OBTAINING REPRESENTATIVE GROUNDWATER QUALITY DATA FROM HETEROGENEOUS AQUIFER SYSTEMS

As part of Los Alamos National Laboratory's Hydrogeologic Workplan (1998-2005), 33 wells were drilled in and around the facility's 43 square-mile area to characterize the hydrogeologic setting of the Pajarito Plateau (PP), to understand potential contaminant pathways, and eventually to become part of a site-wide monitoring network. The complex hydrogeologic setting of the PP, consisting of ash flow tuffs, interbedded basalts, Rio Grande Rift-basin sediments, ancient river gravels, and alluvial fan materials, leads to heterogeneous flow paths within alluvial, perched intermediate, and regional flow systems and greatly increases the difficulty of drilling, necessitating the use of drilling fluids and additives. Groundwater data quality and detection monitoring of contaminants have been compromised in some of the well screens as a result of incomplete removal of drilling fluids during well development and possibly also because of low-flow sampling methods that prevent effective purging. To restore representative groundwater quality at these wells, a multiple-year rehabilitation program has been initiated. This includes field-testing hydrologic properties around the screens over time, modeling “skin” effects of reduced hydraulic conductivity at the screens, testing aquifer materials for the presence of secondary minerals formed by drilling-fluid induced changes in redox and speciation chemistry, modeling reaction kinetics, and re-qualifying data utilizing knowledge of specific effects on indicator solutes. Initial results of these studies show that single-screen wells produce groundwater data with the highest confidence. Site-specific biogeochemical conditions control reaction kinetics for reestablishing predrilling conditions at each well. In multiple-screen wells, tritium can always be detected, and nonconservative solutes can be detected in the more hydraulically conductive units. Aggressive redevelopment of some saturated intervals, combined with different sampling systems, holds promise for establishing reliable predrilling chemistry of the varied aquifer systems beneath the PP.