Noble gases in aquitard provide insight into underlying subsurface stratigraphy and free gas formation

Abstract

Biogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas–water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH4 production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH4−gas‐producing zone of an As‐contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air‐like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure—a main control of free gas formation (i.e., CH4) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard–aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core.