Improved geostatistical simulation of the spatial heterogeneity of deltaic hydrofacies for environmental applications

Abstract

The spatial geological heterogeneity of an aquifer significantly affects groundwater storage, flow and the transport of solutes. In the particular case of coastal aquifers, spatial geological heterogeneity is also a major determining factor of the spatio-temporal patterns of water quality (salinity) due to seawater intrusion. The hydraulics of coastal hydrogeology can be modelled effectively by variable density flow equations but the aquifer geology is highly uncertain. A stochastic solution to the problem is to generate numerical models of the geology using sequential stratigraphy, geophysical models or geostatistical approaches. The geostatistical methods (two-point geostatistics, Markov chain models and multiple-point geostatistics) have the advantage of minimal data requirements, e.g., when the only data available are from cores from a few sparsely located boreholes. Advances in the geostatistical simulation of hydrofacies (categorical variables) have included more realistic models and more flexibility in variogram modelling. The variogram model used in the simulation of hydrofacies significantly influences the degree of connectivity of the hydrofacies in the simulated model. The choice of model is critical as connectivity determines the amount and extent of seawater intrusion and hence the environmental risk. The methodology is illustrated with a case study of the Andarax river delta, a coastal aquifer in south-eastern Spain. This is a semi-arid Mediterranean region in which the increasing use of, and demand for, groundwater is exacerbated by a transient tourist population that reaches its peak in the summer when the demand for the permanent population is at its highest. The work reported her contributes to the design of optimal management systems for groundwater resources.