Envlron. Scl. Technol. 1991, 25, 1708-1715
(18) Zief, M.; Mitchell, J. W. Contamination Control i n Trace Element Analysis; Wiley: New York, 1976. (19) Deveral, S. J.; Millard, S. P. Enuiron. Sci. Technol. 1988,
22, 697.
(20) U S . Environmental Protection Agency. National interim primary drinking water regulations; Environmental Protection Agency 57019-76-003;Washington, DC, U.S. Environmental Protection Agency Office of Water Supply,
(21) (22)
(23) (24)
(25)
1976. National Academy of Science-National Academy of Engineering. Water Quality Criteria; National Academy of Science: Washington DC, 1972; p 594. Dye, W. B.; O’Hara,J. L. Bull.-Nev., Agric. Exp. Stn. 1959, 208. Chapman, H. D.; Vanselow, A. P. Calif. Citrogr. 1955,40, 455. Bradford, G. R. Soil. Sci. 1963, 96, 77. McNeely, R. N.; Neimanis, V. P.; Dwyer, H. Water Quality Source Book: A Guide to W a t e r Quality Parameters; Inland Waters Directorate, Water Quality Branch Ottawa, ON, Canada, 1978; p 18.
(26) Letey, J.; Roberts, C.; Perberth, M.; Vasek, C. Special
Publication 3319. Agriculture and Natural Resources, Univ. of California, Oakland, CA, 1986; p 21. (27) Westcot, D. W.; Rosenbaum, S. E.; Grewell, B. J.; Beldin, K. K. Water and Sediment Quality in Evaporation Basins Used for the Disposal of Agricultural Subsurface Drainage Water in the San Joaquin Valley, California. Report to the
Central Valley Regional Water Quality Control Board, 1988. (28) Hem. J. D. US.Geol. Surv. Water-Supply Pap. 1985,No. 2254. (29) U.S. Environmental Protection Agency. Water Pollution Control National primary drinking water regulations, radio-nuclides. Advance notice of proposed rulemaking. Fed. Regist. 1986, 51(348),36-34862. Received for review September 21, 1990. Revised manuscript received February 22,1991. Accepted M a y 28,1991. W e express our appreciation to the California Regional Quality Control Board, Central Valley Region, for their financial and other assistance i n this study.
Factors Influencing Organic Contaminant Diffusivities in Soil-Bentonite Cutoff Barriers Henry V. Mott*,t and Walter J. Weber, Jr.t Department of Civil Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, and Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48 109
rn Quasi-steady-state and transient diffusion experiments indicate that the effective diffusivities of low molecular weight organic contaminants in saturated soil-bentonite media are but several factors less than corresponding free aqueous diffusivities. Evidence suggests that bentonite forms a fractured gel within the pore spaces of the soil matrix, thus contributing little to the diffusive resistance of a barrier. Reduction of the “free” volume for diffusion due to the presence of the solid phase and tortuosity of diffusion paths through the pore system account for observed reductions in contaminant diffusivities. The relationship between effective and “free” aqueous diffusivities can in fact be described satisfactorily by a simple power function of porosity.
Introduction Soil-bentonite barriers are frequently employed to prevent or minimize subsurface migration of spilled or otherwise improperly disposed contaminants. Such barriers commonly comprise little more than nonrigid, vertical dikes surrounding areas containing environmental contaminants. The barriers generally extend downward from the ground surface to an underlying impermeable stratum, the objective being to isolate the contaminated area from the surrounding subsurface environment. Contaminants having low molecular weights and exhibiting little, if any, permanent electrovalent character are of particular concern because of their relatively high mobility. Such compounds, which include halogenated aliphatics of one to three carbons and single-ringed aromatics, both substituted and unsubstituted, are commonly found at both hazardous and nonhazardous waste disposal sites (1-3). Contaminants ‘South Dakota School of Mines and Technology. *TheUniversity of Michigan. 1708
Environ. Sci. Technol., Vol. 25, No. IO, 1991
that are only slightly soluble and that adsorb readily to solids, such as PCBs and other multiringed compounds, are relatively immobile in subsurface environments and are thus of secondary concern with respect to containment systems. Design practice typically has focused on minimizing the hydraulic conductivity, Kh, of containment barriers to levels of approximately lo-’ cm/s or lower. Several authors (4-7) have suggested that restricting hydraulic transport suffices to curtail migration of contaminants. Gray and Weber (8), however, advanced the hypothesis that contaminant migration by molecular diffusion may be significant under conditions for which Kh is sufficiently low to render convection (advection) by bulk flow to be insignificant. Johnson et al. (9) subsequently supported this hypothesis, noting with respect to a 5-year-old hazardous waste landfill in Ontario, Canada, that observed solute migration into the glacial till below that landfill was much greater than could be attributed to convective transport alone. Millington (10) developed an empirical relationship describing a hindrance (or formation) factor, H , for gas diffusion through fused-silica cylinders, leading to the following expression relating the effective diffusion coefficient, De,i (L2t-’),to the porosity, E, of a porous medium and the free liquid diffusion coefficient, Daq,i:
De,i = D,,,i/Hi = e4J3Daq,,
(1)
The relationship has been extended to diffusion in both saturated (11,12) and unsaturated aqueous systems (13, 14). The experimental data of Gillham et al. (15)and the field measurements of Johnson et al. (9) for chloride diffusion in clay soils were analyzed for conformance with eq 1, and reasonable agreement was observed (16). The properties of the voids of these types of soils are likely similar to those of consolidated soil-bentonite backfill
0013-936X/91/0925-1708$02.50/0
0 1991 American Chemical Society
Table I. Organic Solutes and Associated Properties at 25 "C
solute
Henry's const, log atm KO,"
aq sol,* mg/L
l,4-dichlorobenzene 138e 3.39 79 4-chlorophenol 0.05e 2.39 271008 lindane 3.72 7.29
I
MASS FRACTION BENTONITE
',;1
aq diffus; mol cm2/s radius,"
x lo6
A
>
0.920 0.930 0.635
3.79 3.67 4.68
v)
t
a
002
I
0 04
*
0 06
008
I
*
0 K
:
*
I
I
OHansch and Leo (27). bVerschuren (18). CHaydukand Laudie correlation (19). dLeBas volume (19). "Calculated from vapor Dressure and solubilitv data. 'Mackav and Leinonen (20). 920 "C.
U
D
13
mixtures. A logical extension, then, is that eq 1 may also in soil-bentonite be applicable to the description of D e i mixtures. The molecular diffusivities of organics in soil-bentonite media may be dependent upon several factors, namely: (1) the porosity, or area available for free diffusion; (2) the constriction resulting from alternately small and large pores in the transport path; (3) the constriction resulting from the very close approach of the boundaries of the limiting pore within the transport path; and (4) the tortuosity imparted by the solid matrix of the soil. The objectives of this work were to identify and evaluate those factors that most significantly affect the diffusivities of low molecular weight organic contaminants in cutoff barriers composed of mixtures of soil and bentonite and to test the applicability of the power function of Millington (9) for the estimation of effective diffusion coefficients in such barriers. Three types of transport experiments were conducted, namely: (1) quasi-steady-state experiments in which solute diffusion was allowed to proceed a t a nearly constant rate through thin soil-bentonite barriers, similar to the type of transport process expected to control mass transfer well after complete solute breakthrough has occurred; (2) transient experiments in which unsteady solute diffusion occurred into relatively thick barriers, simulating the type of transport process expected during the early stages of the performance period of a barrier, well before solute breakthrough; and (3) hydraulic conductivity experiments. The first two types of experiments were used to evaluate the effective diffusion coefficients of three organic solutes in saturated soil-bentonite mixtures. The hydraulic conductivity experiments were used to aid in the definition of certain properties of the porous medium as well as to verify that the soil mixtures used approximated the performance of mixtures employed in operating installations.
Experimental Methods Materials. The target organic solutes employed in this investigation are listed in Table I, along with selected properties pertinent to the present investigation. All chemicals used were of reagent grade or better. The background soil consisted of silica sand (Agsco Corp., Wheeling, IL, special order), silica flour (Agsco No. 140), and kaolinite (Georgia Kaolin Co., Elizabeth, NJ). Selection of the components of the background soil mixture was predicated on the suggestion ( 4 , 5 ) that a well-graded soil containing a significant fraction of colloidal size ma-
1
10
100
1000
CONFINING STRESS (psi)
Figure 1. Relationships between porosity and confining stress for soil-bentonite mixtures.
terial will result in a minimum hydraulic conductivity when mixed with bentonite. Well-characterized soils of low organic matter were desired for use in these experiments to enable isolation of the diffusion process. The components were mixed in the ratio of 77.5 parts silica sand, 10.5 parts silica flour, and 12.5 parts kaolinite. The resulting soil was well graded with 9% of the material finer than 0.001 mm. Sodium bentonite was Slurry Ben 90 (American Colloid Co., Skokie, IL). All soil materials were used as received without modification. Relationships between porosity and confining stress were determined experimentally for low confining stresses with a 3.91-in.-diameter consolidometer fitted with porous stones, and for high confining stresses with a standard slurry consolidometer. Known values of initial dry mass, combined with volumetric measurements under equilibrated, monotonically increased loads yielded the porosity-confining stress relationships shown in Figure 1. Time rate of consolidation was observed during these procedures and used for estimation of the hydraulic conductivity of the various mixtures. Specific procedures are discussed later. Methods of Solute Assay. Aqueous concentrations of 1,4-dichlorobenzene (DCB) and lindane were assayed by using hexane extraction and electron capture gas chromatography. Aqueous 4-chlorophenol (PCP) samples were assayed by reverse-phase high-performance liquid chromatography (RPHPLC). Operating conditions are given in Table 11. Quasi-Steady-StateDiffusion Experiments. As illustrated in the schematic of the quasi-steady-state diffusion cell shown in Figure 2, four major modifications were incorporated into the basic configuration of a diaphragm cell (21,22) to facilitate steady diffusion of volatile organic solutes through thin soil-bentonite barriers: (1) Mininert valves were installed in the upper and lower reservoirs to allow periodic sampling with a 1-mL syringe; (2) the system was fitted with glass displacement rods equipped with compression O-ring seals to permit volume adjustment without atmospheric exposure while sampling; (3) a confinement system consisting of perforated stainless
Table 11. Columns and Conditions for GC and RPHPLC Assay
solute column/mobile phase temp, "C detector DCB 6 f t X 2 mm glass, 6% OV-11 + 4% OV-101 on Chromosorb W-HP 100/120, Ar/CH, mobile phase 75 ECD lindane 6 ft X 2 mm glass, 3% OV-1 on Gas Chrom Q 80/100, Ar/CH, mobile phase 190 ECD C-18, 80/20 acetonitrile/water mobile phase PCP 150 mm X 4.6 mm, 5 - ~ m ambient UV 235 nm Environ. Sci. Technol., Vol. 25, No. 10, 1991
1709
n
I 118' BRASS ROD
UDBRASSDISC
\
STAINLESS STEEL ~ m i o n A T B 0s u m
&4VICNETIC *TIP.
