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Vertical migration of chlorinated organic compounds in porous media
Wat. Res. Vol. 22, No. 4, pp. 481-484, 1988 Printed in Great Britain. All rights reserved
0043-1354/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc
VERTICAL MIGRATION OF CHLORINATED ORGANIC COMPOUNDS IN POROUS MEDIA T. HIRATA and K. MURAOKA The National Institute for Environmental Studies, Tsukuba Science City, Ibaraki 305, Japan (Received April 1987; accepted in revised form November 1987) Abstract--The migration behavior of trichloroethylene through porous media made of glass beads of 5, 3 and 1 mm dia is described. Experiments were carried out in three hydraulic conditions: (1) unsaturated (completely dry), (2) saturated and (3) two layers (the upper portion of the porous media being unsaturated and the lower portion being saturated). In the case of the dry condition (1) trichloroethylene easily flows through the porous media regardless of the size of the glass beads, while it remains in the pore space in the saturated medium in the saturated condition (2). The stagnation of test liquid in the saturated medium increases with decreasing glass bead diameter. With 5 mm glass beads, in the case of condition (3), all the liquid coming from the upper portion moved into the saturated zone, whereas with 3 and 1 mm glass beads, the liquid seemed to stagnate on the surface of the saturated zone. With 1 mm glass beads, especially, almost all the liquid remained stagnant in the boundary region between the unsaturated and the saturated zone. The difference of the trichloroethylene behavior in the capillary zone can be qualitatively explained from the results of the visualized testing by means of the Hele-Shaw model. Key words--groundwater pollution, chlorinated organic compounds, trichloroethylene, migration, infiltration, porous media, Hel~Shaw model
INTRODUCTION
VERTICAL MIGRATION EXPERIMENT USING POROUS MEDIA
Trichloroethylene a n d tetrachloroethylene, chemicals used for cleaning metal parts a n d for dry cleaning, are frequently detected in the g r o u n d w a t e r of u r b a n areas (Burmaster, 1982). M i g r a t i o n of such organochlorines from the g r o u n d surface d o w n to the g r o u n d w a t e r zone is t h o u g h t to be a primary process closely related to the c o n t a m i n a t i o n of groundwater. However, the m o v e m e n t of these substances c a n n o t be detected visually, n o r is it difficult to precisely follow their m i g r a t i o n to the actual sites. In addition, the hydraulic properties of these substances are still uncertain, until n o w little has been investigated concerning the m i g r a t i o n of organic chemicals t h r o u g h porous media (Schwille, 1984). As a result, even when the source of a suspected p o l l u t a n t has been specified, this does n o t always lead to the establishment of a m e c h a n i s m for g r o u n d w a t e r pollution. U n d e r the circumstances, it would be of urgent necessity to characterize the m i g r a t i o n b e h a v i o r of organo-chlorines t h r o u g h p o r o u s media with different porosity a n d different degrees of water content. In this study, the m i g r a t i o n behaviors of trichloroethylene a n d tetrachloroethylene were experimentally investigated using p o r o u s media m a d e of glass beads. F u r t h e r m o r e , the H e l e - S h a w model was used to observe a n d discuss the vertical m i g r a t i o n of these chlorinated c o m p o u n d s in the capillary zone a n d " fir vertical m i g r a t i o n in the b o u n d a r y region between the u n s a t u r a t e d a n d saturated zones.
Apparatus and method of experiment Three types of glass beads were used in the migration experiment, the particle sizes being 5, 3 and 1 mm and the porosity being 39.2, 38.0 and 36.3%, respectively. These beads were washed with pure water, dried in an oven and placed in measuring cylinders to make porous media 6.5 cm dia and 30 cm in depth. The experiment was conducted under three different conditions: (1) unsaturated (completely dry), (2) saturated and (3) two layers (the upper portion of porous medium being unsaturated and the lower portion being saturated). In each case, trichloroethylene was added, as a test liquid, to the surface of the porous medium by means of a syringe and allowed to flow down along the wall of the measuring cylinder. The volume added was 2(~30 ml. Migration in the unsaturated zone Under experimental condition (1) (completely dry), the liquid added readily migrated, with no substancial resistance, regardless of the size of the glass beads and finally collected at the bottom, for the most part, with only a little of the test liquid being left inside the porous medium. It flowed down in the form of a narrow band, with little tendency of extending in the lateral direction. The migration velocities were 9.0, 6.0 and 4.5 cm s -~ in decreasing order of glass bead diameter. Migration in the saturated zone Under experimental condition (2) (saturated), the migration velocity was low because of buoyancy and fluid resistance, being 1.I cms -~ for 5 mm glass beads and 0.42 cm s-~ for 3 mm beads. The test liquid was left in the pore spaces among the glass beads unlike previously in the completely dry condition. With the 5 mm glass beads, the liquid reached the bottom of the measuring cylinder, but 481
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conditions most exactly. The same behavior of trichloroethylene was observed in the upper portion of the porous medium as under experimental condition (1), and the behavior in the saturated zone was as described above. However, the test liquid behaved differently in the boundary region between the unsaturated and saturated zones. With the 5 mm glass beads, all the test liquid coming from the upper portion moved smoothly to the saturated zone without stagnating on the surface of saturated zone, while it tended to stagnate on that surface with the 3 and 1 mm glass beads. With the 1 mm glass beads, in particular, almost all the liquid added remained stagnant in that region and would not easily move toward the saturated zone. This is because the liquid coming from the unsaturated zone must overcome the surface tension o f water, and other resistances inside the porous medium, in order to work its way through the water surface into the lower portion; it is forced to stagnate in that region, if the resistance of the porous medium is high enough, due to the small pore space. Such a behavior may be clearly seen in Fig. 2(a) and (b). Figure 2(a) shows the state in which trichloroethylene has reached the surface of the saturated zone and just started migrating into the aqueous layer, and Fig. 2(b) illustrates
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Fig. 1. Migration of trichloroethylene in saturated porous media. The numerals in (b) indicate glass bead diameter. The syringe injecting trichloroethylene from the top can just be seen in the upper portions of the 5 and 3 mm glass bead tests.
the amount left inside the porous medium increased with decreasing diameter, most of the liquid being left inside the medium with 1 mm glass beads. These results are shown in Fig. l(a) and (b). As trichloroethylene is a colorless, transparent liquid, it was colored with an organic dye (Fat Red) in this experiment. The dark particles which can be seen in Fig. l(a) and (b) represent trichloroethylene remaining in the porous medium. Figure l(a) shows a state in which trichloroethylene has migrated over a distance of about 10 cm from the surface and Fig. l(b) shows the state of complete migration. In each of these photographs, the amount of trichloroethylene added is 20 ml and the sizes o f glass beads used are 5, 3 and 1 mm from left to right.
Migration in the two layers (with the upper half being unsaturated and the lower half saturated) Experimental condition (3) (with the upper portion of the porous medium being unsaturated and the lower portion being saturated) seems to simulate actual underground
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Fig. 2, Migration of trichloroethylene in the boundary region between the saturated and the unsaturated zone. In this case the upper portion of the porous medium is completely dry and the lower half is saturated.
Chlorinated organic compounds the state of complete migration. It may be seen that, as described above, almost all the test liquid remains stagnant on the surface of aqueous layer with the 1 mm glass beads, and that migration into the aqueous layer progresses with the 3 and 5 mm glass beads, the amount of test liquid remaining in the pore spaces in the saturated zone being larger with the 3mm glass beads as in the case with experimental condition (2). MIGRATION EXPERIMENT USING HELE-SHAW MODEL
The Hele-Shaw model is an apparatus consisting of two sheets of glass plates superposed one upon another at a certain distance, which is suitable for simulating two-dimensional, migrational flow near a capillary zone. In this study, glass plates measuring 35 x 50 cm were used and the plate distance was set at 0.1 and 0.2 mm by the insertion of one or two sheets of 0.1 mm thick stainless steel plates. A capillary zone can be formed by standing the Hele-Shaw model and dipping its lower part in a water pan. Since the model used in this study had its periphery kept open, the capillary rise was greatest in the central portion and this tendency was more marked with a plate distance of 0.1 mm. The capillary height in the central portion was 6.9 cm from the water surface for a plate distance of 0.1 m m and 3.1 cm for a plate distance of 0.2mm. Trichloroethylene and tetrachloroethylene were used for the test and no significant difference in migration behavior was observed between the two. Figure 3(a)-(d) show the migration behavior of trichloroethylene at a plate distance of 0.1 mm. The test liquid (0.24).3 ml) was added at the top. The
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space above the capillary zone in this model is hereinafter referred to as the unsaturated zone and the aqueous layer below the capillary zone as the saturated zone. In this case, trichloroethylene rapidly flowed through the unsaturated zone, collected in the capillary zone [Fig. 3(a) and (b)] and gradually extended in a lateral direction as the deposited layer became thicker [Fig. 3(c) and (d)]. Moreover, even if the liquid was added successively, there was no case when stagnant liquid broke through the upper interface of capillary zone into the saturated zone. This experiment, at a plate distance of 0.1 mm, may be regarded as corresponding to migration in porous media of small pore space. Figure 4(a)-(f) shows the migration behavior of trichloroethylene at a plate distance of 0.2 mm. Unlike the case of Fig. 3, the test liquid injected at the top did not widely extend in the two-dimensional plane, but migrated downward in the form of a winding path. The test liquid, which reached the upper interface of capillary zone, first showed a slight rise [Fig. 4(a)] and then extended in a lateral direction [Fig. 4(b)]. This was followed by the development of "fingers" [Fig. 4(c)] at locations where the equilibrium between the gravity of the accumulated trichloroethylene and the surface tension of capillary zone was destroyed and the test liquid finally began to intrude into the saturated zone [Fig. 4(d)]. When the migration into the saturated zone ended, the interface was immediately restored to reconstruct the capillary zone. Further addition of trichloroethylene repeated the same processes as above, as shown in Fig. 4(e) and (f). A phenomenon like this probably
Fig. 3. Migration of trichloroethylene in the capillary zone made by the Hele-Shaw model. Plate distance in this case is 0.1 ram. The dark portion in each photograph corresponds to trichloroethylene and the mesh interval in the photographs is 5 cm.
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Fig. 4. Migration of trichloroethylene in the capillary zone (as in Fig. 3). Plate distance in this case is 0.2 ram, and trichloroethylene is continuously fed from the top of the model. takes place in porous media of large pore space. Schwille (1981) also showed similar experiments on the migration of tetrachloroethylene by using the HeW-Shaw model, but the results obtained were somewhat different.
CONCLUSION
This study deals with the migration of chlorinated organic compounds through porous media made of glass beads. It appears that trichloroethylene readily migrated in the completely dry condition, on the other hand, in the saturated condition it migrated, but part of the test liquid remained stagnant as isolated particles in the pore space of the media. Experiments using two layers, with the upper portion being unsaturated and the lower portion being saturated. reveal that in the case of large pore spaces, trichloroethylene coming from the unsaturated zone readily intruded into the saturated zone, whereas in the case of small pore spaces, almost all of the test liquid remained stagnant on the surface of the saturated zone. These phenomena in the capillary zone can be qualitatively interpreted by using the Hele-Shaw model.
The results obtained above suggest that trichloroethylene, once allowed to enter the soil, will migrate through the unsaturated soil layer at a rather high speed down to the surface of the groundwater. It diffuses along this surface until it reaches locations where the equilibrium between the gravity of trichloroethylene and the surface tension of capillary zone no longer exists, followed by migration into the groundwater layer. The test liquid may remain in the pore spaces in the saturated soil, as observed in the migration experiment in the saturated zone, and then gradually come into solution in the groundwater (Fried et al., 1979). If this occurs, pollution can persist for long periods because groundwater flow is extremely slow. REFERENCES
Burmaster D. E. (1982) The new pollution. Groundwater contamination. Environment 242, 7 13. Fried J. J., Muntzer P. and Zilliox L. (1979) Ground-water pollution by transfer of oil hydrocarbons. Groundwater 17, 586-594. Schwille F. (1981) Groundwater pollution in porous media by fluids immiscible with water. Sci. Total Envir. 21, 173-185. Schwille F. (1984) Migration'of organic fluids immiscible with water in the unsaturated zone. In Pollutants in Porous Media (Edited by Yaron B., Dagan G. and Goldshmid J.), pp. 27-48. Springer, Tokyo, Japan.
0043-1354/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc
VERTICAL MIGRATION OF CHLORINATED ORGANIC COMPOUNDS IN POROUS MEDIA T. HIRATA and K. MURAOKA The National Institute for Environmental Studies, Tsukuba Science City, Ibaraki 305, Japan (Received April 1987; accepted in revised form November 1987) Abstract--The migration behavior of trichloroethylene through porous media made of glass beads of 5, 3 and 1 mm dia is described. Experiments were carried out in three hydraulic conditions: (1) unsaturated (completely dry), (2) saturated and (3) two layers (the upper portion of the porous media being unsaturated and the lower portion being saturated). In the case of the dry condition (1) trichloroethylene easily flows through the porous media regardless of the size of the glass beads, while it remains in the pore space in the saturated medium in the saturated condition (2). The stagnation of test liquid in the saturated medium increases with decreasing glass bead diameter. With 5 mm glass beads, in the case of condition (3), all the liquid coming from the upper portion moved into the saturated zone, whereas with 3 and 1 mm glass beads, the liquid seemed to stagnate on the surface of the saturated zone. With 1 mm glass beads, especially, almost all the liquid remained stagnant in the boundary region between the unsaturated and the saturated zone. The difference of the trichloroethylene behavior in the capillary zone can be qualitatively explained from the results of the visualized testing by means of the Hele-Shaw model. Key words--groundwater pollution, chlorinated organic compounds, trichloroethylene, migration, infiltration, porous media, Hel~Shaw model
INTRODUCTION
VERTICAL MIGRATION EXPERIMENT USING POROUS MEDIA
Trichloroethylene a n d tetrachloroethylene, chemicals used for cleaning metal parts a n d for dry cleaning, are frequently detected in the g r o u n d w a t e r of u r b a n areas (Burmaster, 1982). M i g r a t i o n of such organochlorines from the g r o u n d surface d o w n to the g r o u n d w a t e r zone is t h o u g h t to be a primary process closely related to the c o n t a m i n a t i o n of groundwater. However, the m o v e m e n t of these substances c a n n o t be detected visually, n o r is it difficult to precisely follow their m i g r a t i o n to the actual sites. In addition, the hydraulic properties of these substances are still uncertain, until n o w little has been investigated concerning the m i g r a t i o n of organic chemicals t h r o u g h porous media (Schwille, 1984). As a result, even when the source of a suspected p o l l u t a n t has been specified, this does n o t always lead to the establishment of a m e c h a n i s m for g r o u n d w a t e r pollution. U n d e r the circumstances, it would be of urgent necessity to characterize the m i g r a t i o n b e h a v i o r of organo-chlorines t h r o u g h p o r o u s media with different porosity a n d different degrees of water content. In this study, the m i g r a t i o n behaviors of trichloroethylene a n d tetrachloroethylene were experimentally investigated using p o r o u s media m a d e of glass beads. F u r t h e r m o r e , the H e l e - S h a w model was used to observe a n d discuss the vertical m i g r a t i o n of these chlorinated c o m p o u n d s in the capillary zone a n d " fir vertical m i g r a t i o n in the b o u n d a r y region between the u n s a t u r a t e d a n d saturated zones.
Apparatus and method of experiment Three types of glass beads were used in the migration experiment, the particle sizes being 5, 3 and 1 mm and the porosity being 39.2, 38.0 and 36.3%, respectively. These beads were washed with pure water, dried in an oven and placed in measuring cylinders to make porous media 6.5 cm dia and 30 cm in depth. The experiment was conducted under three different conditions: (1) unsaturated (completely dry), (2) saturated and (3) two layers (the upper portion of porous medium being unsaturated and the lower portion being saturated). In each case, trichloroethylene was added, as a test liquid, to the surface of the porous medium by means of a syringe and allowed to flow down along the wall of the measuring cylinder. The volume added was 2(~30 ml. Migration in the unsaturated zone Under experimental condition (1) (completely dry), the liquid added readily migrated, with no substancial resistance, regardless of the size of the glass beads and finally collected at the bottom, for the most part, with only a little of the test liquid being left inside the porous medium. It flowed down in the form of a narrow band, with little tendency of extending in the lateral direction. The migration velocities were 9.0, 6.0 and 4.5 cm s -~ in decreasing order of glass bead diameter. Migration in the saturated zone Under experimental condition (2) (saturated), the migration velocity was low because of buoyancy and fluid resistance, being 1.I cms -~ for 5 mm glass beads and 0.42 cm s-~ for 3 mm beads. The test liquid was left in the pore spaces among the glass beads unlike previously in the completely dry condition. With the 5 mm glass beads, the liquid reached the bottom of the measuring cylinder, but 481
482
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65
E 0 0
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(a)
conditions most exactly. The same behavior of trichloroethylene was observed in the upper portion of the porous medium as under experimental condition (1), and the behavior in the saturated zone was as described above. However, the test liquid behaved differently in the boundary region between the unsaturated and saturated zones. With the 5 mm glass beads, all the test liquid coming from the upper portion moved smoothly to the saturated zone without stagnating on the surface of saturated zone, while it tended to stagnate on that surface with the 3 and 1 mm glass beads. With the 1 mm glass beads, in particular, almost all the liquid added remained stagnant in that region and would not easily move toward the saturated zone. This is because the liquid coming from the unsaturated zone must overcome the surface tension o f water, and other resistances inside the porous medium, in order to work its way through the water surface into the lower portion; it is forced to stagnate in that region, if the resistance of the porous medium is high enough, due to the small pore space. Such a behavior may be clearly seen in Fig. 2(a) and (b). Figure 2(a) shows the state in which trichloroethylene has reached the surface of the saturated zone and just started migrating into the aqueous layer, and Fig. 2(b) illustrates
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(ram) (a)
Fig. 1. Migration of trichloroethylene in saturated porous media. The numerals in (b) indicate glass bead diameter. The syringe injecting trichloroethylene from the top can just be seen in the upper portions of the 5 and 3 mm glass bead tests.
the amount left inside the porous medium increased with decreasing diameter, most of the liquid being left inside the medium with 1 mm glass beads. These results are shown in Fig. l(a) and (b). As trichloroethylene is a colorless, transparent liquid, it was colored with an organic dye (Fat Red) in this experiment. The dark particles which can be seen in Fig. l(a) and (b) represent trichloroethylene remaining in the porous medium. Figure l(a) shows a state in which trichloroethylene has migrated over a distance of about 10 cm from the surface and Fig. l(b) shows the state of complete migration. In each of these photographs, the amount of trichloroethylene added is 20 ml and the sizes o f glass beads used are 5, 3 and 1 mm from left to right.
Migration in the two layers (with the upper half being unsaturated and the lower half saturated) Experimental condition (3) (with the upper portion of the porous medium being unsaturated and the lower portion being saturated) seems to simulate actual underground
;ii3Oi
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3 (b)
1
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Fig. 2, Migration of trichloroethylene in the boundary region between the saturated and the unsaturated zone. In this case the upper portion of the porous medium is completely dry and the lower half is saturated.
Chlorinated organic compounds the state of complete migration. It may be seen that, as described above, almost all the test liquid remains stagnant on the surface of aqueous layer with the 1 mm glass beads, and that migration into the aqueous layer progresses with the 3 and 5 mm glass beads, the amount of test liquid remaining in the pore spaces in the saturated zone being larger with the 3mm glass beads as in the case with experimental condition (2). MIGRATION EXPERIMENT USING HELE-SHAW MODEL
The Hele-Shaw model is an apparatus consisting of two sheets of glass plates superposed one upon another at a certain distance, which is suitable for simulating two-dimensional, migrational flow near a capillary zone. In this study, glass plates measuring 35 x 50 cm were used and the plate distance was set at 0.1 and 0.2 mm by the insertion of one or two sheets of 0.1 mm thick stainless steel plates. A capillary zone can be formed by standing the Hele-Shaw model and dipping its lower part in a water pan. Since the model used in this study had its periphery kept open, the capillary rise was greatest in the central portion and this tendency was more marked with a plate distance of 0.1 mm. The capillary height in the central portion was 6.9 cm from the water surface for a plate distance of 0.1 m m and 3.1 cm for a plate distance of 0.2mm. Trichloroethylene and tetrachloroethylene were used for the test and no significant difference in migration behavior was observed between the two. Figure 3(a)-(d) show the migration behavior of trichloroethylene at a plate distance of 0.1 mm. The test liquid (0.24).3 ml) was added at the top. The
483
space above the capillary zone in this model is hereinafter referred to as the unsaturated zone and the aqueous layer below the capillary zone as the saturated zone. In this case, trichloroethylene rapidly flowed through the unsaturated zone, collected in the capillary zone [Fig. 3(a) and (b)] and gradually extended in a lateral direction as the deposited layer became thicker [Fig. 3(c) and (d)]. Moreover, even if the liquid was added successively, there was no case when stagnant liquid broke through the upper interface of capillary zone into the saturated zone. This experiment, at a plate distance of 0.1 mm, may be regarded as corresponding to migration in porous media of small pore space. Figure 4(a)-(f) shows the migration behavior of trichloroethylene at a plate distance of 0.2 mm. Unlike the case of Fig. 3, the test liquid injected at the top did not widely extend in the two-dimensional plane, but migrated downward in the form of a winding path. The test liquid, which reached the upper interface of capillary zone, first showed a slight rise [Fig. 4(a)] and then extended in a lateral direction [Fig. 4(b)]. This was followed by the development of "fingers" [Fig. 4(c)] at locations where the equilibrium between the gravity of the accumulated trichloroethylene and the surface tension of capillary zone was destroyed and the test liquid finally began to intrude into the saturated zone [Fig. 4(d)]. When the migration into the saturated zone ended, the interface was immediately restored to reconstruct the capillary zone. Further addition of trichloroethylene repeated the same processes as above, as shown in Fig. 4(e) and (f). A phenomenon like this probably
Fig. 3. Migration of trichloroethylene in the capillary zone made by the Hele-Shaw model. Plate distance in this case is 0.1 ram. The dark portion in each photograph corresponds to trichloroethylene and the mesh interval in the photographs is 5 cm.
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Fig. 4. Migration of trichloroethylene in the capillary zone (as in Fig. 3). Plate distance in this case is 0.2 ram, and trichloroethylene is continuously fed from the top of the model. takes place in porous media of large pore space. Schwille (1981) also showed similar experiments on the migration of tetrachloroethylene by using the HeW-Shaw model, but the results obtained were somewhat different.
CONCLUSION
This study deals with the migration of chlorinated organic compounds through porous media made of glass beads. It appears that trichloroethylene readily migrated in the completely dry condition, on the other hand, in the saturated condition it migrated, but part of the test liquid remained stagnant as isolated particles in the pore space of the media. Experiments using two layers, with the upper portion being unsaturated and the lower portion being saturated. reveal that in the case of large pore spaces, trichloroethylene coming from the unsaturated zone readily intruded into the saturated zone, whereas in the case of small pore spaces, almost all of the test liquid remained stagnant on the surface of the saturated zone. These phenomena in the capillary zone can be qualitatively interpreted by using the Hele-Shaw model.
The results obtained above suggest that trichloroethylene, once allowed to enter the soil, will migrate through the unsaturated soil layer at a rather high speed down to the surface of the groundwater. It diffuses along this surface until it reaches locations where the equilibrium between the gravity of trichloroethylene and the surface tension of capillary zone no longer exists, followed by migration into the groundwater layer. The test liquid may remain in the pore spaces in the saturated soil, as observed in the migration experiment in the saturated zone, and then gradually come into solution in the groundwater (Fried et al., 1979). If this occurs, pollution can persist for long periods because groundwater flow is extremely slow. REFERENCES
Burmaster D. E. (1982) The new pollution. Groundwater contamination. Environment 242, 7 13. Fried J. J., Muntzer P. and Zilliox L. (1979) Ground-water pollution by transfer of oil hydrocarbons. Groundwater 17, 586-594. Schwille F. (1981) Groundwater pollution in porous media by fluids immiscible with water. Sci. Total Envir. 21, 173-185. Schwille F. (1984) Migration'of organic fluids immiscible with water in the unsaturated zone. In Pollutants in Porous Media (Edited by Yaron B., Dagan G. and Goldshmid J.), pp. 27-48. Springer, Tokyo, Japan.