Approach Used in this Compilation


Environmental Tracer Data


Tracer data collected from FY 2006 to FY 2010 from 812 Land-Use Study (LUS), Major-Aquifer Study (MAS), Flowpath Study (FPS), and Reference (REF) sites were identified and compiled (table A1, appendix A). Analytical methods are described elsewhere (Busenberg and Plummer, 1992; Plummer and Mullin, 1997; Busenberg and Plummer, 2000). For internal consistency, CFC concentrations, which were originally measured over a range of analysis dates using a variety of calibration standards, were rescaled to the Scripps Institution of Oceanography 2005 scale of atmospheric CFC mixing ratios (Walker and others, 2009). All SF₆ data were scaled to the National Oceanic and Atmospheric Administration 2000 scale of atmospheric SF₆ mixing ratios (National Oceanic and Atmospheric Administration, 2008). 


For tracer data interpretation, the processes used are documented in detail in Hinkle and others (2010) and are not repeated here. The same procedure was used in this compilation in order to make the two reports consistent. One minor difference was that SF₆ concentrations from repeat samples from the same well were averaged to determine an age, instead of averaging the apparent ages of the two samples. This approach was used to be consistent with the way that the apparent CFC ages were determined for multiple samples from the same well. The tracer-based ages may approximate time-of-travel in certain physical settings; however, a more detailed knowledge of the hydrogeologic environment, as well as the development of mixing models, typically are needed to provide more realistic age interpretations. This kind of effort is beyond the scope of this report. 


Ancillary Chemistry, Water Level, Well Construction, and Tritium Data


Ancillary chemistry, water level, well construction, and ³H data for individual sites were obtained from the NAWQA Data Warehouse (DWH) (U.S. Geological Survey, 2011).


Water level and ancillary chemistry data typically were from the same sampling date as the environmental tracers, but in those cases where the date was not the same, a notation was made in the tables in appendix B. The ancillary chemistry data provided insight into redox conditions in the aquifer, particularly if major dissolved-gas data were not available.


Estimates of spatial and temporal variations in ³H in precipitation were used for reconstructed ³H analysis. Reconstructed ³H analysis is based on back-decaying measured ³H concentrations in groundwater, using tracer- based piston-flow ages to estimate the time-dependent amount of ³H decay, and comparing these undecayed or original ³H concentrations to historical ³H inputs (see for example: Dunkle and others, 1993; Ekwurzel and others, 1994). Estimates of ³H in precipitation were based on International Atomic Energy Agency data from 10 long- term stations in the conterminous United States. Missing temporal records were replaced with values based on correlation using long-term datasets from Ottawa, Canada (August 1953–December 1987) and Vienna, Austria (January 1988– December 2001). Concentrations prior to August 1953 were estimated using graphical pre-bomb distributions of Thatcher (1962).


First posted July 31, 2012