Effect of dissolved organic matter and bacteria on contaminant transport in riverbank filtration.

A mathematical model for the transport of hydrophobic organic contaminants in an aquifer under simplistic riverbank filtration conditions is developed. The model considers a situation where contaminants are present together with dissolved organic matter (DOM) and bacteria. The aquifer is conceptualized as a four-phase system: two mobile colloidal phases, an aqueous phase, and a stationary solid phase. An equilibrium approach is used to describe the interactions of contaminants with DOM, bacteria, and solid matrix. The model is composed of bacterial transport equation and contaminant transport equation. Numerical simulations are performed to examine the contaminant transport behavior in the presence of DOM and bacteria. The simulation results illustrate that contaminant transport is enhanced markedly in the presence of DOM and bacteria, and the impact of DOM on contaminant mobility is greater than that of bacteria under examined conditions. Sensitivity analysis demonstrates that the model is sensitive to changes of three lumped parameters: K+1 (total affinity of stationary solid phase to contaminants), K+2 (total affinity of DOM to contaminants), and K+3 (total affinity of bacteria to contaminants). In a situation where contaminants exist simultaneously with DOM and bacteria, contaminant transport is mainly affected by a ratio of K+1/K+2/K+3, which can vary with changes of equilibrium distribution coefficient of contaminants and/or colloidal concentrations. In riverbank filtration, the influence of DOM and bacteria on the transport behavior of contaminants should be accounted to accurately predict the contaminant mobility.

Heat and mass transfer in the vadose zone with plant roots.

The vadose zone is the intermediate medium between the atmosphere and groundwater. The modeling of the processes taking place in the vadose zone needs different approaches to those needed for groundwater transport problems because of the marked changes in environmental conditions affecting the vadose zone. A mathematical model to simulate the water flow, and the fate and transport of recalcitrant contaminants was developed, which could be applied to various bioremediation methods such as phytoremediation and natural attenuation in the vadose zone. Two-phase flow equations and heat flux models were used to develop the model. Surface energy, balance equations were used to estimate soil surface temperature, and root growth and root distribution models were incorporated to represent the special contribution of plant mots in the vegetated soils. Interactions between the roots and environmental conditions such as temperature and water content were treated by incorporating a feedback mechanism that made allowance for the effects of water and temperature stresses on root distribution and water uptake by roots. In conducting the modeling study, Johnson grass and unplanted soil were simulated to compare the effect of root water uptake on soil water content. After the numerical experiments were conducted to investigate model behavior, the proposed model was applied to estimate actual water flow and heat flow in field lysimeter experiments over a 1-year period. Root growth and distribution for Johnson grass and rye grass were simulated to compare the warm season grass to the cold season grass. A significant agreement was observed between the simulations and measured data.

The use of box lysimeters with freshly contaminated soils to study the phytoremediation of recalcitrant organic contaminants.

The potential for phytoremediation of soil contaminated by trinitrotoluene (TNT) and 2,2',5,5'-tetrabromobiphenyl (PBB was used as a surrogate for PCBs) was examined in a 2-year study using box lysimeters under field conditions. The treatments were a warm season grass, Johnson grass, and a cool season grass, Canadian wildrye, and a rotation of Johnson grass and wildrye plus a fallow condition. The experiment was conducted using 12 large (1.50 m x 1.50 m x 0.75 m), in-ground, box lysimeters filled with a Weswood silt-loam soil freshly spiked with the TNT and PBB compounds to a concentration of 10 mg kg(-1) for each chemical. The lysimeters were sheltered to permit controlled applications of water. A total of five sampling rounds were conducted where soil, herbage, and leachate samples were collected for laboratory analysis. TNT and PBB concentrations were determined using the U.S. EPA approved immunoassay test procedures. In the soil, TNT concentrations dropped below the detection limit of 0.25 mg kg(-1) by day 92 and PBB concentrations dropped below the detection limit of 0.50 mg kg(-1) by day 184. There were no significant differences in chemical concentrations among any of the vegetated or fallow treatments at a significance level of alpha < 0.05. However, PBB soil concentrations rebounded above the 0.50 mg kg(-1) level by day 720 for all treatments. No detectable concentrations of TNT or PBB were found in any of the herbage samples or in the leachate.

Plant contamination by organic pollutants in phytoremediation.

Phytoremediation is a remediation technique that involves plant uptake, transformation, accumulation, and/or volatilization of soil- and aqueous-phase contaminants or by the stimulation of microbial cometabolic activity in the rhizosphere of the plant. Even when the principal mechanism is by stimulation of bacteria, any resultant plant contamination cannot be overlooked. For the purpose of modeling, a two-compartment plant model has been developed. The model divides the plant into the shoot compartment (which can be harvested) and the root compartment (into which contaminants can accumulate). Numerical experiments were conducted to investigate model behavior and to determine important parameters affecting plant contamination. Johnsongrass [Sorghum halepense (L.) Pers.] was used to evaluate the model behavior. The contaminants TNT (2,4,6,-trinitrotoluene) and chrysene were selected on the basis of their contrasting aqueous-phase solubilities. The results indicate that plant contamination and soil remediation by plants depend on soil properties such as soil organic carbon content, the physicochemical properties of the contaminants such as the octanol-water partition coefficient, and plant properties. The most important factor affecting plant contamination is bioavailability. As bioavailability increased, the concentrations in root and shoot compartments were predicted to increase. Microbial activities and plant contamination are closely related, which suggests that plants and microorganisms can have complementary roles in phytoremediation.