GROUNDWATER MODELING OF A COMPLEX HYDROLOGIC SYSTEM IN SOUTH CAROLINA THROUGH THE USE OF ANALYTIC ELEMENTS M. Tolika 1 and E. K. Paleologos 2,∗ 1 Department of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece; 2 Department of Geological Sciences, University of South Carolina, Columbia,U.S.A. (*author for correspondence: e-mail: epal@geol.sc.edu; Phone: 803-777-8125; Fax: 803-777-6610) Abstract. The objective of this study is to evaluate the use of the Analytical Element Method (AEM) toward multiobjective, multiscale, ongoing modeling needs at complex hydrologic sites such as those managed by the US Department of Energy. This method presents several advantages over the tradi- tional numerical methods that include absence of grid, natural incorporation of hydrologic features, and generation of an exact solution at every point in a flow field. The AEM with its semi-analytical formulation is particularly efficient in addressing what-if scenarios, the resolution of boundary con- ditions, and the incorporation of new data all of which are important aspects of remediation efforts in complex sites. Our model accounted for important hydrologic features in an area of the Savannah River Site, South Carolina that included river branches, artificial surface basins, monitoring wells, and the existence of heterogeneities. Our simulated heads were found to be in excellent agreement with the measured heads, with over 90% of the wells exhibiting a maximum discrepancy of less than 10 ft. The AEM was found to be a very efficient and fast method for the analysis of a flow field even when a limited number of elements was considered. The AEM was seen to lead to better physical understanding and resolution of the critical components of a groundwater system and it can offer significant advantages in using models to guide site characterization and remediation efforts. Keywords: analytic element method, groundwater flow, regional model 1. Introduction This paper presents a regional model of the groundwater flow field at the Gen- eral Separations Area(GSA), South Carolina that is based on the Analytic Element Method (AEM). The AEM (Strack, 1989; Haitjema, 1995) is an object-oriented, semi-analytical method that resembles the classical boundary element methods with solutions not necessarily being Cauchy integrals but closed-form analytical expressions (Strack, 1999). The method does not make use of a mesh to obtain the hydraulic head or flux at specific points of the grid but rather utilises the superposi- tion of analytical elements, which approximate the influence of various hydrologic features of a region on a background, uniform field, to obtain the overall solution at any point in the hydraulic system. Thus, point analytical elements may repre- sent wells, line elements can approximate creeks, rivers, or geologic contacts, and lakes or aquifer properties (hydraulic conductivity, recharge, and leakage) are rep- resented by area elements (Jankovic and Barnes, 1999a,b; Strack and Jankovic, Water, Air, and Soil Pollution: Focus 4: 215–226, 2004. C  2004 Kluwer Academic Publishers. Printed in the Netherlands.