In-Well Degassing of Monitoring Wells Completed in Gas-Charged Aquifers

Area of Science:

• Hydrogeology
• Environmental Science
• Geochemistry

Background:

• Accurate in situ dissolved gas concentration measurements in groundwater are crucial.
• In-well degassing presents a significant challenge to obtaining precise measurements.
• The underlying mechanisms of in-well degassing remain unclear.

Purpose of the Study:

• To investigate the mechanisms of in-well degassing in a gas-charged aquifer.
• To understand how groundwater degassing occurs in a recently pumped well.
• To elucidate the processes affecting total dissolved gas pressure (P_TDG) in groundwater wells.

Main Methods:

• Monitoring of vertical P_TDG and dissolved gas concentration profiles in a groundwater well.
• Measurements conducted over 7 days before and 15 days after well pumping.
• Analysis of groundwater behavior in the standing water column (SWC) of a gas-charged aquifer.

Main Results:

• P_TDG increased significantly after pumping, exceeding calculated bubbling pressure (P_BUB).
• Post-pumping, P_TDG decreased below P_BUB, indicating active degassing.
• Observed downward mixing of atmospheric/tracer gases and upward mixing of groundwater gases.

Conclusions:

• In-well degassing is driven by bubble exsolution when P_TDG exceeds P_BUB during well recovery.
• Microadvection due to bubble buoyancy and thermally driven vertical convection also contribute to degassing.
• These mechanisms are critical for understanding and mitigating degassing in groundwater monitoring.