First International Conference on Saltwater Intrusion and Coastal Aquifers Monitoring, Modeling, and Management. Essaouira, Morocco, April 2325, 2001 Effects of aquifer heterogeneity on the intrusion of sea water H. Gotovac 1 , R. Andričević 2 and M. Vranje 1 1 Civil Engineering Department, University of Split, Croatia 2 Desert Research Institute, Las Vegas, Nevada, USA ABSTRACT In this study we examined the effects of geologic heterogeneity upon the seawater intrusion in coastal aquifers. Subsurface formations display spatial variability of the permeability such that it changes in an irregular manner in space over scales much larger than the pore scale. We describe hydraulic conductivity as random and characterize it statistically. The study domain is hypothetical aquifer with the properties similar to the actual case study of the Nitzanim aquifer in Israel. 100 realizations of the heterogeneous aquifer are generated and seawater intrusion problem using numerical code SUTRA [Voss, 1984] is solved. The results are statistically analyzed and compared to the common deterministic solution with homogeneous domain and macrodispersion coefficient. We examined the shape of the transition zone, intrusion length and salt concentration uncertainty as a result of aquifer heterogeneity. The results indicate that aquifer heterogeneity does affect the shape of the transition zone and shifts the zone inland compared to the deterministic solution with macrodispersion. The concentration uncertainty is more pronounced in the front of the transition zone and increases with depth. INTRODUCTION Seawater intrusion problems are becoming very important water management issues in coastal regions. As salt water intrudes the coastal aquifer, the groundwater abstraction, especially close to the coastline, becomes saline and remediation measures have to be taken. Since the saline and brackish water cannot be used directly for water supply and agriculture, often desalinization or artificial recharge is attempted to resolve the situation. These actions, particularly desalinization, are very expensive measures directly related to the salinity level in the pumped groundwater. When designing the exploitation of fresh groundwater in coastal aquifers it is important to predict the possible salt concentration level for different pumping scenarios. The salt concentration in the abstracted water will depend, besides system dynamics and considered processes, also on the aquifer geologic heterogeneity through which all physical processes are taking place. The recent advances in solute transport in aquifers are not directly applicable in saltwater intrusion cases because of coupled flow and transport problem. The saltwater intrusion problems are commonly treated either using the sharp interface models often employing analytical tools or applying transition zone models using numerical modeling. Subsurface geologic variability and dual density between fresh water and salt water in reality creates the transition zone between seawater and fresh inland water. In coastal regions, within that transition zone most groundwater management is usually taking