Formation mechanisms of hydraulic circulation and its effects on the nitrobenzene migration in homogeneous aquifers with different medium sizes during air sparging remediation

Abstract

Porewater flow is known to introduce uncertainty in the effectiveness and extent of remediation in aquifers during air sparging (AS). In this study, the hydraulic circulation migration behavior, influencing factors, and its effects on nitrobenzene transport in AS process were investigated using light transmission visualization technology. Experimental results showed that when the air injection rate (Q) exceeded a critical threshold (Qc), porewater flow induced hydraulic circulation in homogeneous aquifers with different medium sizes. The values of Qc were found to be approximately 300 L/h for medium sand, 100 L/h for coarse sand, and 0.5 L/h for gravel aquifers. It was observed that the flow velocity of hydraulic circulation was linearly positively correlated with Q, while the zone of influence (ZOI) area was logarithmically correlated with Q. In aquifers with coarse sand, continuous channelized flow was seen to impede contaminant migration from the left to the right hydraulic circulation zone. In contrast, in gravel aquifers, discontinuous bubbly flow allowed contaminants to migrate between zones, which increased the risk of expanding the contaminant plume's range. Moreover, in aquifers with medium and coarse sand, hydraulic circulation was mainly caused by the chimney effect, whereas in gravel aquifers, oscillating bubble effects were notably observed. This observation explained why the hydraulic circulation effect in gravel aquifers was superior compared to that in medium and coarse sand aquifers. These findings are expected to contribute to expanding the remediation mechanisms, achieving precise remediation, and improving contaminant removal in AS technology.