AIR SPARGING REMEDIATION: A STUDY ON HETEROGENEITY AND AIR MOBILITY REDUCTION 1S.S. Di Julio and 2A.S. Drucker 1California State University, Northridge, Northridge, CA 91330; 1Phone: (805) 667-2496, 1Fax: (805) 667-7062, 1E-mail: [email protected]; 2NFESC, US Navy; 2Phone: (805) 982-4847, 2E-mail: [email protected]. ABSTRACT Contaminated groundwater is a widespread problem often requiring innovative technology to remediate. The purpose of this paper is to present the laboratory results of air sparging models. Initial tests used very fine porous media (glass beads-packed column) to represent a relatively homogeneous soil samples. Subsequent testing employed budded core samples taken from a site of interest to represent more realistic, heterogeneous samples. 1,1,1 Trichloroethane (TCA) was used as the dissolved contaminant to represent BTEX/ gasoline contamination; however, results obtained here can be applied to any NAPL-dissolved phase. A technique based on foam injection is proposed and is demonstrated to reduce air mobility. This reduction in air mobility has potential to improve contaminant removal. Laboratory results are compared with predictions of a numerical model, which is an advection-diffusion air sparge simulation model. Sensitivity analysis of the numerical model provides the range of some key parameters used to screen/evaluate air sparging as the remediation method for a given contaminated site of interest. Eventual scaleup of the model to an actual site application can be justified by the favorable results presented in this paper. Key words: remediation, air sparging, NAPL, foam BACKGROUND scopic capillary air channeling is estimated by The efficiency of air sparging as a ground- Clayton (1998) to occur at air-entry pressure of water remediation process depends to a large about 15 to 20 cm of water. Since this is a low extent on the contact time and contact area of air-entry pressure, it is very likely that air air with contaminated water. A number of channeling occurs, as was seen in all 18 labora- investigators have conducted laboratory or field tory experiments carried out by Clayton (1998). studies, or numerical simulation, in order to It is important to realize that the mecha- better understand air distribution and a few have nism of contaminant removal, in such by-passed conducted studies to measure the removal rate regions both in homogeneous and heteroge- of contaminants from groundwater. A majority neous porous media, is severely diffusion limited of these studies chose mostly homogeneous (Clayton, 1998; Ji and Ahfled, 1993; Clayton porous media, either 2D/3D glass-beads or and Nelson, 1995; Clark, 1996; Choa, 1998; sand packs, or core samples for laboratory Brusseau, 1991). Plummer et al. (1997) studies, and/or homogeneous strata for field observed channeling in their 2-D homogeneous, studies. Ji et al. (1993), Ahfeld et al. (1994), medium-grained glass beads model and the and Clayton (1998) have demonstrated the homogeneous sand pack of comparable perme- tendency of air channels developing in response ability, density, and porosity representing both to heterogeneity at both pore and larger scale in horizontal and vertical well configurations. The coarse to fine homogeneous sand. The transi- air distribution was more uniform for the hori- tion from pore-scale viscous fingering to macro- zontal well, suggesting that more of the porous 256 Proceedings of the 2000 Conference on Hazardous Waste Research media is impacted by air flow. We have also 1998), as well as us; our sensitivity runs, dis- observed (discussed in Results Section) that in cussed later in this paper, indicate that contami- our core studies the contaminant recovery is nant recovery near the interface increases as more efficient in the horizontally cut cores than the air channel density and VOC diffusivity the vertically cut, indicating the adverse effect of increase. However, as our results indicate, the soil stratification on contaminant recovery rate. overall contaminant recovery efficiency McKay and Acomb (1996) and Schima et decreases when air channels or bypassing al. (1996) used neutron moisture probe and occurs. This is mainly due to overall decrease cross-bore hole resistivity, respectively, to in air saturation, and as seen in our laboratory measure percentage of fluid displaced and air results, the contaminant recovery time will distribution during air sparging at two wells in a increase drastically. Hence it is best to reduce homogeneous formation consisting of uniform or eliminate channeling as proposed by foam sands. They observed an initial rapid lateral injection, discussed later in this paper. We also expansion followed by consolidation of the share the observation made by Chao et al. region. They also observed inconsistent read- (1998) that there seem to be an optimum mass ings in less permeable, heterogeneous forma- transfer flow rate. tions, indicating the inconsistent behavior of air Ahlfed et al. (1994) have conceptually flow in such formations. described the air sparging process and they note Chao et al. (1998) have developed water- that in heterogeneous, stratified formations in to-air mass transfer for a number of VOCs which sparging is often applied, the pattern of during air sparging in soil columns packed with air movement through the subsurface is com- coarse, medium, or fine sand or glass bead. plex. This complexity is largely driven by They used a reaction numerical model and variation in grain size, capillary resistance, and assumed concentration in the bulk phase re- intrinsic permeability of the porous media. In mains constant due to slow diffusion of VOC in addition, operating parameters such as airflow the aqueous phase to the air-water interface as rate, injection pressure, and depth and cross- compared to rapid volatilization of VOCs at the sectional area of injection will also affect the air-water interface. Therefore, they have contaminant recovery process. As far as the modeled the interface mass transfer alone. authors know, no laboratory study has specifi- Their results indicated that, depending on the cally involved heterogeneous core studies. VOC sparged, the estimated fraction of total Hence we hope that our laboratory results on volume affected by air sparging varied from 5 to comparison between contaminant recoveries in 20% for fine sand, but may be as high as 50% relatively homogeneous and heterogeneous for coarse sand, where more channels are porous media, and the proposed foam injection, expected to form. This observation has been instead of air injection, will aid in improving the made by others (Ji et al. 1993, and Clayton contaminant recovery for an air sparging pro- Proceedings of the 2000 Conference on Hazardous Waste Research 257 cess. The proposed process may also decrease pressure and pressure drop across the column. the remediation or cleanup time, which is of The water vapor in the air was removed prior to important concern during field operation. injection and its flow rate was controlled and measured accurately by a digital flow system. EXPERIMENTAL AND NUMERICAL MODELS The effluent air was sampled by a gas chro- matograph. At the end of each run, it was a. Experimental Setup and Procedure extremely important to remove residual con- To study contaminant recovery in a rela- taminant. This was done by flushing the tively homogeneous porous media, a glass-bead column with warm water and hot air for a packed column, constructed of clear acrylic number of days. pipe, packed with beads of an average grain To study contaminant recovery in heteroge- diameter of 67 microns, was used. Both ends neous porous media, a composite core model of the column were sealed with recessed end was used. The core samples taken from the site caps around the injection and production ports of interest were composed of silty sand, sandy to prevent leakage. To allow a dispersed flow siltstone, silty sandstone, and fine to coarse- of air through the column, a 40-micron sintered- grained sandstone. The composite core assem- bronze filter, 0.32 cm in diameter and length, bly consisted of three core samples, cut either was positioned on the centerline at the bottom vertically or horizontally. A sleeve of heat- of the glass bead-packed column. This filter shrinkable Teflon™ tubing was slid over the represented the slotted portion of a sparge well, composite cores with screens and end plates at distributing air along its length. Positioned at the the two ends to hold the sand grains in place; the top of the column was a 11.4 cm long acrylic composite cores were then housed in a Hassler pipe, which was sealed with another end cap. pressure cell. Mineral oil was used for applica- This void space was created to allow the water tion of overburden pressure. Properties of all to swell (rise due to displaced volume by air). three laboratory models are given in Table 1. A positive pressure transducer and a differential pressure transducer were used to measure inlet Table 1. Porous Media Properties Porous Media Glass Bead Vertical Horizontal Properties Pack Composite Core Composite Core L1ength (cm) 64164.1 15.0 D2iameter (cm) 54.7 24.5 2.5 P9ermeability, k (md) 02.3 115. 52. Porosity, φ (3%) 346. 2535. P8ore volume, ml 414 256. 23. 258 Proceedings of the 2000 Conference on Hazardous Waste Research Benzene, toluene, ethyl benzene, and and the experimental results can be applied to xylene are all constituents of BTEX, the largest both LNAPL and DNAPL remediation. regulated component of gasoline contamination. Both the glass-bead pack and the com- In an effort to reduce the number of variables in posite cores were saturated with a solution of the laboratory study, focus was primarily given water and TCA at three different concentrations to the remediation of benzene, the most strin- of 10, 25, and 50 ppm. Low-pressure air of gently regulated contaminant.