EXECUTIVE SUMMARYThe groundwater flow and radionuclide transport model characterizing the Shoal underground nuclear test has been accepted by the State of Nevada Division of Environmental Protection. According to the Federal Facility Agreement and Consent Order (FFACO) between the U.S. Department of Energy and the State of Nevada, the next steps in the closure process for the site are model validation (or postaudit), proof of concept, and long-term monitoring. This report addresses the development of the monitoring strategy for Shoal, which is needed for preparing the subsurface Corrective Action Decision Document/Corrective Action Plan (CADD/CAP). The proposed monitoring plan builds on three different, yet complementary, approaches (or tools) for locating the monitoring wells around the site with the main objective being detection monitoring and the secondary objective being data collection for model validation. The purpose of a detection-based monitoring system is the identification of groundwater contamination before a plume traverses a regulatory boundary located hydraulically downgradient of the contamination source. The design of such a system entails locating monitoring wells in the areas likely to encounter plume migration.The first tool is applied to select a number of potential siting horizons to which monitoring wells could be allocated. Based on plume geometry, this tool is used to determine the efficiency of each siting horizon and the minimum number of wells needed to span each horizon for detection monitoring. Different siting horizons can thus be ranked for detection efficiency by evaluating, for each horizon, the ratio of the maximum well spacing to the width of the potential zone of contaminant migration. A large value of this ratio indicates an effective horizon because the migration zone can be traversed with fewer wells. When a large number of monitoring wells are planned, a mathematical programming model that allocates a specified number of monitoring sites throughout the model domain can then be used. For Shoal, however, the number of monitoring wells is expected to be relatively small thereby allowing one to allocate the potential wells to the siting horizons with the highest efficiency rankings, provided that other constraints are being considered in this allocation process.Five siting horizons or control planes (CPs) have been selected for analysis. The five CPs are oriented perpendicular to the mean flow direction, which is not parallel to the model's y-coordinate. The selection of the location of these CPs is aimed at providing the necessary distances from the compliance boundary for a reaction time of 50 years. The farthest CP (CP #5) passes through the western edge of the maximum contaminant level (MCL)-based contaminant boundary (assumed here to be the compliance boundary). CP #4 is located at a distance equivalent to a 50-year reaction time (about 60 m) from the farthest point on the MCL boundary. CP #3 is at a distance of 60 m from CP #5. The next CP (CP #2) passes through the ea...