Summary and Conclusions


CFC, SF₆, and ³H/³He tracer data collected from 812 NAWQA Program groundwater sites across the United States were compiled in this report. The data were from Land- Use Study (LUS), Major-Aquifer Study (MAS), Flowpath Studies (FPS) and Reference (REF) networks. The time period focused on Federal fiscal years 2006–10. Tracer data from other NAWQA Program components were not compiled here.


Tracer data were evaluated using established methods and are presented as aqueous concentrations, equivalent atmospheric concentrations (for CFCs and SF₆), and tracer- based piston-flow ages, and also include selected ancillary data, such as redox data, well-construction data, and major dissolved-gas (N₂, O₂, Ar, CH₄, and CO₂) data. Brief summaries of each tracer dataset also are included.


Recharge temperature often is inferred using climate data (approximated by MAAT +1ºC); however, these temperatures typically only represent an average recharge temperature, around which there can be significant variation as determined using the N₂-Ar-based data. For aquifers composed of sediments in this compilation, differences between N₂-Ar-based recharge temperatures and recharge temperatures based on MAAT+1°C were, on average, within about 0.54°C; however, the standard deviation of these differences was 4.3°C. In this compilation, not only did N₂-Ar-based temperatures show significantly more variation than climate-based temperatures, but they also showed entire networks where recharge is limited to the winter months when evapotranspiration is limited and the climate data are significantly warmer than the N₂-Ar-inferred recharge temperatures. The N₂-Ar-based temperatures also showed effects of degassing resulting from highly reduced conditions in numerous networks. The degassing, usually by excess N₂, can partially strip CFCs, SF₆, and ³He resulting in an old bias in CFC and SF₆ ages and young bias in ³H/³He ages.


An understanding of groundwater age can be used to (1) infer groundwater flowpaths and rates of recharge or biogeochemical reactions, (2) reconstruct contaminant loading histories, (3) explain trends in groundwater quality, (4) gain insight into groundwater susceptibility to contamination, and (5) constrain groundwater flow and transport models. Characterizing groundwater age using tracers, however, is a complex undertaking. As evidenced in this compilation, as well as that of Hinkle and others (2010), collection of tracer data does not necessarily lead to determination of groundwater age. Collecting tracer data is only part of the investigation, but can be of greater use if multiple tracers are utilized. In addition, the outcome can be improved if the selected tracers are relevant to the area that is sampled (that is, are not degraded, do not have additional unconstrained sources, etc.), additional tracers are included that would provide age estimates on the timescale of centuries or millennia for wells with mixtures of very old and very young water, ancillary geochemical data are collected, and the study design is carefully considered (attempting to avoid wells with large open holes, heavily pumped wells, etc.). 


First posted July 31, 2012