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Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer
Author(s) -
McLeod Heather C.,
Roy James W.,
Smith James E.
Publication year - 2014
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/gwat.12203
Subject(s) - dissolution , aquifer , water table , environmental science , atmospheric pressure , geology , groundwater , environmental chemistry , soil science , hydrology (agriculture) , chemistry , geotechnical engineering , oceanography
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 ( P TDG ) 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 P TDG 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 P TDG , with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). P TDG 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 P TDG 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 P TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.