Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

Abstract

Past studies of entrapped air dissolution have focused on one-dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180 × 240 × 600 cm(3) ) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (PTDG ) probes. Dissolution occurred as a diffuse wedge-shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by PTDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi-saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and PTDG , with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). PTDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower PTDG of the source water. The observed dissolution pattern has long-term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of PTDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.

Total Dissolved Gas Pressure Measurements for Assessing Contaminant Degradation Processes in Groundwater

“…Subsequently, the P TDG declined to P* G , and from then on it closely mimicked the P* G , which fluctuated as a result of changing P atm in the laboratory. The coincident fluctuations of P TDG and P* G is suggestive of equilibration between the dissolved gases and a gas phase in the aquifer, as was noted by McLeod et al () for dissolution of entrapped air (with no active dissolved gas production). A ruptured P TDG sensor membrane would also give this matching pattern, but observation and tests during probe maintenance indicated this had not occurred.…”

“…The presence of a gas phase in the aquifer, by maintaining P TDG at P* G , appears to have masked some of the changes in the nature of gas production occurring in the aquifer. For instance, the coincident fluctuating pattern of P TDG and P* G was observed from the outset of the experiment for sensors in the zone with trapped gas (air) remaining from the previous experiment (McLeod et al 2014 ), that is, both depths for MW3 (Figure 3 C) and the shallow depth for MW2 (Figure 3 B). Thus, any onset of fermentation-and/ or denitrification-associated ethanol degradation, or even the absence of O 2 in the groundwater entering from the HT, was not apparent at those locations.…”

“…Such an effect might also be caused by upward flow in the monitoring well, bringing water from greater depth (higher P* G ) and, thus, with potentially higher P TDG , across the P TDG probe. However, this seems unlikely because permeability generally increased with depth due to the declining presence of entrapped air (McLeod et al 2014 ), which would favour downward flow in the well, and there was no indication of preferential gas phase formation at greater depths following the hydrocarbon injection (based on TDR data; not shown). In addition, the custom rubber baffles installed with the P TDG probes would impede vertical flow between the top and bottom sections of the well.…”

Quasi‐Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

“…To exemplify, the concentration of noble gases below the WT at levels above equilibrium with atmospheric air (also known as “excess air”) have been linked experimentally and numerically to entrapped air, in which the gases are transferred from entrapped air to groundwater by diffusional movement (Heaton and Vogel ; Aeschbach‐Hertig et al , ; Holocher et al ; Mächler et al ). Likewise, Williams and Oostrom (), Mächler et al (), Mcleod et al (), and Teramoto and Chang () demonstrated that the presence of entrapped air delivers oxygen to groundwater. Aquifer oxygenation by entrapped air dissolution can have important implications to biogeochemical process in the saturated zone.…”

“…Although the quasi‐saturated zone is ephemeral and dissipates when the WT moves down, its recurrent formation when the WT moves up (Figure ) allows one to consider this zone as a separate (seasonal) layer of the aquifer. Moreover, entrapped gases within this zone may persist for a decade or longer, even without WTFs at this depth (Ryan et al ; McLeod et al ). Although the pore system is filled with different proportions of air and water, the main feature distinguishing the quasi‐saturated zone from the capillary fringe is that the quasi‐saturated zone has a positive pressure head, similar to the fully saturated zone.…”