Effective management of available groundwater resources and strategies for remediation of water impacted by past nuclear testing practices depend on knowledge about the migration of radionuclides in groundwater away from the sites of the explosions. A primary concern is to assess the relative mobilities of the different radionuclide species found near sites of underground nuclear tests and to determine the concentration, extent, and speed of this movement. Ultimately the long term transport behavior of radionuclides with half-lives long enough that they will persist for decades, their interaction with groundwater, and the resulting flux of these contaminants is of paramount importance. As part of a comprehensive approach to these assessments, more than three decades of site-specific sites studies have been undertaken at the Nevada Test Site (NTS) which have focused on the means responsible for the observed or suspected movement of radionuclides away from underground nuclear tests (RNM, 1983). More recently regional and local models of groundwater flow and radionuclide transport have been developed as part of a federal and state of Nevada program to assess the long-term effects of underground nuclear testing on human health and environment (e.g., U.S. DOE/NV, 1997a; Tompson et al., 1999; Pawloski et al., 2001). Necessary to these efforts is a reliable measure of the hydrologic source term which is defined as those radionuclides dissolved in or otherwise transported by groundwater (Smith et al., 1995). Measurement of radionuclides in waters sampled near the sites of underground nuclear test provides arguably the best opportunity to bound the hydrologic source term. This empirical approach was recognized early and concentration data has been collected annually since mid-1970's. Initially three sites were studied at the NTS; over the years the program has been expanded to include more than fifteen study locations. As part of various field programs, Lawrence Livermore National Laboratory and Los Alamos National Laboratory have annually returned water samples from wells in near-field locations at the NTS for radiochemical analyses. This report makes the distinction between samples taken in the near-field and the far field. The near-field includes the area extending radially {approx}300 meters from surface ground zero (the firing point of an underground nuclear test projected upwards on ground surface). Over the years this sampling program has also been refereed to as the ''hot-well monitoring program'' because these water samples contained concentrations of tritium above natural background (tritium concentrations in southern Nevada precipitation are 0.5 to 2.0 Bq/L, Farmer et al., 1998). A majority of the hot wells contain tritium in excess of the 741 Bq/L (20,000 pCi/L) drinking water standard (Smith et al., 1996a; Smith et al., 1997). The sites which comprise our current hot well sampling network are plotted on a map of the NTS in Figure 1.
