.9-44.7 108 m3.a-1 (224Ra) and 6.9 108 m3.a-1 (223Ra, in winter). Water level effect, rather than tidal pumping, was identified as the main driver for recirculated saline groundwater, while fresh submarine groundwater discharge from the aquifer was about 1% of total SGD. As the case study is an extreme case of anthropization, these results present a wide interest that is not limited to the Murcia region or the Campo de Cartagena aquifer. The methods developed in this thesis might be used in other Mediterranean sites where groundwater exploitation seems to follow a continuous and inevitable increase.
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Paul Baudron. Anthropization of a semiarid Mediterranean multi-layer aquifer system (Campo de Cartagena, SE Spain) : hydrodynamic, geochemical and isotopic approaches. Earth Sciences. Université Montpellier II - Sciences et Techniques du Languedoc, 2013. English. ⟨NNT : 2013MON20070⟩. ⟨tel-01003775⟩

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