Impacts of Unsaturated Zone Properties on Oxygen Transport and Aquifer Reaeration

This study investigated the unsaturated zone properties that affect ground water reaeration (i.e., the diffusive flux of oxygen through the unsaturated zone and into an aquifer system). Laboratory column experiments were undertaken to quantify oxygen flux into anaerobic ground water as a function of soil type, soil water content, soil oxygen demand, and unsaturated zone thickness. Soils used in these studies included coarse sand, sand, loamy fine sand, fine sandy loam, silt, silty clay loam, bentonite, kaolin, and peat. The results showed that reaeration flux rates ranging from 11,000 to 12,000 mg/m2-day were possible under conditions typical of the natural environment. Soil water content provided the greatest resistance to oxygen transport in the unsaturated zone while the remaining factors of soil type, soil oxygen demand, and unsaturated zone thickness did not significantly inhibit reaeration flux into ground water. An unsaturated zone transport model based on Fick's second law and gas-liquid interfacial mass transport was developed and showed good agreement with the experimental results derived from the column studies. The results of this study suggest that unsaturated zone reaeration of anaerobic ground water may be a contributing factor in controlling the steady–state size and shape of hydrocarbon plumes and incorporating reaeration into ground water models—based on unsaturated zone properties—may allow for a more accurate representation of bioattenuation reactions taking place in subsurface environments.