Reliability assessment of groundwater monitoring networks at landfill sites N. Buket Yenigu¨l a, * , Amro M.M. Elfeki a,1 , Johannes C. Gehrels a,2 , Cees van den Akker a , Andre´ T. Hensbergen b , F. Michel Dekking b a Faculty of Civil Engineering and Geosciences, Section of Hydrology and Ecology, TU Delft, P.O. Box 5048, 2600 GA Delft, The Netherlands b Faculty of Electrical Engineering, Mathematics and Computer Science TU DELFT, P.O. Box 5031, 2600 GA Delft, The Netherlands Received 19 May 2003; revised 18 October 2004; accepted 25 October 2004 Abstract Landfills represent a significant threat to groundwater contamination due to their nature of operation and their abundance. Monitoring well networks at these sites are of vital importance in detecting leakage plumes. This study presents a reliability assessment to estimate the performance of groundwater monitoring systems at landfill sites. A hypothetical problem is presented where the detection probability of several monitoring systems is compared. A Monte–Carlo approach is used to incorporate uncertainties due to subsurface heterogeneity and the leak location. Hydraulic conductivity and leak location are considered as random variables with prescribed probability density functions. A finite difference groundwater model coupled with a random walk particle-tracking model simulates a contaminant plume released from the landfill for each Monte–Carlo realization. The analysis shows that lateral dispersivity of the medium has a significant influence on the reliability of the monitoring system, since it is the primary parameter controlling the width of the contaminant plume. Furthermore the number and the location of the monitoring wells are dependent on the heterogeneity of the medium and size of the contaminant leak. It is concluded that the reliability of the common practice of three downgradient monitoring wells is inadequate from the point of view of prevention of groundwater contamination due to landfills. q 2004 Elsevier B.V. All rights reserved. Keywords: Detection probability; Groundwater monitoring; Landfill; Contaminant plume; Monte–Carlo analysis 1. Introduction Landfills represent a widespread and significant threat to groundwater quality, human health and even some of the ecosystems due to their nature of operation and abundance. In communal language landfill means waste disposal on land. However, technically the International Solid Wastes Association (Bagchi, 1994) defines landfill as ‘the engineered 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.10.017 Journal of Hydrology 308 (2005) 1–17 www.elsevier.com/locate/jhydrol * Corresponding author. Fax: C31 15 2785915. E-mail addresses: n.b.yenigul@citg.tudelft.nl (N.B. Yenigu¨l), a.m.elfeki@citg.tudelft.nl (A.M.M. Elfeki), j.gehrels@nitg.tno.nl (J.C. Gehrels), j.m.dejong@citg.tudelft.nl (C. van den Akker), a.t.hensbergen@ewi.tudelft.nl (A.T. Hensbergen), f.m.dekking@ ewi.tudelft.nl (F.M. Dekking). 1 On leave from Faculty of Engineering, Mansoura University, Egypt. 2 TNO-NITG. P.O. Box 80015, 3508 TA Utrecht, The Netherlands.