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On the relation between viscosity and hydraulic conductivity for volatile organic liquid mixtures in soils
JOURNAL OF
Contaminant Hydrology ELSEVIER
Journal of Contaminant Hydrology 25 (1997) 113-127
On the relation between viscosity and hydraulic conductivity for volatile organic liquid mixtures in soils Jerker Jarsj~5 a,*, Georgia Destouni a, Bruno Yaron b Department of Civil and Environmental Engineering, Royal Institute of Technology, S-1 O0 44 Stockholm, Sweden b Institute of Soils and Water, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan, 50 250, Israel a
Received 20 January 1995; revised 19 March 1996; accepted 24 June 1996
Abstract
Changes in the volatile organic liquid mixture (VOLM) hydraulic conductivity in different soils are compared with corresponding changes in VOLM viscosity through an extended analysis of results from three previous experimental studies. The conductivity with regard to four different kerosene mixtures, corresponding to different degrees of volatilisation of the original kerosene, was determined in one set of soils; an increasing degree of volatilisation implies less lighter kerosene compounds, changing both kerosene viscosity and its chemical composition. In another set of soils, kerosene conductivity measurements were conducted at two temperatures, which provided two different viscosities but did not affect the kerosene chemical composition. Both volatilisation- and temperature-induced changes in kerosene viscosity and conductivity were studied in two of the soils. In all the soils that were used in the temperature experiments, the changes in kerosene conductivity could be successfully predicted by scaling the original kerosene conductivity value based on the observed viscosity ratio. For the chemically different kerosene mixtures, the changes in conductivity agreed with the corresponding viscosity changes only in inert sands. For a montmorillonitic loam, a montmorillonitic clay and a peat soil, considerable deviations were found between the conductivity ratio and the viscosity ratio; for the peat, which was also used in temperature experiments, no such deviations were observed at different temperatures. The deviations between the conductivity ratio and the viscosity ratio were also found to increase with increasing differences in kerosene chemical composition. These results indicate that chemical composition may be of major importance for VOLM hydraulic conductivity in interacting soils, apart from the effect that the composition has on viscosity. The viscosity ratios
* Corrresponding author. 0169-7722/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 7 7 2 2 ( 9 6 ) 0 0 0 3 6 - 8
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were shown to deviate more than 300% from observed conductivity ratios for the chemically most different kerosene mixtures. © 1997 Elsevier Science B.V.
Keywords: Hydraulic conductivity; Kerosene; NAPL; Organic liquid; Permeability; Retardation; Selective volatilisation; Soil; Viscosity 1. Introduction
Quantification of the flow of volatile organic liquid mixtures (VOLMs) through the unsaturated zone is important for understanding the fate of hazardous organic contaminants. Such contaminants may, for instance, be spilled accidentally on the soil surface or leak from underground storage tanks. Unsaturated flow in porous media is often quantified by relative conductivity relationships for each mobile phase present, i.e. the aqueous and non-aqueous liquid phases and the gas phase. The relative conductivity function has its maximum value at full saturation and reaches zero at residual saturation (Mercer and Cohen, 1990). Generally, the conductivity at full saturation determines the maximum VOLM conductivity value, and for a two-phase system, the non-wetting residual saturation determines the zero point of the two-phase VOLM conductivity. However, in more complex three-phase systems, a non-aqueous liquid may exist in the residual state both completely surrounded by water (as in the two-phase system) and as a residual film between the air and the water. A technique for extending two-phase saturation-pressure relations to three-phase systems was described by Lenhard and Parker (1988) and was based on two-phase air-oil and oil-water capillary pressuresaturation measurements. Conductivity relationships for the water and VOLM phases are commonly assumed to be related to an intrinsic permeability that is a characteristic property of the porous medium and is independent of the fluid flowing through this medium (this assumption was adopted by, for example, Ahriola and Pinder (1985), Osborne and Sykes (1986), Pinder and Abriola (1986), Corapciogtu and Baehr (1987) and Kuppusamy et al. (1987)). The saturated fluid conductivity is then assumed to be related to the intrinsic permeability as K~ =
kpg
(1)
/z
where K s is the saturated conductivity of the porous medium with respect to the specific fluid, k is the intrinsic permeability of the porous medium, p and /z are the fluid density and viscosity, respectively, and g is the gravitational constant. For multi-phase conditions, relationships between immiscible fluid conductivities that are based on Eq. (1) are often used in the literature (e.g. Cary et al., 1989a, Cary et al., 1989b) and may be expressed as K o =Ksw
/Xw Po
f(O,~p;e)
/Xo Pw
(2)
in which K o, /-to, Po and q~ are the unsaturated conductivity, viscosity, density and
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
115
volume fraction of the VOLM, respectively, and Ksw, /~w, Pw and 0 are the saturated conductivity, viscosity, density and volume fraction of water, respectively. Furthermore, the function f in Eq. (2) quantifies the relative VOLM conductivity based on 0, ~p and a soil parameter set e that includes soil porosity and pore-size-distribution related soil coefficients. The function f is often derived from relative water conductivity relationships where the actual form of f depends on the conceptualisation of phase distribution within the pore space (Cary et al., 1989b). Relative water conductivity relationships are typically derived from capillary water retention functions, which are obtained by fitting analytical expressions to experimental capillary water retention data (Durner, 1994). Regardless of the form of f, Eq. (2) is based on the assumption that Eq. (1) is valid, such that there exists an intrinsic permeability k that can be used to scale the saturated conductivity Ks2 of any fluid to a known saturated fluid conductivity Ks1 as /-tl ,0 2
Ks2
=
K~I-
--
11"2 Pl
(3)
in which /z i and pi are the viscosity and density, respectively, of fluid i = 1,2. From a practical point of view, it is convenient to express the hydraulic conductivity of a soil through an intrinsic permeability, because water conductivity data, or conductivity data for non-aqueous liquids, then can be translated to apply to any other liquid. It is therefore of interest to investigate under which conditions Eq. (3) is applicable. In this paper, we investigate the applicability of the conductivity scaling relationship (Eq. (3)) for VOLMs in different soils by analysing experimental results. The experimental results include conductivity measurements in various soils for chemically different kerosenes, where the difference is due to different degrees of volatilisation. Besides chemical composition, these kerosenes also differ from each other with regard to their viscosity. The experimental conductivity data with respect to these kerosenes have previously been published by Galin et al. (1990) and Gerstl et al. (1994). In this paper we extend the analysis of these data with the purpose of investigating the applicability of Eq. (3). For the same purpose we use and report previously unpublished data from the experimental series of Jarsjti et al. (1994) on both the temperature and chemical composition dependence of kerosene viscosity and conductivity in different soils.
2. Materials and methods 2.1. Soils and VOLMs
The 11 soils used in the experiments were classified as coarse, medium and fine sands, sandy loams, loam, clays, and peat. The properties of these soils are presented in Table 1, along with the references to the publications where these data were first reported. Each soil was used in one of the following experimental series: (1) chemical composition experiments (indicated with C in Table 1), where the conductivity was determined with regard to four different kerosene mixtures; (2) temperature experiments (indicated with T in Table 1), where the conductivity was determined for the same kerosene mixture at two temperatures; (3) both temperature and chemical composition
6
J. Jarsji~ et a l . / J o u r n a l o f Contaminant Hydrology 25 (1997) 1 1 3 - 1 2 7
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~-~ © 'r"
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.=.
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z
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117
experiments (indicated with T + C in Table 1). In Case (3), the conductivity was determined for the same kerosene mixture at two temperatures and for an additional kerosene mixture, with a different chemical composition, at one of the temperatures. The coarse sand 1, the medium and fine sands, and the montmorillonitic loam and clay originate from the coastal area of Israel; further details have been reported by Galin et al. (1990) and Gerstl et al. (1994). The rest of the soils were sampled from the Ap-horizon at four sites in the county of Uppland, north of Stockholm, and represent common soil types in central Sweden below the highest post-glacial marine level (Jarsj6 et al., 1994). Before use, all soils were air-dried, ground and sieved through a screen with 0.25 mm openings. The kerosene used in all of the considered experimental investigations is characterised by low viscosity and medium volatility relative to heavy and residual fuel oils, and consists of about 100 hydrocarbons. Seven major components of the kerosene were identified, ranging from C 9 to C]5 and representing 37% of the total kerosene (Fig. 1). Selective volatilisation of the original kerosene results in changes of the kerosene composition, with an increase in kerosene viscosity as a result (Galin et al., 1990; Gerstl et al., 1994; Jarsj6 et al., 1994). The changes in kerosene viscosity caused by preferential volatilisation of the light fractions is a gradual, irreversible process. Four different kerosene mixtures, corresponding to different degrees of volatilisation of the original kerosene, were used in the chemical composition experiments (indicated
~U
o
~
~
u
U
.E-
U
,"
_
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I
I
I
I
I
I
I
I
I
0
5
lO
15
2_0
z5
30
35
40
Retention time (minutes) Fig. 1. G a s ¢ h r o m a t o g r a m o f the original kerosene used in the experiments.
118
J. Jarsji5 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
with C in Table 1). The mass percentages of volatilised kerosene in the different mixtures were 0% (i.e. original kerosene), 20%, 40% and 60%, and the dynamic viscosities of these liquids were 1.32mPas, 1.48mPas, 1.78mPas and 1.96mPas, respectively. Corresponding densities were 8 0 5 k g m 3, 810kgm-3, 8 1 8 k g m - 3 and 819kgm -3, and the surface tensions were 2 . 7 5 m N m l, 2.78mNm-1, 2.80mNm l and 2.78mNm -1 (Galin et al., 1990; Gerstl et al., 1994). In the following, these mixtures will be referred to as kerosene-0%, kerosene-20%, kerosene-40% and kerosene-60%, respectively. Fluctuations in the VOLM viscosity may also be caused by changes in the ambient temperature. In contrast to volatilisation-induced viscosity changes, this is an instantaneous, reversible process. The kerosene used in the temperature experiments (indicated with T in Table 1) had a viscosity of 1.66mPas at 24°C and 2.33 mPas at 5°C. 2.2. E x p e r i m e n t a l procedures
The conductivity measurements were in all cases performed using the saturated flow method with a constant pressure head. Cylindrical glass tubes of 21 mm i.d. and 120 mm height were packed with soil to a height of 85 mm. The end caps consisted of glass wool and rubber stoppers. The soil columns were connected to a Mariotte bottle containing kerosene. A constant head of 360 mm was kept throughout the experiments. The soil columns were slowly saturated with kerosene from the bottom to minimise air entrapment. The conductivity values were calculated using 10-30 liquid samples fi'om a fraction collector that accumulated mass from the column outlet for a specific time, before changing sample vessels. For each soil, two soil columns were used for determination of the conductivity. In the temperature experiments (T in Table 1), constant flow rates were first established at room temperature, 24°C, then the temperature was changed to 5°C, and after a number of pore volumes of fluid had passed the column, a repeat experiment was conducted at 24°C, to detect any changes in flow rates with time, owing to sample redistribution, etc. In the experiments with different kerosene chemical composition (C in Table 1), kerosene mixtures corresponding to 0%, 20%, 40% and 60% volatilisation were used at 24°C. After the experiment with the first kerosene mixture was finished, the next mixture was introduced into the same pair of soil columns, and the system was allowed to flow until constant flow rates were established. This procedure minimises the risk of having differences in saturation in the experiments with different liquids, as the columns are not dried and resaturated for each liquid. When experiments had been conducted with all four kerosene mixtures, an identical series of experiments with the same mixtures was conducted in the same pair of columns, to detect any deviations that could stem from sample redistribution or successive clogging. On two of the considered soils, both chemical composition and temperature experiments were conducted (T + C experiments in Table 1). In addition to the temperature experiments, the kerosene-60% liquid used by Galin et al. (1990) was then used on the two soils (coarse sand 2 and peat from the experimental series of Jarsj~3 et al. (1994)) at
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
119
24°C in two columns per soil type. A repeat experiment within the same column was also performed for each soil type and column. The kerosene viscosity was measured by a 'Schott Gerate' (Germany), type 53001/01 viscosity meter in a water bath (five replicates per kerosene composition). The kerosene surface tension was determined using a platinum-iridium test ring method (Cambridge Du Nouy tensiometer, Cambridge Instrument Co. Ltd., UK).
3. Experimental results 3.1. Statement of the problem The changes in kerosene viscosity in the experiments were due to either (1) temperature changes or (2) changes in the chemical composition of the kerosene, caused by selective volatilisation. In the following, we will use the conductivity values for the kerosene used in the temperature experiments at 24°C (Ksk(24°C); Table 2(a)) as a reference value in the temperature experiments, and the conductivity values for kerosene-0% at 24°C (Ksk(0%); Table 2(b)) as a reference value in the chemical composition experiments. The objective is to analyse the impact of kerosene-viscosity changes on the saturated kerosene conductivity for the different soils. Provided that the intrinsic permeability k (in Eq. (1)) is a parameter that depends only on the properties of the porous medium, Eq. (3) is valid and quantifies the fluid conductivity ratio Ks2/Ksl between two fluids with known densities and viscosities. In the present experiments, the difference in density between the studied kerosene mixtures was less than 2% for all cases and could, for the sake of simplicity, be neglected compared with the corresponding differences in viscosity. In the following discussion, the conductivity ratio Ks2/Ks1 will therefore be related only to the corresponding viscosity ratio as K~z/Ksl = tzl/ix z.
3.2. Temperature experiments The obtained mean kerosene conductivities and associated standard deviations among the experiments at 24°C and 5°C are summarised in Table 2(a); this temperature dependence is based on the experimental series of Jarsj5 et al. (1994), but the data have not previously been published. The standard deviation reported in Table 2(a) quantifies the combined effect of (l) the variability between the two soil columns and (2) the variability between the initial and the repeat experiment in each column. The reason for including both variability (1) and (2) in the reported standard deviation (i.e. n = 4) is that the variability (2) in a few cases exceeded the variability (1). However, the actual outcome of the statistical analysis with n = 4 (see Section 3.4) is the same as if one only accounts for variability between the two columns, i.e. variability (1) with n = 2. Differences between the initial and the repeat experiments in the same soil column may reflect disturbances such as, for instance, initial redistribution of the soil sample. For the coarse sand 2, the original and repeat experiment conductivity values deviated 11% from their common mean value reported in Table 2(a). In the sandy loam 2 and the
20
J. Jarsji5 et al. / J o u r n a l o f C o n t a m i n a n t
Hydrology
25 (1997) 113-127
e~
.=.
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1
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eq
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eq ,-2 ~
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J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
121
1.0--
0.8
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O" o eq
0.6
o
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t/3
0.2
0.0
[_
I
I
0
0
I
I
15
19
I 44
I
1
51
Clay Content (%) Fig. 2. Ratio betweeen the kerosene viscosities at 24°C and 5°C (continuous line) and corresponding mean conductivity ratios (bars).
peat, the corresponding deviations were 8% and 2%, respectively. For the other soils in Table 2(a), the conductivity values of the repeat experiment coincided with the conductivity values of the original experiment. Thus, the conductivity experiments at 24°C are generally reproducible, exhibiting only small differences between them, which is also reflected in the corresponding standard deviations. For the temperature experiments listed in Table 2(a), the ratio between the mean kerosene viscosities at 24°C and 5°C was 0.71, and is illustrated in Fig. 2 (continuous line), along with the ratios between the corresponding mean kerosene conductivities for the different soils (bars). For peat and the sandy soils (coarse sand 2, sandy loam 1 and sandy loam 2, which are characterised by a sand fraction of more than 65% according to the USDA system of classification), the conductivity ratio Ksk(5°C)/Ksk(24°C) deviates less than 2% from the viscosity ratio /x(24°C)/~(5°C). For the soils with a higher clay content, the corresponding deviation reached a maximum of 18%.
3.3. Chemical composition experiments In Table 2(b), we present the mean conductivity values and standard deviations of all conductivity experiments with different kerosene mixtures in each soil. The statistics are for each soil obtained from observed conductivities in two soil columns during an initial and a repeat experiment. These experiments were performed and reported by Galin et al. (1990) and Gerstl et al. (1994), with the exception of the previously unpublished data for
122
J. Jarsji5 et al. / Journal of" Contaminant I4vdrology 25 (1997) 113-127 Eq. (3), p neglected Least square fit to experimental data 10000
-~
A
Coarse sand 1 Jt
_A m m
•
C o a r s e s a n d 2.
Medium sand
I o o o -= A
f_
E
g
Fine s a n d
1oo
A
v
=
Peat U
"~ cO
1,9
Loam
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M. C l a y5 ~
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I
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I
I
i
I
I
1,3
1.4
1.5
1.6
1.7
1.8
1.9
2..0
Viscosity (mPa s) Fig. 3. Observed (symbols) and predicted (thick continuous lines) conductivities for kerosene mixtures with different viscosities and chemical composition.
the coarse sand 2 and the peat from the experimental series of Jarsji5 et al. (1994). Also for these experiments, the deviations between the initial and the repeat experiment conductivities were relatively small, as reflected in the standard deviations of Table 2(b). Fig. 3 illustrates the predictions of the mean kerosene conductivity according to Eq. (3), based on the viscosity ratios and the reference kerosene conductivity K~k(0%), along with the mean conductivity values for the kerosene mixtures in the soils of Table 2(b). The reference conductivity (Ksk(0%)) for the coarse sand 2 and the peat was obtained with a kerosene used by Jarsji5 et al. (1994). This kerosene had a higher viscosity at 0% volatilisation than the kerosene-0% of the other five soils in Fig. 3, which originate from the experimental series of Galin et al. (1990) and Gerstl et al. (1994). The thick lines of Fig. 3 represent the predictions of Eq. (3) and the thin trend lines are the least-squares fits to the observed conductivities for the kerosene mixtures (coefficient of determination: R 2 = 0.92 + 0.08). The first points on the trend lines represent the conductivity for the original kerosene, Ksk(0%), and the last points on the lines represent the conductivity for kerosene-60%, K~k(60%). The standard deviations are indicated in Fig. 3 with
J. Jarsjii et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
123
20% d i f f e r e n c e in v o l a t i l i s e d mass 1.o
(a)
(b)
1.o-
0 . 8 ~
1.o-
(c)
-
12(40%)/1.1~60%)
--
o.o
~ 0.6-
0.2
:2
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~ 0.4-
~
S~
~ 0.2-
uo
I~ ..-
0.0
0.0 -
0.0
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16
0
40
Clay Content (%)
0 O 16 40 Clay Content (%)
1.0a~
0.8-
o
40% difference "~ 0.6in v o l a t i l i s e d
mass
(d)
% ~.o'
"
"
",U{20~60%)
o.6-
~
--
"~ ~0
0~0"4-
=o-o
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.
0.0'
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0 0 16 40 Clay Content (%)
o~
0 0 16 40 Clay Content (%)
(f)
1.0-
0.8-
L., •'D
•,~ 0.6-
60% difference in volatilised mass
o
(e)
o~ 0 . 8 N
-
b
Clay Content (%)
l.O-
P(O%),/IJ{4(
~ 0.4-
"~0.20.0-
b
E
~o%)/I-t(6o%]
0.4•-
~
o
--I
_
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0.20.0-
0
0
0
16
40
i
Clay Content (%) Fig. 4. Ratio between the kerosene viscosities (continuous lines) and corresponding ratios between the mean conductivities (bars) for kerosene mixtures with: (a)-(c) 20% difference in volatilised mass; (d) and (e) 40% difference in volatilised mass; (f) 60% difference in volatilised mass.
error bars (however, for the sands, the deviations are too small to be seen in the figure). For the inert sands the difference between observed and predicted conductivity is relatively small. For the peat, the prediction of the kerosene-60% conductivity through Eq. (3) is considerably higher than the observed kerosene-60% conductivity. Further-
124
J. Jarsji5 et a l . / Journal of Contaminant Hydrology 25 (1997) 113-127
more, for the montmorillonitic loam and clay, the continuous increase in viscosity of the kerosene mixtures with increasing volatilised mass percentage seems to explain only a smaller part of the observed changes in hydraulic conductivity (note the logarithmic scale on the y-axis in Fig. 3). To show the effect of volatilisation-induced changes in chemical composition, we have plotted in Fig. 4 the viscosity ratios of different fluid pairs (continuous lines, which coincide with the prediction of Eq. (3)) along with the corresponding observed mean conductivity ratios (bars). Hereby, the comparison between observed and predicted conductivities of Fig. 3 is extended to include also Ksk(20%) and Ksk(40%) as reference points for the soils used in systematic chemical composition experiments (C in Table 1). In Fig. 4, the plots are arranged after difference in kerosene chemical composition (reflected in the difference in volatilised mass percentage). Kerosenes of relatively similar chemical composition (20% difference in volatilised mass) are compared in Fig. 4(a), Fig. 4(b) and Fig. 4(c), where the viscosity ratios between kerosene-0% and kerosene-20% (Fig. 4(a)), kerosene-20% and kerosene-40% (Fig. 4(b)) and kerosene-40% and kerosene-60% (Fig. 4(c)) are compared with corresponding ratios of the hydraulic conductivities of the soils. The viscosity ratios (continuous lines in Fig. 4(a), Fig. 4(b) and Fig. 4(c)) show good agreement with the conductivity ratios (bars) in the three sands, whereas greater differences can be seen for the montmorillonitic loam and clay. The corresponding analysis for kerosene mixtures with greater differences in chemical composition (40-60% difference in volatilised mass, Fig. 4(d), Fig. 4(e) and Fig. 4(f)) shows that the disagreement between the viscosity ratio and conductivity ratio in the montmorillonitic soils increases with increasing difference in volatilisation degree. For a 40% difference in volatilised mass the viscosity ratio was about 1.9 times greater than the mean conductivity ratio (Fig. 4(d) and Fig. 4(e)), and for a 60% difference in volatilised mass the viscosity ratio was 3.0 times greater than the mean conductivity ratio (Fig. 4(f)). For the inert sands, however, the differences remain small, with the mean conductivity ratio being 1.1 times greater than the viscosity ratio both for both a 40% and a 60% difference in volatilised mass (Fig. 4(d), Fig. 4(e) and Fig. 4(0).
3.4. Significance testing The significance of the deviations between experimental data and the theoretical predictions of Eq. (3) was evaluated statistically. Specifically, the null hypothesis that the differences between experimental observations and predictions of Eq. (3) are insignificant was tested by studying the 95% confidence interval for these differences. For determination of this confidence interval, the t-distribution was used with (n E - 1) + ( n p - 1) degrees of freedom, where n E is the number of experimental observations and np is the number of realisations of the prediction. In this case, np is equal to the number of experimental observations of the reference conductivity (i.e. K~k(24°C) for the temperature experiments (Table 2(a)) and K~k(0%) for the chemical composition experiments (Table 2(b))) because the theoretical predictions are linearly related to the reference conductivities through Eq. (3). A comparison of the conductivities for kerosene at 24°C with corresponding conductivities at 5°C showed that the null hypothesis was accepted, i.e. differences between
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
125
observations and predictions were found to be insignificant, at the 0.05 significance level, for all the temperature experiments (Table 2(a)). Thus, on this significance level, the data do not provide a basis for doubting the validity of Eq. (3) for conductivity changes caused by temperature-induced changes in VOLM viscosity. In contrast, a comparison of the conductivities for kerosene-0% with corresponding conductivities for kerosene-60% showed that the null hypothesis was rejected, i.e. differences between observations and predictions were found to be significant, at the 0.05 significance level, for four soils in the chemical composition experiments. These four soils are the coarse sand 1, the peat, the montmorillonitic loam and the montmorillonitic clay. For these soils, the data thus provide strong indications that Eq. (3) does not accurately describe changes in VOLM conductivity that are caused by volatilisation-induced changes in VOLM viscosity. However, for all the sandy soils, including the coarse sand 1 where differences were statistically significant at the 0.05 significance level, the deviation between observation and prediction is small in comparison with the deviations seen in the peat and the clay soils, where considerably larger discrepancies are observed (Fig. 3).
4. Discussion and conclusions
For temperature-induced changes in kerosene viscosity, the resulting saturated conductivity ratios were consistent with the corresponding viscosity ratios, i.e. with the predictions of Eq. (3), to within the range of experimental deviations. For volatilisationinduced viscosity changes, which reflect irreversible changes in kerosene chemical composition, the theoretical scaling relationship, Eq. (3), showed relatively small deviations from corresponding viscosity ratios for inert, sandy soils (coarse, medium and fine sand). However, for volatilisation-induced viscosity changes in soils containing clay or organic matter the observed saturated conductivity ratios deviated significantly from the theoretical predictions of Eq. (3). A comparison between four different kerosene mixtures in two montmorillonitic soils furthermore showed that the discrepancies between these two ratios increased with increasing differences in liquid chemical composition (Fig. 4). Thus, the results indicate that other properties than the fluid viscosity may be of importance for conductivity of potentially interacting soils with respect to VOLM residues. The study of Fernandez and Quigley (1988) demonstrated that for pure organic solvents, fluid viscosity may not be the controlling parameter for conductivity. In addition, for single non-aqueous organic compounds (heptane and xylene), Yong et al. (1992) observed that there may be significant differences between the conductivity ratio and the viscosity ratio of the organic compound and water in clay soils because of phase partitioning. Based on these observations on single compounds, Yong et al. (1992) suggested that a change in proportions of the chemical constituents in a multi-component liquid mixture could render the original laboratory conductivity values invalid. However, Yong et al. (1992) lacked supporting observations for this suggestion. The laboratory observations on VOLMs reported in this study support the hypothesis that chemical composition of a liquid mixture may be critical to its conductivity value.
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Furthermore, they demonstrate that volatilisation of the liquid phase is capable of changing the VOLM chemical composition sufficiently to yield considerable conductivity changes, which are not attributed to viscosity changes, in interacting soils. It has been recognised (Yong, 1992) that conductivity in soils may decrease with increasing hydrophobicity of organic compounds. Heavier organic compounds are generally characterised by lower solubility and higher octanol-water partitioning coefficients (i.e. they are more hydrophobic) than are lighter organic compounds. The preferential volatilisation of lighter kerosene components changes the composition of the kerosene such that the relative amount of heavy compounds increases with increasing volatilised mass percentage (Galin et al., 1990; Jarsji5 et al., 1994). The remaining liquid kerosene thus becomes more hydrophobic with increasing volatilisation, which may, in turn, result in greater adsorption and possibly explain the observed reductions of conductivity of interacting soils. For swelling clays, another possible explanation for changes in conductivity as a result of changes in chemical composition of the liquid mixture may be given by the diffusive double layer theory that relates swelling with the dielectric constant of the liquid (e.g. Yong et al., 1992), and the extended solution theory (Graber and Mingelgrin, 1994). However, the relations between solvent properties, clay swelling and resulting hydraulic conductivity are complex, and intrinsic permeability measurements have often yielded results that conflict with the diffusive double layer theory (e.g. Green et al., 1981). In this study, we have not attempted to provide explicit cause-and-effect relationships between VOLM conductivity changes and the actual mechanisms causing such changes. However, the reported deviations of up to 300% between conductivity ratio and viscosity ratio for kerosene mixtures with different chemical composition have provided some information on the limitations of the relatively well-established conductivityviscosity scaling relation, Eq. (3). This information may be useful for future studies of the influence of chemical composition on the VOLM conductive properties of potentially interacting soils.
Acknowledgements Financial support for this work was provided by the Swedish Environmental Protection Agency (SNV). The reported research was also partially supported by a grant from Water Research Institute (WRI)-Technion Haifa, Israel. The second author acknowledges the financial support of the Swedish Natural Science Research Council (NFR).
References Abriola, L.M. and Pinder, G.F., 1985. A multiphase approach to the modeling of porous media contamination by organic compounds. 1. Equation development. Water Resour. Res., 2 1 : 1 1 - 1 8 . Cary, J.W., McBride, J.F. and Simmons, C.S., 1989a. Observations of water and oil infiltration into soiL: some simulation challenges. Water Resour. Res., 25: 73-80.
J. Jarsji~ et al. / Journal of Contaminant Hydrology 25 (1997) 113-12 7
127
Cary, J.W., Simmons, C.S. and McBride, J.F., 1989b. Predicting oil infiltration and redistribution in unsaturated soils. Soil Sci. Soc. Am. J., 53: 335-342. Corapeioglu, M.Y. and Baehr, A.L., 1987. A compositional multiphase model for groundwater contamination by petroleum products. 1. Theoretical considerations. Water Resour. Res., 23: 191-200. Durner, W., 1994. Hydraulic conductivity estimation for soils with heterogeneous pore structure. Water Resour. Res., 30:211-223. Fernandez, F. and Quigley, R.M., 1988. Viscosity and dielectric constant controls on the hydraulic conductivity of clayey soils permeated with water soluble organics. Can. Geotech. J., 25: 582-589. Galin, Ts., McDowell, C. and Yaron, B., 1990. The effect of volatilization on the mass flow of a non-aqueous pollutant liquid mixture in an inert porous medium: experiments with kerosene. J. Soil Sci., 41:631-641. Gerstl, Z., Galin, Ts. and Yaron, B., 1994. Mass flow of a volatile organic liquid mixture (VOLM) in soils. J. Environ. Qual., 23: 487-493. Graber, E.R. and Mingelgrin, U., 1994. Clay swelling and regular solution theory. Environ. Sci. Technol., 28: 2360-2365. Green, W.J., Lee, G.F. and Jones, A., 1981. Clay-soils permeability and hazardous waste storage. J. Water Pollut. Control Fed., 53: 1347-1354. Jarsj~, J., Destouni, G. and Yaron, B., 1994. Retention and volatilisation of kerosene: laboratory experiments on glacial and post glacial soils. J. Contam. Hydrol., 17: 167-185. Kuppusamy, T., Sheng, J., Parker, J.C. and Lenhard, RJ., 1987. Finite-element analysis of multiphase immiscible flow through soils. Water Resour. Res., 23: 625-631. Lenhard, R.J. and Parker, J.C., 1988. Experimental validation of the theory of extending two-phase saturation-pressure relations to three-phase systems for monotonic drainage paths. Water Resour. Res., 24: 373-380. Mercer, J.W. and Cohen, R.M., 1990. A review of immiscible fluids in the subsurface: properties, models, characterisation and remediation. J. Contam. Hydrol., 6: 107-163. Osborne, M. and Sykes, J., 1986. Numerical modeling of immiscible organic transport at the Hyde Park landfill. Water Resour. Res., 22: 25-33. Pinder, G.F. and Abriola, L.M., 1986. On the simulation of nonaqueous phase organic compounds in the subsurface. Water Resour. Res., 22: 109S-119S. Yong, R.N., Mohamed, A.M.O. and Waretkin, B.P., 1992. Principles of Contaminant Transport in Soils. Elsevier, Amsterdam.
Contaminant Hydrology ELSEVIER
Journal of Contaminant Hydrology 25 (1997) 113-127
On the relation between viscosity and hydraulic conductivity for volatile organic liquid mixtures in soils Jerker Jarsj~5 a,*, Georgia Destouni a, Bruno Yaron b Department of Civil and Environmental Engineering, Royal Institute of Technology, S-1 O0 44 Stockholm, Sweden b Institute of Soils and Water, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan, 50 250, Israel a
Received 20 January 1995; revised 19 March 1996; accepted 24 June 1996
Abstract
Changes in the volatile organic liquid mixture (VOLM) hydraulic conductivity in different soils are compared with corresponding changes in VOLM viscosity through an extended analysis of results from three previous experimental studies. The conductivity with regard to four different kerosene mixtures, corresponding to different degrees of volatilisation of the original kerosene, was determined in one set of soils; an increasing degree of volatilisation implies less lighter kerosene compounds, changing both kerosene viscosity and its chemical composition. In another set of soils, kerosene conductivity measurements were conducted at two temperatures, which provided two different viscosities but did not affect the kerosene chemical composition. Both volatilisation- and temperature-induced changes in kerosene viscosity and conductivity were studied in two of the soils. In all the soils that were used in the temperature experiments, the changes in kerosene conductivity could be successfully predicted by scaling the original kerosene conductivity value based on the observed viscosity ratio. For the chemically different kerosene mixtures, the changes in conductivity agreed with the corresponding viscosity changes only in inert sands. For a montmorillonitic loam, a montmorillonitic clay and a peat soil, considerable deviations were found between the conductivity ratio and the viscosity ratio; for the peat, which was also used in temperature experiments, no such deviations were observed at different temperatures. The deviations between the conductivity ratio and the viscosity ratio were also found to increase with increasing differences in kerosene chemical composition. These results indicate that chemical composition may be of major importance for VOLM hydraulic conductivity in interacting soils, apart from the effect that the composition has on viscosity. The viscosity ratios
* Corrresponding author. 0169-7722/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 7 7 2 2 ( 9 6 ) 0 0 0 3 6 - 8
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
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were shown to deviate more than 300% from observed conductivity ratios for the chemically most different kerosene mixtures. © 1997 Elsevier Science B.V.
Keywords: Hydraulic conductivity; Kerosene; NAPL; Organic liquid; Permeability; Retardation; Selective volatilisation; Soil; Viscosity 1. Introduction
Quantification of the flow of volatile organic liquid mixtures (VOLMs) through the unsaturated zone is important for understanding the fate of hazardous organic contaminants. Such contaminants may, for instance, be spilled accidentally on the soil surface or leak from underground storage tanks. Unsaturated flow in porous media is often quantified by relative conductivity relationships for each mobile phase present, i.e. the aqueous and non-aqueous liquid phases and the gas phase. The relative conductivity function has its maximum value at full saturation and reaches zero at residual saturation (Mercer and Cohen, 1990). Generally, the conductivity at full saturation determines the maximum VOLM conductivity value, and for a two-phase system, the non-wetting residual saturation determines the zero point of the two-phase VOLM conductivity. However, in more complex three-phase systems, a non-aqueous liquid may exist in the residual state both completely surrounded by water (as in the two-phase system) and as a residual film between the air and the water. A technique for extending two-phase saturation-pressure relations to three-phase systems was described by Lenhard and Parker (1988) and was based on two-phase air-oil and oil-water capillary pressuresaturation measurements. Conductivity relationships for the water and VOLM phases are commonly assumed to be related to an intrinsic permeability that is a characteristic property of the porous medium and is independent of the fluid flowing through this medium (this assumption was adopted by, for example, Ahriola and Pinder (1985), Osborne and Sykes (1986), Pinder and Abriola (1986), Corapciogtu and Baehr (1987) and Kuppusamy et al. (1987)). The saturated fluid conductivity is then assumed to be related to the intrinsic permeability as K~ =
kpg
(1)
/z
where K s is the saturated conductivity of the porous medium with respect to the specific fluid, k is the intrinsic permeability of the porous medium, p and /z are the fluid density and viscosity, respectively, and g is the gravitational constant. For multi-phase conditions, relationships between immiscible fluid conductivities that are based on Eq. (1) are often used in the literature (e.g. Cary et al., 1989a, Cary et al., 1989b) and may be expressed as K o =Ksw
/Xw Po
f(O,~p;e)
/Xo Pw
(2)
in which K o, /-to, Po and q~ are the unsaturated conductivity, viscosity, density and
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
115
volume fraction of the VOLM, respectively, and Ksw, /~w, Pw and 0 are the saturated conductivity, viscosity, density and volume fraction of water, respectively. Furthermore, the function f in Eq. (2) quantifies the relative VOLM conductivity based on 0, ~p and a soil parameter set e that includes soil porosity and pore-size-distribution related soil coefficients. The function f is often derived from relative water conductivity relationships where the actual form of f depends on the conceptualisation of phase distribution within the pore space (Cary et al., 1989b). Relative water conductivity relationships are typically derived from capillary water retention functions, which are obtained by fitting analytical expressions to experimental capillary water retention data (Durner, 1994). Regardless of the form of f, Eq. (2) is based on the assumption that Eq. (1) is valid, such that there exists an intrinsic permeability k that can be used to scale the saturated conductivity Ks2 of any fluid to a known saturated fluid conductivity Ks1 as /-tl ,0 2
Ks2
=
K~I-
--
11"2 Pl
(3)
in which /z i and pi are the viscosity and density, respectively, of fluid i = 1,2. From a practical point of view, it is convenient to express the hydraulic conductivity of a soil through an intrinsic permeability, because water conductivity data, or conductivity data for non-aqueous liquids, then can be translated to apply to any other liquid. It is therefore of interest to investigate under which conditions Eq. (3) is applicable. In this paper, we investigate the applicability of the conductivity scaling relationship (Eq. (3)) for VOLMs in different soils by analysing experimental results. The experimental results include conductivity measurements in various soils for chemically different kerosenes, where the difference is due to different degrees of volatilisation. Besides chemical composition, these kerosenes also differ from each other with regard to their viscosity. The experimental conductivity data with respect to these kerosenes have previously been published by Galin et al. (1990) and Gerstl et al. (1994). In this paper we extend the analysis of these data with the purpose of investigating the applicability of Eq. (3). For the same purpose we use and report previously unpublished data from the experimental series of Jarsjti et al. (1994) on both the temperature and chemical composition dependence of kerosene viscosity and conductivity in different soils.
2. Materials and methods 2.1. Soils and VOLMs
The 11 soils used in the experiments were classified as coarse, medium and fine sands, sandy loams, loam, clays, and peat. The properties of these soils are presented in Table 1, along with the references to the publications where these data were first reported. Each soil was used in one of the following experimental series: (1) chemical composition experiments (indicated with C in Table 1), where the conductivity was determined with regard to four different kerosene mixtures; (2) temperature experiments (indicated with T in Table 1), where the conductivity was determined for the same kerosene mixture at two temperatures; (3) both temperature and chemical composition
6
J. Jarsji~ et a l . / J o u r n a l o f Contaminant Hydrology 25 (1997) 1 1 3 - 1 2 7
.r"
+
÷
ca
0 e~
8
:©
~-~ © 'r"
e~
&
.=.
o
z
J. Jarsj5 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
117
experiments (indicated with T + C in Table 1). In Case (3), the conductivity was determined for the same kerosene mixture at two temperatures and for an additional kerosene mixture, with a different chemical composition, at one of the temperatures. The coarse sand 1, the medium and fine sands, and the montmorillonitic loam and clay originate from the coastal area of Israel; further details have been reported by Galin et al. (1990) and Gerstl et al. (1994). The rest of the soils were sampled from the Ap-horizon at four sites in the county of Uppland, north of Stockholm, and represent common soil types in central Sweden below the highest post-glacial marine level (Jarsj6 et al., 1994). Before use, all soils were air-dried, ground and sieved through a screen with 0.25 mm openings. The kerosene used in all of the considered experimental investigations is characterised by low viscosity and medium volatility relative to heavy and residual fuel oils, and consists of about 100 hydrocarbons. Seven major components of the kerosene were identified, ranging from C 9 to C]5 and representing 37% of the total kerosene (Fig. 1). Selective volatilisation of the original kerosene results in changes of the kerosene composition, with an increase in kerosene viscosity as a result (Galin et al., 1990; Gerstl et al., 1994; Jarsj6 et al., 1994). The changes in kerosene viscosity caused by preferential volatilisation of the light fractions is a gradual, irreversible process. Four different kerosene mixtures, corresponding to different degrees of volatilisation of the original kerosene, were used in the chemical composition experiments (indicated
~U
o
~
~
u
U
.E-
U
,"
_
"E
I
I
I
I
I
I
I
I
I
0
5
lO
15
2_0
z5
30
35
40
Retention time (minutes) Fig. 1. G a s ¢ h r o m a t o g r a m o f the original kerosene used in the experiments.
118
J. Jarsji5 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
with C in Table 1). The mass percentages of volatilised kerosene in the different mixtures were 0% (i.e. original kerosene), 20%, 40% and 60%, and the dynamic viscosities of these liquids were 1.32mPas, 1.48mPas, 1.78mPas and 1.96mPas, respectively. Corresponding densities were 8 0 5 k g m 3, 810kgm-3, 8 1 8 k g m - 3 and 819kgm -3, and the surface tensions were 2 . 7 5 m N m l, 2.78mNm-1, 2.80mNm l and 2.78mNm -1 (Galin et al., 1990; Gerstl et al., 1994). In the following, these mixtures will be referred to as kerosene-0%, kerosene-20%, kerosene-40% and kerosene-60%, respectively. Fluctuations in the VOLM viscosity may also be caused by changes in the ambient temperature. In contrast to volatilisation-induced viscosity changes, this is an instantaneous, reversible process. The kerosene used in the temperature experiments (indicated with T in Table 1) had a viscosity of 1.66mPas at 24°C and 2.33 mPas at 5°C. 2.2. E x p e r i m e n t a l procedures
The conductivity measurements were in all cases performed using the saturated flow method with a constant pressure head. Cylindrical glass tubes of 21 mm i.d. and 120 mm height were packed with soil to a height of 85 mm. The end caps consisted of glass wool and rubber stoppers. The soil columns were connected to a Mariotte bottle containing kerosene. A constant head of 360 mm was kept throughout the experiments. The soil columns were slowly saturated with kerosene from the bottom to minimise air entrapment. The conductivity values were calculated using 10-30 liquid samples fi'om a fraction collector that accumulated mass from the column outlet for a specific time, before changing sample vessels. For each soil, two soil columns were used for determination of the conductivity. In the temperature experiments (T in Table 1), constant flow rates were first established at room temperature, 24°C, then the temperature was changed to 5°C, and after a number of pore volumes of fluid had passed the column, a repeat experiment was conducted at 24°C, to detect any changes in flow rates with time, owing to sample redistribution, etc. In the experiments with different kerosene chemical composition (C in Table 1), kerosene mixtures corresponding to 0%, 20%, 40% and 60% volatilisation were used at 24°C. After the experiment with the first kerosene mixture was finished, the next mixture was introduced into the same pair of soil columns, and the system was allowed to flow until constant flow rates were established. This procedure minimises the risk of having differences in saturation in the experiments with different liquids, as the columns are not dried and resaturated for each liquid. When experiments had been conducted with all four kerosene mixtures, an identical series of experiments with the same mixtures was conducted in the same pair of columns, to detect any deviations that could stem from sample redistribution or successive clogging. On two of the considered soils, both chemical composition and temperature experiments were conducted (T + C experiments in Table 1). In addition to the temperature experiments, the kerosene-60% liquid used by Galin et al. (1990) was then used on the two soils (coarse sand 2 and peat from the experimental series of Jarsj~3 et al. (1994)) at
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
119
24°C in two columns per soil type. A repeat experiment within the same column was also performed for each soil type and column. The kerosene viscosity was measured by a 'Schott Gerate' (Germany), type 53001/01 viscosity meter in a water bath (five replicates per kerosene composition). The kerosene surface tension was determined using a platinum-iridium test ring method (Cambridge Du Nouy tensiometer, Cambridge Instrument Co. Ltd., UK).
3. Experimental results 3.1. Statement of the problem The changes in kerosene viscosity in the experiments were due to either (1) temperature changes or (2) changes in the chemical composition of the kerosene, caused by selective volatilisation. In the following, we will use the conductivity values for the kerosene used in the temperature experiments at 24°C (Ksk(24°C); Table 2(a)) as a reference value in the temperature experiments, and the conductivity values for kerosene-0% at 24°C (Ksk(0%); Table 2(b)) as a reference value in the chemical composition experiments. The objective is to analyse the impact of kerosene-viscosity changes on the saturated kerosene conductivity for the different soils. Provided that the intrinsic permeability k (in Eq. (1)) is a parameter that depends only on the properties of the porous medium, Eq. (3) is valid and quantifies the fluid conductivity ratio Ks2/Ksl between two fluids with known densities and viscosities. In the present experiments, the difference in density between the studied kerosene mixtures was less than 2% for all cases and could, for the sake of simplicity, be neglected compared with the corresponding differences in viscosity. In the following discussion, the conductivity ratio Ks2/Ks1 will therefore be related only to the corresponding viscosity ratio as K~z/Ksl = tzl/ix z.
3.2. Temperature experiments The obtained mean kerosene conductivities and associated standard deviations among the experiments at 24°C and 5°C are summarised in Table 2(a); this temperature dependence is based on the experimental series of Jarsj5 et al. (1994), but the data have not previously been published. The standard deviation reported in Table 2(a) quantifies the combined effect of (l) the variability between the two soil columns and (2) the variability between the initial and the repeat experiment in each column. The reason for including both variability (1) and (2) in the reported standard deviation (i.e. n = 4) is that the variability (2) in a few cases exceeded the variability (1). However, the actual outcome of the statistical analysis with n = 4 (see Section 3.4) is the same as if one only accounts for variability between the two columns, i.e. variability (1) with n = 2. Differences between the initial and the repeat experiments in the same soil column may reflect disturbances such as, for instance, initial redistribution of the soil sample. For the coarse sand 2, the original and repeat experiment conductivity values deviated 11% from their common mean value reported in Table 2(a). In the sandy loam 2 and the
20
J. Jarsji5 et al. / J o u r n a l o f C o n t a m i n a n t
Hydrology
25 (1997) 113-127
e~
.=.
c5~
~
1
II II II
II
II
II
eq
"
" ,..4 ,,+,
e~
S o
II
=o o~
+1
e4
~+ ~
©
E H .=_
~E ¢q
&o
II II
II
II
I
II II
+1 +1 tt~
C
II
eq ,-2 ~
+, +,
¢5 rq. ~
~ +, ~, +' -~ ~
! ,o
2~
d
J. Jarsj6 et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
121
1.0--
0.8
~z4oc) / ~ s o c )
O" o eq
0.6
o
0.4
t/3
0.2
0.0
[_
I
I
0
0
I
I
15
19
I 44
I
1
51
Clay Content (%) Fig. 2. Ratio betweeen the kerosene viscosities at 24°C and 5°C (continuous line) and corresponding mean conductivity ratios (bars).
peat, the corresponding deviations were 8% and 2%, respectively. For the other soils in Table 2(a), the conductivity values of the repeat experiment coincided with the conductivity values of the original experiment. Thus, the conductivity experiments at 24°C are generally reproducible, exhibiting only small differences between them, which is also reflected in the corresponding standard deviations. For the temperature experiments listed in Table 2(a), the ratio between the mean kerosene viscosities at 24°C and 5°C was 0.71, and is illustrated in Fig. 2 (continuous line), along with the ratios between the corresponding mean kerosene conductivities for the different soils (bars). For peat and the sandy soils (coarse sand 2, sandy loam 1 and sandy loam 2, which are characterised by a sand fraction of more than 65% according to the USDA system of classification), the conductivity ratio Ksk(5°C)/Ksk(24°C) deviates less than 2% from the viscosity ratio /x(24°C)/~(5°C). For the soils with a higher clay content, the corresponding deviation reached a maximum of 18%.
3.3. Chemical composition experiments In Table 2(b), we present the mean conductivity values and standard deviations of all conductivity experiments with different kerosene mixtures in each soil. The statistics are for each soil obtained from observed conductivities in two soil columns during an initial and a repeat experiment. These experiments were performed and reported by Galin et al. (1990) and Gerstl et al. (1994), with the exception of the previously unpublished data for
122
J. Jarsji5 et al. / Journal of" Contaminant I4vdrology 25 (1997) 113-127 Eq. (3), p neglected Least square fit to experimental data 10000
-~
A
Coarse sand 1 Jt
_A m m
•
C o a r s e s a n d 2.
Medium sand
I o o o -= A
f_
E
g
Fine s a n d
1oo
A
v
=
Peat U
"~ cO
1,9
Loam
10~_ -~ i
M. C l a y5 ~
Ksk(0%)
0.1 1.z
I
I
I
I
I
i
I
I
1,3
1.4
1.5
1.6
1.7
1.8
1.9
2..0
Viscosity (mPa s) Fig. 3. Observed (symbols) and predicted (thick continuous lines) conductivities for kerosene mixtures with different viscosities and chemical composition.
the coarse sand 2 and the peat from the experimental series of Jarsji5 et al. (1994). Also for these experiments, the deviations between the initial and the repeat experiment conductivities were relatively small, as reflected in the standard deviations of Table 2(b). Fig. 3 illustrates the predictions of the mean kerosene conductivity according to Eq. (3), based on the viscosity ratios and the reference kerosene conductivity K~k(0%), along with the mean conductivity values for the kerosene mixtures in the soils of Table 2(b). The reference conductivity (Ksk(0%)) for the coarse sand 2 and the peat was obtained with a kerosene used by Jarsji5 et al. (1994). This kerosene had a higher viscosity at 0% volatilisation than the kerosene-0% of the other five soils in Fig. 3, which originate from the experimental series of Galin et al. (1990) and Gerstl et al. (1994). The thick lines of Fig. 3 represent the predictions of Eq. (3) and the thin trend lines are the least-squares fits to the observed conductivities for the kerosene mixtures (coefficient of determination: R 2 = 0.92 + 0.08). The first points on the trend lines represent the conductivity for the original kerosene, Ksk(0%), and the last points on the lines represent the conductivity for kerosene-60%, K~k(60%). The standard deviations are indicated in Fig. 3 with
J. Jarsjii et al. / Journal of Contaminant Hydrology 25 (1997) 113-127
123
20% d i f f e r e n c e in v o l a t i l i s e d mass 1.o
(a)
(b)
1.o-
0 . 8 ~
1.o-
(c)
-
12(40%)/1.1~60%)
--
o.o
~ 0.6-
0.2
:2
0.2-
~ 0.4-
~
S~
~ 0.2-
uo
I~ ..-
0.0
0.0 -
0.0
0
0
0
16
0
40
Clay Content (%)
0 O 16 40 Clay Content (%)
1.0a~
0.8-
o
40% difference "~ 0.6in v o l a t i l i s e d
mass
(d)
% ~.o'
"
"
",U{20~60%)
o.6-
~
--
"~ ~0
0~0"4-
=o-o
o2"-3
0
.
0.0'
0
0 0 16 40 Clay Content (%)
o~
0 0 16 40 Clay Content (%)
(f)
1.0-
0.8-
L., •'D
•,~ 0.6-
60% difference in volatilised mass
o
(e)
o~ 0 . 8 N
-
b
Clay Content (%)
l.O-
P(O%),/IJ{4(
~ 0.4-
"~0.20.0-
b
E
~o%)/I-t(6o%]
0.4•-
~
o
--I
_
U
0.20.0-
0
0
0
16
40
i
Clay Content (%) Fig. 4. Ratio between the kerosene viscosities (continuous lines) and corresponding ratios between the mean conductivities (bars) for kerosene mixtures with: (a)-(c) 20% difference in volatilised mass; (d) and (e) 40% difference in volatilised mass; (f) 60% difference in volatilised mass.
error bars (however, for the sands, the deviations are too small to be seen in the figure). For the inert sands the difference between observed and predicted conductivity is relatively small. For the peat, the prediction of the kerosene-60% conductivity through Eq. (3) is considerably higher than the observed kerosene-60% conductivity. Further-
124
J. Jarsji5 et a l . / Journal of Contaminant Hydrology 25 (1997) 113-127
more, for the montmorillonitic loam and clay, the continuous increase in viscosity of the kerosene mixtures with increasing volatilised mass percentage seems to explain only a smaller part of the observed changes in hydraulic conductivity (note the logarithmic scale on the y-axis in Fig. 3). To show the effect of volatilisation-induced changes in chemical composition, we have plotted in Fig. 4 the viscosity ratios of different fluid pairs (continuous lines, which coincide with the prediction of Eq. (3)) along with the corresponding observed mean conductivity ratios (bars). Hereby, the comparison between observed and predicted conductivities of Fig. 3 is extended to include also Ksk(20%) and Ksk(40%) as reference points for the soils used in systematic chemical composition experiments (C in Table 1). In Fig. 4, the plots are arranged after difference in kerosene chemical composition (reflected in the difference in volatilised mass percentage). Kerosenes of relatively similar chemical composition (20% difference in volatilised mass) are compared in Fig. 4(a), Fig. 4(b) and Fig. 4(c), where the viscosity ratios between kerosene-0% and kerosene-20% (Fig. 4(a)), kerosene-20% and kerosene-40% (Fig. 4(b)) and kerosene-40% and kerosene-60% (Fig. 4(c)) are compared with corresponding ratios of the hydraulic conductivities of the soils. The viscosity ratios (continuous lines in Fig. 4(a), Fig. 4(b) and Fig. 4(c)) show good agreement with the conductivity ratios (bars) in the three sands, whereas greater differences can be seen for the montmorillonitic loam and clay. The corresponding analysis for kerosene mixtures with greater differences in chemical composition (40-60% difference in volatilised mass, Fig. 4(d), Fig. 4(e) and Fig. 4(f)) shows that the disagreement between the viscosity ratio and conductivity ratio in the montmorillonitic soils increases with increasing difference in volatilisation degree. For a 40% difference in volatilised mass the viscosity ratio was about 1.9 times greater than the mean conductivity ratio (Fig. 4(d) and Fig. 4(e)), and for a 60% difference in volatilised mass the viscosity ratio was 3.0 times greater than the mean conductivity ratio (Fig. 4(f)). For the inert sands, however, the differences remain small, with the mean conductivity ratio being 1.1 times greater than the viscosity ratio both for both a 40% and a 60% difference in volatilised mass (Fig. 4(d), Fig. 4(e) and Fig. 4(0).
3.4. Significance testing The significance of the deviations between experimental data and the theoretical predictions of Eq. (3) was evaluated statistically. Specifically, the null hypothesis that the differences between experimental observations and predictions of Eq. (3) are insignificant was tested by studying the 95% confidence interval for these differences. For determination of this confidence interval, the t-distribution was used with (n E - 1) + ( n p - 1) degrees of freedom, where n E is the number of experimental observations and np is the number of realisations of the prediction. In this case, np is equal to the number of experimental observations of the reference conductivity (i.e. K~k(24°C) for the temperature experiments (Table 2(a)) and K~k(0%) for the chemical composition experiments (Table 2(b))) because the theoretical predictions are linearly related to the reference conductivities through Eq. (3). A comparison of the conductivities for kerosene at 24°C with corresponding conductivities at 5°C showed that the null hypothesis was accepted, i.e. differences between
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observations and predictions were found to be insignificant, at the 0.05 significance level, for all the temperature experiments (Table 2(a)). Thus, on this significance level, the data do not provide a basis for doubting the validity of Eq. (3) for conductivity changes caused by temperature-induced changes in VOLM viscosity. In contrast, a comparison of the conductivities for kerosene-0% with corresponding conductivities for kerosene-60% showed that the null hypothesis was rejected, i.e. differences between observations and predictions were found to be significant, at the 0.05 significance level, for four soils in the chemical composition experiments. These four soils are the coarse sand 1, the peat, the montmorillonitic loam and the montmorillonitic clay. For these soils, the data thus provide strong indications that Eq. (3) does not accurately describe changes in VOLM conductivity that are caused by volatilisation-induced changes in VOLM viscosity. However, for all the sandy soils, including the coarse sand 1 where differences were statistically significant at the 0.05 significance level, the deviation between observation and prediction is small in comparison with the deviations seen in the peat and the clay soils, where considerably larger discrepancies are observed (Fig. 3).
4. Discussion and conclusions
For temperature-induced changes in kerosene viscosity, the resulting saturated conductivity ratios were consistent with the corresponding viscosity ratios, i.e. with the predictions of Eq. (3), to within the range of experimental deviations. For volatilisationinduced viscosity changes, which reflect irreversible changes in kerosene chemical composition, the theoretical scaling relationship, Eq. (3), showed relatively small deviations from corresponding viscosity ratios for inert, sandy soils (coarse, medium and fine sand). However, for volatilisation-induced viscosity changes in soils containing clay or organic matter the observed saturated conductivity ratios deviated significantly from the theoretical predictions of Eq. (3). A comparison between four different kerosene mixtures in two montmorillonitic soils furthermore showed that the discrepancies between these two ratios increased with increasing differences in liquid chemical composition (Fig. 4). Thus, the results indicate that other properties than the fluid viscosity may be of importance for conductivity of potentially interacting soils with respect to VOLM residues. The study of Fernandez and Quigley (1988) demonstrated that for pure organic solvents, fluid viscosity may not be the controlling parameter for conductivity. In addition, for single non-aqueous organic compounds (heptane and xylene), Yong et al. (1992) observed that there may be significant differences between the conductivity ratio and the viscosity ratio of the organic compound and water in clay soils because of phase partitioning. Based on these observations on single compounds, Yong et al. (1992) suggested that a change in proportions of the chemical constituents in a multi-component liquid mixture could render the original laboratory conductivity values invalid. However, Yong et al. (1992) lacked supporting observations for this suggestion. The laboratory observations on VOLMs reported in this study support the hypothesis that chemical composition of a liquid mixture may be critical to its conductivity value.
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Furthermore, they demonstrate that volatilisation of the liquid phase is capable of changing the VOLM chemical composition sufficiently to yield considerable conductivity changes, which are not attributed to viscosity changes, in interacting soils. It has been recognised (Yong, 1992) that conductivity in soils may decrease with increasing hydrophobicity of organic compounds. Heavier organic compounds are generally characterised by lower solubility and higher octanol-water partitioning coefficients (i.e. they are more hydrophobic) than are lighter organic compounds. The preferential volatilisation of lighter kerosene components changes the composition of the kerosene such that the relative amount of heavy compounds increases with increasing volatilised mass percentage (Galin et al., 1990; Jarsji5 et al., 1994). The remaining liquid kerosene thus becomes more hydrophobic with increasing volatilisation, which may, in turn, result in greater adsorption and possibly explain the observed reductions of conductivity of interacting soils. For swelling clays, another possible explanation for changes in conductivity as a result of changes in chemical composition of the liquid mixture may be given by the diffusive double layer theory that relates swelling with the dielectric constant of the liquid (e.g. Yong et al., 1992), and the extended solution theory (Graber and Mingelgrin, 1994). However, the relations between solvent properties, clay swelling and resulting hydraulic conductivity are complex, and intrinsic permeability measurements have often yielded results that conflict with the diffusive double layer theory (e.g. Green et al., 1981). In this study, we have not attempted to provide explicit cause-and-effect relationships between VOLM conductivity changes and the actual mechanisms causing such changes. However, the reported deviations of up to 300% between conductivity ratio and viscosity ratio for kerosene mixtures with different chemical composition have provided some information on the limitations of the relatively well-established conductivityviscosity scaling relation, Eq. (3). This information may be useful for future studies of the influence of chemical composition on the VOLM conductive properties of potentially interacting soils.
Acknowledgements Financial support for this work was provided by the Swedish Environmental Protection Agency (SNV). The reported research was also partially supported by a grant from Water Research Institute (WRI)-Technion Haifa, Israel. The second author acknowledges the financial support of the Swedish Natural Science Research Council (NFR).
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