Environ. Sci. Techno/. 1995, 29,2198-2207
ln Situ Mixed Region Vapor Striming in Low-Permeability Media. -2. Full-Scale Field Experiments ROBERT L. S I E G R I S T , ' , ' OLIVIA R . WEST,+ MICHAEL I. MORRIS,* DOUG A. PICKERING,$ DENNIS W. GREENE,+ CHRIS A. M U H R , $ D O U G D . DAVENPORT,Il A N D J O H N S. GIERKEl Environmental Sciences, Chemical Technology, and Health Sciences Research Divisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, Environmental Restoration Division, Martin Marietta Energy Systems, Inc., Piketon, Ohio 45661, and Department of Geological Engineering, Michigan Technological University, Houghton, Michigan 49931
This paper is the second in a three-part series that describes mixed region vapor stripping (MRVS) for in situ treatment of fine-grained soils contaminated by volatile organic compounds (VOCs) including trichloroethene (TCE), l,l,l-trichloroethane (TCA), and related halocarbons. As described in this paper, MRVS processes were studied during full-scale field experiments wherein ambient or heated air was injected at high volumetric flow rates during in situ soil mixing, and VOCs were volatilized and advectively removed from the subsurface, captured in a shroud covering the mixed region, and then treated on-site. The field test was conducted at an inactive land disposal site in southern Ohio where dense silty clay soils were contaminated by VOCs at concentrations in the 10500 mg kg-l range. During the field studies, seven columns, each 3.0 m diameter and 4.6 or 6.7 m deep, were treated with ambient air (-15-25 "C) or heated air (-120-130 "C) injected at flow rates of 28-40 m3 min-'. Intensive monitoring and measurement activities defined contaminant behavior and key MRVS operation and performance parameters. The field testing revealed that MRVS could rapidly reduce the concentrations of VOCs (Le., TCE, TCA, ...) in dense silty clay soil by 88-98%. The rate and extent of reduction was somewhat higher with the injection of heated air as compared to ambient air. Regardless of injection air temperature, as treatment progressed, the rate of VOC removal became increasingly mass transfer limited.
2198
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 9. 1995
Introduction Soils and sediments contaminated by organic solvents and petroleum hydrocarbons are common environmental problems at private industrial sites and federal facilities in the United States and abroad ( I ) . In situ treatment of these sites has increasingly been accomplished by soil vapor extraction (SVE) processes ( 2 , 3 ) . However, application of SVE in fine-grained soils and sediments is normally infeasible due to low pneumatic permeability caused by fine and disconnected pores and high water contents. An emerging approach to rapid in situ treatment for these sites involves the use of mixed region vapor stripping (MRVS) (4-9). In this process, compressed gases are injected at high volumetric flow rates [e.g., (1m3 of air) (m-3 of soil) min-'1 and organics are stripped from the subsurface, captured in a hood, and managed by an appropriate offgas treatment process (e.g., carbon adsorption, catalytic oxidation). Soil mixing technology has been used for years in the construction industry and can include various auger designs while gas injection can be accomplished through orifices along the auger blade(s1 or out the bottom end of a mixing shaft. In prior work by these authors, MRVS removal efficiency was found to be dependent on contaminant/media properties (e.g.,pore size and continuity,water content, sorption) and injected gas properties (e.g.,flow rate, energy content) (8).However, these findings were based on early process modeling and laboratory pilot-scale experiments. To corroborate and extend these findings, a full-scalefield test was conducted at an inactive land disposal unit (a.k.a. X-231B) located at a U.S. Department of Energy (DOE)site in southern Ohio (9, 10). The X-231B unit was used from 1976 to 1983 as a land disposal site for waste oils and solvents, and dense silty clay deposits beneath the unit (K,,, < cm s-l) were contaminated with TCE, TCA, and other chlorinated aliphatics at concentrations ranging from 10 to 100 mg kg-' (Figure 1, Table 1). Low levels of heavy metals and radionuclides were also present. The shallow groundwater (saturated zone at -3.6-4.2 m depth) was also contaminated, with TCE well above drinking water standards. During May 1992, field experiments were conducted within a test area of the X-231B site to compare the operation and performance of vapor stripping, chemical oxidation, and solidification processes coupled with auger mixing (9, 10). For the MRVS process, seven columns were treated (each 3.0 m diameter and 4.6 or 6.7 m deep) while intensive monitoring and measurement activities defined contaminant behavior and key operation and performance parameters. This paper presents a synopsis of the MRVS field experiments while further details may be found elsewhere (9-11). The previous paper in this issue introduced the * Corresponding author present address: Colorado School of Mines, Golden, CO 80401-1887; e-mail address: rsiegris@mines. colorado.edu; fax: 303-273-3413. ' Environmental Sciences Division, Oak Ridge National Laboratory. Chemical Technology Division, Oak Ridge National Laboratory. 0 Health Sciences Research Division, Oak Ridge National Laboratory. I' Martin Marietta Energy Systems, Inc. Michigan Technological University.
*
0013-936X/95/0929-2198$09.00/0
E 1995 American Chemical Society
FIGURE 1. Photographs of (a, top) the test site during the field experiments in May 1992 and (b. bonoml the auger injection blade and air injection paltern used in all tests.
MRVS concept and described laboratory experiments and preliminary analytical modeling ( I 2) while a subsequent paper describesthe development ofaheat and mass transfer model and its application to the field experimental data (13).
Experimental Methods MRVS Proteas Opwation. During the field studies, replicated tests of MRVS were made using ambient and heated air injected into the subsurface during auger mixing (MecTool, Millgard Environmental Corporation, Livonia, MI). The auguring equipment was comprised of a trackmountedcranewitha hollow, kellybarattachedtoadding toolwithtwo, 1.5 mlong blades, yieldinganeffectivemixing diameter of y 3 m. In the field test, the augers were used
to penetrate the subsurfacewhile simultaneouslyinjecting compressed air at either ambient (15-25 "C) or elevated temperature (120-130 "C) through 0.5-1.25 cm diameter orifices into soil regions, 3 m in diameter by 4.6 or 6.7 m deep (Table 2). The ground surface above the mixed region was covered by a hood maintained under low vacuum (- 1 kPa) to capture air emissions for treatment by activated carbon adsorptionandhigh-efficiencyparticulate filtration. MRVS of fine-grained soil was studied in two columns surrounded by undisturbed soil (IEl and IE2, TEl and TE2) as well as in one column that overlapped the other two (ID,TE3) with a total overlap volume of -39% (Figure 2). Treatment to 4.6 m depth in each of the three columns by MRVS with heated air (-120-130 "C) REI-TU) was completed on May 11-12, and in each of three columns VOL. 29. NO. 9,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY. 2199
TABLE 1
Representative Characteristics of Subsurface within Test Sitea characteristic
representative range of properties in 0-5.4 m depth interval
grain size distribution clay,
