Salt and Water Transfer in Permafrost by Anatoli Brouchkov

PERMAFROST AND PERIGLACIAL PROCESSES Permafrost Peri`lac[ Process[ 00] 042Ð059 "1999#

Salt and Water Transfer in Frozen Soils Induced by Gradients of Temperature and Salt Content Anatoli Brouchkov Geocryology Department\ Moscow State University\ Vorobieyy Gory\ Moscow\ 008788\ Russia

ABSTRACT Experiments were undertaken for 01 years at the Amderma permafrost station on the long!term in~uence of temperature and salinization gradient on frozen soils[ Temperature gradients were maintained at about 9[94 >C:cm[ Salt migration studies were carried out on samples containing two soil parts with di}erent salt content[ There was a change in the cryogenic structure of samples\ notably the form and size of ice lenses\ under the in~uence of the temperature gradient[ However\ only local migration of water and salt occurred and there was no transfer from one side of the sample to the other[ At the salt content gradient used\ there was a migration of salt over the 01!year period[ Copyright Þ 1999 John Wiley + Sons\ Ltd[ RE łSUME ł Des expe riences ont e te poursuivies pendant 01 ans a la station d|e tudes du perge lisol d|Amderma sur l|in~uence a long terme de la tempe rature et du gradient de salinisation dans les sols gele s[ Les gradients de tempe rature ont e te maintenus a environ 9[94 >C:cm[ Des e tudes de la migration du sel ont e te poursuivies sur des e chantillons avec di}e rents contenus en sels[ Il y a eu un changement dans la structure cryoge nique des e chantillons et notamment de la forme et la taille des lentilles de glace sous l|in~uence du gradient de tempe rature[ Toutefois\ seulement une migration locale d|eau et de sel s|est produite et il n|y a eu aucun transfert d|une partie de l|e chantillon a l|autre[ Avec le gradient de contenu en sel utilise \ il y a eu une migration du sel au cours de ces 01 anne es[ Copyright Þ 1999 John Wiley + Sons\ Ltd[ KEY WORDS]

experimental^ frozen soils^ permafrost^ salinity

INTRODUCTION The problem of long!term processes inducing struc! ture changes in frozen soils under mechanical loads or temperature gradients is far from being solved[ Correspondence to] Anatoli Brouchkov\ Geocryology Depart! ment\ Moscow State University\ Vorobieyy Gory\ Moscow\ 008788\ Russia[ E!mail] alinabroÝnetcom[ca

Despite considerable research\ the role and the characteristic of processes causing long!term trans! formation in frozen soils remain under discussion[ In this connection\ any experiments lasting several years are of interest[ From 0876 to the present\ experiments proceeded at the Amderma permafrost station relating to the long!term in~uence on frozen soil "marine silt# of gradients of temperatures and salt content with constant mechanical loads[ Received 8 December 0888 Accepted 29 January 1999

043 A[ Brouchkov

WATER MIGRATION IN FROZEN SOILS UNDER CONSTANT TEMPERATURE GRADIENT Background The question of water transfer in soils frozen under a temperature gradient has been recognized as important despite experimental di.culties in research[ Bouyoucos and McCool "0805# estab! lished that water migration increased with water contents up to some value\ and then decreased[ These conclusions were con_rmed by many other researchers[ The main reason for migration is prob! ably the reduction of water potential during trans! formation to ice[ However\ the nature of migration forces is not completely clear[ According to Derja! guin and Churaev "0867#\ the phenomenon of water migration is connected to ice crystallization[ Exper! imental results\ however\ are rather inconsistent[ For example\ the thermo!osmotic ~ow of water is directed to the cold side in porous walls with an average pore radius of approximately one micron\ and in glass capillaries with a radius of 09Ð04 microns[ However\ this was not observed with jeweller|s carborundum powder with particle sizes of approximately 9[0 microns[ On a surface of glass\ the water thermo!osmotic ~ow was towards the warm side "Yershov and Churaev\ 0858#[ Water migration in frozen soils requires special consideration[ Yershov "0884# explains the water transfer by changes in the surface energy of soil particles[ The existence of such water migration in frozen soils is established in numerous publications "Hoekstra\ 0858^ Chistotinov\ 0862^ Yershov\ 0889\ 0887^ Williams and Smith\ 0878#[ Derjaguin and Churaev "0867# considered a system where the ice plate is between two _rm mineral surfaces\ sep! arated by layers of unfrozen water[ If such a system is removed from a condition of balance\ creating a small di}erence in temperature dT T0−T1 or a small di}erence in pressure dP P1−P0 in layers\ a heat and water transfer between layers will begin\ accompanied by melting of ice on one surface and freezing of water on the other[ According to Derja! guin and Churaev "0867#\ the expression for water ~ow qs is]

qs a00 dP¦rs Q

dT T

where a00 is a transfer factor\ rs is the density of water\ Q is the latent heat of fusion\ T is tem! perature and P is pressure[ This approach appears Copyright Þ 1999 John Wiley + Sons\ Ltd[

valid as the water migration in frozen soils should be conditioned energetically by the phase transition of water to ice and by the latent heat of this phase transition[ Experiments by Yershov "0889# and his associates have shown that a rapid redistribution of water occurs under large "9[5Ð2[9 >C:cm# tem! perature gradients[ Thus the migration of water is theoretically reasonable and is con_rmed experimentally in frozen soils under the in~uence of a temperature gradient\ but the characteristics of this process remain to be established[ Despite obvious technical di.culties\ it is necessary to determine the con! ditions of migration under low temperature gradi! ents and for long periods[ Experimental Techniques In the experiments we were guided by the desire to choose the minimum possible temperature gradi! ents\ but at the same time to ensure that these could be reliably maintained for several years[ The Amderma underground laboratory "Kara Sea Coast# is located about 03 metres below ground and is divided by insulated walls[ A constant tem! perature is maintained over many months and years with the help of electric heaters[ The ~uctuations of tempeature did not exceed 9[2 >C over 2Ð4 years and occurred simultaneously in all parts of the lab! oratory[ We used one of the walls\ which is 09 cm thick\ where special containers with samples were inserted[ The temperature on the {cold| side of the wall averaged −1[6 >C while on the {warm| side it was −1[1 >C^ consequently\ the temperature gradi! ent in the samples was about 9[94 >C:cm[ Marine silt that was obtained from marine deposits near Amderma was used in the exper! iments "Table 0#[ For the experiments the unsalted silt was dried and mixed with water without salts or with a solution of sea salt[ Water content was about 39) and 33)[ The soil was then put into polyvinyl containers "tubes# with a diameter of about 1 cm\ a 0 mm wall thickness and a length of 01 cm[ The containers were placed in a refrigerator in a horizontal position and were frozen at tem! peratures of −8 to −00 >C for three days[ The cryogenic texture produced was chaotic ice lensing^ the thickness of the lenses was up to 9[4Ð0[9 mm[ The ice lenses in the salinized samples were thinner with a maximum thickness of 9[4 mm[ The charac! ter of the initial cryogenic structure of samples is shown in Figure 0a[ Samples were placed in the wall on 04 July 0875[ The measurement of temperatures Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

Salt and Water Transfer in Frozen Soils 044 Table 0 Characteristics of tested soil[ Size and percentage content of particles

Mineral composition

0Ð9[4 mm

9[4Ð9[14 mm

9[14Ð9[0 mm

9[0Ð9[94 mm

9[94Ð9[90 mm

9[90Ð9[994 mm

³9[99 mm

of the samples was conducted weekly^ temperatures on the {cold| side of the wall were −1[629[2 >C\ on the {warm| side −1[129[2 >C[ Changes of tem! perature occurred slowly\ over many months[

Experimental Results Three samples were examined 5 months after the start of the experiment[ Each sample was cut in eight equal parts in length "0[14 cm on average#[ The results of the study of water content are shown

Polymineral

in Table 1[ There was some increase of ice content owing to an increase in the thickness of ice lenses close to the {cold| end of the samples[ Further study of water content distribution was undertaken 4 years and 00 years after the beginning of the exper! iment "Figures 0 and 1#[ As follows from the presented materials\ an appreciable redistribution of water did not occur[ However\ the cryogenic structure did not remain the same[ An overall decrease in moisture content over 00 years is shown in Figure 1[ Examination of the samples with the help of an optical microscope showed that the homogeneous microstructure of

Figure 0 The character of a cryogenic structure of samples "a# before and "b# 5 months\ "c# 4 years and "d# 00 years after the beginning of experiment] {cold| side "−1[6 >C# at the top[

Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

045 A[ Brouchkov Table 1 Distribution of water contents W ")# in a sample[ Distance from the {warm| end of the sample "cm#

W ")# after 5 months

9[5

0[8

2[1

3[4

4[7

6[0

7[3

8[6

31[0

31[6

30[7

39[6

39[7

39[2

30[9

important role belongs to ice lenses[ It is still di.! cult to evaluate movement factors "Yershov\ 0889^ Nakano and Tice\ 0889^ Williams and Smith\ 0878# for the description of long!term processes in such conditions owing to an absence of any signi_cant water movement from one side of our samples to another[ Figure 1 Distribution of water content W ")# in samples of frozen marine silt under the in~uence of a temperature gradient "at the right {warm| side −1[1 >C\ on the left {cold| side −1[6 >C#] "0# initial\ "1# after 0 year\ "2# after 4 years\ "3# after 00 years[

silt samples changed\ and the biggest changes were located in the saline sample with salinization of 9[4)[ The basic changes occurred in the section close to the {warm| end of the sample[ The re! organization of cryogenic microtexture was expre! ssed in increasing ice lensing parallel to the heat ~ow\ and a reduction in the thickness of ice inclusions perpendicular to the ~ow of heat[ A cer! tain ordering of the cryogenic microstructure was observed] a thickening of bigger ice lenses and an increase in their length at the expense of a reduction of quantity and size of smaller ice lenses\ which presented a local redistribution of water[ Changes in the microstructure at the {cold| end were neg! ligible^ this was connected\ apparently\ to lower temperature and a reduction in the opportunity for water movement[ The experiments show that very long!term movement in permafrost is a possible process at the temperatures and temperature gradi! ents applied^ its exact e}ects remain unknown[ The movement of water through soil depends upon the gradient of potential in the water\ or the gradient of water content in this case\ and on a coe.cient which depends on the soil[ The latter is called the permeability "Williams and Smith\ 0878# and has the units cm:s "for the gradient of potential in the water# or cm1:s "for the gradient of water content#[ The permeability of silt was approximately evalu! ated only for the local redistribution of water which occurred in the experiments[ A rough value of the permeability is 0Ð4×09−6 cm1:s and less[ An Copyright Þ 1999 John Wiley + Sons\ Ltd[

MIGRATION OF SALTS IN FROZEN SOILS UNDER CONSTANT TEMPERATURE GRADIENT Background The question of salt migration is important both theoretically and practically for construction on the Arctic coast and in other regions[ The most favour! able conditions for migration of salts in frozen soils under a gradient of temperature are probably at {warmer| negative temperatures when a signi_cant amount of water is contained in a liquid condition[ The chemical potential of any component is expressed for an ideal solution as follows "Isaev\ 0875#]

0 m m9 "T#¦PV"T\ 9# 0− aP ¦RT ln ri 1 "0# where m9 is a constant\ also termed the absolute chemical potential "J:mole#^ V"T\ 9# is the molar volume of component i in a solution at temperature T "K# and pressure P : 9^ a is a factor of isothermal compression^ R is the universal gas constant^ and ri is the molar concentration of component i[ On the other hand\ the decrease of crystallization temperature is proportional to the proportion of dissolved substances and does not depend on their chemical nature] DT ukp s ri

"1#

where ukp is a constant ">C# and ri is the molar Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

Salt and Water Transfer in Frozen Soils 046

concentration of component i[ As salt migration ~ow qc is proportional to a di}erence of chemical potentials\ the value of the ~ow can be expressed as] qc kðm"T0 #−m"T1 #Ł ¼ kR"T0 ln rT0 −T1 ln rT1 # "2# where k is a factor of migration[ For example\ the salt NaCl is dissolved[ Then we can _nd the con! centration of salt in solution at T0 and T1] rT0

T9 −T0 ukp

rT1

T9 −T1 ukp

"3#

The migration ~ow qc will be equal in this case]

qc kR T0 ln

T9 −T0 T9 −T1 −T1 ln ukp ukp

"4#

Thus\ the process of salt migration under a tem! perature gradient is possible\ but migration coe.cients k are probably very small[ Murrmann "0862# found experimentally that the temperature gradient is not signi_cant to the migration of sodium ions[ Some _eld studies showed\ however\ that changes in structure and concentration of soluble salts in soils can occur in the winter "Panin and Kazantchev\ 0875#[ Experi! ments on migration of salts in frozen soils under a gradient of temperature were conducted by Ostrou! mov "0889#[ He observed the migration of water to the {cold| side of samples and the migration of salt to the {warm| side of samples[ It should be noted that the temperature gradient used "about 0 >C:cm# is very high and unusual for permafrost[ Thus\ the e}ect of migration of salts in frozen soils is possible in certain conditions^ but high salinization and large gradients of temperatures are required[ It can be used to explain frozen saline soil formation and salt distribution[

Experimental Techniques The general techniques are described above[ Soil was air!dried for at least a week\ then mixed with the solution of sea salt dissolved in distilled water[ Salinization of samples was about 9[4) by mass and water content was about 39)[ Sea salt was prepared by boiling it out of Kara Sea water[ The samples were frozen at −09 >C[ We used {salinization| to refer to the weight content of salt in dry soil\ rather than {salinity| which is de_ned as salt concentration in pore water[ With a change of temperature\ {salinity| will also change\ so it is a characteristic not only of the soil but also of the temperature and that is inconvenient from our point of view[ We used a quick method employing electrical conductivity for measurement of sali! nization[ The method includes determination of water content of the sample\ measurement of con! ductivity of distilled water for calibration\ mixing 099 g of air!dried soil with 099 ml of distilled water and determination of the electrical conductivity using sea water as the standard[ The initial dis! tribution of salinization is shown in Table 2[

Experimental Results After 5 months the measured water content and the salinization were in essence equal along the sample length "Table 3#[ The ~uctuations of water content and salinization are within the limits of the accuracy of the measurement system[ However\ di}erent _ndings were obtained after 3 years\ when similar samples were investigated and a small redistribution of salinization was observed "Figures 2 and 3#[ Thus\ the salinization in frozen soils at regular temperature conditions can be con! sidered a relatively steady value that changes with increase of gradients of temperature and sali! nization[ This point of view is also con_rmed by our _eld data[ The distribution of salinization in

Table 2 Distribution of water content W ")# and salinization Dsal ")# in a control sample[ Distance from the edge of the sample "m#

W ")# Dsal ")#

39[9 9[40

27[9 9[49

27[9 9[37

26[9 9[37

26[9 9[38

Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

047 A[ Brouchkov Table 3 Distribution of water content W ")# and salinization Dsal ")# in samples[ Distance from the {warm| end of the sample "cm#

W ")# after 5 months Dsal ")# after 5 months W ")# after 3 years Dsal ")# after 3 years

9[5

0[8

2[1

3[4

4[7

6[0

7[3

8[6

28[0 9[37 30[4 9[37

27[1 9[42 32[4 9[49

27[2 9[36 28[0 9[35

26[7 9[35 32[3 9[56

26[3 9[37 31[4 9[38

26[4 9[42 30[1 9[26

27[0 9[36 30[5 9[36

27[1 9[40 28[4 9[38

Figure 2 Distribution of salinization Dsal ")# in samples of frozen marine silt under the in~uence of a temperature gradient "at the right {warm| side −1[1 >C\ on the left {cold| side −1[6 >C#] "0# initial\ "1# after 5 months\ "2# after 3 years[

Figure 3 Distribution of "0# water content and "1# salinization in a sample of frozen marine silt under the in~uence of a tem! perature gradient after 3 years[

permafrost is not equal] its value changes from 9) up to 0) and more over vertical distances of one metre or less[ Velli "0862# showed that the dis! turbances of the salt distribution with depth\ caused by piles in permafrost\ are not restored at tem! peratures of −3 >C and below[ There is also increasing salinization from the sur! face to a depth of about 09 m in frozen soils "Dubi! kov et al[\ 0877^ Hivon\ 0880#[ These trends are typical for Ugorski and Yamal Peninsulas\ and other regions "Figure 4#[ The most likely explanation of the surface to depth salinization trend shown in Figure 4 is salt migration with water ~ow under a temperature gradient[ For example\ a change in salt con! centration occurred in about a month at a high temperature gradient "0 >C:cm# in an experiment by Romanov "0874#[ The migration of both water Copyright Þ 1999 John Wiley + Sons\ Ltd[

Figure 4 Distribution of salinization in di}erent holes drilled in permafrost "presented by marine silt# in Amderma region[

and salts occurs at higher temperatures and at increased temperature gradients[ Salt ~ow accompanies water movement under the in~uence of a temperature gradient[ This more or less cor! responds with results presented in Figure 3[ In this case\ the redistribution of salt is close to water redis! tribution[ The result is an increase of salinization to depths of 7Ð01 metres\ which is observable at many places along the Arctic coast "Brouchkov\ 0887#[ MIGRATION OF SALTS IN FROZEN SOILS UNDER GRADIENT OF SALINIZATION Background Molecular di}usion in frozen soils occurs where there is a gradient of concentration of salt in liquid Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

Salt and Water Transfer in Frozen Soils 048

water[ The concentration of salts in unfrozen water is determined mainly by temperature\ not by salinization[ As salinization increases\ part of the ice changes into water and a constant concentration is maintained[ Thus\ the existence of a gradient of concentration of salts in frozen soils at constant temperature presents a problem[ Nevertheless thicker layers of unfrozen water in a more saline soil are subject to migration to a less saline part of soil[ This moving water transfers salts\ equalizing the value of salinization in di}erent parts of a soil[ Derbeneva "0854# and Murrmann "0862# showed that ions can migrate in frozen soils at constant temperatures[ Romanov "0874# did not _nd notice! able changes of initial water content in similar experiments[ At the same time they showed the presence of ion migration due to concentration gradients in soils[ At a constant negative tem! perature of −2[4 to −3[4 >C in the underground laboratory in Yakutsk\ the change of concentration of ions can be observed only after 7Ð09 months "Romanov\ 0874#[ Similar results were obtained in experiments lasting 89 days "Chuvilin\ 0888# and produced estimated values of di}usion coe.cient between about 09−4 and 09−6 cm−1:s[ However\ long!term experiments at constant temperature have not been conducted\ and it remains important to obtain characteristics of salt transport for the period of several years[ Experimental Techniques The experiments on migration of salts were carried out at a constant temperature of −229[2 >C for almost 01 years[ They were originally prepared and begun at Amderma permafrost station by Y[ S[ Petruhin[ The author of this article continued the experiments[ Some samples of marine silt did not contain salt^ the others had salinization of about 0)[ All of them were initially frozen at −09 >C[ Two samples\ a saline one and an unsalted one\ were placed in contact[ They were isolated from the external environment by two layers of plastic and were placed in a box in the underground laboratory[ The initial distribution of salinization is shown in Figure 5[ Experimental Results With such a sharply non!uniform initial dis! tribution of salinization "a gradient of more than 9[0):cm#\ long!term redistribution occurs in silt Copyright Þ 1999 John Wiley + Sons\ Ltd[

Figure 5 Sea salt transfer in marine silt at temperature −2 >C] "0# an initial distribution of salinization\ "1# after 6 months\ "2# after 00 years of experiment[ Length "horizontal axis# is in centimetres[

"Figure 5#[ The factor of di}usion\ calculated on the basis of these data\ is about 1×09−6 cm1:s[ The initial period is best for salt transport\ which becomes much less later[ Hence\ it is possible to consider a salinization in permafrost under normal temperature conditions only as a conditionally steady value that changes with increases in the gradients of temperature and salinization "see above#[ The increase with depth of salinization in many regions of permafrost is connected to these results[ Here\ as well as in many other cases\ an adequate quantitative thermodynamic analysis is di.cult[ The basic thermodynamic characteristics are incompletely calculated and are not entirely appli! cable for thin liquid water layers[ The existence of salt transfer cannot be excluded at small gradients of salinization and temperature for long periods[ Unfortunately\ it is outside the range of direct experiments[ CONCLUSION Research into frozen soils structure under the in~uence of a temperature gradient of 9[94 >C for 00 years showed that there was a change in the cryogenic structure of samples\ and in the form and size of ice inclusions[ Direct ~ow of water and salts from one side of a sample to the other side was not observed for this temperature gradient\ but there was a small amount of local migration[ The migration of both water and salts probably occurs in frozen soils at higher temperatures and at greater gradients of temperature[ This could explain an increase in salinization that occurs from the surface to depths of about 09 m in permafrost[ A long!term redistribution of salt content occurred in marine silt at a constant temperature Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#

059 A[ Brouchkov

for 00 years[ The factor of di}usion\ calculated on the basis of the experimental data\ is about 1×09−6 cm1:s[

ACKNOWLEDGEMENT I am grateful to Dr Antony Lewkowicz "Ottawa University# for comments and assistance[

REFERENCES Bouyoucos GJ\ McCool MM[ 0805[ Further studies on the freezing point lowering of soils[ Michi`an A`ric! ulture Experimental Station Technical Bulletin no[ 20[ Brouchkov A[ 0887[ Frozen Saline Soils of the Arctic Coast[ Moscow University Press] Moscow "in Russian#[ Chuvilin EM[ 0888[ Migration of ions of chemical elements in freezing and frozen soils[ Polar Record 24"081#] 48Ð55[ Derbeneva MM[ 0854[ Experimental research of transfer of water and ions in frozen soils[ Soils Science no 0] 47Ð51 "in Russian#[ Derjaguin BV\ Churaev NV[ 0867[ The theory of frost heaving[ Journal of Colloid and Interface Science 56"2#] 280Ð285[ Dubikov GI\ Ivanova NV\ Aksenov VI[ 0877[ Pore solu! tion of frozen ground and its properties[ In Proceedin`s

of the Fifth International Conference on Permafrost\ Trondheim\ Tapir Publishers\ Norway^ 222Ð227[ Hivon E[ 0880[ Behaviour of saline frozen soils[ Edmon! ton\ Alberta[ Ph[D Thesis\ McGill University[ Hoekstra P[ 0858[ Water movement and freezing pres! sures[ Soil Science Society of America Proceedin`s 22] 401Ð407[ Murrmann RP[ 0862[ Ionic mobility in permafrost[ In Proceedin`s of the Second International Conference on Permafrost\ Yakutsk[ Nakano Y\ Tice AR[ 0889[ Transport of water due to a temperature gradient in unsaturated frozen clay[ Cold Re`ions Science and Technolo`y 07] 46Ð64[ Ostroumov VE[ 0889[ Salt transfer in frozen soils under temperature gradient[ In Frozen Saline Soils as Bases[ Nauka] Moscow "in Russian#[ Panin PC\ Kazantchev VA[ 0875[ Processes of salt trans! fer in cryogenic soils[ In Successes of Soil Science[ Nauka] Moscow^ 134Ð149 "in Russian#[ Romanov VP[ 0874[ Research of connection between electrical properties and ion transfer in frozen soils[ In Mi`ration of Chemical Elements in Cryolithozone[ Nauka] Novosibirsk "in Russian#[ Velli YY[ 0862[ Stability of Buildin`s in the Arctic[ Stroiiz! dat] Leningrad "in Russian#[ Williams PJ\ Smith MW[ 0878[ The Frozen Earth[ Cambridge University Press] Cambridge[ Yershov ED[ 0889[ General Geocryolo`y[ Nedra] Moscow "in Russian#[ In translation\ Cambridge Uni! versity Press] Cambridge\ 0887[ Yershov ED "ed[#[ 0884[ Foundations of Geocryolo`y\ Vol[ 0] Physical and Chemical Foundations[ Moscow State University] Moscow "in Russian#[ Yershov AP\ Churaev NV[ 0858[ In Theoretical Foun! dations of Chemical Technolo`y 2"3#] 472Ð476[

Permafrost and Peri`lac[ Process[\ 00] 042Ð059 "1999#