Geochemistry of PAHs in the Vadose Zone of Contaminated Alluvial Sediments under Unsaturated Transient Flow Conditions.

Redox and sorption processes in aquifer sediments that are contaminated with landfill leachate are possible mechanisms of natural attenuation of polycyclic aromatic hydrocarbons (PAHs).  We designed a 2 × 2 factorial laboratory experiment that mimics specific hydrological regimes that occur within the capillary fringe (CF) zone of sandy alluvial sediments (Oxyaquic Udifluvents).  We hypothesized that CFs with greater saturated hydraulic conductivity have greater redox potential, and natural attenuation is the greatest in zones with greater availability of bio-available terminal electron acceptors (TEAs) and donors.  Mineral soil samples from the vadose zone (50- to 85-cm depth) of the closed landfill site near Norman, Oklahoma were repacked in Perspex columns (15 cm diameter × 35 cm height) and monitored over a 91-day experiment with four soil-hydrologic regime treatment combinations.  Two levels of soil organic matter (i.e., total C, 3 % and 0.5 %) were combined with two levels of two-line ferrihydrate plus a lactate-containing growth medium, which act as a source of TEA and electron donor, respectively.  Synthetic rain and ground water that contained the targeted PAHs, naphthalene, phenanthrene, and pyrene, were used to mimic transient flow conditions that are observed within the CF.  At four 7.5-cm depth increments, gravimetric water content, matric potential, and Eh voltages were logged at hourly time intervals; while at 0, 7, 14, 28, 49, 70, and 91 days post inundation, soil pore solution samples, extracted by suction lysimeters, and effluent solution samples, collected in a fraction collector, were analyzed for pH, PAHs, dissolved organic C and N, NO₃^-, NH₄⁺, and Fe²⁺.  Intense monitoring of biogeochemical and hydraulic processes will allow a better characterization of the underpinning mechanisms that control natural attenuation of PAHs in a landfill leachate.