Chloride and Tritiated Water Flow in Disturbed and Undisturbed Soil Cores1

M. A. MCMAHON AND G. W. THOMAS2

ABSTRACT

Columns of disturbed and undisturbed Eden, Maury, andPembroke soils were eluted with CaCl2 in tritiated water. Thechloride was used as a substitute for nitrate since it is notaffected biologically. The experiments were carried out toinvestigate the effects of natural soil structure on the flow ofwater and chloride. In all three soils flow was less stable in theundisturbed cores judging from the shape of the curve and thepoints of initial tritiated water and chloride breakthrough.Chloride moved through the undisturbed Eden and Maury soilsfaster than through the disturbed columns. Both soils excludeanions to a significant extent. In the Pembroke soil, whichadsorbs anions, the chloride was retarded in the undisturbedcolumn as compared to the disturbed column. The results sug-gest that field movement of water and anions is not describedvery well by columns of disturbed, packed soils.

Additional Index Words: anion exclusion, anion adsorption,soil structure.

rpHE MOVEMENT of the nitrate ion through soil is of theJ. utmost importance since, of all the major nutrient ions,

it is the most susceptible to loss by leaching and because ofits potential impact on water quality.

Chloride has been shown to be a good tracer of nitrate(Wetselaar, 1962). Therefore in leaching studies chloridecan be used without biological interference; it can be as-sumed that nitrate behaves similarly under well-aeratedconditions.

Recent work by Thomas and Swoboda (1970) and Smith(1972) has been done on disturbed columns. These work-ers found that chloride moved deeper into the soil thanwould be calculated using the assumption that it was mov-ing with the added water. They explained this deeper move-ment by stating that anions were excluded from the watersurrounding the negatively-charged clay platelets and alsofrom extremely small pores where diffusion of the anion isits only means of movement. However, in our experience,these disturbed columns do not adequately describe leach-

728 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974

1 -0

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MAURYUNDISTURBED

.

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600 800 1000NILS. OF EFFLUENT

Fig. 1—Chloride and tritium breakthrough curves for an undis-turbed core of Maury soil eluted with tritiated CaClj.

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Fig. 2—Chloride and tritium breakthrough curves for a dis-turbed core of Maury soil.

ing as experienced under field conditions. Wild (1972)showed that under field conditions rainfall moved throughlarge cracks and not as a distinct front. Elrick and French(1966) compared chloride movement in disturbed and un-disturbed cores. They found that chloride appeared muchearlier in the effluent from the undisturbed columns at highwater content. These differences were attributed to differentpore connectivity patterns. Natural drainage in the undis-turbed column would tend to set up vertical channels andpathways while the disturbed cores would be more homo-geneous and the chloride might be expected to movethrough more nearly as a front. Kissel et al. (1973) foundthat large, continuous pores were important in the down-ward movement of chloride through saturated, swelling claysoils. They compared chloride movement through disturbedand undisturbed soil cores. Chloride appeared in the efflu-ent at less than 0.10 pore volume in the undisturbed core,whereas 0.25 pore volume was eluted before chloride ap-peared in the effluent of the disturbed core. The work ofCorey and Horton (1968) gives evidence that 3H behavesas a good tracer for water. Stewart (1972) found that theratio of 3H concentrations of tenaciously-absorbed waterto respective concentrations of bulk pore water were ap-proximately equal to 1.0 over a range of soil and clay min-erals used.

Krupp et al. (1972) showed that exclusion volume for36C1 and the separation volume for 36C1 and 3H increasedas flow velocity decreased and these changes were relatedto the total ion concentration, the thickness of the diffuse

double layer and the zones of mobile and immobile solution.The present study was designed to determine the effect

of natural soil structure versus packed soil on water flowand on the chemical reactions of anion exclusion and anionadsorption. Three Kentucky soils, differing quite widely inproperties, were used.

MATERIALS AND METHODSThe characteristics of the soils used are presented in Table 1.

All three soils occur in central Kentucky, but are formed fromvery different parent material. Maury is formed in Ordovicianphosphatic limestone, Pembroke in Mississippian limestone,and Eden from Ordovician shale deposits. The clay minerals inthe Maury and Pembroke soils are vermiculite and kaolinitewhereas the Eden soil contains vermiculite and mica. Undis-turbed columns were collected in the field by driving a stainlesssteel column, 7.6 cm in diameter, into the ground to the re-quired depth and then digging it out. The Maury columns wereapproximately 56 cm long and the Pembroke and Eden soil col-umns were 25 cm long. A rubber stopper with an open hole,into which a glass tube was inserted was placed into the lowerend of each column. These columns were then saturated withdistilled water and allowed to drain under the influence of grav-ity. When they had reached equilibrium they were weighed.Then a solution of 0.002JV CaCl2 in tritiated water was appliedto the columns at a constant rate (7.9 ml/hour) by a motor-operated syringe. Effluent fractions, were collected on an LKBultrarac fraction collector. When the experiment was terminatedthe soil was weighed again. The difference between beginningand final weights was negligible in all cores since flow previousto Cl and 3H introduction was the same as that afterwards.Then the soil was pushed from the column and dried in a forceddraft oven at 105C. The difference in the dry (oven dry soil

Table 1—Characteristics of the soils used in the column studyMaury silt loam

Wet weight (g)Dry weight (g)Water content (ml)Bulk density (g/cm3 )Volumetric water content (0)Cation-exchange capacity (surface)Clay percentage (surface)

(meq/100 g)Iron oxide percentage:

Surface soilSubsoil

Length of column (cm)'•-J Depth of surface soil (cm)

Undisturbed3,9093,049

8601.200.338

—

—

——55.935

Disturbed3,1902,324

8660.910.339

14.7

14.1

6.407.60

55.935

Pembroke silt loamUndisturbed1,4571,157

3001.250.324

—

—

——25.45

Disturbed1,368

978390

0.840.335

11.5

13.5

—12.9525.45

Eden silty clay loamUndisturbed1,9641,519

4451.310.384

—

—

——25.410

Disturbed1,4761,013

4630.870.396

20.9

32.0

6.659.45

25.410

MC MAHON & THOMAS: CHLORIDE AND TRITIATED WATER FLOW IN SOIL CORES 729

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Fig. 3—Chloride and tritium breakthrough curves for an undisturbed core of Pembroke soil.

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Fig. 4—Chloride and tritium breakthrough curves for a disturbed core of Pembroke soil.

basis) and the wet weight of the column was assumed to be theweight of pore volume water. Breakthrough curves for CI" andtritiated water were plotted as the fraction C/C0 versus volumeof effluent where C = concentration of effluent and C0 = con-centration of applied solution.

Disturbed Columns

The soil for the disturbed columns was air-dried and thensieved. The 2.0 to 0.5-mm fraction was collected and put into7.6-cm diameter steel columns in small increments. After theaddition of each increment the column was tapped gently on thebench. The soil was added in layers as found in the undisturbedcolumns and made to resemble the undisturbed columns asnearly as possible. This gave an artificial soil profile in eachcolumn. After these preparations all subsequent treatments werethe same as those applied to undisturbed columns.

Chloride Analysis0

The amount of chloride in the effluent was determined on aBuchler-Cotlove Automatic chloride titrator. The titration wascarried out in a 4-ml aliquot of the effluent plus 1 ml of 0.4N

HNO3 in 10% acetic acid solution. Four drops of Gelatin Rea-gent (6 g Knox Unflavored Gelatin no. 1, 0.1 g thymol blue,and 0.1 g thymol dissolved in 1 liter of hot water) were addedas suggested by the manufacturer, to prevent the clumping ofthe AgCl formed. The amount of chloride in the sample wascomputed by comparing the titration time of the sample to thetitration time of a standard chloride solution.

Tritium AnalysisTritium concentrations in tritiated water were measured using

a Packard Tri-Carb liquid scintillation counter. One milliliterof the tritiated water was placed in 15 ml of liquid scintillationcocktail which consisted of: 720 g naphthalene, 24 g PPO, 0.3 gPOPOP, and 6 liters of p-dioxane. This mixture gave a countingefficiency of 33%. The counting was done in special liquid scin-tillation vials supplied by the Packard Company.

RESULTS AND DISCUSSION

Figures 1 to 6 show breakthrough curves for chloride andtritiated water for all soils and treatments in the experiment.If no ion interaction had occurred with the soil and flow had

730 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974

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MLS. OF EFFLUENTFig. 5—Chloride and tritium breakthrough curves for an undisturbed core of Eden soil.

been stable, a symmetrical curve would have passed throughC/C0 = 0.5 at the pore volume line representing the vol-ume of water held in the column. Thomas and Swoboda(1970) using a Houston Black clay soil showed that abreakthrough curve for D2O reached a C/C0 = 0.5 at onepore volume and was symmetrical about this point. Biggarand Nielsen (1962) presented the same principle using trit-ium. These workers used sieved aggregates in all cases.

Another measurement which shows the importance oflarge cracks in the conduction of water and solute in soils isthe position of the first appearance of the solute and watertracer in the effluent. These data are shown for the presentexperiment in Table 2. The pore volume fraction is used asan indicator. Undisturbed and disturbed columns are com-pared. In all cases the chloride and tritiated water appearedmuch ahead in the undisturbed columns as compared withthe disturbed ones indicating a considerable by-passing ofsoil by both water and chloride. The bulk density of the un-disturbed columns was 30 to 40% higher than the bulk den-sity of the disturbed columns and this may explain a partof the by-passing effect. However this still would not occur

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unless relatively large natural channels conducted most ofthe water.

The percentage of volumetric water taking part in chlo-ride movement was calculated by counting squares underthe breakthrough curve. The data are shown in Table 3.Negative values indicate the percentage of volumetric waterexcluding chloride while positive values indicate adsorptionof the anion. In the Maury and Eden soils, which excludedchloride, the percentage of water not participating in chlo-ride movement was greater in the undisturbed samples. Thisapparently was due to the physical effect that some solutewent through large pores and bypassed some of the waterwithin the peds. The difference between the undisturbedand disturbed columns of the Eden is very small, —30.6%compared with —28.5%. Another noticeable feature ofFig. 1 to 6 is that the chloride comes out ahead of triti-ated water in the Eden and Maury soils and behind triti-ated water in the Pembroke soil. This means that there waschloride exclusion in Maury and Eden soils while there wasadsorption of chloride in the Pembroke soil. Exclusion isthought to be due to repulsion of the chloride from thenegatively charged clay particles (De Haan, 1965). Kruppet al. (1972) found chloride eluting ahead of tritium in aPanoche clay loam (41% clay, CEC = 18.2 meq/100 g).Their results are similar to those obtained on the Maury andEden soils. Adsorption in the Pembroke soil apparently isdue to positively-charged iron sesquioxides and a low CEC(Thomas, 1963). Iron oxide content was 7.60 for Maurysubsoil and 9.45 for Eden subsoil as compared to 12.95%

Table 2—Position of initial appearance of chloride and tritiatedwater expressed as pore volume fraction for disturbed

and undisturbed samples of three soils

Fig, 6—Chloride and tritium breakthrough curves for a dis-turbed core of Eden soil.

Position (fraction ofpore volume)

SoilMaury

Pembroke

Eden

TreatmentUndisturbedDisturbedUndisturbedDisturbedUndisturbedDisturbed

Chloride0.340.600.350.560.220.35

Tritiatedwater0.450,660.200.440. 220.48

MC MAHON & THOMAS: CHLORIDE AND TRITIATED WATER FLOW IN SOIL CORES 731

Table 3—Percentage of volumetric water excluding chloride(negative values) and holding chloride in excess of 100%

(positive values) in adsorbing soils

Table 4—Effects of 6 and 9a on depth of chloride movement of1 cm of chloride solution placed on three soils in

undisturbed and disturbed conditions

SoilMaury

Pembroke

Eden

TreatmentUndisturbedDisturbedUndiatrubedDisturbedUndisturbedDisturbed

Percent ofvolumetric

H20-24.4-12.2+16.7+10.3-30.6-28.5

for Pembroke subsoil (Table 1). The iron oxide content ofthe Eden also was rather high, but it also has a higher pro-portion of expanding clays than does either Maury or Pem-broke.

Comparing soils (Table 2), the initial appearance of chlo-ride was always earlier from the Eden than from either ofthe other two soils showing the larger exclusion by the highCEC soil. This was true for both disturbed and undisturbedcolumns. Chloride was excluded from a substantial part ofthe volumetric water in the Maury and Eden soils (Table3). This exclusion was higher in the Eden due to the highercation exchange capacity. The undisturbed Pembrokeshowed a greater amount of chloride adsorption than didthe disturbed sample. This is contrary to the reasoning thatgreater mixing of salt should lead to greater adsorption. Theonly explanation we can offer for this greater amount of ad-sorption in the undisturbed sample is that sesquioxide-coated clay is oriented along the walls of large pores andtherefore may cause more efficient adsorption.

Thomas and Swoboda (1970) presented an equationwhich determined the depth of salt movement during leach-ing. It is as follows:

where D = depth of salt penetration (cm), R = cm of H2Oapplied, 6 = volumetric H2O content at equilibrium, and9ex = volume of H2O which excludes anions.

A more general form of the equation would be

D-R/(6±9a)

where 6a refers to water which either adsorbs or excludesanions, and the sign is positive for adsorption and negativefor exclusion.

In the case of the undisturbed Maury soil, depth of move-ment per cm of H2O added is

D= 17(0.338 — 0.082 = 3.91.

The value for the undisturbed Pembroke is given by

D = 1 / (0.324 + 0.054) = 2.64.

The other values are shown in Table 4.In the two soils which exclude chloride, relative move-

ment was deeper in the undisturbed cores compared withthe disturbed columns. This is due to solute movementaround the peds and exclusion from the leaching process

SoilMaury (undis. )Maury (dis. )Pembroke (undis. )Pembroke (dis. )Eden (undis. )Eden (dis. )

Volumetricwater

content ofcolumn (0)

0.3380.3390.3240.3350.3840.396

Water adsorbing (+)or excluding (-)

anions (0a)-0. 082-0. 041-10.054-(0. 034-0. 118-0. 113

Depth ofCl (cm)

3. 913.362.642.713.763.53

of some water within the larger aggregates in the undis-turbed columns. Comparing soils, it is shown that move-ment was deepest in the undisturbed Maury. In the case ofthe Pembroke, there is retardation in the flow of chloridedue to adsorption as described previously. The undisturbedEden, although it excludes anions more than the Maury(Table 3), has a much greater volumetric water content(Table 4). Therefore, more water has to be displaced in theEden during leaching than in the Maury. The same effectsare seen in these two soils in the disturbed columns. In thiscase, the relative depths of chloride movement are: Eden3.53 cm and Maury 3.36 cm, although the percentage ofvolumetric water excluding chloride was 28.5 and 12.21 forEden and Maury respectively. The volumetric water contentof a soil thus is a very important factor in the leaching proc-ess regardless of the large effects of exclusion, adsorptionand nonstable flow.

SUMMARY AND CONCLUSIONS

A column study was carried out where chloride waspassed through disturbed and undisturbed columns ofMaury silt loam, Eden silty clay loam, and a Pembroke siltloam. In all cases the tritiated water breakthrough curvewas displaced to the left of the pore volume and was notsymmetrical in shape. This lack of symmetry was more pro-nounced in the undisturbed columns as compared with thedisturbed ones and was thought to be due to lack of uni-formity in pore size and geometry. Exclusion volumes werecalculated for the various soils and treatments. In the twosoils which excluded chloride, exclusion was always greaterin the undisturbed columns. This apparently was due to thefact that some solute went through the large pores and by-passed some of the water within the peds. The initial ap-pearance of chloride and tritiated water was calculated forall soils and treatments. In all cases the chloride and triti-ated water appeared much ahead in the undisturbed col-umns compared with the disturbed ones.

Relative depths of chloride movement were calculatedfor all soil types whether disturbed or undisturbed. In theMaury and Eden soils, movement was deepest in the undis-turbed samples.

The chloride breakthrough curves were displaced to theleft of the tritiated water curves in the Eden and Maury soilswhile the reverse was true for the Pembroke. Exclusion ofchloride was highest in the Eden silty clay loam because ofits high cation exchange capacity. Adsorption of chloridein the Pembroke silt loam apparently was due to presence

732 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974

of positively-charged iron sesquioxides and a rather lowcation exchange capacity.

In conclusion, from a number of measurements, it canbe seen that solute movement through undisturbed columnsof soil is very different from that when disturbed columnsare used. The undisturbed columns may not totally de-scribe solute movement under field conditions, but theyparallel it more closely due to the preservation of ped struc-ture and pore geometry as found under field conditions. Theresults obtained with chloride should apply very closely tonitrate movement as well.