The problem of soil degradation through alkalinization/salinization in an irrigated area with a semi-arid climate was examined in the inner delta of the Niger River, Mali, by the study of groundwater hydraulics and hydrochemistry in an area recharged by irrigation water. On the basis of data analysis on various scales, it is concluded that the current extent of the surface saline soils is due to a combination of three factors: (1) the existence of ancient saline soils (solonchaks) resulting from the creation of a broad sabkha west of the former course of the Niger River, now called the Fala of Molodo. These saline crusts were gradually deposited during the eastward tilting of the tectonic block that supports the Niger River; (2) the irrigation processes during the recent reflooding of the Fala of Molodo (river diversion in 1950). These used very poorly mineralized surface water but reintroduced into the alluvial groundwater system -generally of a low permeability (Kp10 -6 m s -1 ) -salts derived from the ancient solonchaks; and (3) the redeposition of the dissolved salts on the surface due to the intense evapotranspiration linked to the present Sahelian climate. In this context, only efficient artificial draining of subsurface alluvial groundwater can eliminate most of the highly mineralized flow and thus reduce the current saline deposits.Résumé En périmètres irrigués sous climat semi aride (delta intérieur du fleuve Niger, Mali), la dégradation des sols par alcalinisation/salinisation est abordée par l'analyse hydrodynamique et hydrogéochimique des eaux souterraines sous-jacentes. A partir d'informations recueillies à différentes échelles, nous proposons un mode de mise en place actuelle des salants en surface sur la base de trois concepts: (1) l'existence de salants anciens résultant de la création d'une vaste zone de sebkhra à l'Ouest du cours ancien du Niger (Fala de Molodo). Cette mise en place se serait faite progressivement au cours du basculement vers l'Est du bloc tectonique supportant le fleuve; (2) au cours de la remise en eau récente du Fala (1950), les processus d'irrigation à partir d'eau de surface très peu minéra-lisée, réintroduisent dans la nappe alluviale globalement peu perméable des sels provenant des mises en solution des salants anciens; (3) par phénomène d'évapotranspiration intense lié au climat actuel, ces sels se redéposent en surface. Dans un tel contexte, seul un drainage artificiel efficace des eaux souterraines de sub-surface (nappe alluviale) permettrait de soustraire le maximum de flux très minéralisé et, en conséquence, de limiter les dépôts salins actuels.Resumen En perimetros irigados bajo un clima arida (delta interior del rio Niger, Mali), la degradacion de los suelos (alcalinisacion/salinisacion) es analizada con respecto a las aguas subterraneas. A partir de recoleccion de dadas en hidrodinamica, geofisica y hidroquimica, proponemonos un modo de formacion actuale de los salinos segun tres episodios sucesivos: (1) salinos antiguos que se deducen de la formacion de una Sebkh...

Numerical groundwater modelling to support mining decisions is often challenging and time consuming. Simulation of open pit mining for model calibration or prediction requires models that include unsaturated flow, large magnitude hydraulic gradients and often require transient simulations with time varying material properties and boundary conditions. This combination of factors typically results in models with long simulation times and/or some level of numerical instability. In modelling practice, long run times and instability can result in reduced effort for predictive uncertainty analysis, and ultimately decrease the value of the decision-support modelling. This study presents an early application of the Iterative Ensemble Smoother (IES) method of calibration-constrained uncertainty analysis to a mining groundwater flow model. The challenges of mining models and uncertainty quantification were addressed using the IES method and facilitated by highly parallelized cloud computing. The project was an open pit mine in South Australia that required predictions of pit water levels and inflow rates to guide the design of a proposed pumped hydro energy storage system. The IES calibration successfully produced 150 model parameter realizations that acceptably reproduced groundwater observations. The flexibility of the IES method allowed for the inclusion of 1493 adjustable parameters and geostatistical realizations of hydraulic conductivity fields to be included in the analysis. Through the geostatistical realizations and IES analysis, alternative conceptual models of fractured rock aquifer orientation and connections could be conditioned to observation data and used for predictive uncertainty analysis. Importantly, the IES method out-performed finite difference methods when model simulations contained small magnitude numerical instabilities.