Introduction Pump-and-treat is still the most common technology for the cleanup of ground water contamination (U.S. National Research Council 1999). However, the experience of recent decades confirms early warnings from Keely (1989) and Travis and Doty (1990) that with conventional practices aquifer restoration is not achievable within a short timeframe or at all if the contaminated source is not removed entirely. This is due to a limitation of mass transfer from residual con- taminant phases into ground water leading to rather low con- taminant release rates and consequently long lifetimes of the source (Hunt et al. 1988a, 1988b; Berglund and Cvetkovic 1995; Grathwohl 2000). It is commonly accepted that a com- plete source removal will only be possible in exceptional cases, even if there exists a substantial potential to tackle the source zone by in situ source treatment technologies. There- fore, in spite of short-term source treatment measures, a long-term plume management will be still required for the majority of contaminated sites. With this background in mind, we regard pump-and-treat as a long-term plume man- agement method to control contaminant spreading, rather than as a remediation technology aiming at full aquifer restoration within a reasonable timeframe. As a long-term measure, operational costs for pump-and-treat must be mini- mized to achieve cost efficiency. Here, efficiency refers to the hydraulic effect of the pump-and-treat measure, i.e., the control of the contamination disregarding the amount of con- taminant mass removed. Since pumping and on-site treat- ment costs are directly related to the pumping rate, a reduction of operational costs requires a reduction of the pumping rate that is necessary to guarantee plume control (capture). In this paper, we examine the potential of partial con- tainment strategies to reduce the pumping rate required for the pump-and-treat measure by the installation of physical barriers such as slurry walls or sheet piles. Different barrier settings (specified by location, shape, and length of the bar- rier) are analyzed with respect to their effect on the pump- ing rate within the framework of a modeling study on a Abstract A detailed analysis is presented of the hydraulic efficiency of plume management alternatives that combine a conventional pump-and-treat system with vertical, physical hydraulic barriers such as slurry walls or sheet piles. Var- ious design settings are examined for their potential to reduce the pumping rate needed to obtain a complete capture of a given contaminated area. Using established modeling techniques for flow and transport, those barrier configura- tions (specified by location, shape, and length) that yield a maximum reduction of the pumping rate are identified assuming homogeneous aquifer conditions. Selected configurations are further analyzed concerning their hydraulic performance under heterogeneous aquifer conditions by means of a stochastic approach (Monte Carlo simulations) with aquifer transmissivity as a random space function. The results show that physical barriers are an appropriate means to decrease expected (mean) pumping rates, as well as the variance of the corresponding pumping rate distri- bution at any given degree of heterogeneity. The methodology presented can be transferred easily to other aquifer sce- narios, provided some basic premises are fulfilled, and may serve as a basis for reducing the pumping rate in existing pump-and-treat systems. 856 Combining Pump- and- Treat and Physical Barriers for Contaminant Plume Control by Peter Bayer 1 , Michael Finkel 2 , and Georg Teutsch 3 1 Center for Applied Geoscience, University of Tuebingen, Sig- wartstrasse 10, 72076 Tuebingen, Germany; ++49 7071–2973178; fax ++49 7071–5059; peter.bayer@uni-tuebingen.de 2 Center for Applied Geoscience, University of Tuebingen, Sig- wartstrasse 10, 72076 Tuebingen, Germany; ++49 7071–2973177; fax ++49 7071–5059 3 UFZ-Umweltforschungszentrum Leipzing-Halle GmbH, Per- moserstrasse 15, 04318 Leipzig; (0341) 235-2242; fax (0341) 235- 2791; gf@ ufz.de Received August 2003, accepted January 2004. Copyright © 2004 by the National Ground Water Association. Vol. 42, No. 6—GROUND WATER—November–December 2004 (pages 856–867)