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

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.