Nonaqueous Phase Liquid Droplets Research Articles

Enhanced Retention of Chemotactic Bacteria in a Pore Network with Residual NAPL Contamination.

Nonaqueous-phase liquid (NAPL) contaminants are difficult to eliminate from natural aquifers due, in part, to the heterogeneous structure of the soil. Chemotaxis enhances the mixing of bacteria with contaminant sources in low-permeability regions, which may not be readily accessible by advection and dispersion alone. A microfluidic device was designed to mimic heterogeneous features of a contaminated groundwater aquifer. NAPL droplets (toluene) were trapped within a fine pore network, and bacteria were injected through a highly conductive adjacent macrochannel. Chemotactic bacteria (Pseudomonas putida F1) exhibited greater accumulation near the pore network at 0.5 m/day than both the nonchemotactic control and the chemotactic bacteria at a higher groundwater velocity of 5 m/day. Chemotactic bacteria accumulated in the vicinity of NAPL droplets, and the accumulation was 15% greater than a nonchemotactic mutant. Indirect evidence showed that chemotactic bacteria were retained within the contaminated low-permeability region longer than nonchemotactic bacteria at 0.25 m/day. This retention was diminished at 5 m/day. Numerical solutions of the bacterial-transport equations were consistent with the experimental results. Because toluene is degraded by P. putida F1, the accumulation of chemotactic bacteria around NAPL sources is expected to increase contaminant consumption and improve the efficiency of bioremediation.

Cleanup of fractured rock aquifers. II. Effects of matrix diffusion and nonaqueous phase liquid

Model calculations are used to explore the effects of the kinetics of diffusion of dissolved organic compounds into and out of low-permeability porous materials and of the rate of solution of nonaqueous phase liquid (NAPL) droplets (into the aqueous phase) on the rate of cleanup of contaminated aquifers. Two models are presented: (1) the flushing of organic compounds initially distributed as NAPL droplets in a fracture in a porous rock aquifer, and (2) the removal of organic compounds initially present as NAPL in an aquifer containing low-permeability porous clay lenses. NAPL droplet size is found to be of much less importance than the spacing of the fractures in the porous rock in the first model or the thickness of the clay lenses in the second.

Groundwater Cleanup by In-Situ Sparging. VI. A Solution/Distributed Diffusion Model for Nonaqueous Phase Liquid Removal

Abstract A microcomputer model for the sparging of aquifers contaminated with volatile organic compounds (VOCs) is presented which includes the kinetics of solution of nonaqueous phase liquid (NAPL) droplets and of diffusion from low-permeability porous layers. The well configuration modeled is a horizontal slotted pipe. Modeling results lead to the following conclusions. 1) The presence of low-permeability porous lenses of clay, till, silt, etc. results in marked increases in cleanup times. The extent of the increases depends strongly on the thickness of the structures. NAPL droplets of large size also result in marked increases in cleanup time. 2) Increases in air flow rate do not yield corresponding decreases in cleanup time if the system is limited by solution/diffusion kinetics. 3) The rate of induced water circulation plays a minor role in determining cleanup time. 4) Wells should be sufficiently deep and operated at an air flow rate such that air is delivered to the entire zone of contamination. 5) The spatial distribution of the VOC has little effect on the cleanup time as long as air is delivered to the entire contaminated zone. 6) Cleanup times increase roughly proportionally to increasing initial VOC concentration. 7) The terminal phase of cleanup typically shows substantial tailing as water containing VOC must circulate into the zone of aeration for the VOC to be stripped. 8) VOCs having Henry's constants of 0.05 or larger can readily be removed by sparging.

Soil Cleanup by In-Situ Aeration. XVIII. Field-Scale Models with Diffusion from Clay Structures

Abstract Mathematical models for soil vapor extraction (SVE) are developed which model solution of nonaqueous phase liquid (NAPL) and mass transport of volatile organic compounds (VOCs) through low-permeability lumps, lenticular structures, and discontinous layers of clay by means of a distributed diffusion approach. The well configurations modeled are that of a single buried horizontal slotted pipe and that of a single vertical well screened along a short length near its bottom. The models yield high off-gas VOC concentrations initially, followed typically by vary rapid drop-offs to relatively long plateaus, followed in turn by terminal tailing, the length of which is highly variable and determined by the thickness of the low-permeability layers from which diffusion is occurring and by the size of the NAPL droplets (if NAPL is present). The results are in agreement with a previous, simpler model; they indicate that it will be impossible to accurately predict SVE cleanup times from data taken in short-ter...