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Bull. Environ. Contam. Toxicol. (2000) 64: l-7 ? 2000 Springer-Verlag New York Inc. Effects of Two-Dimensional Mass Transport Modeling on Groundwater Monitoring Configurations P. F. Hudak Department of Geography and Environmental Science Program, University of North Texas, Denton, TX 76203-5279, USA Received: 3 August 1999/Accepted: 17 October 1999 Landfills are a principal source of contamination to aquifers. With the proliferation of environmental laws over the past few decades, there has been a shift from small, poorly-designed landfills to large waste storage facilities equipped with liners and leachate collection systems. Although most modem landfills contain measures to prevent subsurface contamination, they are still susceptible to leaking in time. For example, clogged leachate collection pipes and deteriorated liners can lead to groundwater contamination. Large landfills have significant contamination potential due to a substantial volume of stored waste. Consequently, they warrant sound strategies for detecting contaminant releases. Groundwater monitoring is a key component of contaminant release detection systems. Well-designed groundwater monitoring configurations can identify pollutants before they inundate broad areas of aquifers and contaminate water supplies. Previous authors have devised sampling strategies for toxic material buried in abandoned landfills (Parkhurst 1984), contaminated aquifers (Rouhani 1985; Loaiciga 1989; Hudak and Loaiciga 1992), and clean aquifers at risk of contamination (Meyer and Brill 1988; Wilson et al. 1992; Hudak 1994). The latter problem, positioning wells to detect future contamination, is difficult because the contaminant-release locations are unknown. An efficient configuration of detection wells must intersect plumes initiating anywhere within the footprint of a landfill. Furthermore, the plumes should be detected before reaching a downgradient compliance boundary (Domenico and Palcia