Mathematical Geology, Vol. 23, No. 8, 1991 Monitoring Well Placement Using Conditional Simulation of Hydraulic Head I Dale H. Easley, 2 Leon E. Borgman, 3 and Dennis Weber 4 A theoretical technique for conditioning simulations in frequency domain is developed and then applied to a hydraulic-head data set from Pittman, Nevada. Frequency-domain simulation rapidly generates simulations while requiring minimal computer memory. This makes it possible by using a personal computer to create large numbers of simulations of a physical parameterfieldfor use in studying stochastic processes. In our application, groundwater flowlines are constructed from the simulations of the hydraulic head field. Then, the crossings of the flowlines at a transect down- stream from a contaminant point source generate histograms for predicting the probability of plume interception by groundwater monitoring wells. The simulation process is discussed in detail for the Pittman study site. KEY WORDS: conditional simulation, contaminant migration, geostatistics, plume interception. INTRODUCTION Geostatistical simulation shows great promise in the study of geological and hydrological problems where the heterogeneity of the subsurface often cannot be adequately characterized for use in deterministic models (Weber et al., 1991 ; King and Smith, 1988; Journel and Isaaks, 1984). It has been used in these areas less often than estimation techniques such as kriging (Aboufirassi and Marino, 1984; Neuman and Jacobson, 1984) because the simulation algorithms require larger amounts of computer memory and time. The goal of the research reported in this paper was to develop and demonstrate faster and less memory- intensive algorithms for geostatistical conditional simulation (GCS) for use in earth science applications. The method presented here uses the computationally a Received 30 April 1990; accepted 15 April 1991. 2Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70122. 3Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82070. 4Environmental Research Center, Las Vegas, Nevada. 1059 0882-8121/91 / I 100-1059506.50/I © 1991 International Association for M athcmatical Geology