Cationic Polymer Effects on Infiltration Rates with a Rainfall SimulatorAWAD M. HELALIA AND J. LETEY*

ABSTRACTThe cationic polymer "CP-14" was applied to three soils at con-

centrations of 0, 5,10, 20, and 50 mg L ' in synthesized canal water(CW) (0.05 dS m ') and well water (WW) (0.7 dS m ') through arainfall simulator. This first cycle was followed after drying by twocycles with untreated water. In some cases, a fourth cycle was con-ducted with the same polymer concentration as the first cycle. Theinfiltration rate (IR) was measured in each case. With one exception,the IR increased as the polymer concentration increased for all soilsand both waters. The IR was higher with WW than with CW in allcases. The IR decreased successively from the first through the thirdcycle due to dispersion at the soil surface caused by drop impactand chemical effects of the applied water. Although the IR increasedduring the first cycle with increasing polymer concentration, thehighest effects were obtained at 5 and 10 mg L ' concentrationswith diminishing effects at higher concentrations. In most cases,benefits of the higher concentrations were observed during the twocycles when polymers were not added to the water. The IR duringthe fourth cycle with polymer addition was higher than for the sec-ond and third cycle but lower than for the first cycle. The clay con-centration in the effluent from CW did not change with polymeraddition but the IR changed significantly. In addition, the clay con-centration in the effluent from WW was very much lower than forCW. The IR values in the present study were correlated with theresults of a flocculation test at 5% significance. Therefore, the floc-culation test appears to be a useful, quick technique for determiningthe relative effectiveness of water quality and polymer addition onIR.

Additional Index Words: dispersion, water quality, soil structure,crust formation, soil conditioners, guar product CP-14.

THE INFILTRATION RATE (IR) of water into soil canbe greatly influenced by soil factors such as tex-

ture, structure, and degree of compaction. In addition,water quality, especially its salt concentration and itssodium level, can influence IR (Ayers and Westcot,1985).

At low sodium adsorption ratio (SAR) and low so-lution electrolyte concentration, dispersion and claymigration into pores caused reduction in the hydraulicconductivity of soils (Pupisky and Shainberg, 1979).In addition, the data of Frenkel et al. (1978) showedthat plugging of pores by dispersed clay particles is amajor cause of reduced hydraulic conductivity for sur-face soils irrigated with waters of SAR 10 to 30 andsalt concentration 0 to 10 mol m-3.

Oster and Schroer (1979) concluded that irrigationwater composition has a greater influence on IR thandoes the chemistry of the soil itself for the conditionsof their study. Their results suggested that total cationconcentration of the irrigation water is a more im-portant parameter for the prediction of IR than is SAR.Evaluating water quality for irrigation, Shainberg andLetey (1984) stated that the infiltration process is sen-Department of Soil and Environmental Sciences, Univ. of Califor-nia, Riverside, CA 92521. Research was supported by the Univ. ofCalifornia Kearney Foundation of Soil Science. Received 22 May1987. "Corresponding author.

Published in Soil Sci. Soc. Am. J. 52:247-250 (1988).

sitive to sodicity and electrolyte concentration andsuggested that the effect of water quality on the IR isan important consideration.

Soil crusts of low permeability can result from theslaking and dispersion of unstable aggregates and canreduce IR. The major effect of raindrops striking a soilsurface is from physical impact, which creates a crustand restricts water infiltration. Agassi et al. (1981)studied the effect of electrolyte concentration and soilsodicity on IR and crust formation of two loamy soilsusing a rainfall simulator. They reported that IR wasmore sensitive than hydraulic conductivity to the elec-trolyte concentration of applied water. They stated thatin soils with low exchangeable sodium percentage(ESP) <5, an increase in electrolyte concentration to0.5 dS m~' (5 meq L~') reduced soil dispersion sharply.In soils with moderate to high ESP, a gradual changein IR occured as the electrical conductivity (EC) ofthe applied water was increased from 0.1 to 5.6 dSm-'.

Synthetic chemical polymers improve soil physicalproperties (Gardner, 1972). In recent years new chem-icals with very large molecular weight have been man-ufactured. As the molecular weight increases, theamount required to achieve a comparable extent ofaggregation decreases (Ueda and Harada, 1968; Carrand Greenland, 1972).

The effect at low concentration (10 mg L~') of 10synthetic chemicals with varying chemical propertieson flocculation of three California soils was reportedby Helalia and Letey (1988). In addition, the cationicderivatized guar product CP-14 was tested with twowaters synthesized to represent canal and well watersof the San Joaquin Valley of California. All productswere found to be effective in enhancing soil floccula-tion (Helalia and Letey, 1988).

Terry and Nelson (1986) reported that the IRs ofpolyacrylamide (PAM) and sprinkle-irrigated treat-ments were approximately twice those of flood-irri-gated controls. In another study, Cook and Nelson(1986) found that PAM solutions applied to the sur-face of properly prepared seedbeds significantly re-duced aggregate breakdown and soil crust formation,thereby maintaining good infiltration and aerationcharacteristics.

The purpose of this research was (i) to test the ef-fectiveness of CP-14 on infiltration rate with two watersusing a rainfall simulator in a laboratory experiment,and (ii) to determine if the results of flocculation tests(Helalia and Letey, 1988) could be used to infer poly-mer effects on IR.

MATERIALS AND METHODSThe three soils used were Arlington, Pachappa, and Fall-

brook. Classification, organic C content, and clay mineral-ogy of the soils are given in Helalia and Letey (1988). The<2-mm fraction of air-dried soil was packed into trays andplaced in the rainfall simulator. Each tray was 20 by 12 cmand had two openings; one at the bottom below the perfo-rated base to collect the effluent water and the second 2.2

247

248 SOIL SCI. SOC. AM. J., VOL. 52, 1988

cm above the perforated base and at soil level to collectrunoff. The trays were designed with a 5% slope to preventaccumulation of water on the soil surface. Two sheets offiberglass were placed on the perforated base to retain thesoil. Soils were packed in the trays at bulk densities of 1.47,1.55, and 1.49 Mg m-1 for Arlington, Pachappa, and Fall-brook, respectively. Each treatment was run in duplicate.

The rainfall simulator (Fig. 1) was designed by Kleijn etal. (1979). It had a disk at the top rotating with a speed of7 rpm. There were 10 slots in the disk, each containing aunit holding two hypodermic needles. This unit slid in andout as the disk rotated to prevent drops from continuouslyhitting the same spot.

The solution was applied under controlled air pressurethrough the hypodermic needles. The rainfall simulator hadan intensity of 23 mm h ', as determined by measuring thevolume of water collected during a known time-interval foreach tray. The positions of the trays inside the simulatorwere adjusted until each received equal intensity. These po-sitions were marked and uniformity of application was pe-riodically checked.

The drop diameter was determined in triplicate by col-lecting 100 drops in a weighing bottle under four differentpressures. From the average drop weight, the drop diameter(d) was calculated assuming a spherical drop and unit waterdensity. A standard curve was constructed by plotting thecalculated drop diameters against the applied pressures (P).The working pressure was a little higher than the highestpressure, which produced drops at a rate that could becounted. Therefore, the mean drop diameter was deter-mined by extrapolation, using the standard curve equation(d = 1.56 + 6.03 P, f- = 0.92), to be 3.5 mm. The dropfinal velocity, V, was determined from V = (2gh)l/2 (Bueche,1977) (where h is the distance between the needle and soilsurface and g is the gravitational constant) to be 4.0 m s~'.The kinetic energy was 185.;' m~2 h~ ' .

The trays were placed in the simulator and the desiredsolution was applied until steady-state flow through the soillayer was reached. Two waters were synthesized to representcanal and well waters from San Joaquin Valley California.

82cm

The canal water (CW) has a very low salt concentration (EC= 0.05 dS m •') and the well water (WW) has moderatesalinity (EC = 0.7 dS m - ' ) . Both have very low SAR values.The chemical composition of the two waters are reported inHelalia and Letey (1988). The polymer concentrations of thesolutions used for the first cycles were 0, 5, 10, 20, and 50mg L '. After attaining steady state the trays were removedand dried at 45 °C for 48 h between rainfall cycles.

The second and third irrigation cycles provided no poly-mer addition. In some cases a fourth cycle was done withsolutions containing the same polymer concentration as thefirst cycle. The volume of effluent was recorded for each trayin 15-min intervals until steady-state flow occurred. Opticaltransmittance (T °/o) and EC measurements were made onall effluent solutions collected.

RESULTS AND DISCUSSIONA summary of the steady-state IR values for all

treatments and soils during the first cycle is given inFig. 2. Untreated IR values were in the order of: Pa-chappa > Fallbrook > Arlington for both waters. TheIR increased as the polymer concentration increasedfor all soils and both waters, except for the 50 mg L~'treatment in CW and Fallbrook soil. The IR was higherfor WW than CW for all treatments and all soils. TheIR differences between the canal and well waters with-out polymer addition for the three soils are not great(between 10 and 20%); nevertheless, the results illus-trate that differences in IR can result from differencesin electrolyte concentration in waters of very low SAR.

The IR decreased in going from the first to the thirdcycle with both waters and the three soils. That resultis comparable to those of Ben-Hur et al. (1985), wheredrying of the soil after the first rainfall event produceda decrease in the steady-state soil permeability duringthe second rainfall event. These authors observed for-mation of cracks and new structure at the soil surface,whereas we did not. That result can be due to mont-morillonite content in the soil used by Ben Hur et al.(1985), whereas our soils were dominated by nonex-pandable minerals (Helalia and Letey, 1988). There-fore, the lower IR values at the second and third cyclein our work can be explained as successive dispersionat the soil surface not remedied by drying. Dispersion

-̂ 20

EE

|S 15

I10

5

rt

S CW • WW

RRL. Pfl

1v-B

1 ^

\1 ^ ^

I

B| |

11 |^ ^ ^^ ^ ^^ ^ s\J S \

>S s ^-

CH. FflLL.v

1 iii il

^ f

Fig. 1. Schematic design for rainfall simulator.

0 5 102050 0 5 102050 05 102050CP-14 CONCENTRHTION mg.L^

Fig. 2. The effect of CP-14 at different concentrations in two waterson infiltration rate of three soils.

HELALIA & LETEY: CATIONIC POLYMER EFFECTS ON INFILTRATION RATES 249

Table 1. Steady-state infiltration rate and clay content with canal water.

Cycleno.

123123

12312341234

CP-14cone.

mg 1~'0.00.00.05.00.00.0

10.00.00.0

20.00.00.0

20.050.00.00.0

50.0

IR

mm h-5.04.13.67.34.33.38.04.84.29.86.04.18.3

10.86.74.89.3

Arlington

RIRt

%

14610592

160117117196146114

216163133

Effluentclay

gL-0.970.880.880.950.920.890.930.900.920.940.920.81

0.890.770.74

IR

mm h"1

9.18.88.3

12.511.89.3

14.111.210.2

15.012.810.813.615.815.012.213.4

Pachappa

RIRT

%

137134112155127123165145130

174170147

Effluentclay

gL-0.970.940.920.880.890.860.790.820.820.860.840.79

0.670.620.82

IR

mm h"1

7.26.35.9

12.29.45.5

14.88.66.6

15.97.65.6

14.814.512.09.8

11.8

Fallbrook

RIRt

%

16914993

206137112

22112195

201190166

Effluentclay

gL-0.990.930.930.910.780.790.890.850.730.890.770.74

0.790.630.61

t RIR = the infiltration rate of each cycle relative to its respective cycle without polymer addition.

was caused by the beating force of drops on the soilsurface and chemical effects of the water applied.

Steady-state IR values for each cycle, soil, and treat-ment are presented in Tables 1 and 2 for CW andWW, respectively. The infiltration rates of each cyclerelative to its respective cycle without polymer addi-tion (RIR) are also presented in these tables. The poly-mer was more effective in increasing IR of WW thanof CW on the Arlington soil. For the other two soils,the effectiveness of CP-14 was about the same for bothwaters. This result was consistent with the results offlocculation tests using CP-14, where differences inflocculation in the two waters were greater for Arling-ton than for the other two soils (Helalia and Letey,1988). Although RIR for the first cycle consistently(with one exception) increased as polymer concentra-

tion increased, the greatest effects were observed at 5and 10 mg L~' treatments with diminishing effects athigher concentrations.

In most cases, the effectiveness of the initial poly-mer treatment was carried over to the. next two cycleswhen polymer was not added to the water. The effec-tiveness, however, tended to decrease with successivecycles. An important question arises. Does a fourthcycle with the same polymer concentration as the firstcycle restore the IR reduced during the second andthird cycles? In every case, the IR of the fourth cyclewas higher than that of the third cycle and in almostevery case was higher than that of the second cycle.However, IR of the fourth cycle was lower than thatof the first cycle. These results are significant becausethey indicate that IR rates can be restored at least

Table 2. Steady-state infiltration rate and clay content with well water.

Arlington Pachappa Fallbrook

Cycleno.

12312341234

12341234

CP-14cone.mg 1-'

0.00.00.05.00.00.05.0

10.00.00.0

10.020.00.00.0

20.050.00.00.0

50.0

IR

mm h"'6.05.04.7

11.46.15.86.8

14.87.26.9

11.115.27.06.7

12.2

15.67.66.7

12.2

RIRt

%

190122123

247144147

253140143

260152143

Effluentclay

gL-0.100.080.090.120.150.11

0.050.120.06

0.090.120.11

0.090.100.16

IR

mm h-10.39.88.8

15.611.311.413.317.013.613.416.717.313.711.312.318.011.89.8

12.2

RIRt%

151115130

165137153

168140128

175120111

Effluentclay

gL-0.0040.0060.010.060.060.03

0.030.020.02

0.030.050.04

0.030.060.06

IR

mm h"1

8.07.16.6

15.28.09.1

14.815.88.89.5

15.2

17.19.68.6

13.717.613.38.4

13.4

RIRt%

190113138

198124144

214135130

220187127

Effluentclay

gL-0.180.130.150.210.390.05

0.150.130.03

0.130.220.09

0.170.090.10

t RIR = the infiltration rate of each cycle relative to its respective cycle without polymer addition.

250 SOIL SCI. SOC. AM. J., VOL. 52, 1988

20

ic.

g|5

;io

o RRLIN6TON A PRCHflPPfl o FRLLBROOKr- O.7o r- O.87 r- O.BZ

0 10 20 30 40 50 BO 70 80 90 100TRflNSMITTRNCE PERCENT

Fig. 3. The correlation between the transmittance percent in testtubes and the infiltration rate of the three soils.

partially by CP-14 treatment without mechanical dis-ruption of the soil.

The T % was converted to clay concentration in theeffluent by using a standard curve previously reported(Helalia and Letey, 1988).

The amounts of clay and water collected in each 15-min interval were added for the total cycle period tocompute average clay concentrations in the effluent(Tables 1 and 2).

Clay concentrations in the effluent were very muchlower for WW compared to CW regardless of polymeraddition. The low clay concentration with WW sug-gests that very little dispersion occurred, or that if dis-persion did occur, the fine particles were retained inthe soil. If WW limited dispersion, then one wouldexpect much higher IR values for WW than for CW,where considerable dispersion and clay migration oc-curred. The effluent clay concentration from polymertreated waters were not greatly different from un-treated CW waters so that one would not expect largedifferences in IR between these treatments. The ob-served IR values were not consistent with expectedresults based on effluent clay concentrations, suggest-ing that clay migration was not a major determinantfor IR under these conditions. The results suggest thatwater quality and polymer treatment effects on thedegree of aggregate breakdown and status of the clayretained in the soil are the predominate factors af-fecting IR.

Under the experimental conditions, migrating claywas removed from the system and would not haverestricted water flow. It is possible that different resultswould have occurred for systems where translocatedclay would be trapped and clog soil pores. This aspectwas pursued in a subsequent investigation (Helalia etal., 1988).

The average effluent clay concentrations reported inTables 1 and 2 do not reflect the time sequence of claymigration. Generally the clay concentration was rel-atively constant with time without polymer treat-ments, whereas a trend was observed of reduced clayconcentration with effluent volume for the polymer-treated water.

The EC of the effluent from CW decreased with in-creasing effluent volume but was always higher thanthe EC of the CW. The average steady-state EC valueswere 0.14, 0.11, and 0.08 dS m~' for Arlington, Pa-chappa, and Fallbrook, respectively. These effluent ECvalues remained relatively constant during the suc-ceeding cycles. In the case of WW, the effluent EC wasabout the same for all soils and approximated the ECof the applied WW.

One objective of this study was to determine if re-sults from flocculation tests as conducted by Helaliaand Letey (1988) would apply to IR. The relationshipsbetween IR and T % in the flocculation tubes are pre-sented in Fig. 3 for each soil. Data from both CW andWW are plotted for a given soil. The correlation coef-ficients between IR and T% were 0.70, 0.87, and 0.92for Arlington, Pachappa, and Fallbrook, respectively.The flocculation test appears to be a useful, quick tech-nique for determining the relative effectiveness of waterquality and polymer addition on infiltration rates forT % between 0 and 80 but not for T % greater thanapproximately 80.