Department of Geoscience, University of Calgary, Calgary, Alberta, Canada.
Ground Water. 2023 Jan;61(1):86-99. doi: 10.1111/gwat.13238. Epub 2022 Aug 22.
Total dissolved gas pressure (P ) measurements are useful to measure accurate in situ dissolved gas concentrations in groundwater, but challenged by in-well degassing. Although in-well degassing has been widely observed, its cause(s) are not clear. We investigated the mechanism(s) by which gas-charged groundwater in a recently pumped well becomes degassed. Vertical P and dissolved gas concentration profiles were monitored in the standing water column (SWC) of a groundwater well screened in a gas-charged aquifer for 7 days before and 15 days after pumping. Prior to pumping, P values remained relatively constant and below calculated bubbling pressure (P ) at all depths. In contrast, significant increases in P were observed at all depths after pumping was initiated, as fresh groundwater with elevated in situ P values was pumped through the well screen. After pumping ceased, P values decreased to below P at all depths over the 15-day post-pumping period, indicating well degassing was active over this time frame. Vertical profiles of estimated dissolved gas concentrations before and after pumping provided insight into the mechanism(s) by which in-well degassing occurred in the SWC. During both monitoring periods, downward mixing of dominant atmospheric and/or tracer gases, and upwards mixing of dominant groundwater gases were observed in the SWC. The key mechanisms responsible for in-well degassing were (i) bubble exsolution when P exceeded P as gas-charged well water moves upwards in the SWC during recovery (i.e., hydraulic gradient driven convection), (ii) microadvection caused by the upward migration of bubbles under buoyancy, and (iii) long-term, thermally driven vertical convection.
总溶解气体压力 (P) 的测量有助于测量地下水的准确原位溶解气体浓度，但受到井下脱气的挑战。尽管已经广泛观察到井下脱气现象，但其原因尚不清楚。我们研究了在最近抽水井中，充气体地下水如何发生脱气的机制。在一个充气体含水层中，监测了一个经过筛选的地下水井的静止水柱 (SWC) 中的垂直 P 和溶解气体浓度剖面，在抽水前的 7 天和抽水后的 15 天内进行监测。在抽水前，P 值相对稳定，并且在所有深度都低于计算的鼓泡压力 (P)。相比之下，在抽水开始后，所有深度的 P 值都显著增加，因为具有较高原位 P 值的新鲜地下水通过井筛被抽出。在抽水停止后，在 15 天的抽水后期间，所有深度的 P 值都降至 P 以下，这表明在这段时间内，井脱气是活跃的。在抽水前后测量的估计溶解气体浓度的垂直剖面为理解 SWC 中发生的井下脱气机制提供了线索。在两个监测期间，在 SWC 中观察到主导大气和/或示踪气体的向下混合以及主导地下水气体的向上混合。导致井下脱气的关键机制是：(i) 当充气体井水在 SWC 中向上移动时，P 超过 P，气泡释放（即水力梯度驱动对流）；(ii) 气泡在浮力作用下向上迁移引起的微对流；以及 (iii) 长期的、热驱动的垂直对流。
