Zone of flow: A new finding on the characteristics of airflow within the zone of influence during air sparging in aquifers.

Air sparging (AS) is a popular technology for the in-situ remediation of groundwater contaminated by volatile organic compounds. The scope of the zone within which injected air exists, i.e., zone of influence (ZOI) and the airflow characteristics within ZOI are of great interest. However, few studies have shed light on the scope of the zone within which air flows, namely, the zone of flow (ZOF) and its relation with the scope of ZOI. This study focuses on the ZOF characteristics and its relation with ZOI based on quantitative observations of ZOF and ZOI using a quasi-2D transparent flow chamber. The relative transmission intensity obtained by the light transmission method presents a rapid and continuous increasing near the ZOI boundary, providing a criterion for the quantitative determination of ZOI. An integral airflow flux approach is proposed to determine the scope of ZOF based on the airflow flux distributions through aquifers. The ZOF radius decreases with the growth of particle sizes of aquifers; while it increases first and then keeps constant with the increase of sparging pressure. The ZOF radius is around 0.55- 0.82 times of the ZOI radius, which depends on air flow patterns related to particle diameters d, that is, 0.55- 0.62 for channel flow (d < 1- 2 mm), while 0.75- 0.82 for bubble flow (d > 2- 3 mm). The experimental results show that the sparged air is entrapped with little flowing inside ZOI regions that are outside the ZOF, which should be considered carefully in the design of AS.