RUNOFF CHEMISTRY: AN UNDEVELOPED BRANCH OF SOIL SCIENCE1

HELMUT KdHNKE2

RUNOFF water carries away great amounts ofsuspended and dissolved materials from the

land, but to date most investigators have concernedthemselves only with the solid components of runoffand generally have neglected the less spectacular dis-solved matter. This, however, deserves the close at-tention of the soil man as runoff chemistry promisesto be a valuable aid in both pure and appliedpedology.

i LITERATURE REVIEW

Duley (7),3 Duley and Miller (8), Miller (16), and Mil-ler and Krusekopf (17) studied the amounts of variouschemicals present in runoff from erosion experiment plotsand found that some plant nutrients were lost in greateramounts in this form than through cropping. They usedsamples of the runoff water collected in tanks. The samplesfor several storms were composited for analysis.

Middleton, Slater, and Byers (15) studied the erodedmaterial from the runoff plots of the Erosion ExperimentStations. Where erosion was severe, as in the case of culti-vated crops, the composition of the eroded material wassimilar to the whole soil. Where erosion was slight, theeroded material carried a much larger percentage of or-ganic matter and of fine mineral particles. They concludedthat "slight erosion may be relatively more detrimental tofertility than more severe erosion".

Daniel, Elwell, and Harper (6) showed that the runoffwater from small plots contained very little nitrate nitrogen.The higher the infiltration capacity of the soil and the shorterthe plot, the lower was the amount of nitrate nitrogenwashed off.

Scarseth and Chandler (20) determined the phosphatecontent of plots that had been fertilized for 26 years andfound that 60% of the superphosphate and 82% of the rockphosphate could not be accounted for as residue in the soilor as component of the crops harvested and hence must havebeen washed off.

Kohnke and Dreibelbis (12), in surveying methods ofmeasuring soil erosion, point out that "the most seriousimmediate effect of soil erosion is the loss of available plantnutrients. While the removal of gravel, sand, and silt maygreatly reduce the very substance of the soil over a periodof years, it is the washing away of organic and inorganiccolloidal matter, and of plant nutrients in solution thatrapidly decreases soil fertility."

Wilson and Schubert (23) found that "for each gram ofsolid material there were many more microorganisms in the

runoff than there were in the soil from which the runoffcame". They attribute this to the fact that the finer soilparticles and the organic material are washed off in a muchgreater proportion than coarser soil particles.

Rogers (19) determined the loss of phosphate and lime inrunoff from pasture plots that received artificial rainfall.The loss of phosphate due to the first rainfall was 13% ofthe freshly applied fertilizer, while later rains were unableto remove much phosphate as it had formed insoluble com-pounds and as the amount of erosion was small. The rateof removal of lime was more nearly uniform for the vari-ous rain applications.

Information concerning the concentration of chemicals inrunoff water from small natural watersheds was not foundin the literature, while it abounds with papers concerningthe chemical composition of river waters. A very interestingpaper on this subject is a summary by Clarke (4), con-taining many tables. These show definite seasonal variationsof several ions and also the influence of runoff rate on com-position. Breazeale (3) has made a study of the chemicalconstituents of the Colorado River water and their effecton the suspended solids. He found that when the calciumcontent was highest, the river carried very little colloid andwhen calcium was lowest, the river carried a heavy load.The total salt content was about seven times as high at lowwater in February and March as it was during high floodsin May or June.

Information on the chemical composition of river wateris of value in the study of runoff chemistry, as knowledge onthe geographical distribution of the various types of watershas been compiled, seasonal variations have been found, andmethods of analysis and presentation of the data have beenworked put. But owing to the large size of the watersheds ofrivers, the waters of the various tributaries become blendedso that it is difficult to distinguish the effect of the "variouscauses upon the composition.

Possibly of greater value in the study of runoff • chemis-try are the findings of the numerous lysimeter investigations,as percolate water contributes to a great extent to runoff.A review of the literature on lysimeter research was re-cently prepared by Kohnke, Dreibelbis, and Davison (13).

As all runoff water originates from rain or other typesof precipitation, chemical composition of rain water shouldbe studied in connection with runoff chemistry investiga-tions. Some compounds are found in rain water in suchamounts as to account for a considerable part of their oc-currence in surface runoff water (9, n, 18, 22). <

EXPERIMENTAL, The work presented in this paper represents the

Report on the Indiana Agricultural Hydrologic Studies Project, Lafayette, Ind., Soil Conservation Service and PurdueAgricultural Experiment Station, cooperating.2Associate Soil Scientist. The author wishes to express his appreciation for the assistance received in collecting and analyzingrunoff samples and in the calculation of the hydrologic data from E. N. Roth, Student Aide; D. D. Pittman, Assistant Agri-cultural Aide; R. B. Hickok, Project Supervisor; and F. E. Fleming, Assistant Engineering Aide, all of the Soil ConservationService, Lafayette, Ind.3Figures in parenthesis refer to "Literature Cited", p. 500.

492

KOHNKE : RUNOFF CHEMISTRY 493

initial stage of an investigation carried out by theSoil Conservation Service in cooperation with thePurdue Agricultural Experiment Station at Lafay-ette, Ind., on the effect of land use and treatment onthe losses of plant nutrients and of soil colloidsthrough runoff. The purpose of this work was todetermine in what way runoff should be sampled andanalyzed to yield the most reliable information con-cerning the amounts of plant nutrients lost in runoffwater. While this question has been essentiallyanswered, it seems that the data obtained point to awider applicability of runoff chemistry. All samplesused in this study have been taken manually. Whilethe general discussions of runoff chemistry in thispaper are not limited geographically, it should beremembered that the research was carried out ex-clusively in northwestern Indiana in an area of graybrown podzolic soils developed on calcareous glacialtill.

Figs, i and 2 show the chemical" composition ofrunoff water from a small pastured woodland water-shed (watershed No. 33, Indiana Agricultural Hy-drologic Studies, Size: 3.5 acres) on June 28, 1941.All ions for which determinations have been majleshow a somewhat higher concentration in the begin-ning of the runoff than while the runoff is at its peak

rate. In the case of calcium and bicarbonate there isa gradual rise in concentration as the rate of runoffdecreases. Chloride decreases steadily until it ispresent only in traces. Sulfate, nitrate, and potassiumshow a very uniform concentration from the peak tothe end of the runoff. The increase of calcium andbicarbonate seems to be due to a proportional in-crease of seepage water in the runoff.

As runoff from small watersheds (the experimen-tal watersheds of the Indiana Agricultural Hydrolo-gic Studies range from 1.5 to 3.9 acres) continuesonly for a few hours, making it impossible to studythe gradual change of composition of runoff as itchanges from surface runoff to ground water flow,samples were taken for reconnaissance studies fromthe main tile of a 32-acre watershed on the PurdueUniversity dairy farm. This is an old tile in poor re-pair which follows the main channel of the water-shed, and surface water enters it at several pointswhenever runoff occurs. After surface runoff hasceased, percolate water continues to flow for a con-siderable period of time. The lower two-thirds of this

, watershed are in bluegrass pasture, while the upperthird is in corn and farmstead lots. The compositionof the water issuing from the tile during and afterthe rain of August 24, 1941, is shown in Figs. 3 and 4.

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.5

PIG. i.—Anions in runoff from pastured woodland, June 28, 1941.

494 SOIL SCIENCE SOCIETY PROCEEDINGS 1941

3-Q°/V^

600/6s //fere //>c/>300

FIG. 2.—Cations in runoff from

These data afford an opportunity to study thegradual change from surface runoff to percolate.Throughout the duration of the runoff, calcium andbicarbonate are the predominant ions and they arethe ones showing the greatest changes in concentra-tion. The most striking fact shown in the data ofthe runoff from the tile is the steady change of con-centration of the bicarbonate. A comparison withthe runoff rate curve shows that the bicarbonateconcentration is lowest near the runoff peak andincreases gradually toward the end of the runoff. Adecrease in amount of total solids occurs simul-taneously with the increase in bicarbonate content.It should be noted, however, that the decline of theconcentration of total solids is more abrupt. It ap-pears rather likely to assume that the content ofbicarbonate is an expression of the relative amountof ground water in the runoff and of the time whichthis water has had to become charged with this ion.The relatively high concentration of bicarbonate inthe early part of the runoff may be explained by thefact that part of the loosely held soil water is re-placed by rain and thus contributes to the concen-tration of the ions in the runoff.

The calcium concentrations are similar in trendand amounts to the bicarbonate concentrations, how-

pastured woodland, June 28, 1941.ever, saturation is evidently reached sooner. Calci-um in runoff water is present in two forms, vis., assoluble salt, mostly bicarbonate, and in the colloidalexchange complex. Even during periods of high run-off velocities, when the soil load is large, the "ex-changeable" calcium represents only a small portionof the total calcium, while later on, when only clearwater percolates, the calcium is present exclusivelyas soluble salt. This explains why the concentrationof calcium in surface water was always found to beas high or higher than the bicarbonate content, whilein percolating water this relationship is reversed, aspart of the bicarbonate occurs as magnesium bi-carbonate.

The amount of potassium found in runoff wateris relatively so large that exchangeable potassiumcan account for only a minor portion of it, hence byfar the greater part must have been present as solublesalt. The rather uniform concentration of potassiumthroughout the periods of runoff, in spite of greatvariations of the amounts of total solids present,points to that same conclusion. It is planned in thefuture to analyze exchangeable and dissolved cationsseparately in order to establish their relative oc-currence in runoff.

Only a few determinations of magnesium and

KOHNKE: RUNOFF CHEMISTRY

FIG. 3.—Anions in runoff from tile open to surface water, August 24, 1941.

sodium in runoff water were carried out, hence noconclusions can be drawn on their concentrations.

Sulfate, nitrate, and chloride performed some-what similarly to the bicarbonate. Their lowest con-centrations occur near the runoff peak. However,they reach their maximum concentrations very muchfaster than the bicarbonate. As soon as surface run-off ceased, the concentration of these three ions didnot vary very much.

Only a limited number of determinations of thehydrogen ion concentration of runoff water was car-ried out. They showed that the pH of surface runoffwater was slightly higher that the pH of the surfacesoil of the watershed. Percolating water had about thepH of the calcareous parent material. A gradual in-crease in pH of runoff water from 6.5 to 7.0 wasnoticed from the beginning to the end of a 45-min-ute runoff from a pastured woodland watershed.

Data obtained in electrometric neutralization titra-tions of surface runoff water and of tile drainagewater are presented in Fig. 5. They show for the per-

colate a much higher content of weak acid radicles.This has been found to consist almost exclusively ofbicarbonate.

PHENOMENA RESPONSIBLE FOR BRINGINGABOUT VARIOUS CONCENTRATIONS OF

CHEMICALS IN RUNOFF WATERIf a hard beating rain hits an unprotected field,

the resulting runoff consists essentially of a suspen-sion of soil in rain water. As rain water usuallycarries only a low concentration of chemicals, sucha type of runoff will be little different from a suspen-sion of the soil in distilled water. Normally, however,the materials carried in surface runoff water arepredominantly the finer and more valuable particlesof the original soil. These carry with them microor-ganisms, exchangeable cations, and absorbed phos-phorus. If runoff occurs quickly after the start of arain, soluble salts accumulated in the surface duringa dry period may be washed off before infiltrationis able to carry them info the body of the soil.

496 SOIL SCIENCE SOCIETY PROCEEDINGS 1941

600

FIG. 4.—Cations in runoff from tile open to surface water, August 24, 1941.

Rain water that enters the soil becomes chargedwith chemical ingredients. Part of this water isforced out of the ground by additional rain water.This replacement of soil water may occur in variousforms. It may pass through the complete profile andleave the soil as "percolate" or it may reach a rela-tively impermeable soil horizon and travel laterallydown the slope as subsurface seepage until it reachesthe surface.

These considerations show that three componentsmake up runoff water, vis., surface runoff, lateralseepage or subsurface runoff, and percolate. Lateralseepage and percolate may be grouped togetheras ground water flow. Whether and in what relativeproportions these components are present in a givenrunoff water depends upon type and time of rainfall,soil conditions and vegetative cover, and size, con-figuration, and topography of the watershed. Obvi-ously no sharp boundary exists between surface run-off and subsurface seepage and between subsurfaceseepage and percolate.

Typically surface runoff water, assuming erosionto occur, is high in solid soil particles, especially clayand organic matter, high in total nitrogen, high inabsorbed phosphorus, but low in soluble salts.

Percolation water contains a relatively high con-centration of soluble salts, but little or no organicmatter, phosphorus, and colloids.

O / Z . 3 4 5 6 7 3 9 / OAc/a' /Watec? fc / //r or" ' Mm-oW

FIG. 5. — Electrometric titration of runoff water, w-Ln HZSO4.10

KOHNKE : RUNOFF CHEMISTRY 497

Subsurface seepage may be high both in solublesalts and colloids, especially organic colloids, if theconditions are favorable, but the data obtained areinsufficient to warrant any generalizations.

APPLICATION OF RUNOFF CHEMISTRY IN SOILSCIENCE AND HYDROLOGY

FERTILITY

A knowledge of the concentration of chemicalcomponents of runoff water is useful for severalpurposes. The primary and most important one isthe maintenance of soil fertility.. In the balance sheetof plant nutrients in the United States compiled byLipman and Conybeare (14) which summarizes datafrom all cropped land in the United States, it ispointed out that phosphorus, potassium, calcium,and sulfur are lost to a greater extent through erosionthan in the harvested crops. As the data of the pres-ent study do not represent the total annual runofffrom any given watershed, it is impossible to calcu-late the annual losses of plant nutrients throughrunoff. In order, however, to convey a picture of themagnitude of these losses under the conditions ofthis study, the maximum and minimum concentra-tions of several ions in surface runoff water sampled

during the summer of 1941 near Lafayette, Ind.,have been calculated in terms of pounds per acre-inch and are shown in Fig. 6.

The data represent runoff from woods, bluegrasspasture, meadow, and corn land. In comparing theseamounts with those normally applied as fertilizers, ithas to be kept in mind that the annual amount ofsurface runoff varies from a fraction of an acre-inchto 10 or 15 acre-inches, depending on weather, crop,and cultural practices. It is interesting to note thatthe relative variation of the concentration of the ex-changeable metallic cations is distinctly smaller thanthat of the anions forming soluble salts. The leastspread in data, however, was found in the case ofphosphate. Due to the great insolubility of mostphosphates occurring in the soil, it is evident thatthe phosphate in runoff is present mostly as adsorbedto soil colloids and possibly as solid calcium or ironphosphates that are carried bodily as colloids in thewater. The high concentrations of phosphate insurface runoff water reveal a cause of soil depletionthat is generally not yet recognized, especially aspercolate water is known to contain only traces ofphosphate.

Only a few quantitative determinations of organic

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a7

6tes/Ac.5

4

3

2

nFIG. 6.—Maximum and minimum concentration of ions in surface runoff

of 1941.

C/ M>3from small watersheds, Lafayette, Ind., summer

498 SOIL SCIENCE SOCIETY PROCEEDINGS 1941

matter in runoff water were carried out. These andobservations of the water have shown that the ma-jority of the samples contain soluble organic matter.This is particularly noticeable in water from the tile,which remains yellow for about half a day after thelast evidence of surface runoff has disappeared. Thepercolate becomes colorless only after calcium andsulfate are present in their maximum amounts, indi-cating the flocculating influence of these ions. Organ-ic colloids are the most active part of organic matter,and one should attempt to retain them in the soil.It might be best to plow under lime or gypsum sothat the organic colloids be flocculated at a depthwhere soil moisture does not vary as much as in thesurface and where losses due to oxidation and erosionwill not be as rapid. In this way the organic matterwould increase the water-holding capacity and thefertility of a soil layer, that ordinarily is the zoneof greatest root accumulation. Extensive researchon these problems of soil fertility is definitely needed.

As the various crops have specific influences uponthe rate of runoff and the. amount of erosion, theyundoubtedly also have specific influences upon thechemical composition of runoff water. The same maybe said about tilling and fertilizing practices. Athorough realization of the factors involved in theremoval of nutrient elements will undoubtedly helpin finding the method and season best suited to applymanure and commercial fertilizers with the minimumloss by runoff.

GENESISIt is a rather general assumption of soil men that

the weathering products of the surface soil arewashed out and carried into the underlying strata.In this way A and B horizons originate. Withoutquestion this is true, but it is evidently only part ofthe picture, as frequently a part of these products ofsoil decomposition are leached out laterally throughsubsurface flow, as described above. The relativeamount of vertical and lateral leaching depends uponclimatic conditions, slope, and parent material.Wherever subsurface flow carries off all or nearlyall of the weathering products formed, an AC soilprofile results.

HYDROLOGY

One of the problems of the study of rainfall runoffrelationships, is to establish the amount of groundwater flow or "base flow" of a stream during stormperiods. In other words, it is desired to determine towhat proportion the stream water is made up ofsurface runoff and of seepage water. This is a rath-

er difficult task, as it is impossible merely to extrapo-late the ground water flow data previous to the storm.The ground water flow increases greatly duringrainfall of longer duration. The data of this studymake it seem possible to determine the proportion ofseepage water in a stream by means of chemicalanalysis. The ion best suited for this purpose is bi-carbonate. Its concentration generally increases withthe proportion of ground water. It is present ingreater quantities than all other ions. Its determina-tion is very simple and can even be carried out ac-curately in the field. Under the conditions studied itseems that water containing two or more M.E. ofbicarbonate per liter is made up completely ofground water. It should not be deducted from thisstatement that correspondingly smaller amounts willcarry proportionally smaller percentages of groundwater. This could not be the case, since even rainwater contains measurable amounts of bicarbonate(0.05 to 0.2 M.E. per liter) and as even the surfaceof the soil will impart some bicarbonate to the runoffwater. The determination of bicarbonate concentra-tion may also be a simple and effective way of com-paring the relative erosiveness of runoff water fromwatersheds in different land use or under differentcultural treatments.

It must be pointed out that a high content of bi-carbonate in ground water exists only in an area ofcalcareous parent material, as is the case at Lafayette,Irid. In noncalcareous watersheds another ion mightbe used for this purpose, probably sulfate.

METHODOLOGY

The methods used in studying the chemical com-position of runoff water must depend on the purposeof the investigation. This refers both to the samplingand to the analyses. If the object of the work is thedetermination of the losses of plant nutrients, therunoff should be sampled at the outlet of a relative-ly small watershed of a single type of land use. Onemajor handicap in the study of the composition ofrunoff water from small watersheds is the difficultyof sampling. Where the samples are to be takenmanually, many trips to the field have to "be madebefore a series of useful samples can be obtained, asit is impossible to predict the occurrence of runoff-producing rains. In case of larger watersheds thetime of concentration of runoff is longer and it iseasier to reach the watershed on time.

If a satisfactory aliquote sampler is available, acompounded sample can be used for each storm,thus greatly reducing the number of analyses needed.

KOHNKE: RUNOFF CHEMISTRY 499

An aliquote divisor runoff sampler is now under in-vestigation at the Indiana Agricultural HydrologicStudies Project at Lafayette, Ind. Otherwise, sam-ples have to be taken manually. The intervals be-tween sampling should be the shortest near the timeof peak runoff, as here the changes of concentrationsare the greatest. The magnitude of the time intervaldepends on the type of storm, the characteristics ofthe watershed, and the land use. As it is practicallyimpossible to mix the samples in proportion to theamount of runoff represented, it will be necessary toanalyze the samples individually.

, For fertility studies it may be best to attempt todetermine the amount of "available" plant nutrients.This has been attempted in the present study by thefollowing technic. The samples were treated with i %concentrated HC1 and allowed to digest on thesteam bath. The HC1 dissolves any solid carbonate,it replaces the cations, it brings part of the phos-phorus into solution, and it flocculates the colloids.After filtering off the solids, the filtrate is used forthe determination of calcium, magnesium, potassium,sodium, phosphate, and sulfate. For the determina-tion of chloride the colloid is coagulated with alumand separated from the solution with the centrifugeand decantation. Organic matter has been determinedby the wet oxidation technic on a few samples ofrunoff water. Ammonia and nitrate are determinedas the total amounts present. These and total nitro-gen were determined by the Kjehldahl method. Thetotal bicarbonate is determined by titrating the run-off against standard acid in the presence of "modi-fied methyl orange" (10). If an aliquote.of 100 ml isused and if the acid (either HC1 or H2SO4) is i/ionormal, the number of milliliters of acid used cor-responds directly to the M.E. of bicarbonate per literof runoff water. This titration can easily be carriedout in the field. The accuracy of this determinationhas been checked against the CO2 absorption methodand it was found to be very reliable. The value of asimple analysis of bicarbonate lies in the fact that theknowledge of the concentration of bicarbonate at thetime of the runoff may help in selecting the propertime intervals between samples.

In the study of soil development the determina-tion of the composition of surface runoff is merelyone phase of the research needed. It seems advisableto sample surface water, laterally flowing water atvarious depths of the profile, and deep percolate.The analyses should include silica and sesquioxides,besides those used in fertility investigations. For thedetermination of the proportion of surface water in

runoff the analysis for one ion will generally be.suffi-cient.

Several publications (i, 2., 5, 21) are availableas aids in analyzing runoff water. It is necessary tostate whether only dissolved chemicals or dissolvedand exchangeable ions were determined or whethera total analysis was carried out. Data obtained inthe study of runoff chemistry should be reported asconcentrations and if possible also in annual amountsper unit area. It is immaterial whether the concentra-tions are expressed as parts per million, M.E. perliter, or pounds per acre inch. However, if a stand-ard were adopted, it would simplify a comparison ofthe results of different investigators.

DISCUSSIONIt is the purpose of this paper to point out the

problems and possibilities of runoff chemistry. Thehigh concentrations of the essential plant nutrients inrunoff water show the necessity of reducing theselosses if soil fertility is to be maintained. A study ofthe effect of the various crops and agricultural prac-tices upon the chemical composition of runoff waterand of the seasonal trends is required if we are todeal with this problem intelligently.

No panacea exists to prevent the losses of plantnutrients in runoff. Any method, however, tendingto decrease the amount of surface runoff will prob-ably also decrease the amount of valuable chemicalslost, as total nitrogen, organic matter, and phos-phorus are high in surface runoff. A reduction of theamount of surface runoff tends to increase percola-tion, but with the exception of nitrate the chemicalsabundant in percolate are of little commercial value.

Other problems that present themselves to thestudent of runoff chemistry are as follows:

To what extent will the surface tension of runoffwater be decreased by the organic matter dissolvedin it and how will this change affect the capillarityif such water enters the soil ?

Is part of the beneficial effect of an organic mulchdue to the fact that organic substances are washedinto the ground and that they help retain moistureand nutrients?

The chief problem for the conservationist, how-ever, is, What can be done to decrease the loss ofvaluable nutrients in runoff from the soil?

SUMMARYThe chemical composition of runoff water has in

the past received only scant attention. The stress ofrunoff analyses has been mainly on the amount and

5°° SOIL SCIENCE SOCIETY PROCEEDINGS IQ4I

the condition of the eroded soil present in the water.It is pointed out that some ions may be lost in greateramounts from the soil in surface runoff than in thecrops harvested. The concentration of chemicals inrunoff vary greatly from storm to storm and fromthe beginning to the end of runoff for a given storm.An ion of prime importance in runoff water is bi-carbonate. Under the conditions studied it occurredin the greatest concentration of the anions and onlycalcium outranked it occasionally. The concentrationof bicarbonate was found to be associated .with theproportion of ground water in the runoff water. Asits analysis is very simple it may be possible to useit as a field method in estimating the amount of baseflow in a stream.

The studies carried out point to the fact that leach-ing of the A horizon of the soil does not occur ex-clusively in a vertical direction, but it also takesplace laterally removing soil constituents without acorresponding enrichment of the B horizon.

The need for further research on the effect of soilconditions, plant growth, and season upon the com-position and amount of runoff is pointed out.