Tracer-Based Piston-Flow Ages and Related Information 


This section documents newly interpreted tracer data for 812 sites evaluated as part of this compilation. In addition, some insights about recharge temperatures, derived from analysis of major dissolved-gas data, are briefly discussed.


Interpretations of tracer data are given as tracer- based piston-flow ages and as tracer-based piston-flow recharge dates. As addressed in Hinkle and others (2010), a groundwater sample collected during calendar year 2000 and attributed with a tracer-based piston-flow age of 10 years would have a tracer-based piston-flow recharge date of (calendar year) 1990. Tracer-based piston-flow ages and tracer-based piston-flow recharge dates often are censored with a “>” (greater than) or “<” (less than). For example, if CFC degradation appeared to be present in a sample, the tracer-based piston-flow age and tracer-based piston- flow recharge date could have an old bias. The tracer-based piston- flow recharge date could be accompanied by a “>” to indicate a greater (more recent) date, and the tracer-based piston-flow age by a “<” to indicate a smaller (younger) age. Sites with age-dating results that have been censored with “>” or “<” do not have discrete tracer-based piston-flow ages or discrete tracer-based piston-flow recharge dates.


Environmental Tracer Data


Environmental tracer data from 812 sites in 27 Study Units were interpreted here as tracer-based piston-flow ages. The data are organized by Study Unit and network.


The measured tracer concentration data, interpreted tracer-based piston-flow ages, and ancillary data for each network are reported in appendix B. The derived tracer-based piston-flow ages are examined for consistency with local age gradients and tritium data, and also are compared against each other when multiple tracers are available for a given site. A summary of the tracer-based piston-flow ages by sample is given in table 1 (at back of report). Where tracer-based piston-flow ages from more than one tracer type (CFCs, SF₆, ³H/³He) were available for a given site, interpretations from all tracers are provided.


Major-Dissolved-Gas-Based and Climate-Based Recharge Temperatures


A compilation of major dissolved-gas data that were used for estimation of recharge temperatures is included wherever such samples were taken. Recharge temperature often is assumed to be close to either the MAAT (Andrews, 1992) or the MAAT +1ºC (Stute and Schlosser, 2000) (MAAT +1ºC was used in this report). In many networks, the recharge temperatures based on major dissolved-gas data are comparable to MAAT +1ºC (fig. 2). Considering sites from aquifers composed of sediments, the differences between the recharge temperatures based on major dissolved-gas data and those based on MAAT +1ºC were, on average, about 0.54^oC (n=449). However, the standard deviation of these differences was 4.3^oC. As noted by Hinkle and others (2010), climate data are useful for estimation of average recharge temperatures; however, recharge temperatures vary greatly around this average, and characterization of site-specific recharge temperatures benefits from site-specific data, such as N₂-Ar- data, as long as these data are not affected by highly reduced conditions and gas-stripping. The uncertainty in the estimation of recharge temperature can lead to uncertainty in the tracer-based piston-flow ages that varies in magnitude depending on the age of the water sample (Plummer and Busenberg, 2000).


In this compilation, the climate data are significantly warmer for seven networks (ACAD, ALBE, CONN, LINJ, NECB, POTO, and SANA) than the N₂-Ar-inferred recharge temperatures and are shifted to the left of the 1:1 line in figure 2. For these seven networks, samples are taken from wells in locations where recharge likely occurs in the winter months when evapotranspiration is minimized. For these locations, the N₂-Ar-inferred recharge temperatures would be colder than the MAAT+1^oC. In locations that are not dominated by seasonal growth, N₂-Ar-inferred recharge temperatures and climate- based temperatures are more comparable. The large spread in N₂-Ar-inferred recharge temperatures for a network, such as UMIS, likely results from the mixture of oxic and suboxic conditions [identified where water contained O₂ < 1 mg/L, Mn > 50 μg/L, Fe >100 μg/L, and(or) CH₄ > 1 μg/L as discussed in Hinkle and others (2010)], as well as probable gas stripping issues. 


First posted July 31, 2012