Introduction


The age of groundwater is defined as the time required for a water molecule to travel from a point of recharge to a measurement point such as a well. An understanding of groundwater age can be used to infer groundwater flowpaths and rates of recharge or biogeochemical reactions, reconstruct contaminant loading histories, explain trends in groundwater quality, gain insight into groundwater susceptibility to contamination, and constrain groundwater flow and transport models. Environmental tracers are ideal tools to estimate ages because they have a global signature, and their input histories to aquifers typically can be constrained. In this report, reference to environmental tracers pertains specifically to the use of chlorofluorocarbons (CFCs) (Busenberg and Plummer, 1992; Plummer and Busenberg, 2000; International Atomic Energy Agency, 2006; Hinkle and others, 2010), sulfur hexafluoride (SF₆) (Busenberg and Plummer, 2000; International Atomic Energy Agency, 2006), and the combination of tritium/helium-3 (³H/³He) (Schlosser and others, 1989; Solomon and Cook, 2000). Tracer ages are interpreted using the assumption of piston-flow conditions and can provide an initial interpretation of age structure in an aquifer, but also provide a practical means of reporting tracer concentration data. The use of tracer-based piston flow ages is discussed in detail in Hinkle and others (2010). The approach used here is identical to that of Hinkle and others (2010).


The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) is tasked with: (1) describing the status of and trends in water quality of large, representative portions of the Nation’s water resources, and (2) providing an understanding of natural and anthropogenic factors affecting the quality of these resources (Gilliom and others, 1995). Tracer-based piston-flow ages are an important component in the effort to achieve these goals.


The NAWQA Program is composed of geographically and hydrologically distinct Study Units located throughout the United States. Study Units and bibliographies of Study-Unit publications are described in detail in U.S. Geological Survey (2010). 


Groundwater assessments within Study Units include networks (groups related by commonality of targeted resource) of 20–30 randomly distributed wells. These networks include wells from Major-Aquifer Studies (MASs), Land-Use Studies (LUSs), Flowpath Studies (FPSs), and also include Reference (REF) wells. These networks are described in detail in Hinkle and others (2010).


Purpose and Scope


Tracer data have been collected from NAWQA groundwater sites since 1992. The NAWQA Program has a need for: (1) a comprehensive listing of known tracer datasets, and (2) a compilation of estimates of groundwater age for these groundwater samples. One such compilation was published by Hinkle and others (2010), and includes NAWQA data from fiscal years 1992–2005. [The Federal fiscal year (FY) begins October 1 and ends September 30 of the year with which it is numbered.] This report summarizes tracer data that were collected from NAWQA LUS, MAS, FPS, and REF wells from FY 2006–10 during the second decadal cycle of the NAWQA Program. A total of 812 sites are included in this report: 437 LUS, 281 MAS, 72 FPS, and 21 REF sites, plus 1 dual-purpose LUS/FPS site. 


Only samples from wells were assembled for this report; samples from springs were not included because of an almost exclusive programmatic focus on wells for groundwater sampling (Gilliom and others, 1995).


The tracers of interest include CFCs (CFC-11, CFC- 12, and CFC-113), SF₆, and ³H/³He, which are useful in determining tracer-based piston-flow ages on the order of years to decades. In addition, data on the major dissolved gases (N₂, O₂, Ar, CH₄, and CO₂) were assembled where available because they are used to constrain recharge temperatures and excess air concentrations.


The tracer data compiled in this report represent the CFC, SF₆, and ³H/³He data associated with the targeted (LUS, MAS, FPS, and REF) wells and collected during the targeted timeframe (FY 2006–10) from NAWQA Study Units. These Study Units are shown in figure 1 and are listed in table A1 (appendix A).


In the following sections, the tracer datasets that were assembled for this report are described, and the tracer-based piston-flow ages are provided. In addition, some insights on recharge temperatures that resulted from this compilation are briefly discussed. 


First posted July 31, 2012