Impacts of unsaturated zone reaeration on the bioattenuation of organic contaminants in groundwater systems
Neale, Charles Nelson
Hughes, Joseph B.; Ward, C. H.
Doctor of Philosophy
Reaeration, or the diffusion of O2 through the unsaturated zone and into an aquifer system, is a mechanism that influences the extent of hydrocarbon contaminant bioattenuation in ground water. A series of laboratory and numerical modeling experiments were completed to determine the amount of O2 supplied to an anaerobic aquifer by reaeration under various unsaturated zone conditions and to quantify the impacts of reaeration on the migration of hydrocarbon plumes in ground water. Results from laboratory experiments indicated that reaeration flux rates exhibited a bimodal distribution and either approached 12,000 Mg/M2-day or were less than 2,000 Mg/M2-day. Soil water content significantly affected the reaeration flux rate while soil type, soil O2 utilization rate, and unsaturated zone thickness did not influence the reaeration rate. In some cases, the capillary rise impeded O2 transport into the bulk ground water due to high water content near the base of the capillary region. An unsaturated zone transport model was developed to predict reaeration flux based on the effective diffusion coefficient of soil (Ds), liquid-side mass transfer coefficient (KL), Soil O2 utilization rate (Rsoil), and unsaturated zone thickness (z). Good agreement was generally found between the predicted and experimental flux results. Results from numerical modeling experiments indicated that reaeration reduced the steady-state length of hydrocarbon plumes in ground water for aquifer hydraulic conductivities of K = 10--2 cm/s and K = 10--4cm/s. Important groundwater reaeration parameters having an influence on the steady-state hydrocarbon plume length included the concentration of O2 in the overlying soil gas (C O2(g)), the ground water mass transfer coefficient (KL,GW), the vertical dispersivity in the saturated zone (alphav), and the zone of reaeration or the thickness of the top model layer (ZR). The relative importance of each of these parameters was on the order of alphav > > KL,GW > CO2(g) > ZR.
Environmental science; Environmental engineering