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Soil pollution by petroleum products, I. Multiphase migration of kerosene components in soil columns
Journal of Contaminant Hydrology, 4 (1989) 333-345 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
333
SOIL P O L L U T I O N BY P E T R O L E U M P R O D U C T S , I. M U L T I P H A S E M I G R A T I O N OF K E R O S E N E C O M P O N E N T S IN SOIL C O L U M N S
A.J. ACHER, P. BODERIE and B. YARON
Department of Organic and Residues Chemistry, Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet-Dagan 50250, Israel (Received February 10, 1988; revised and accepted January 25, 1989)
ABSTRACT Acher, A.J., Boderie, P. and Yaron, P., 1989. Soil pollution by petroleum products, I. Multiphase migration of kerosene components in soil columns. J. Contam. Hydrol., 4: 333-345. A laboratory study of soil contamination by a synthetic "kerosene" is reported. Soil (Mediterranean red sandy clay) samples with different moisture contents (0.0, 0.8, 4.0, and 12%, w/w) were contaminated by vapors and/or liquid from a mixture containing 5 kerosene components (m-xylene, pseudo-cumene, t-butylbenzene, n-decane and n-dodecane). The contribution of the different kerosene components to the adsorption, volatilization and transport processes is described. Vapor adsorption was found to be dependent on the vapor concentration of each component (except for the n-decane), and on the soil moisture content. The sorption coefficients of the kerosene components decreased with increasing temperature but showed only a very slight variability between 20 and 34°C, in air-dried soil. The volatilization from soil was high: more than 90% of the aromatic components were desorbed in less than 2 h. The transport of the kerosene, in liquid and vapor phases, through the soil columns, was studied using amounts of kerosene which were less (1 mL) or more (10 mL) than the retention capacity of the soil columns. The increase in the moisture content of the soil increased the rate and the depth of kerosene downward penetration. It stopped however, the vapor movement (at 4%) and the upward liquid movement (at 12%). Among the properties of the kerosene components, volatility seems to be the prime factor which determines kerosene movement once liquid phase movement has ceased.
INTRODUCTION T h e e v e r i n c r e a s i n g u s e of p e t r o l e u m p r o d u c t s r e q u i r e s h u g e s t o r a g e a n d t r a n s p o r t f a c i l i t i e s . T h e t h r e a t of p o l l u t i o n i m p o s e d b y t h e p r e s e n c e of s u c h i n s t a l l a t i o n s a n d t h e r e p o r t e d c a s e s of soil a n d g r o u n d w a t e r p o l l u t i o n by a c c i d e n t a l s p i l l a g e h a s p r o m p t e d a n i n c r e a s e d i n t e r e s t i n t h e fate of s u c h p r o d u c t s i n soil a n d w a t e r . I n h i s r e v i e w o n m i g r a t i o n i n t h e v a d o s e z o n e of o r g a n i c fluids i m m i s c i b l e w i t h w a t e r , S c h w i l l e (1984) p o i n t e d o u t t h a t ~ ' g a s o l i n e p o s s e s s e s a h i g h v o l a t i l i t y t h e effect of w h i c h h a s b e e n l a r g e l y u n d e r e s t i m a t e d u p u n t i l n o w " . I n a d d i t i o n , s o m e c a s e s t u d i e s r e c e n t l y d e s c r i b e d i n t h e l i t e r a t u r e (e.g., R e i c h m u t , 1984; M a r r i n a n d T h o m p s o n , 1984; B i s q u e , 1984; H i n c h e e a n d
0169-7722/89/$03.50
~ 1989 Elsevier Science Publishers B.V.
334
Reisinger, 1985; Wittman et al., 1985) described the effect of volatile organic compounds in the contamination of porous media and groundwater. Bossert and Bartha (1984), suggested that infiltration through porous media limits and reduces the rate of evaporative losses of volatile hydrocarbons. The impact of volatilization in determining the fate of petroleum constituents in soils was reported in the review of McGill et al. (1981). Abriola and Pinder (1985 a,b) introduced a simulation model for multiphase flow in porous media. The same authors comprehensively reviewed the existing data on the transport of waterimmiscible compounds in the subsurface (Pinder and Abriola, 1986). A recent study of Corapcioglu and Baehr (1987) includes a comprehensive mathematical model which describes the fate and transport of a petroleum product in the subsurface environment. In this model, the transport of the contaminant can occur as a solute in soil water, as a vapor in soil air and as a component within the immiscible liquid phase. Finally, Baehr (1987) utilized a two-dimensional radially symmetric transport model to analyze vapor dissipation to the atmosphere and the role of vapor transport in defining the contamination potential of an oil spill. Despite the major importance that the gaseous phase seems to play in the transport and redistribution of petroleum components in porous media, its effect has been little studied experimentally. For example, the recent studies on the movement of hydrocarbons in the unsaturated zone carried out by Barbee and Brown (1986) did not consider the simultaneous liquid and gaseous transport. The work reported here, is part of a study carried out in our laboratory on soil pollution by petroleum products. The aim of this study is to accumulate knowledge on physical and chemical processes acting in the unsaturated zone and at its interface with the groundwater. The final aim is to learn how to handle and minimize the adverse impacts from an accidental spill or release of petroleum products in soils or groundwater. The paper described the contribution of the different kerosene components (in liquid and/or gaseous phases) toward the physico-chemical processes involved in the pollution of soil. MATERIALS AND METHODS
Soil The soil used in the experiments was a Mediterranean Red sandy-clay containing 12% clay, 4% silt, 84% sand and 0.5% organic matter with a specific surface area of 50m2g 1, from Bet-Dagan. Before use, the soil was ground, sieved (50 mesh) and oven-dried, at 110°C for 24 hours (0.0% moisture), or air-dried at 22°C until the moisture content equilibrated (0.8%). Subsequently, part of the air dried soil was wet up to 4.0% or to its field capacity (12%, 0.33 bar). The water content figures are gravimetric values (w/w).
335 TABLE 1 Chemical composition and physical properties of "kerosene". Component
m-Xylene
ps-Cumene
t-but-Benzene
n-Decane
n-Dodecane
mol. wt, (g) boil. pt. (°C) density (g cm-3) vol. ratio (%) mol. ratio (%)
106.2 139.0 0,864 6.67 10.28
120.2 169.0 0,877 6.67 9.22
134.2 169,0 0,867 6.67 8.17
142.3 173.0 0,730 40.0 38.91
170.3 216.0 0.750 40.0 33.41
8.48 0.98 1.9
2.94 0.34 1.0
44.16 5.10 1.46
1.12 0.13 0.23
0.04 0.8
0.03 0.5
sat. gas atmosphere a relative vol. ratio conc. (pLL 1) vapor pres. (mmHg)
43.4 5.0 6.8
water solubility~ sing. comp. (pLL 1) in mixture (#LL 1)
160.0 10.0
56.0 3.5
23.0 1.5
a at 27°C, ~at 22°C.
"Kerosene" Kerosene consists of a mixture of more than 100 different hydrocarbons which are difficult both to identify and quantify. We chose to use a synthetic "kerosene" having the same ratio of the aromatic and aliphatic components (20:80) as natural kerosene but comprising only five well defined components. The following considerations were taken into account in the selection of "kerosene" components: their presence in natural kerosene; the penetration rate in soil columns relative to that of natural kerosene; and good chromatographic resolution. The chemical composition and the physical properties of the "kerosene" components used in this study are given in Table 1.
Oil blue liquid This organic dye solution (in xylene, product of Du Pont De Nemours, Petroleum Chem. Div., Wilinghton, USA, catalog # DR-9399-C), was used in column experiments filled with soil having 4 or 12% moisture content.
Experimental procedure (a) Adsorption experiments with the "kerosene" vapor were carried out in Erlenmeyer flasks (250 mL) fitted with ground-glass stoppers containing ovenor air-dried soil samples (10 g). Each flask contained an open test tube with 2 mL "kerosene" whose vapors saturated the atmosphere above the soil. The flasks were tightly closed and kept for 7 days at different temperatures (7, 17, 27 and 34°C). The experiments were carried out in 3 replications. (b) Volatilization experiments. After the "kerosene" constituents were adsorbed from the vapor phase onto samples of air-dried soil (as described
336 above) the Erlenmeyer flasks were opened to the atmosphere. The test tubes were removed and the soil samples analyzed for hydrocarbons content after 2 and 16 hours. (c) Transport experiments. These experiments were carried out in soil columns, using 2 different amounts of "kerosene". When the amount applied was less (c.1) or larger (c.3) than the "oil retention capacity" of the soil (Schwille, 1984), we called the "kerosene" movement in unsaturated or saturated conditions, respectively (the terms unsaturated and saturated conditions are not used here in the sense of groundwater hydrology). The term "oil retention capacity" of the soil indicates maximum sorptive capacity and fluid retention (in pores spaces). All the transport experiments were carried out in duplicate. (c.1) "Kerosene" movement in soil columns - - unsaturated conditions. Transport experiments were performed in glass columns (35 cm length, 2.16 cm I.D.). The soil (155g) was introduced into the glass column using a funnel connected with a rubber tube (30 cm). By gradually raising the funnel, the soil was packed into the column from the bottom upward, avoiding the gravitational fractionation of its particles. This resulted in a packing density of 1.40gcm 3. Four different levels of soil moisture content were used in the column experiments: 0.0, 0.8, 4.0 and 12.0%. To obtain columns with soil at field capacity, the air-dried soil columns were water saturated from below and left on a dried sand bed to equilibrate for a week. The columns were vertically levelled and closed with rubber stoppers at both ends. The "kerosene" (1 mL) was applied dropwise from a syringe by piercing the upper stopper. This amount of 1.0mL was experimentally determined to be lower than the "kerosene" retention capacity of the soil column at any moisture content of the soil. The columns were kept at constant temperature (22°C). The advance of the liquid fronts was visually observed on the columns packed with oven- and air-dried soils. In the columns filled with moist soil (4 and 12% moisture content), it was not possible to see the advance of the liquid "kerosene" front. To visualize the front, the kerosene dye, previously described, was applied to the inner wall of the glass column as lines on opposing sides of the column. After drying in an oven (150°C), for 10 rain, the columns were packed with soil. As the liquid "kerosene" moved through the moist soil it dissolved the dye on the wall of the column enabling one to mark its advance. (c.2) Redistribution of the "kerosene" components. The effect of soil moisture (0.0, 0.8 and 12%) on the redistribution of "kerosene" components in soil columns was studied at different times (5, 7, 14, 21, 25, 30 and 46 days) after the initial "kerosene" addition. By the end of the experiments, the soil from each column was extruded and divided into 8 layers, which were extracted and analyzed separately, as described in the following analytical procedure. (c.3) "Kerosene" movement in soil columns - - saturated conditions. An amount of "kerosene" (10 mL), which was previously found to be greater than the "kerosene retention capacity" of the soil column, was applied on the soil
337 surface. The experiments were carried out using soil at two different moisture contents: air-dried and field capacity. The preparation of columns and the experiments were carried out as described above (c.1). (c.4) The upward movement of "kerosene" in soil columns. Soil columns [prepared as described above (c.1)], were filled with soil at different moisture contents (0.0, 0.8, 4.0 and 12%) and "kerosene" (1 mL) was added. The columns were then arranged vertically with the pollutant layer at the bottom, so that the advancement of the ~kerosene" was opposite to the gravitational pull. The experiments using these columns were performed in a manner similar to those investigating the downward movement of ~'kerosene" (c.1).
Analytical procedure Volatization losses of ~'kerosene" constituents during sample handling can lead to erroneous analytical results. Therefore, the handling of samples containing '~kerosene" (soil, water, air) was performed using methods to prevent such losses.
"Kerosene" extraction from the soil Soil samples from the adsoprtion, volatization and transport experiments were extracted in Erlenmayer flasks (250mL). For the soil samples from the adsorption experiments the following procedure was used: immediately after opening the flask, the test tube containing "kerosene" was removed. Water ( l m L per g soil), and then carbon tetrachloride (2 mL per g soil), were added and the flask tightly closed (with a ground glass stopper and teflon tape) and placed in a laboratory shaker for 2 hours. Then, sodium chloride (0.5 g per g soil) was added and the flask again closed and shaken for another 5 minutes. An aliquot of the carbon tetrachloride layer (1-2mL) was transferred into a 5-mL screw cap vial (with aluminium liner) to which anhydrous sodium sulfate and aluminium oxide (H, basic, type E) were added (to remove water and extractable organic, respectively). This sample was then used for "kerosene" analytical quantifications. The soil samples from volatilization and transport experiments were extracted using the same procedure.
Gas chromatographic determination The carbon tetrachloride extract of the '~kerosene" from the soil sample was analyzed chromatographically. Aliquots (3 ttL) were injected into a Varian 3300 Gas Chromatograph equipped with a 30-M DB-1 column (i.d. 0.53mm). The chromatography was performed in a temperature programmed regime, from 35 to 150°C with a heating rate of 5°C min 1. The carrier gas was nitrogen, while hydrogen and air provided the combustion mixture for the flame ionization detector (FID); the flow rates for nitrogen, hydrogen and air, were 30, 30 and 300mLmin -1, respectively. The FID was connected to a Merck D-2000 integrator, which quantified the chromatogram peaks.
338 TABLE 2 V a p o r c o n c e n t r a t i o n of " k e r o s e n e " c o m p o n e n t s in j a r s a t m o s p h e r e at different t e m p e r a t u r e s S a t u r a t e d a t m o s p h e r e composition ( p L L 1)
"Kerosene" components
m-xylene ps-cumene t-but.-benzene n-decane n-dodecane
7°C
17°C
27°C
34°C
1.20 0.17 0.04 0.74 n.d.
2.90 0.65 0.24 2.65 0.08
5.00 0.98 0.34 5.10 0.13
7.80 1.50 0.62 7.10 0.24
R E S U L T S AND D I S C U S S I O N
The most important physico-chemical processes involved in the pollution of soil by "kerosene" such as adsorption from the vapor phase, volatilization from soil, transport and redistribution through soil column, are described.
(a) Adsorption from the vapor phase Adsorption of "kerosene" components from the vapor phase onto soil as affected by environmental factors such as temperature and soil moisture content was studied in a closed system. The "kerosene" vapor concentrations in jars atmosphere is given in Table 2. The amount of hydrocarbons adsorbed per gram of soil after 7 days of contact with a saturated vapor atmosphere at different temperatures (7, 17, 27 and 34°C) and for two moisture contents: ovenand air-dried soils, is given in Table 3. It may be observed that the adsorption of the hydrocarbons from both aliphatic and aromatic groups increases with the increase of ambient temperature. The increase in temperature increases the vapor pressure of the hydrocarbons thus affecting their concentrations in the TABLE 3 A d s o r p t i o n of v a p o r " k e r o s e n e " c o m p o n e n t s on soil, as affected by soil m o i s t u r e and t e m p e r a t u r e ( p g g 1) Soil m o i s t u r e (%)
T = 7°C
T = 17°C
T = 27°C
T = 34°C
0.0 a
0,8 h
0.0 a
0.86
0.0 ~
0.8 h
0.0 a
0.86
200 130 50 430 30
40 40 15 100 10
390 185 120 510 55
90 80 47 130 20
800 345 210 440 130
110 100 60 130 35
1150 490 330 500 230
120 120 100 110 80
Components m-xylene ps-cumene t-butyl-benzene n-decane n-dodecane "oven-dried; b air-dried.
339
n-Decane ' '
°°°F\ z i,i o
c.)
n - Dodecane
/
1 ,oo
o
il,oo o
m-Xylene 2 0 0 lq.__. ~
, o
' o
!
I O0 t,m__..___.. 0 / e: o o3
, -
8 0 0 ~o
ps-Curnene , ,
,le
t- butyl - Benzene
5
15 25 TEMPERATURE (°C)
7,5
MOISTURE CONTENT: 00.0% eO.8 % 5
15 25 35 T E M P E R A T U R E (°C)
Fig. 1. Sorption coefficient of the "kerosene" components in the soil, as affected by temperature and soil moisture content.
atmosphere above soil surfaces and hence their adsorption. The amount adsorbed on the oven-dried soil at the highest temperature (34°C) showed the following adsorption order: m-xylene > n-decane > pseudo-cumene > tbutylbenzene > n-dodecane. With the exception of n-decane (for air- and ovendried soil), all the hydrocarbons studied exhibited the tendency to be more highly adsorbed on the soil surface at higher temperatures. The adsorption of n-decane reaches its maximum at about 20°C and no increase in the adsorption was observed up to a temperature of 34°C. The influence of the moisture content of the soil on the adsorption of "kerosene" components on soil surface can also be seen from the Table 3. The adsorption of hydrocarbons on the air-dried soil was drastically decreased in comparison to the adsorption on oven-dried soil. It may be observed, for example, that the adsorption of aromatic and aliphatic compounds investigated at 34°C on the air-dried soil is diminished by more than a factor of 3 compared with their adsorption on oven-dried soil. This is probably because the "kerosene" components (low solubility in water and low polarity) are unable to compete with water molecules for the adsorption sites of the soil solid phase. This behavior was also observed by Chiou and Soup (1985) for benzene and its related compounds and for xylene by Barbee and Brown (1986) on a Texas soil. The competition for soil adsorption sites between the ~'kerosene" constituents and water decreases the oil retention capacity of the soil, thus increasing the penetration depth into soil. Figure 1 shows the effect of temperature and soil moisture on sorption coefficient of "kerosene"
340 TABLE 4 Relative volatilization of "kerosene" components from air dried soil "Kerosene" components m-xylene ps-cumene t-butyl-benzene n-decane n-dodecane
Lossesa (at 22°C) after 2h
16h
99.0 94.0 92.5 71.5 14.3
99.3 99.7 99.7 89.5 61.3
aIn percentage from the amount adsorbed. components. The sorption coefficient (component concentration on soil/vapor concentrations in air) is lowered by a temperature increase, having the highest value at 7°C. For all the components, the presence of moisture in air-dried soil (0.8%) drastically decreased the sorption coefficient, especially at lower temperatures; in the temperature range of 20-34°C the sorption coefficients showed very small variability.
(b) Volatilization from the adsorbed phase " K e r o s e n e " constituents volatilize from the adsorbed phase after relatively short periods of time. Table 4 shows the relative volatilization of the " k e r o s e n e " constituents from soil into the atmosphere after 2 and 16 hours, at 22°C. It may be observed t ha t in the open system more than 99% of the adsorbed aromatic compounds (m-xylene, pseudo-cumene and t-butylbenzene) volatilize in 2 hours. The aliphatic compounds have a lower rate of loss, especially n-dodecane of which 40% remains after 16 hours.
(c.) Transport experiments in soil columns The use of two sets of conditions (c.1, saturated and c.3, unsaturated), for " k e r o s e n e " movement into soil columns enabled us to not only to study the liquid and the vapor phase movement, but the dynamic redistribution of the " k e r o s e n e " components as well.
(c.1) "Kerosene" movement in soil columns - - unsaturated conditions During infiltration of liquid " k e r o s e n e " into the soil column there was simultaneous, but faster, vapor movement of the " k e r o s e n e " components producing a p ene t r at i on front in advance of the liquid. However, contamination by " k e r o s e n e " vapor was important only after the liquid movement stopped. Twenty one days after " ke r os e ne " application to the oven-dried soil
341
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~e
" 5 tl I /
0
•
3
nt
I -o.o % 2 - o.8",,
It'
O
"
3-
2
4
~
4.0"/,
6 8 I0 12 TIME (doys)
14
16
Fig. 2. T h e p e n e t r a t i o n of t h e " k e r o s e n e " - l i q u i d f r o n t in soil c o l u m n s , as affected by soil m o i s t u r e c o n t e n t a n d time.
columns, the liquid front (observed visually) reached a depth of 9.4 cm, while the vapor movement reached 25.0 cm. (as determined by soil extraction). For the same period, in the column with air-dried soil, the above values were 12.3 cm and 32.0 cm, respectively. In the experiments in which the moisture contents of the soil were 4 and 12%, the liquid front depths were 17.5 cm and 19.2 cm, respectively. In these cases there was no vapor movement in advance of the liquid fronts. These results display a general behavior of "kerosene" transport through the soil columns: an increase in the moisture content of the soil decreased the ~kerosene" retention capacity, resulting in a deeper and faster penetration of the liquid ~kerosene". However, for vapor-'kerosene" movement, the increase in the soil moisture content above 4% completely inhibited vapor penetration. These results are similar to those of Barbee and Brown (1986) who reported that when xylenes were applied to a sandy loam soil with a moisture content near field capacity, the hydrocarbons were retained by the soil and their movement was significantly attenuated by the presence of water in soil. Figure 2 shows the depth of the wetting front of the liquid ~kerosene" in soil columns of different moisture contents (0.0, 0.8, 4.0 and 12.0%, w/w), with time. The data show a decreasing rate of penetration with time. At about 12 days following application no further advance of the wetting front was observed as the amount of liquid '~kerosene" (lmL), originally applied, depleted by soil retention capacity and the gravitational pressure head became negligible. The data do not fit the classical infiltration equation (a linear relationship between depth and xfi), since a constant "kerosene" head was not maintained. As expected, from the slope of the infiltration curves, it can be seen that the increase of the moisture content of the soil resulted in a deeper and faster penetration of the liquid.
(c.2) "Kerosene" components redistribution in soil columns When the liquid "kerosene" (1 mL) applied on the surface of the column was
342
0
0.1 0.2 0
C/C o OJ 0.2 0
o.I
0.2
,o °I~I~' ~,~, ~/~~' 20 ~o
J
0
'
,~
30
E
0
T
20
n
30
III
CI
I//
°2(}2 , '
I
7
t
,~' , V
,b2 ,
i
i
L
I
o3
ii
,b3
,t
k
ii
0
Io 2o 3o Fig. 3. Redistribution of "kerosene" components in soil columns as affected by soil moisture and time. Numbers on curves represent days after '~kerosene" application. C ~ hydrocarbon amount in each soil layer. CO - total hydrocarbon amount in the column. (a) = m-xylene; (b) n-decane; (c) ~ ps-cumene; (d) t-butylbenzene and (e) = n-dodeeane at (1) 0.0%, (2) 0.8% and (3) 12.0% soil moisture content. depleted and the wetting front ceased to advance, the vapor and the liquid trapped in soil pores or adsorbed on the soil produced a redistribution of the d i f f e r e n t " k e r o s e n e " c o m p o n e n t s a c c o r d i n g to a d y n a m i c s o i l - v a p o r a d s o r p tion-desorption equilibrium. This vapor movement resulted in a redistribution o f t h e m o r e v o l a t i l e c o m p o n e n t s , h e n c e to a c h a n g e i n t h e r e s i d u a l " k e r o s e n e " composition. Figure 3 shows the redistribution in soil columns of different soil m o i s t u r e c o n t e n t s (0.0, 0.8 a n d 12%) o v e r d i f f e r e n t t i m e p e r i o d s ( u p t o 46 d a y s ) o f a l l t h e " k e r o s e n e " c o m p o n e n t s (a - - m - x y l e n e , b - - p s - c u m e n e , c - - t - b u t benzene, d -- n-decane, e -- n-dodecane). The amount of each component (C) (from each layer of the soil column), divided by the total amount of that c o m p o n e n t i n t h e e n t i r e c o l u m n (Co) w a s p l o t t e d v e r s u s t h e c o r r e s p o n d i n g
343 column layer depth (cm). The redistribution of '~kerosene" components mostly depends on their volatility. So, the redistribution of m-xylene is very similar to t h at of n-decane. The same can be observed between t-butylbenzene and ndodecane curves, while ps-cumene redistribution curve is somewhere between a and c curves. In the soil column at field capacity it was observed that both aliphatic and aromatic hydrocarbons remained in the upper 20 cm layer and their redistribution with time occurs only to this depth. The movement of '~kerosene" in oven and air dried soil columns is different. It may be observed t ha t independent of their properties the ~kerosene" components moved faster and deeper in air-dried rat her than in the oven-dried soil columns. This behavior agrees with the adsorption pattern already described above. The extent of redistribution with depth is affected by the h y d r o c a r b o n properties. In the case of oven dried soils, the penetration of the ~kerosene" products follows the order: m-xylene > n-decane > pscumene > t-butylbenzene > n-dodecane. This behavior is in accordance with the vapor pressure (Table 1), the vapor concentration and the adsorption order of the ~kerosene" constituents. In the case of the columns packed with air dried soil the redistribution of the ~'kerosene" components is only slightly affected by their volatility. (c.3) "Kerosene" movement in soil columns - - saturated conditions The high amount of ~'kerosene" applied (10mL) to the soil columns increased the gravitational pressure head, hence the rate of penetration. In these experiments, the penetration of " ke rosene" through the whole soil column took only 3.0 and 1.8 hours for air-dried and field capacity soils, respectively. The leachates and the samples from different soil layers were chromatographically analyzed but did not show any clear change in their composition. As expected an increase in soil moisture content increases the downward p e n e t r a t i o n rate of ~kerosene" into the soil columns. (c.4) U p w a r d movement of "kerosene" in soil columns This series of experiments was carried out in order to simulate the pollution of the soil when a '~kerosene" lens lying upon the groundwater is the source of pollution. Since the direction of kerosene penetrations is against gravitational forces, only upward force acting is capillarity. In this case the rate of the advance of the liquid front was slower by about 20% than that for downward movement. An increase in the moisture content of the soil resulted in a decrease of the upward rate of penetration. For samples taken 21 days after ~kerosene" application the heights of infiltration were 7.0, 4.5 and 0.5 cm for 0.0%, 0.8% and 4% moisture contents, respectively. On the other hand the gas phase extended upwards 23.0 and 30.0 cm for oven- and air-dried soils, respectively. The 4% moisture content also proved to be a barrier for upwards vapor penetration, while soil at field capacity prevented any ~kerosene" upward movement at all.
344 CONCLUSIONS
In all the physico-chemical processes involved in the soil contamination by "kerosene", the most influential factor was the moisture content of the soil. Its increase resulted in a drastic decrease in the adsorption of the vapor components of the "kerosene", an increase in the rate and depth of the downward penetration of the liquid-"kerosene" through the soil columns, a decrease of the upward movement of the liquid "kerosene" and a decrease of vapor "kerosene" movement. The increase in vapor concentration brought about by higher temperatures increased the amount of vapor "kerosene" components adsorbed by the soil, however, the sorption coefficient was decreased. The volatility of the "kerosene" components seems to be the prime factor in the redistribution process once liquid phase movement has ceased. ACKNOWLEDGMENTS
This study was supported by a grant from the National Council for Research and Development, Israel, and the Deutsche Forschungs- and Versuchsanstald fuer Luft- und Raumfahrt e.V., Koeln-Porz, Germany. REFERENCES Abriola, L.M. and Pinder, G.F., 1985a. A multiphase approach to the modeling of porous media contamination by organic compounds. 1. Equation development. Water Resour. Res., 21:11-18. Abriola, L.M. and Pinder, G.F., 1985b. A multiphase approach to the modeling of porous media contamination by organic compounds. 2. Numerical simulation. Water Resour. Res., 21: 19-26. Baehr, A.L., 1987. Selective Transport of Hydrocarbons in the Unsaturated Zone due to Aqueous and Vapor Phase Partitioning. Water Res., 21: 192~38. Barbee, G.C. and Brown, K.W., 1986. Movement of xylene through unsaturated soils following simulated spills. Water Air and Soil Pollution, 29:321 333. Bisque, R.E., 1984. Migration rate of volatiles from buried hydrocarbon sources through soil media. In: Proc. Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA/API First Conference, Houston, USA. National Water Well Assoc., Worthington, OH, pp. 267-274. Bossert, I. and Bartha, R., 1984. The Fate of Petroleum in Soil Ecosystems. In: R.M. Atlas (Editor), Petroleum Microbiology. MacMillan, New York, NY, pp. 435~475. Chiou, C.T. and Soup, T.D., 1985. Oil sorption of organic vapors and effect of humidity on sorption mechanism and capacity. Environ. Sci. Technol., 19: 119~1200. Corapcioglu, M.Y. and Baehr, A.L., 1987. A computational multiphase model for groundwater contamination by petroleum products. 1. Theoretical consideration. Water Resour. Res., 23: 191 200. Hinchee, R.E. and Reisinger, H.J., 1985. Multiphase transport of petroleum hydrocarbons in the subsurface environment: Theory and practical application. In: Proc. Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA/API First Conference Houston USA. National Water Well Assoc., Worthington, OH, pp. 58- 77. Marrin, D.L. and Thompson, G.M., 1984. Contaminants in the ground water via shallow soil gas sampling. In: Proc. Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA/ API First Conference Houston USA. National Water Well Assoc., Worthington, OH, pp. 172 188.
345 McGill, W.B., Rowell, M.J. and Westlake, D.W., 1981. Biochemistry, ecology and microbiology of petroleum components in soil. In: E.A. Paul and J.N. Laad (Editors), Soil Biochemistry, Vol. 5. Marcel Dekker, New York, NY, pp. 229-296. Pinder, G.F. and Abriola, L.M., 1986. On the simulation of nonaqueous phase organic compounds in the subsurface. Water Resour. Res., 22:109 119 (August supplement). Reichmuth, D.R., 1984. Subsurface gasoline migration perpendicular to groundwater: A Case Study. In: Proc. Petroleum Hydrocarbons and Organic Chemicals in Ground Water, NWWA/ API First Conference, Houston USA. National Water Well Assoc., Worthington, Ohio, pp. 43 53. Schwille, F., 1984. Migration of organic fluids immiscible with water in the unsaturated zone. In: B. Yaron et al. (Editors), Pollutants in Porous Media. Springer-Verlag, Berlin-Heidelberg- New York, pp. 27-50. Wittman, S.G., Quinn, K.J. and Lee, R.D., 1985. Use of soil gas sampling techniques for assessment of ground water contamination. In: Proc. Petroleum Hydrocarbons and Organic Chemicals in Ground water, NWWA/API, Second Conference Houston, USA. National Water Well Assoc., Worthington, OH, pp. 291-310.
333
SOIL P O L L U T I O N BY P E T R O L E U M P R O D U C T S , I. M U L T I P H A S E M I G R A T I O N OF K E R O S E N E C O M P O N E N T S IN SOIL C O L U M N S
A.J. ACHER, P. BODERIE and B. YARON
Department of Organic and Residues Chemistry, Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet-Dagan 50250, Israel (Received February 10, 1988; revised and accepted January 25, 1989)
ABSTRACT Acher, A.J., Boderie, P. and Yaron, P., 1989. Soil pollution by petroleum products, I. Multiphase migration of kerosene components in soil columns. J. Contam. Hydrol., 4: 333-345. A laboratory study of soil contamination by a synthetic "kerosene" is reported. Soil (Mediterranean red sandy clay) samples with different moisture contents (0.0, 0.8, 4.0, and 12%, w/w) were contaminated by vapors and/or liquid from a mixture containing 5 kerosene components (m-xylene, pseudo-cumene, t-butylbenzene, n-decane and n-dodecane). The contribution of the different kerosene components to the adsorption, volatilization and transport processes is described. Vapor adsorption was found to be dependent on the vapor concentration of each component (except for the n-decane), and on the soil moisture content. The sorption coefficients of the kerosene components decreased with increasing temperature but showed only a very slight variability between 20 and 34°C, in air-dried soil. The volatilization from soil was high: more than 90% of the aromatic components were desorbed in less than 2 h. The transport of the kerosene, in liquid and vapor phases, through the soil columns, was studied using amounts of kerosene which were less (1 mL) or more (10 mL) than the retention capacity of the soil columns. The increase in the moisture content of the soil increased the rate and the depth of kerosene downward penetration. It stopped however, the vapor movement (at 4%) and the upward liquid movement (at 12%). Among the properties of the kerosene components, volatility seems to be the prime factor which determines kerosene movement once liquid phase movement has ceased.
INTRODUCTION T h e e v e r i n c r e a s i n g u s e of p e t r o l e u m p r o d u c t s r e q u i r e s h u g e s t o r a g e a n d t r a n s p o r t f a c i l i t i e s . T h e t h r e a t of p o l l u t i o n i m p o s e d b y t h e p r e s e n c e of s u c h i n s t a l l a t i o n s a n d t h e r e p o r t e d c a s e s of soil a n d g r o u n d w a t e r p o l l u t i o n by a c c i d e n t a l s p i l l a g e h a s p r o m p t e d a n i n c r e a s e d i n t e r e s t i n t h e fate of s u c h p r o d u c t s i n soil a n d w a t e r . I n h i s r e v i e w o n m i g r a t i o n i n t h e v a d o s e z o n e of o r g a n i c fluids i m m i s c i b l e w i t h w a t e r , S c h w i l l e (1984) p o i n t e d o u t t h a t ~ ' g a s o l i n e p o s s e s s e s a h i g h v o l a t i l i t y t h e effect of w h i c h h a s b e e n l a r g e l y u n d e r e s t i m a t e d u p u n t i l n o w " . I n a d d i t i o n , s o m e c a s e s t u d i e s r e c e n t l y d e s c r i b e d i n t h e l i t e r a t u r e (e.g., R e i c h m u t , 1984; M a r r i n a n d T h o m p s o n , 1984; B i s q u e , 1984; H i n c h e e a n d
0169-7722/89/$03.50
~ 1989 Elsevier Science Publishers B.V.
334
Reisinger, 1985; Wittman et al., 1985) described the effect of volatile organic compounds in the contamination of porous media and groundwater. Bossert and Bartha (1984), suggested that infiltration through porous media limits and reduces the rate of evaporative losses of volatile hydrocarbons. The impact of volatilization in determining the fate of petroleum constituents in soils was reported in the review of McGill et al. (1981). Abriola and Pinder (1985 a,b) introduced a simulation model for multiphase flow in porous media. The same authors comprehensively reviewed the existing data on the transport of waterimmiscible compounds in the subsurface (Pinder and Abriola, 1986). A recent study of Corapcioglu and Baehr (1987) includes a comprehensive mathematical model which describes the fate and transport of a petroleum product in the subsurface environment. In this model, the transport of the contaminant can occur as a solute in soil water, as a vapor in soil air and as a component within the immiscible liquid phase. Finally, Baehr (1987) utilized a two-dimensional radially symmetric transport model to analyze vapor dissipation to the atmosphere and the role of vapor transport in defining the contamination potential of an oil spill. Despite the major importance that the gaseous phase seems to play in the transport and redistribution of petroleum components in porous media, its effect has been little studied experimentally. For example, the recent studies on the movement of hydrocarbons in the unsaturated zone carried out by Barbee and Brown (1986) did not consider the simultaneous liquid and gaseous transport. The work reported here, is part of a study carried out in our laboratory on soil pollution by petroleum products. The aim of this study is to accumulate knowledge on physical and chemical processes acting in the unsaturated zone and at its interface with the groundwater. The final aim is to learn how to handle and minimize the adverse impacts from an accidental spill or release of petroleum products in soils or groundwater. The paper described the contribution of the different kerosene components (in liquid and/or gaseous phases) toward the physico-chemical processes involved in the pollution of soil. MATERIALS AND METHODS
Soil The soil used in the experiments was a Mediterranean Red sandy-clay containing 12% clay, 4% silt, 84% sand and 0.5% organic matter with a specific surface area of 50m2g 1, from Bet-Dagan. Before use, the soil was ground, sieved (50 mesh) and oven-dried, at 110°C for 24 hours (0.0% moisture), or air-dried at 22°C until the moisture content equilibrated (0.8%). Subsequently, part of the air dried soil was wet up to 4.0% or to its field capacity (12%, 0.33 bar). The water content figures are gravimetric values (w/w).
335 TABLE 1 Chemical composition and physical properties of "kerosene". Component
m-Xylene
ps-Cumene
t-but-Benzene
n-Decane
n-Dodecane
mol. wt, (g) boil. pt. (°C) density (g cm-3) vol. ratio (%) mol. ratio (%)
106.2 139.0 0,864 6.67 10.28
120.2 169.0 0,877 6.67 9.22
134.2 169,0 0,867 6.67 8.17
142.3 173.0 0,730 40.0 38.91
170.3 216.0 0.750 40.0 33.41
8.48 0.98 1.9
2.94 0.34 1.0
44.16 5.10 1.46
1.12 0.13 0.23
0.04 0.8
0.03 0.5
sat. gas atmosphere a relative vol. ratio conc. (pLL 1) vapor pres. (mmHg)
43.4 5.0 6.8
water solubility~ sing. comp. (pLL 1) in mixture (#LL 1)
160.0 10.0
56.0 3.5
23.0 1.5
a at 27°C, ~at 22°C.
"Kerosene" Kerosene consists of a mixture of more than 100 different hydrocarbons which are difficult both to identify and quantify. We chose to use a synthetic "kerosene" having the same ratio of the aromatic and aliphatic components (20:80) as natural kerosene but comprising only five well defined components. The following considerations were taken into account in the selection of "kerosene" components: their presence in natural kerosene; the penetration rate in soil columns relative to that of natural kerosene; and good chromatographic resolution. The chemical composition and the physical properties of the "kerosene" components used in this study are given in Table 1.
Oil blue liquid This organic dye solution (in xylene, product of Du Pont De Nemours, Petroleum Chem. Div., Wilinghton, USA, catalog # DR-9399-C), was used in column experiments filled with soil having 4 or 12% moisture content.
Experimental procedure (a) Adsorption experiments with the "kerosene" vapor were carried out in Erlenmeyer flasks (250 mL) fitted with ground-glass stoppers containing ovenor air-dried soil samples (10 g). Each flask contained an open test tube with 2 mL "kerosene" whose vapors saturated the atmosphere above the soil. The flasks were tightly closed and kept for 7 days at different temperatures (7, 17, 27 and 34°C). The experiments were carried out in 3 replications. (b) Volatilization experiments. After the "kerosene" constituents were adsorbed from the vapor phase onto samples of air-dried soil (as described
336 above) the Erlenmeyer flasks were opened to the atmosphere. The test tubes were removed and the soil samples analyzed for hydrocarbons content after 2 and 16 hours. (c) Transport experiments. These experiments were carried out in soil columns, using 2 different amounts of "kerosene". When the amount applied was less (c.1) or larger (c.3) than the "oil retention capacity" of the soil (Schwille, 1984), we called the "kerosene" movement in unsaturated or saturated conditions, respectively (the terms unsaturated and saturated conditions are not used here in the sense of groundwater hydrology). The term "oil retention capacity" of the soil indicates maximum sorptive capacity and fluid retention (in pores spaces). All the transport experiments were carried out in duplicate. (c.1) "Kerosene" movement in soil columns - - unsaturated conditions. Transport experiments were performed in glass columns (35 cm length, 2.16 cm I.D.). The soil (155g) was introduced into the glass column using a funnel connected with a rubber tube (30 cm). By gradually raising the funnel, the soil was packed into the column from the bottom upward, avoiding the gravitational fractionation of its particles. This resulted in a packing density of 1.40gcm 3. Four different levels of soil moisture content were used in the column experiments: 0.0, 0.8, 4.0 and 12.0%. To obtain columns with soil at field capacity, the air-dried soil columns were water saturated from below and left on a dried sand bed to equilibrate for a week. The columns were vertically levelled and closed with rubber stoppers at both ends. The "kerosene" (1 mL) was applied dropwise from a syringe by piercing the upper stopper. This amount of 1.0mL was experimentally determined to be lower than the "kerosene" retention capacity of the soil column at any moisture content of the soil. The columns were kept at constant temperature (22°C). The advance of the liquid fronts was visually observed on the columns packed with oven- and air-dried soils. In the columns filled with moist soil (4 and 12% moisture content), it was not possible to see the advance of the liquid "kerosene" front. To visualize the front, the kerosene dye, previously described, was applied to the inner wall of the glass column as lines on opposing sides of the column. After drying in an oven (150°C), for 10 rain, the columns were packed with soil. As the liquid "kerosene" moved through the moist soil it dissolved the dye on the wall of the column enabling one to mark its advance. (c.2) Redistribution of the "kerosene" components. The effect of soil moisture (0.0, 0.8 and 12%) on the redistribution of "kerosene" components in soil columns was studied at different times (5, 7, 14, 21, 25, 30 and 46 days) after the initial "kerosene" addition. By the end of the experiments, the soil from each column was extruded and divided into 8 layers, which were extracted and analyzed separately, as described in the following analytical procedure. (c.3) "Kerosene" movement in soil columns - - saturated conditions. An amount of "kerosene" (10 mL), which was previously found to be greater than the "kerosene retention capacity" of the soil column, was applied on the soil
337 surface. The experiments were carried out using soil at two different moisture contents: air-dried and field capacity. The preparation of columns and the experiments were carried out as described above (c.1). (c.4) The upward movement of "kerosene" in soil columns. Soil columns [prepared as described above (c.1)], were filled with soil at different moisture contents (0.0, 0.8, 4.0 and 12%) and "kerosene" (1 mL) was added. The columns were then arranged vertically with the pollutant layer at the bottom, so that the advancement of the ~kerosene" was opposite to the gravitational pull. The experiments using these columns were performed in a manner similar to those investigating the downward movement of ~'kerosene" (c.1).
Analytical procedure Volatization losses of ~'kerosene" constituents during sample handling can lead to erroneous analytical results. Therefore, the handling of samples containing '~kerosene" (soil, water, air) was performed using methods to prevent such losses.
"Kerosene" extraction from the soil Soil samples from the adsoprtion, volatization and transport experiments were extracted in Erlenmayer flasks (250mL). For the soil samples from the adsorption experiments the following procedure was used: immediately after opening the flask, the test tube containing "kerosene" was removed. Water ( l m L per g soil), and then carbon tetrachloride (2 mL per g soil), were added and the flask tightly closed (with a ground glass stopper and teflon tape) and placed in a laboratory shaker for 2 hours. Then, sodium chloride (0.5 g per g soil) was added and the flask again closed and shaken for another 5 minutes. An aliquot of the carbon tetrachloride layer (1-2mL) was transferred into a 5-mL screw cap vial (with aluminium liner) to which anhydrous sodium sulfate and aluminium oxide (H, basic, type E) were added (to remove water and extractable organic, respectively). This sample was then used for "kerosene" analytical quantifications. The soil samples from volatilization and transport experiments were extracted using the same procedure.
Gas chromatographic determination The carbon tetrachloride extract of the '~kerosene" from the soil sample was analyzed chromatographically. Aliquots (3 ttL) were injected into a Varian 3300 Gas Chromatograph equipped with a 30-M DB-1 column (i.d. 0.53mm). The chromatography was performed in a temperature programmed regime, from 35 to 150°C with a heating rate of 5°C min 1. The carrier gas was nitrogen, while hydrogen and air provided the combustion mixture for the flame ionization detector (FID); the flow rates for nitrogen, hydrogen and air, were 30, 30 and 300mLmin -1, respectively. The FID was connected to a Merck D-2000 integrator, which quantified the chromatogram peaks.
338 TABLE 2 V a p o r c o n c e n t r a t i o n of " k e r o s e n e " c o m p o n e n t s in j a r s a t m o s p h e r e at different t e m p e r a t u r e s S a t u r a t e d a t m o s p h e r e composition ( p L L 1)
"Kerosene" components
m-xylene ps-cumene t-but.-benzene n-decane n-dodecane
7°C
17°C
27°C
34°C
1.20 0.17 0.04 0.74 n.d.
2.90 0.65 0.24 2.65 0.08
5.00 0.98 0.34 5.10 0.13
7.80 1.50 0.62 7.10 0.24
R E S U L T S AND D I S C U S S I O N
The most important physico-chemical processes involved in the pollution of soil by "kerosene" such as adsorption from the vapor phase, volatilization from soil, transport and redistribution through soil column, are described.
(a) Adsorption from the vapor phase Adsorption of "kerosene" components from the vapor phase onto soil as affected by environmental factors such as temperature and soil moisture content was studied in a closed system. The "kerosene" vapor concentrations in jars atmosphere is given in Table 2. The amount of hydrocarbons adsorbed per gram of soil after 7 days of contact with a saturated vapor atmosphere at different temperatures (7, 17, 27 and 34°C) and for two moisture contents: ovenand air-dried soils, is given in Table 3. It may be observed that the adsorption of the hydrocarbons from both aliphatic and aromatic groups increases with the increase of ambient temperature. The increase in temperature increases the vapor pressure of the hydrocarbons thus affecting their concentrations in the TABLE 3 A d s o r p t i o n of v a p o r " k e r o s e n e " c o m p o n e n t s on soil, as affected by soil m o i s t u r e and t e m p e r a t u r e ( p g g 1) Soil m o i s t u r e (%)
T = 7°C
T = 17°C
T = 27°C
T = 34°C
0.0 a
0,8 h
0.0 a
0.86
0.0 ~
0.8 h
0.0 a
0.86
200 130 50 430 30
40 40 15 100 10
390 185 120 510 55
90 80 47 130 20
800 345 210 440 130
110 100 60 130 35
1150 490 330 500 230
120 120 100 110 80
Components m-xylene ps-cumene t-butyl-benzene n-decane n-dodecane "oven-dried; b air-dried.
339
n-Decane ' '
°°°F\ z i,i o
c.)
n - Dodecane
/
1 ,oo
o
il,oo o
m-Xylene 2 0 0 lq.__. ~
, o
' o
!
I O0 t,m__..___.. 0 / e: o o3
, -
8 0 0 ~o
ps-Curnene , ,
,le
t- butyl - Benzene
5
15 25 TEMPERATURE (°C)
7,5
MOISTURE CONTENT: 00.0% eO.8 % 5
15 25 35 T E M P E R A T U R E (°C)
Fig. 1. Sorption coefficient of the "kerosene" components in the soil, as affected by temperature and soil moisture content.
atmosphere above soil surfaces and hence their adsorption. The amount adsorbed on the oven-dried soil at the highest temperature (34°C) showed the following adsorption order: m-xylene > n-decane > pseudo-cumene > tbutylbenzene > n-dodecane. With the exception of n-decane (for air- and ovendried soil), all the hydrocarbons studied exhibited the tendency to be more highly adsorbed on the soil surface at higher temperatures. The adsorption of n-decane reaches its maximum at about 20°C and no increase in the adsorption was observed up to a temperature of 34°C. The influence of the moisture content of the soil on the adsorption of "kerosene" components on soil surface can also be seen from the Table 3. The adsorption of hydrocarbons on the air-dried soil was drastically decreased in comparison to the adsorption on oven-dried soil. It may be observed, for example, that the adsorption of aromatic and aliphatic compounds investigated at 34°C on the air-dried soil is diminished by more than a factor of 3 compared with their adsorption on oven-dried soil. This is probably because the "kerosene" components (low solubility in water and low polarity) are unable to compete with water molecules for the adsorption sites of the soil solid phase. This behavior was also observed by Chiou and Soup (1985) for benzene and its related compounds and for xylene by Barbee and Brown (1986) on a Texas soil. The competition for soil adsorption sites between the ~'kerosene" constituents and water decreases the oil retention capacity of the soil, thus increasing the penetration depth into soil. Figure 1 shows the effect of temperature and soil moisture on sorption coefficient of "kerosene"
340 TABLE 4 Relative volatilization of "kerosene" components from air dried soil "Kerosene" components m-xylene ps-cumene t-butyl-benzene n-decane n-dodecane
Lossesa (at 22°C) after 2h
16h
99.0 94.0 92.5 71.5 14.3
99.3 99.7 99.7 89.5 61.3
aIn percentage from the amount adsorbed. components. The sorption coefficient (component concentration on soil/vapor concentrations in air) is lowered by a temperature increase, having the highest value at 7°C. For all the components, the presence of moisture in air-dried soil (0.8%) drastically decreased the sorption coefficient, especially at lower temperatures; in the temperature range of 20-34°C the sorption coefficients showed very small variability.
(b) Volatilization from the adsorbed phase " K e r o s e n e " constituents volatilize from the adsorbed phase after relatively short periods of time. Table 4 shows the relative volatilization of the " k e r o s e n e " constituents from soil into the atmosphere after 2 and 16 hours, at 22°C. It may be observed t ha t in the open system more than 99% of the adsorbed aromatic compounds (m-xylene, pseudo-cumene and t-butylbenzene) volatilize in 2 hours. The aliphatic compounds have a lower rate of loss, especially n-dodecane of which 40% remains after 16 hours.
(c.) Transport experiments in soil columns The use of two sets of conditions (c.1, saturated and c.3, unsaturated), for " k e r o s e n e " movement into soil columns enabled us to not only to study the liquid and the vapor phase movement, but the dynamic redistribution of the " k e r o s e n e " components as well.
(c.1) "Kerosene" movement in soil columns - - unsaturated conditions During infiltration of liquid " k e r o s e n e " into the soil column there was simultaneous, but faster, vapor movement of the " k e r o s e n e " components producing a p ene t r at i on front in advance of the liquid. However, contamination by " k e r o s e n e " vapor was important only after the liquid movement stopped. Twenty one days after " ke r os e ne " application to the oven-dried soil
341
g,o ~
~e
" 5 tl I /
0
•
3
nt
I -o.o % 2 - o.8",,
It'
O
"
3-
2
4
~
4.0"/,
6 8 I0 12 TIME (doys)
14
16
Fig. 2. T h e p e n e t r a t i o n of t h e " k e r o s e n e " - l i q u i d f r o n t in soil c o l u m n s , as affected by soil m o i s t u r e c o n t e n t a n d time.
columns, the liquid front (observed visually) reached a depth of 9.4 cm, while the vapor movement reached 25.0 cm. (as determined by soil extraction). For the same period, in the column with air-dried soil, the above values were 12.3 cm and 32.0 cm, respectively. In the experiments in which the moisture contents of the soil were 4 and 12%, the liquid front depths were 17.5 cm and 19.2 cm, respectively. In these cases there was no vapor movement in advance of the liquid fronts. These results display a general behavior of "kerosene" transport through the soil columns: an increase in the moisture content of the soil decreased the ~kerosene" retention capacity, resulting in a deeper and faster penetration of the liquid ~kerosene". However, for vapor-'kerosene" movement, the increase in the soil moisture content above 4% completely inhibited vapor penetration. These results are similar to those of Barbee and Brown (1986) who reported that when xylenes were applied to a sandy loam soil with a moisture content near field capacity, the hydrocarbons were retained by the soil and their movement was significantly attenuated by the presence of water in soil. Figure 2 shows the depth of the wetting front of the liquid ~kerosene" in soil columns of different moisture contents (0.0, 0.8, 4.0 and 12.0%, w/w), with time. The data show a decreasing rate of penetration with time. At about 12 days following application no further advance of the wetting front was observed as the amount of liquid '~kerosene" (lmL), originally applied, depleted by soil retention capacity and the gravitational pressure head became negligible. The data do not fit the classical infiltration equation (a linear relationship between depth and xfi), since a constant "kerosene" head was not maintained. As expected, from the slope of the infiltration curves, it can be seen that the increase of the moisture content of the soil resulted in a deeper and faster penetration of the liquid.
(c.2) "Kerosene" components redistribution in soil columns When the liquid "kerosene" (1 mL) applied on the surface of the column was
342
0
0.1 0.2 0
C/C o OJ 0.2 0
o.I
0.2
,o °I~I~' ~,~, ~/~~' 20 ~o
J
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7
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o3
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,b3
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k
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0
Io 2o 3o Fig. 3. Redistribution of "kerosene" components in soil columns as affected by soil moisture and time. Numbers on curves represent days after '~kerosene" application. C ~ hydrocarbon amount in each soil layer. CO - total hydrocarbon amount in the column. (a) = m-xylene; (b) n-decane; (c) ~ ps-cumene; (d) t-butylbenzene and (e) = n-dodeeane at (1) 0.0%, (2) 0.8% and (3) 12.0% soil moisture content. depleted and the wetting front ceased to advance, the vapor and the liquid trapped in soil pores or adsorbed on the soil produced a redistribution of the d i f f e r e n t " k e r o s e n e " c o m p o n e n t s a c c o r d i n g to a d y n a m i c s o i l - v a p o r a d s o r p tion-desorption equilibrium. This vapor movement resulted in a redistribution o f t h e m o r e v o l a t i l e c o m p o n e n t s , h e n c e to a c h a n g e i n t h e r e s i d u a l " k e r o s e n e " composition. Figure 3 shows the redistribution in soil columns of different soil m o i s t u r e c o n t e n t s (0.0, 0.8 a n d 12%) o v e r d i f f e r e n t t i m e p e r i o d s ( u p t o 46 d a y s ) o f a l l t h e " k e r o s e n e " c o m p o n e n t s (a - - m - x y l e n e , b - - p s - c u m e n e , c - - t - b u t benzene, d -- n-decane, e -- n-dodecane). The amount of each component (C) (from each layer of the soil column), divided by the total amount of that c o m p o n e n t i n t h e e n t i r e c o l u m n (Co) w a s p l o t t e d v e r s u s t h e c o r r e s p o n d i n g
343 column layer depth (cm). The redistribution of '~kerosene" components mostly depends on their volatility. So, the redistribution of m-xylene is very similar to t h at of n-decane. The same can be observed between t-butylbenzene and ndodecane curves, while ps-cumene redistribution curve is somewhere between a and c curves. In the soil column at field capacity it was observed that both aliphatic and aromatic hydrocarbons remained in the upper 20 cm layer and their redistribution with time occurs only to this depth. The movement of '~kerosene" in oven and air dried soil columns is different. It may be observed t ha t independent of their properties the ~kerosene" components moved faster and deeper in air-dried rat her than in the oven-dried soil columns. This behavior agrees with the adsorption pattern already described above. The extent of redistribution with depth is affected by the h y d r o c a r b o n properties. In the case of oven dried soils, the penetration of the ~kerosene" products follows the order: m-xylene > n-decane > pscumene > t-butylbenzene > n-dodecane. This behavior is in accordance with the vapor pressure (Table 1), the vapor concentration and the adsorption order of the ~kerosene" constituents. In the case of the columns packed with air dried soil the redistribution of the ~'kerosene" components is only slightly affected by their volatility. (c.3) "Kerosene" movement in soil columns - - saturated conditions The high amount of ~'kerosene" applied (10mL) to the soil columns increased the gravitational pressure head, hence the rate of penetration. In these experiments, the penetration of " ke rosene" through the whole soil column took only 3.0 and 1.8 hours for air-dried and field capacity soils, respectively. The leachates and the samples from different soil layers were chromatographically analyzed but did not show any clear change in their composition. As expected an increase in soil moisture content increases the downward p e n e t r a t i o n rate of ~kerosene" into the soil columns. (c.4) U p w a r d movement of "kerosene" in soil columns This series of experiments was carried out in order to simulate the pollution of the soil when a '~kerosene" lens lying upon the groundwater is the source of pollution. Since the direction of kerosene penetrations is against gravitational forces, only upward force acting is capillarity. In this case the rate of the advance of the liquid front was slower by about 20% than that for downward movement. An increase in the moisture content of the soil resulted in a decrease of the upward rate of penetration. For samples taken 21 days after ~kerosene" application the heights of infiltration were 7.0, 4.5 and 0.5 cm for 0.0%, 0.8% and 4% moisture contents, respectively. On the other hand the gas phase extended upwards 23.0 and 30.0 cm for oven- and air-dried soils, respectively. The 4% moisture content also proved to be a barrier for upwards vapor penetration, while soil at field capacity prevented any ~kerosene" upward movement at all.
344 CONCLUSIONS
In all the physico-chemical processes involved in the soil contamination by "kerosene", the most influential factor was the moisture content of the soil. Its increase resulted in a drastic decrease in the adsorption of the vapor components of the "kerosene", an increase in the rate and depth of the downward penetration of the liquid-"kerosene" through the soil columns, a decrease of the upward movement of the liquid "kerosene" and a decrease of vapor "kerosene" movement. The increase in vapor concentration brought about by higher temperatures increased the amount of vapor "kerosene" components adsorbed by the soil, however, the sorption coefficient was decreased. The volatility of the "kerosene" components seems to be the prime factor in the redistribution process once liquid phase movement has ceased. ACKNOWLEDGMENTS
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