Comparison of 4He ages and 14C ages in simple aquifer systems: Implications for groundwater flow and chronologies

Maria Clara Castro, Martin Stute, Peter Schlosser

Research output: Contribution to journalArticlepeer-review

106 Scopus citations


4He concentrations in excess of the solubility equilibrium with the atmosphere by up to two to three orders of magnitude are observed in the Carrizo Aquifer in Texas, the Ojo Alamo and Nacimiento aquifers in the San Juan Basin, New Mexico, and the Auob Sandstone Aquifer in Namibia. A simple 4He accumulation model is applied to explain these excess 4He concentrations in terms of both in situ production and a crustal flux across the bottom layer of the aquifer. Results from the model simulations suggest variability in the 4He fluxes, ranging from 6 x 10-6 cm3 STP cm-2 yr-1 for the Auob Sandstone Aquifer to 3.6 x 10-7 cm3 STP cm-2 yr-1 for the Carrizo aquifer. For the Ojo Alamo and Nacimiento aquifers an intermediate value of 3 x 10-6 cm3 STP cm-2 yr-1 was estimated. The contribution of in-situ produced 4He to the measured concentrations was also estimated. This contribution is negligible for the Auob Sandstone Aquifer as compared with both the concentrations measured at the top and bottom of the aquifer for most of the pathway. In the Carrizo aquifer, in-situ produced 4He contributes 27.5% and 15.4%, to the total 4He observed at the top and bottom of the aquifer, respectively. For both aquifers of the San Juan Basin in-situ production almost entirely dominates the 4He concentrations at the top of the aquifer for most of the pathway. In contrast, the internal production is negligible as compared with the measured concentrations at the bottom of these aquifers, reaching, at most, 1.1%. The model simulations require an exponential decrease in the horizontal velocity of the water with increasing recharge distance to reproduce the distribution of 4He in these aquifers. For the Auob Sandstone Aquifer the highest range in the velocity values is obtained (25 to 0.4 m yr-1). The simulations for the Carrizo aquifer and both aquifers located in the San Juan Basin require velocities varying from 4 to 0.1 m yr-1, and from 2 to 0.3 m yr-1, respectively. For each aquifer, average permeability values were also estimated. They are generally in agreement with results obtained from pumping tests, hydrodynamic modeling and previous 14C measurements. On the basis of the results obtained by calibrating the model with the measured 4He concentrations, the mean water residence times were estimated. They agree reasonably well with 14C ages. When applied as chronologies for noble gas temperatures in the same aquifers, the calculated 4He ages allow the identification of three different climate periods similar to those previously identified using 14C ages: (1) the Holocene period (0-10 Ka BP), (2) the Last Glacial Maximum (κ 18 Ka BP), and (3) the preceeding period (30-150 Ka BP).

Original languageEnglish (US)
Pages (from-to)1137-1167
Number of pages31
JournalApplied Geochemistry
Issue number8
StatePublished - 2000
Externally publishedYes

ASJC Scopus subject areas

  • Environmental Chemistry
  • Pollution
  • Geochemistry and Petrology


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