Estimating the impact of a discontinuous confining layer on the seawater intrusion using a stochastic approach: the Oristano coastal aquifer site (Sardinia, Italy)

The study site covers an area of 273 km in the Oristano plain on the western coast of central Sardinia (Italy). In the coastal area, since 1950, intensive agriculture and subsequent urbanization have resulted in overexploitation of the aquifer system and significant deterioration of groundwater quality due to seawater encroachment. The groundwater basin, hosted in the Quaternary alluvia released by the Tirso river, is formed of a shallow phreatic aquifer and a deeper semi-confined aquifer, separated by a possibly discontinuous thin clayey aquitard. Hydrogeological and hydrochemical data [Pala & Cossu, 1994; Barroccu et. al, 1995] do not allow for a precise characterization of this confining layer, but suggest that partial hydraulic connection between the upper and lower aquifer units occurs. In this study we assess the impact of the aquitard on the salt dispersion process by running simulations based on different realizations of its hydraulic conductivity (K) distribution. The proposed methodology consists in three steps: (1) a stochastic approach to generate synthetic realizations of random spatially correlated parameters K, (2) a modeling study, based on a three dimensional coupled flow and transport groundwater model, to simulate for each realization pressure heads and solute concentrations in the aquifer system, and (3) a statistical analysis of the simulated fields. With this strategy we attempt to cover the uncertainty of the model parameter K in the prediction of fluid and solute mass exchange between upper and lower aquifer units in the framework of a rational development on groundwater resources for the study site. DESCRIPTION OF THE STUDY AREA Adequate water supply is one of the major problems afflicting Sardinia due to the absence of permanent water resources and the prolonged periods of drought, which have caused in recent years rationing measures on urban, agricultural and industrial uses. As a consequence, a substantial increment of groundwater withdrawal has been observed especially along coastal areas. The study site is located in the coastal zone of the Oristano plain (Figure 1) in the western part of central Sardinia (Italy). The aquifer basin is formed of two main units, a shallow phreatic aquifer (few tens of meters) and a deeper aquifer (few hundreds of meters), separated by a variable thickness, possibly vanishing, aquitard. The groundwater reservoir is one of the more productive in the region: it is estimated that more then 25000, between used and disused, agricultural wells are present in the plain. Because of inadequate policies of water resource management, groundwater has been heavily contaminated by seawater intrusion due to aquifer overexploitation. Over the last 5 years an increase of about 20% in the electric conductivity measurements has been found in the wells located close to the sea [Cau et. al., 2002(b)]. These measurements also showed high contamination levels both in the phreatic and confined aquifers, suggesting the existence of partial hydraulic connection between these units. The effect is presumably connected to the leakage of water through sandy lenses and corroded and failed casings of abandoned wells. Furthermore these preferential paths through the aquitard have shown to have a strong influence on the groundwater dynamics when subjected to intensive pumping [Cau et al., 2002(a)]. Figure 1. Location of the Oristano plain. Hydrography and topography maps are also shown. THE MONTE CARLO SIMULATION A preliminary modeling study [Cau et al., 2002(b)] showed that the existence of localized areas of severe upconing or salt buildup largely depends, for the given spatial distribution of wells and their exploitation rates, on the heterogeneity of the aquitard hydraulic conductivity field K. In this study Monte Carlo simulation has been used to quantify the impact of the confining layer on the salt dispersion process in order to evaluate the potential risk of contamination due to different exploitation schemes. To this end we compare two modeling schemes for the aquifer system that differ only for the degree of spatial variability of the aquitard K field: the homogeneous and the heterogeneous cases. The former approach assumes an impervious confining layer with an isotropic conductivity of 10 m/s. The latter approach is based on stochastic realizations of the aquitard conductivity that mimic the random occurrence of spatial discontinuities in the confining layer. The K field generation has been carried out with the HYDRO_GEN computer code [Bellin & Rubin, 1996]. Let Y be a function with a Gaussian distribution of mean ) ln(K = Y and variance , Y was modeled as a stationary random space function with an exponential covariance function , where 2 Y σ r Y Y e C − = 2 σ 2 2         +       =