Distribution of Nonionic Surfactant in Soil Columns Following Application and Leaching1

W. W. MILLER AND J. LETEYZ

ABSTRACT

The distribution of 14C-tagged surfactant (Soil Penetrant)was observed in soil columns following application of variousconcentrations and leaching under unsaturated flow on wettable(Pachappa) and water-repellent (Morris Dam) soils. The maxi-mum depth of surfactant penetration for a given leaching periodwas greater for Pachappa than for Morris Dam. Followingleaching there was a more uniform distribution of surfactantthroughout the column of Pachappa soil. Water drop penetra-tion time (WDPT) experiments showed good correlation be-tween experimental distribution as determined by 14C tracingand actual reduction of water repellency to a given depth.Consequently, the movement and distribution characteristicsof a non-14C-tagged surfactant (Aqua Gro) on the Morris Damsoil following application and leaching was qualitatively exam-ined by the WDPT method. A theoretical model was tested forits ability to qualitatively predict experimental Soil Penetrantdistributions. There was reasonable agreement between experi-mental and calculated distributions. Specific adsorptive charac-teristics at low equilibrium concentrations were found to bevery important to surfactant distribution in a given soil.

Additional Index Words: water repellency, surfactants, waterdrop penetration time.

MATERIALS USED to overcome water repellency in soils areorganic compounds known as "wetting agents" or

"surfactants." Hence, the distribution of surfactant mate-rials in soil after irrigation or rainfall is important for sev-eral reasons: (i) the effectiveness of surfactants in reducingsoil water repellency is dependent in part, upon its positionwithin the soil profile since water-repellent layers are notalways located at the soil surface; (ii) the quantity or con-centration of surfactant that must be applied per unit areato render a repellent soil wettable, and the depth to whichit must travel to reach the repellent layer is important envi-ronmentally as well as economically; and (iii) adsorptionand desorption characteristics influence not only the initialsurfactant penetration following application but also theultimate distribution of wetting agent in the soil and thepotential hazard of ground-water contamination.

A previous investigation (4) has shown that the additionof two nonionic surfactant materials to a soil system af-fected the hydraulic conductivity of that system. The inten-sity of the conductivity effect was influenced by type ofsurfactant, the concentration of the surfactant, adsorption

1 Contribution of the Department of Soil Science and Agri-cultural Engineering, University of California, Riverside 92502.The research leading to this report was supported by the Officeof Water Resources Research, USDI, under the MatchingFunds program of Public Law 88-379, as amended, and by theUniversity of California, Water Resources Center. It is a partof Office of Water Resources Research Projects no. B-072-CALand B-141-CAL (Water Resources Center Project W-332).Received 15 April 1974. Approved 28 Aug. 1974.2 Research Assistant and Professor of Soil Physics, respec-tively, Univ. of Calif., Riverside-. The senior author is currentlyAssistant Professor of Soil Ecology and Extension Specialist inNatural Resource Development, University of Nevada, Reno.

characteristics of the material, and soil type. Soil Penetrantwas found to be more effective for deeper penetration in ashorter period of time and has a greater potential towardsdeep percolation. On the other hand, Aqua Gro was morestrongly adsorbed and less subject to leaching, indicatingthat Aqua Gro might have a more lasting wettability effecton surface layers. Finally, the behavior and movement ofsurfactants in soil were found to be a function of the charac-teristics of adsorption isotherms, mixing or dispersion dueto flow velocities, solute concentration, and the physical andchemical characteristics of the porous medium itself.

The previous investigation (4) was conducted under satu-rated water flow conditions and surfactant concentrations ofthe effluent were measured. No measurements were madeof the distribution of surfactant within the soil profile in thefirst investigation. A supply of 14C-labeled Soil Penetrant3685 was subsequently supplied by Emery Industries, Inc.,Santa Fe Springs, California and allowed measurementswithin the soil profile. This paper reports the results of sur-factant distribution in the soil column under unsaturatedwater flow conditions.

The purpose of this investigation was: (i) to examinespecific distribution characteristics of nonionic surfactantin a soil medium following application and leaching and theimplications thereof; (ii) to determine the amount of sur-factant necessary to effectively reduce soil water repellency;and (iii) to compare measured and calculated surfactantdistributions in a soil medium following application andleaching.

MATERIALS AND METHODSTwo soils were used in this investigation: (i) Pachappa

sandy loam (a wettable soil) and (ii) a water-repellent soil alsoof sandy loam texture which will be referred to as Morris Damsoil. Both soils were air dried and passed through a 1-mm sieveprior to use. The nonionic surfactants used in this study wereHC-labeled Soil Penetrant 3685, an alkyl polyoxyethylene eth-anol, and nonlabeled Aqua Gro, a mixture of polyoxyethyleneester and polyoxyethylene ether. Aqueous solutions were pre-pared by mixing various amounts of tap water with the non-ionics to obtain surfactant concentrations of 0, 500, 1,000,1,500, and 2,000 ppm. The 500-ppm solution concentration isonly slightly above normal field application rates.

Surfactant solutions and leaching water were applied to thetop of vertical soil columns by a positive pressure pump (9.5cmVhour) at a constant Darcy flux of 1.44 cm/hour whichwas lower than the saturated conductivity of the respective soils.The surfactant solution was applied for 1 hour followed byleaching periods of 2, 4, or 6 hours. A glass centre of mediumpore size was placed at the top of the soil column to insure auniform distribution of liquid at the soil surface. The top 10 cmof the column (2.9 cm in diameter) was composed of 20 sec-tions each of 0.5 cm thickness which were partitioned at the endof each experiment. The bottom of the column consisted of acontinuous tube. The bulk density was 1.4 and 1.0 g/cm3 forPachappa and Morris Dam soils, respectively.

The soil contained in each section treated with 14C-labeledSoil Penetrant was placed in small glass counting vials. Twentymilliliters of liquid scintillation solution consisting of 10.4 gof PPO and 166 g of napthalene dissolved in 800 ml ofxylene, 800 ml of dioxane, and 473 ml of absolute ethanol were

17

18 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

.10 .20 30 .40 .50

II-Q.UJQ

APPLICATION

——— PACHAPPA-SOIL PENETRANT

——— MORRIS DAM-SOIL PENETRANT

—o— MORRIS DAM-AQUA GRO

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

inn

_ 1

_ • 2 HR. LEACHING [

_ X 4 HR. LEACHING ***

_ 0 6 HR. LEACHING **c»>

- ——— PACHAPPA SOIL !

° ' x3 ' -i'VY

i 1 -- —— MORRIS DAM SOIL r* \° ^t *?

I i i 1 1 1 ! I i I.40 .50

VOLUMETRIC WATER CONTENT 6Fig. 1—Volumetric water content vs. depth for Pachappa and

Morris Dam soil-surfactant systems following applicationand leaching.

added to the vials. 14C activity of Soil Penetrant was determinedin the presence of soil, similar to the method as described byEhlers, Letey, Spencer, and Farmer (2), and Huggenberger,Letey, and Farmer (3). Counting efficiencies were determinedby adding an aliquot of 14C-labeled surfactant of known activityto counting vials containing a known weight of soil. The mois-ture content of each section was determined by concurrentexperiments in which nonlabeled Soil Penetrant was added tothe column for the same periods of time. The combined concen-tration of surfactant in solution and adsorbed in each sectionwas reported as Mg/g of oven-dry soil.

Water drop penetration time (WDPT) measurements wereconducted in an effort to obtain supporting data with respectto surfactant distribution within the column and also to test theeffectiveness of surfactant in reducing water repellency as afunction of concentration and depth. If a drop of water (72dynes/cm) penetrated the Morris Dam soil in < 5 sec, the soilwas considered wettable. WDPT data correlated very well withthe distribution of Soil Penetrant as determined by 14C tracing.As a result, the movement and distribution characteristics of anonlabeled surfactant (Aqua Gro) on the Morris Dam soil fol-lowing application and leaching was qualitatively examined bythe WDPT method.

The model as presented by Miller et al. (4) was used to pre-dict Soil Penetrant distribution within the soil columns. Calcu-lated solution and adsorbed concentrations were added to givethe total predicted surfactant concentration in each section.These values were compared to measured values.

RESULTS AND DISCUSSION

The moisture content as a function of depth for a givensoil was similar for all concentrations of surfactant solu-tions. Figure 1, therefore, represents the average trend inmoisture distribution for each soil system for all solution

PACHAPPA SOIL

SOLUTION CONCENTRATION• 2000ppm

MORRIS DAM SOIL

SOLUTION CONCENTRAIieff• 2000ppm

I 500ppm

I 000ppm

500ppm

17.0-22.0 IT.O-25.0--*-" . -*-^

DEPTH (cm)

Fig. 2—Combined concentration of surfactant in solution andadsorbed vs. depth for Soil Penetrant application experiments.Numbers represent the time (sec) for penetration of waterdrop.

concentrations. During the application experiments, thePachappa soil had a lower volumetric water content, adeeper penetration of moisture, and a more uniform watercontent with depth than was observed for the Morris Damsoil. Since the application rate was below the saturated con-ductivity of both soils, the differences in moisture distribu-tion between the two soil systems was due primarily to thefact Morris Dam, being water repellent, had restricted watermovement within the column once all of the surfactant hadbeen adsorbed out of solution. Thus, it is not surprising tofind that the application moisture distributions observed forthe Morris Dam experiments appear to be tending towardswhat one would expect from a ponded surface. Note thatfor the Morris Dam-Aqua Gro experiments the moisturedistribution pattern as a function of depth was similar to theMorris Dam-Soil Penetrant experiments except that therewas a slightly shallower penetration and slightly highervolumetric water content in the upper part of the column.(These slight differences in water distribution for the twosurfactants were consistently reproducible). This differenceis reasonable since Aqua Gro has been observed to be morereadily adsorbed by the Morris Dam soil than Soil Penetrant(4, 5). Consequently, Aqua Gro was adsorbed out of solu-tion nearer the surface of the soil column and "surfactantfree" water movement was restricted at a shallower depth.

For the leaching experiments both soils exhibited a fairlyuniform moisture distribution with depth which increasedonly slightly with increasing leaching period. The volumet-ric water content for the Pachappa soil was lower than thatof the Morris Dam. There was no significant difference inmoisture distribution during leaching for either surfactanttreatment of the Morris Dam soil. Once again, moisture dis-tribution for the Morris Dam soil seemed to exhibit thecharacteristics of ponded infiltration.

Calculations of surfactant distribution were made usingwater contents measured at the end of each experimentalrun. It was assumed the water content in each section didn'tchange with time. This assumption is not completely validas pointed out by Braester (1).

The surfactant distribution following application (Fig.2) as determined by 14C tracing, indicates that the depth

MILLER & LETEY: DISTRIBUTION OF NONIONIC SURFACTANT IN SOILS COLUMNS 19

1500 PACHAPPA SOIL 1500 ppm SOLUTION CONCENTRATION• 2 HR. LEACHING PERIOD

£ 4 HR. LEACHING PERIOD

o 6 HR. LEACHING PERIOD

x——x APPLICATION DISTRIBUTION

500 -

3.0 4.0DEPTH (cm)

Fig. 3—Combined concentration of surfactant in solution and adsorbed vs. depth for Soil Penetrant leaching experiments, 1,500-ppmapplication solution concentration. Numbers represent the time (sec) for penetration of water drop.

of surfactant penetration is independent of the applied solu-tion concentration for the respective soils. The concentra-tion at a given depth, however, increases with increasingsolution concentration.

The distributions of 1,500 ppm Soil Penetrant followingleaching are presented in Fig. 3. Considering the Pachappasoil results first, note that after 2 hours of leaching the sur-factant has begun to move as a wave through the soil col-umn. As leaching time is increased, however, the wavemotion begins to dissipate and the surfactant materialapproaches a uniform distribution within the column. Also,the amount of surfactant "wash-out" from the first section

isoo

1000

o>500

PACHAPPA SOIL

is greater between the 2nd and 4th hour than between the4h and 6th hour leaching periods. This indicates that after4 hours of leaching the point of irreversibility of the ad-sorbed surfactant is being approached. The same generaltrends are observed during leaching for the 1,000 and 500ppm solution applications (Fig. 4 and 5). There is a moreuniform distribution of surfactant throughout the columnwith increasing leaching period. Results seem almost identi-cal except presented on a smaller scale because of lowerapplication concentrations.

For the Morris Dam leaching experiments (Fig. 3, 4, and5), there is an increase of surfactant penetration with in-

1000 ppm SOLUTION CONCENTRATION• 2 HR. LEACHING PERIOD

A 4 HR. LEACHING PERIOD

o 6 HR. LEACHING PERIOD

x—x APPLICATION DISTRIBUTION

3.0 4.0DEPTH (cm)

5.O 6.0 7.0

Fig. 4—Combined concentration of surfactant in solution and adsorbed vs. depth for Soil Penetrant leaching experiments, 1,000-ppmapplication solution concentration. Numbers represent the time (sec) for penetration of water drop.

20 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

ISOOr

£-?CP

PACHAPPA SOIL 500 ppm SOLUTION CONCENTRATION

• 2HR. LEACHING PERIOD

a 4HR. LEACHING PERIOD

o 6 HR. LEACHIMG PERIOD

x— — * APPLICATION DISTRIBUTION

1000 -

5.O 6.0 7.03.0 4.0DEPTH (cm)

Fig. 5—Combined concentration of surfactant in solution and adsorbed vs. depth for Soil Penetrant leaching experiments, 500-ppmapplication solution concentration. Numbers represent the time (sec) for penetration of water drop.

creasing leaching period. The maximum depth of surfactantpenetration in Morris Dam is not as great as for Pachappafor a given leaching period. For the two highest solutionapplications (Fig. 3 and 4) the position of maximum con-centration within the column did not change for the 2- and4-hour leaching periods but was greater for the 6-hourleaching period. The position of maximum concentrationfor the lowest solution application (Fig. 5) did not changeas the leaching time was increased.

During leaching, the depth to the wetting front for Pa-chappa soil was about 2.5 times, and for Morris Dam wasabout 3.5 to 4 times the depth of surfactant penetration.The fact that there was little or no wave motion of the sur-factant within the column with increase in the leachingperiod indicates some degree of irreversible adsorption forboth soils. The irreversibility being less in the Pachappathan in the Morris Dam. This effect contributed to a moreuniform distribution of surfactant throughout the columnof Pachappa soil.

The results of WDPT tests for the Soil Penetrant-MorrisDam system are illustrated in Fig. 2, 3, 4, and 5. The num-bers represent the time (sec) for penetration of a drop of

Table 1—Depth of soil wettability following application andleaching of various concentrations of Soil Penetrant

and Aqua Cro on Morris Dam soil

Soil Penetrant

Solutioncone.

2, 000 ppm(32 gal/acre)1,500 ppm(24 gal/acre)1,000 ppm(16 gal/ acre)

500 ppm( 8 gal/acre)

Aqua GroLeaching period,

hoursApplication 2

1.0

1.0 1.5

1.0 1.5

1.0 1.5

4 6

3.0 3.0

2. 5 2. 5

2.5 2.5

Application

1.0

0.5

0.5

0.5

Leaching period,hours

2

0.5

0.5

0.5

4

1.0

0.5

0.5

6

1.0

0.5

0.5

water (72 dynes/cm). Recall that the soil is consideredwettable if the droplet penetrated the soil in < 5 sec. Fol-lowing application, note that the Morris Dam soil hasbecome wettable down to 1 cm (or two sections of the soilcolumn) for all concentrations of solution application. Withthe exception of one low point, it appears that the minimumamount of surfactant required to make the soil wettable isabout 200-300 ppm in the soil on a dry weight basis. Dur-ing leaching with the 1,500-ppm solution application MorrisDam became wettable to 1.5 cm for the 2-hour leaching,2.5 cm for the 4-hour leaching, and 3 cm for the 6-hourleaching. Very similar results are observed for the 1,000-and 500-ppm solution applications. Note the good correla-tion between low and high application concentrations interms of depth of soil wettability for a given leaching periodand the minimum amount of Soil Penetrant required tomake the Morris Dam soil wettable, i.e., 200-300 ppm inthe soil on a dry weight basis. WDPT data, therefore, indi-cate that when applied as a solution low concentrations ofSoil Penetrant 3685 are as effective as higher concentrationsin reducing soil water repellency of the Morris Dam soil toa given depth, and that the minimum amount required forwettability is approximately 38.3 liters/ha-cm (10.4 gal/acre-inch) adsorbed. However, if the surfactant materialcannot be applied as a solution, it is conceivable that higherquantities might be required for effective wetting.

There was good agreement between experimental distri-bution as determined by 14C tracing and actual reductionof soil water repellency to a given depth. Hence, the move-ment and distribution characteristics of a non-14C-taggedsurfactant (Aqua Gro) on the Morris Dam soil followingapplication and leaching was qualitatively examined by theWDPT method. Comparative results of these experimentsto those of the Soil Penetrant-Morris Dam system are givenin Table 1. Aqua Gro is shown to be strongly and irre-versibly adsorbed within the top 1 cm of the column. Higher

MILLER & LETEY: DISTRIBUTION OF NONIONIC SURFACTANT IN SOILS COLUMNS 21

- PACHAPPA-SOIL PENETRANT

1.0CONCENTRATION

— ——— CURVE FITTED VALUES

VALORAS_1_

1000PACHAPPA-SOIL PENETRANT

1000 ppm

2.0(mg/ml)

3.0

MORRIS DAM-SOIL PENETRANT

——— — CURVE FITTED VALUES

VALORAS

1.0 2.0 3.0CONCENTRATION (mg/ml)

Fig. 6—Curve fitted values for respective adsorption isothermsas compared to those reported by Miller et al. (4) andValoras et al. (5) .

concentrations of Aqua Gro appear slightly more effectivein terms of the maximum depth of effective wettability fol-lowing application and leaching than do lower concentra-tions. Consequently, it would take a larger solution appli-cation of Aqua Gro than it would of Soil Penetrant toachieve a given depth of wettability for the Morris Dam soilfollowing a given period of leaching. On the other hand,Aqua Gro might have a more lasting effect due to itsstrongly irreversible adsorption characteristics. These re-sults are consistent with previous investigations (4, 5).

Initially, for the theoretical prediction of the experimen-tal distributions following application and leaching the iso-therm values as calculated by Miller et al. (4) were usedfor the respective soil-surfactant systems. Calculated distri-bution indicated greater adsorption than actual distribution.The problem could be one of overestimation of adsorptionin the adsorption isotherms at low concentrations as previ-ously reported, and/or one of non-equilibrium adsorption.Isotherm values were thus manipulated (Fig. 6) until a"best fit" relationship between calculated and experimentaldistribution was obtained. The shapes of the fitted isothermcurves were similar to those of earlier investigations, butvaried in magnitude at the low equilibrium concentrations.The fitted isotherm curves, therefore were used in thisinvestigation for the calculated distribution of Soil Penetrantin the respective soil systems. Adsorption for both soils wasconsidered reversible to 0.1 mg/g adsorbed.

Figure 7 shows typical calculated and experimental sur-factant distributions as a function of depth for the varioussolution concentrations following 2, 4, and 6 hours of leach-ing. Since surfactant material does not desorb as readily aswas assumed by the model, experimental data show a higherconcentration of organic in the upper part of the soil col-

lOOOr-

lOOOr

I.O 2.0 3.0 4.0 5.0 6.0 7.0

DEPTH (cm)CE

CO 2000

-5" 1000

MORRIS DAM-SOIL PENETRANT1000 ppm

i ^&tg_ ,i i i i

2000-

1000

2000

1000

1.0 2.0

——•—— EXPERIMENTAL—— o—— CALCULATED

6.0 7.03.0 4.0 5.0DEPTH (cm)

Fig. 7—Combined concentration of surfactant in solution andadsorbed vs. depth for typical calculated and experimentaldistribution of Soil Penetrant leaching experiments.

umns than that predicted by theory. Hence, the observedmaximum concentration at a greater depth within the col-umn is lower than predicted since less organic was availablefor transfer. Similar observations were reported by Huggen-berger et al. (3). Despite several assumptions which areonly partially met, the model is fairly accurate in its predic-tion of where the bulk of the surfactant will be located asa function of depth, the spreading characteristics of a partic-ular surfactant in an adsorbing medium, and the maximumdepth of surfactant penetration following a given amountof leaching. Furthermore, the calculated distribution agreeswith WDPT data with respect to maximum depth of soilwettability (considering 200-300 ppm adsorbed requiredfor wetting) for a given leaching period for all appliedconcentrations.

Two important factors in possible ground-water contami-nation of organic chemicals are the rate and duration ofwater flow through the soil profile, and the adsorption ofthe organic by the soil. The predictive model presented maythus be considered reliable from a qualitative and practicalpoint of view. Applied to the general situation, one shouldbe able to interpret basic movement and distribution char-acteristics of any organic compound in an adsorbing me-dium once adsorption isotherm relationships have beendetermined. A more quantitative approach would be toinclude an "adsorption-desorption" function as suggestedby van Genuchten et al. (6). However, desorption data foran organic and specific adsorbing medium are not as readilyavailable since desorption is also a function of its startingposition on the adsorption isotherm. Furthermore, field ap-

22 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

plications of an organic substance are generally low solutionconcentrations, i.e., concentrations at which the calculatedsolute distribution of substances possessing an atypical iso-therm is highly influenced by specific isotherm values.