Comparison of 4He ages and 14C ages in simple aquifer systems: implications for groundwater flow and chronologies (original) (raw)

2000, Applied Geochemistry

He 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 4 He accumulation model is applied to explain these excess 4 He concentrations in terms of both in situ production and a crustal¯ux across the bottom layer of the aquifer. Results from the model simulations suggest variability in the 4 He¯uxes, ranging from 6 Â 10 À6 cm 3 STP cm À2 yr À1 for the Auob Sandstone Aquifer to 3.6 Â 10 À7 cm 3 STP cm À2 yr À1 for the Carrizo aquifer. For the Ojo Alamo and Nacimiento aquifers an intermediate value of 3 Â 10 À6 cm 3 STP cm À2 yr À1 was estimated. The contribution of in-situ produced 4 He 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 4 He contributes 27.5% and 15.4%, to the total 4 He 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 4 He 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 4 He in these aquifers. For the Auob Sandstone Aquifer the highest range in the velocity values is obtained (25 to 0.4 m yr À1 ). 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 À1 , and from 2 to 0.3 m yr À1 , 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 14 C measurements. On the basis of the results obtained by calibrating the model with the measured 4 He concentrations, the mean water residence times were estimated. They agree reasonably well with 14 C ages. When applied as chronologies for noble gas temperatures in the same aquifers, the calculated 4 He ages allow the identi®cation of three dierent climate periods similar to those previously identi®ed using 14 C ages: (1) the Holocene period (0±10 Ka BP), (2) the Last Glacial Maximum (118 Ka BP), and (3) the preceeding period (30±150 Ka BP). 7