Experimental investigation on airflow characteristics during air injection into broadly graded aquifers

Air injection into aquifers is commonly used in the remediation of aquifers contaminated with volatile or aerobically biodegradable organic compounds. Most focus has been directed to airflow characteristics for air injection into homogeneous porous media aquifers, while broadly graded (heterogeneous) aquifers are seldom involved in previous research. This study focuses on air injection characteristics in both narrowly and broadly graded aquifers using a customized apparatus. A transformation from channel to hybrid bubble-channel flow patterns is observed with the increase in gradation broadness. The zone of influence (ZOI) increases, while both the zone of flow (ZOF) and the ratio of ZOF to ZOI radius decrease with increasing gradation broadness. A further insight into the constriction size distributions (CSD) of aquifers indicates that a broader gradation leads to a larger difference in constriction size, and thereby promotes a larger ZOI but a smaller ZOF. The ratio of ZOF to ZOI radius is closely related to airflow patterns in both narrowly and broadly graded aquifers with various effective particle diameters and uniformity coefficients; that is, it decreases from 0.82 to 0.55 with the transition from channel to bubble flow. This study presents new understanding on air injection into broadly graded aquifers that is often encountered at field scale.