Inverse modeling of groundwater flow in the semiarid evaporitic closed basin of Los Monegros, Spain

• Published in: Hydrogeology journal Vol. 6; no. 1; pp. 33 - 49
• Main Authors: Samper-Calvete, F J, Garcia-Vera, MA
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01-06-1998
• Subjects: Anisotropy; Aquifers; Basins; Field tests; Fluid flow; Freshwater; Geology; Groundwater; Groundwater discharge; Groundwater flow; Hydraulics; Hydrogeology; Hydrology; Irrigation; Mathematical models; River basins; Structural basins; Water quality
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s100400050132

Only minor attention has been given in the past to the study of closed-basin hydrogeology in evaporitic environments, because these basins usually contain poor-quality groundwater. The motivation for hydrogeological research in the Los Monegros area in northeastern Spain was the approval in 1986 of a large irrigation project in the Ebre River basin. The irrigation of 60,000 ha is planned, partly in an evaporitic closed basin containing playa lakes. The project has given rise to environmental concerns. The evaluation of the hydrologic impacts of irrigation requires quantifying properly the hydrogeology of the area. With the available information, a conceptual hydrogeological model was formulated that identifies two main aquifers connected through a leaky aquitard. On the basis of the conceptual model, a numerical model was calibrated under steady-state conditions using the method of maximum-likelihood automatic parameter estimation (Carrera and Neuman, 1986a). The calibrated model reproduces the measured hydraulic heads fairly well and is consistent with independent information on groundwater discharge. By the solution of the inverse problem, reliable parameter estimates were obtained. It is concluded that anisotropy plays a major role in some parts of the lower aquifer. The geometric average of model conductivity is almost two orders of magnitude larger than the average conductivity derived from small-scale field tests. This scale effect in hydraulic conductivity is consistent with the findings of Neuman (1994) and Sanchez-Vila et al. (1996).Original Abstract: Dans le passe, on s'est peu interessea l'hydrogeologie des bassins fermes en milieu evaporitique, parce que ces bassins possedent en general de l'eau souterraine de qualite mediocre. L'interet porte aux recherches hydrogeologiques dans la region de Los Monegros, dans le nord-est de l'Espagne est dua l'approbation en 1986 d'un vaste projet d'irrigation dans le bassin de l'Ebre. L'irrigation de 60000 ha a ete programmee, en partie dans un bassin evaporitique ferme ou existent des lacs de playa. Le projet a fait emerge des preoccupations environnementales. L'evaluation des impacts hydrologiques de l'irrigation implique de quantifier correctement l'hydrogeologie de cette region. A partir des informations disponibles, un modele hydrogeologique conceptuel a eteetabli; il identifie deux aquiferes principaux interconnectes par drainance au travers d'un impermeable. Un modele numerique, etabli sur la base de ce modele conceptuel, a ete calibre en conditions de regime permanent, en utilisant la methode d'estimation automatique du maximum de vraisemblance des parametres (Carrera and Neuman, 1986a). Le modele calibre reproduit correctement la piezometrie mesuree et est en accord avec les informations concernant l'ecoulement des nappes. La resolution du probleme inverse a fourni des estimations acceptables des parametres. On en a conclu que l'anisotropie joue un role essentiel dans certaines parties de l'aquifere inferieur. La moyenne geometrique de la conductivite hydraulique est d'environ deux ordres de grandeur plus elevee que la conductivite hydraulique moyenne obtenue par des essais de terrain a l'echelle locale. Cet effet d'echelle sur la conductivite hydraulique est conforme aux resultats obtenus par Neuman (1994) et Sanchez-Vila et al. (1996).