T136 Innovative Tracer Applications in Hydrogeology: New Techniques, Design and Interpretation Methods, and Case Studies I

Tuesday, 9 November 2004
8:00 AM - 12:00 PM

A COMPARISON OF SF6 AND 3H/3HE DERIVED GROUNDWATER AGES IN ARAIHAZAR, BANGLADESH

Paper number: T136-12
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

This study explores whether sulfur hexafluoride, SF6, can be used as a transient tracer for determining groundwater ages in a shallow aquifer in Bangladesh, either independently or in conjunction with 3H/3He. Samples for the tracer measurements were collected from four depth levels from each of two well nests in Araihazar Upazilla, Bangladesh (23.7°E, 90.6°N). Good agreement between the SF6 and 3H/3He derived ages (ranging 0.8 to >35 yrs) was obtained at one site characterized by high 3H/3He derived recharge rates (63 cm yr-1). However, at the other site, characterized by lower 3H/3He derived recharge rates (6 cm yr-1), the SF6 ages were 14-17 years older than the apparent 3H/3He ages (ranging 3.5 to >35 yrs). He and Ne concentrations of these particular samples are affected by degassing, which reduces the SF6 concentrations and results in higher apparent ages.

No bubbles were detected during sampling and there is good agreement between replicate samples. Therefore, degassing during the sampling process or in the aquifer at depth are unlikely mechanisms. Degassing might have occurred during the recharge process while water was percolating through sediments that produce CO2 and/or CH4. After applying a solubility-controlled degassing correction of the dissolved noble gas and SF6 concentrations at site with low recharge, good agreement (R2=0.99) is found between the SF6 and 3H/3He estimated ages in the shallow aquifer.

An additional complication to the use of SF6 as a transient tracer in Bangladesh is the presence of SF6 in the deeper aquifer that should be free of SF6 according to 3H and radiocarbon data. Tests of well screen and sampling material showed no evidence of SF6 contamination from these sources. This suggests that there is a natural source slowly releasing SF6.

The study concludes that SF6 can be used as a transient tracer estimating groundwater ages in the shallow Bangladesh aquifers, but only if combined with 3H/3He data. Other gas measurements (e.g. He, Ne, Ar, or N2) are required to identify and quantify potential degassing of the groundwater.