YDROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN3RESTED WATERSHEDS OF THE SOUTHERN APPALACHIANS

Michael Anthony Velbel

Dept. of Geology and Geophysics, Yale University (now at:Dept. of Geological Sciences, Michigan State University,East Lansing, MI 48824-1115, U.S.A.)

BSTRACT

Two variables, parent rock type and flushing rate (the amount ofater flushed through the weathering profile per hectare per year),>ntrol the long-term average dissolved load of streams in forestedatersheds of southwestern North Carolina, U.S.A. The same variablescplain qualitative stability relations, as shown by stability fieldagrams which are, in turn, consistent with the hydrology and kaolinite-bbsite clay mineralogy of the profiles. Tardy's Re, a simplesmiquantitative mass-balance tool, ranges from 1.36 to 1.65, againjalitatively consistent with the known clay mineralogy of the systems,ic consistency of hydrology, aqueous geochemistry and clay mineralogyaces useful constraints on more sophisticated geochemical mass-ilance models.

ITRODUCTION

Most geochemical research on weathering has been oriented towardcplanations of the chemical composition of soil- and groundwaters that•ain areas of active rock-water interactions. Many ear'y studies:tempted to interpret the composition of groundwater (and resultingreamwater) in terms of chemical reactions between parent mineralsid weathering products in near-surface weathering environments. Theudies suggest or necessarily assume that soil- and groundwaters are injuilibrium with observed or inferred weathering products or withfpothetical metastable phases. However, a recent study undertaken inle Absaroka Mountains by Miller and Drever (23) has demonstrated thatream waters there are not in equilibrium with weathering products>und in the nearby soil. Miller and Drever suggest, after Paces (27,28),

231

r. Drever (ed.). The Chemistry of Weathering, 231-247.1985 by D. Reidel Publishing Company.

232 M. A. VELBEL

that these waters reflect deep incipient alteration of large volumes ofrock and that the composition of deep weathering waters are controllednot by equilibrium with weathering products, but by the relative rates ofdissolution, precipitation, and water movement through the deepweathering zone. This idea represents an innovative departure fromearlier work on chemical weathering, both in its emphasis on non-equilibrium (kinetic) factors, and in its emphasis on the deep sub-soilportion of the weathering profile.

Determining rates and mechanisms of mineral weathering in naturehas recently become a major goal of low temperature geochemistry. Itis generally agreed that geochemical mass-balance studies are the mostreliable approach to determining mineral weathering rates in nature (4).Mass-balance modeling, however, requires careful constraints on mineralreaction stoichiometries, and input and output fluxes.

The purpose of this report is to discuss the effect of lithology andhydrology on the nature and composition of dissolved output fluxes(effluxes) from forested watersheds of the southern Blue RidgeMountains of eastern North America. The major emphasis of this reportis on using hydrological, mineralogical and aqueous geochemicalinformation to constrain the stream output (efflux) terms in moregeneral geochemical mass-balance models of mineral weathering rates indeeply-weathered crystalline rocks (saprolites). Mineral weathering inthis area has been or will be discussed elsewhere as will the overallmass-balance model based on these accumulated studies (35-42).

THE STUDY AREA

Topography and Climate

The Coweeta Hydrologic Laboratory of the U.S. Forest Service islocated in the Nantahala Mountains 15 km (10 miles) southwest ofFranklin, North Carolina. The physiographic Coweeta Basin totals some1625 hectares, ranging in altitude from over 1585 meters (5200 feet) atits western limit to about 670 meters (2200 feet) in the valley ofCoweeta Creek in the east. Slopes within individual watersheds averageabout 45% (24 degrees). Average annual rainfall is among the highest ineastern North America, from 170 centimeters (80 inches) at lowerelevations to 250 centimeters (100 inches) on the upper slopes; thiscorresponds roughly to decreasing rainfall with distance east from thewestern boundary of the Laboratory. Less than 5% of the annualprecitation falls as snow. More important than precipitation, however, iswhat Duchaufour (8) calls "climatically controlled drainage," the amountof water actually flushed through the weathering profile, which is alwaysless than rainfall due to evapotranspiration. Annual climaticallycontrolled drainage at Coweeta ranges from 106 to 210 cm (10.6 to 21.0million liters per hectare per year). The mean annual temperature is12.8 degrees Celsius (55 degrees F); average maxima and minima are33°C (92°F) and -17°C (1°F), respectively (6,16,32).

DROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 233

drock Geology

Two major lithostratigraphic units occur in the study area. Thellulah Falls Formation (9) consists of metagraywackes, pelitic schists,i metavolcanic rocks, which were derived mainly from sedimentaryitoliths of low mineralogical maturity (e.g., graywackes). Theweeta Group (10,12) consists of biotite gneisses, metaarkoses,itasandstones, quartzites, and pelitic and biotite schists, which were•ived predominantly from sedimentary protoliths of intermediate to;h mineralogical maturity (e.g., arkoses, quartzarenites). Theiracteristic minerals of these two units include quartz, plagioclasedspar, biotite and muscovite micas, and almandine garnet, along withnor amounts of staurolite, kyanite and other "heavy" minerals (11,12).jm a practical point of view, the qualitative difference between theD main lithostratigraphic units must suffice; the heterogeneity of rock>es, combined with the extreme structural complexity of the area,,ke it almost impossible to estimate quantitatively the absolute orative abundance of the different minerals and rock types.

drology

"There is another world under this, and it is like ours ineverything - animals, plants, and people - save that theseasons are different. The streams that come down from themountains are the trails by which we reach this underworld,and the springs at their heads are the doorways by which weenter it, but to do this one must fast and go to water andhave one of the underground people for a guide. We knowthat the seasons in the underworld are different from ours,because the water in the springs is always warmer in winterand cooler in summer than the outer air."

Cherokee Origin Myth (25)

Base flow is apparently sustained by prolonged drainage of moistt unsaturated soil and saprolite (15,17). During the "hydrologic survey"Coweeta in the early 1930's, 28 wells were dug to bedrock (by hand) toDths of 1.5 to 11 meters (5 to 35 feet); average regolith depth atweeta is around 6.1 meters (20 feet) (6,32). When pumped dry, 21led to recover until heavy rains occurred. From this behaviour, it wasicluded that water in these wells reflected "cistern" storage, ratherin water levels in an areally extensive aquifer. The remaining seven11s were located near stream channels or in mountain flood plains, andiy be connected to local bodies of "groundwater." Hewlett (15)icluded that a saturated groundwater-table-like aquifer does not existsupply base flow to Coweeta streams.

In a simple and elegant experiment designed to test a hypotheticaljrce of base flow, Hewlett and Hibbert (15,17) constructed gianticrete troughs, which were filled with "subsoil" (C horizon soil),:ked to approximately its natural bulk density; moisture tensiometersd neutron-scattering probes were empiaced, as was a "spigot" at the

234 M. A. VELBEL

lower end. The experimental hillslopes were then saturated with waterto simulate intense precipitation, covered to prevent evaporation, andpermitted to drain under the influence of gravity. The results suggestedthat drainage coninued long after the pores became unsaturated, andthat the unsaturated soil zone could contribute sufficient water tosustain observed rates of base flow even after 60 days without recharge.Because recharge is usually much more frequent at Coweeta than in theexperiment, Hewlett concluded that prolonged drainage of unsaturatedpores is the primary source of base flow to Coweeta streams. There isno evidence that the mountain streams of Coweeta are fed by waterfrom the permanently-saturated water-table.

Storm flow at Coweeta is also dominated by drainage in theunsaturated zone. Hewlett and Hibbert (18) attribute the "flashy"response of Coweeta streams to storms as a result of "subsurfacetranslatory flow, or the rapid displacement of stored water by newrains." (p. 275). "Above the zone of saturation, we may regard suchmovement as due to thickening of the water films surrounding soilparticles and a resulting pulse in water flux as the saturated zone isapproached. The process under rainfall is varying everywhere in a mostcomplex way but such movement can be verified in an elementarymanner by allowing a soil column to drain to field capacity in thelaboratory and slowly adding a unit of water at the top. Some water willflow from the bottom almost immediately, but it will be apparent that itis not the same water added at the top." (p.279). Hewlett and Hibbert(18) cite experimental work using tritium-labelled water to support thisnotion. Horton and Hawkins (19) found that 87% of the water originallyheld in pores was "pushed" out of the soil by a plug of tritium-taggedwater before any tritium appeared in the effluent. High-runoff episodestherefore involve "the rapid displacement of stored water by new rain"rather than interflow or overland flow. Observations on hydrogen,oxygen and radon isotopes in other natural systems led Sklash andFarnvolden (30) to the same conclusion regarding the major contributionof subsurface water in high-runoff episodes. Winner (M) determined thatsimilar subsurface flow characteristics probably apply to much of theNorth Carolina Blue Ridge.

A large body of observation, experiment, and theory, based on workat Coweeta and elsewhere, suggests that both base flow and storm flowin streams draining deeply-weathered, saprolitic landscapes are sustainedby water from depth in the subsurface (saprolite), and that neitheroverland flow not interflow contributes directly (or significantly) to thestreams.

Clay Mineralogy

Clay minerals in soils and saprolites at the Coweeta area arekaolinitey gibbsite, hydrobiotite, and goethite (35,37,38,40, in prep.). Ingeneral, weathering profiles developed on the mineralogically immatureTallulah Falls Formation have a higher kaolinite/gibbsite ratio than thoseon Coweeta Group rocks. In addition to this lithologic influence on claymineralogy, there is also a climatic influence -watersheds in the wetter

PROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 235

stern part of the basin have a lower kaolinite/^ibbsite ratio than doer watersheds further east on similar rock types. Gibbsite isquitous in soil and shallow saprolite: gibbsite also occurs widely atith in the saprolite, especially in the more thoroughly flushed westerntersheds, but was not detected at depth in poorly fl-ushed watersheds, in prep.). Hydrology also exerts an influence: kaolihite increases inative abundance with depth in all profiles (see Figure 3). Incipientlyathered bedrock in outcrop and at depth contains expandable clays). Neither these nor the hydrobiotite weathering products of mica>w any systematic variability with rock type or flushing rate,37,38,40, in prep.)

UEOUS GEOCHEMISTRY

Thus far, this discussion has dealt primarily with evidence fromIrology and clay minerals. There is, however, an other aspect ofleral-water interactions to be considered; the chemistry of Coweetaters. The dissolved content of the waters often determines the naturethe solid weathering products precipitated from it, and waters carry:h them the dissolved products of weathering (all the material released

weathering which is not incorporated in the solid weatheringducts).

Since mineral nutrient cycling studies were initiated at Coweeta ini late 1960's, a wealth of information has been gathered on elementalicentrations in, and fluxes via, precipitation and stream water. Thest comprehensive reports of the results to date is Swank and Douglass), wherein are reported elemental concentrations and fluxes for allweeta watersheds which had ever been studied up to the date ofiparation of the paper. The stream chemistry data suggested to SwankJ Douglass (32,33) that higher concentrations of sodium and calcium inith-facing watersheds north of Shope Fork might be due t.o lithologiciations in bedrock, based on preliminary geologic mapping (10).

The data of Swank and Douglass (33) are weighted means for studyervals of various duration (depending on watershed and element)ween 1969 and 1976.

eamwater Chemistry as an Indicator of Groundwater Chemistry

If we are to use streamwater chemistry data to model weatheringictions in the saprolite, we must first be sure that the streamwaterlects the chemistry of water which has in fact percolated throughirolite. Table 1 compares the chemistry of soil solutions (collected byimeter at 25 cm depth; unpublished Coweeta Hydrologic Laboratory:a, Swank, written communication), "groundwater" (subsurface waterlected from a well at the rock-saprolite interface; unpublished data,ank, written communication), and streamwater (33) from watershed 6.thin the range of analytical errors and possible minor natural•iations, the major-element chemistry of streamwater isistinguishable from that of subsurface water which has percolated

236 M. A. VELBEL

TABLE 1: COMPARISON OF SOIL-, WELL-, ANDSTREAMWATER FOR WATERSHED 6

Concentration (ppm)

SoilWellStream

K

.921

.604

.591

Na

.2711.0811.094

Ca

2.4181.1231.063

Mg

1.268.651.643

PH

6.125.106.64

through the saprolite (Swank, written communication). Furthermore, thestreamwater and subsurface water chemistry are profoundly differentfrom those of soil solutions. For the balance of this discussion ofmajorelements, it is considered established that streams are merelysamples of subsurface water which have undergone no significantchemical change (except re-equilibration with atmospheric gases whichwould affect pH) on leaving the saprolite to feed the streams. Thechemical character of streams is apparently determined by the processeswhich alter the composition of water as it percolates through thesaprolite, and the streams effectively sample the solutions which resultfrom weathering processes in the saprolite.

Hydrologic and Lithologic Influences on Stream Chemistry

Johnson and Swank (21) found that, within an individual watershed,concentration is independent of discharge; in other words, high-runoffepisodes do not reflect dilution of base flow by interflow or overlandflow (see above). High-runoff episodes are caused by addition of rain tothe top of the solum, pushing "old" water out of lower levels of theprofile (18). This "old" water has been in prolonged contact with soil androck minerals, and may have reached a "steady-state" composition, aswas experimentally shown (3). Consequently, base flow and storm flowhave essentially the same composition; storms merely "force out" moreof the same water which normally sustains base flow.

Each watershed, however, is different; several different rock unitsare weathering at Coweeta, and the "mean annual flushing rate" varieswithin the Coweeta basin. Consequently, each individual watershedpossesses an "average" or "steady-state" composition (3) that differsfrom the "steady-state" composition of other watersheds on differentrock types or with different flushing rates. Furthermore, inter-watershed variability in "average" streamwater composition issystematic with respect to rock type and discharge.

Shown in Figure la are long-term average concentrations of Na,Ca, K, Mg, silica, and pH of streamwater for four control (undisturbed,unmanaged) watersheds underlain dominantly or exclusively by TallulahFalls rocks, plotted as a function of long-term average discharge (i.e.,flushing rate). Several elements (sodium, postassium, and silica)decrease in concentration with increased flushing rate, whereas calcium

Oopa

1.3-1

1.2-

1.1-

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|.6H

§ -5HS,H

.3-

.2-

.1-

01.0 1.2

1a-Tal lu lahFal ls Fm.

1.4 1.6Discharge (m yr" ' )

-6.7

1.8 2.0 2.1

K E Y

Na •K x

Mg .Ca +

SiO0

1 0PH

1.0-

.8-

.6-

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1.0

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-6.6

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no

1oz

JO

Io3SHw

Figure 1: Dissolved Concentration vs. Discharge

238 M. A. VELBEL

and magnesium remain approximately constant. Similar data for controlwatersheds underlain dominantly or exclusively by Coweeta Group rocks(Figure Ib) show decreases in long-term average pH and concentrationsof most major cations, with increasing long-term average discharge. Thetrends suggest that, as more water is flushed through a unit volume ofwatershed per unit time, the chemical weathering reactions by whichcations are added to solution do not proceed "as far"; the higher thewater/rock ratio, the less opportunity the water has to acquire solutes.There appears to be an important interplay between the rate at whichsolutions percolate through a watershed, and the rate at whichweathering reactions contribute cations to the percolating solution.

The importance of lithology in influencing streamwater chemistry,which was first speculated upon for Coweeta by Swank and Douglass (33),is clearly shown by comparing Figures la and Ib. The greatermineralogical maturity of the Coweeta Group protoliths results in theirhaving a lesser abundance of weatherable minerals than the TallulahFalls Formation. At any given flushing rate, the concentration of anyelement is therefore lower in waters draining Coweeta Group watershedsthan in waters draining Tallulah Falls watersheds. In other words, agiven volume of Coweeta Group rock delivers fewer cations per unittime to solutions than a given volume of Tallulah Falls rocks, at thesame flushing rate.

Thermodynamic Relations

Data for dissolved aluminum, iron, and manganese are not availableat Coweeta, so estimating the ion activity product (IAP) and saturationstate for primary and secondary minerals is impossible. Enough data are,however, available to plot the compositions of Coweeta waters onthermodynamic stability field diagrams.

A large number of studies of chemical weathering have usedthermodynamic stability diagrams to interpret water chemistry. Manyof these studies have concluded that equilibrium between solutions andclay-mineral weathering products is attained and determines the nature

Figure 2: Stability Field Diagram for the System K2O-Al2O,-SiO2-H2Oat 25 C and 1 atm. Thermodynamic properties from (29). On the rigfit,a region of the kaolinite stability field is enlarged to show the variationof Coweeta streamwater composition with rock type and flushing rate.The number to the left of any data point is the watershed number; thenumber in parentheses to the right of any point is the correspondingmean annual discharge in millions of liters per hectare per year. Notethat a) the two major rock-type groups plot as separate lines, and b)that, within either rock-type group, watersheds with lower discharge(flushing rate) plot higher and to the right of better-flushed watersheds,indicating that less-thoroughly flushed systems approach equilibriummore closely than better-flushed systems.

OROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 239

")

<%>- O

CN"0

_ o

240 M. A. VELBEL

of the clay mineral formed (7,26). At least one study has usedthermodynamic properties to argue against equilibrium between soilsolution and minerals, invoking atmospheric deposition as the source ofthe clay-mineral in question (5). A number of recent studies, however,have considered the implications of open-systems on interpretingstability-field relations, and are increasingly suggesting that what isusually present in natural systems is a "steady-state" or dynamic balancebetween the rate of approach to thermodynamic equilibrium and the rateof flushing through the open system (3,20,31). Solutions may be inpartial equilibrium (13,14,22) with one of the weathering products, butthis is only an intermediate stage in the evolution of the solution towardequilibrium. Water leaves the open system at this intermediate stage,which is why we see only partial equilibrium.

Stability-field relations for aqueous solutions at Coweeta areshown in Figure 2. Three important features of the streamwaterchemistry at Coweeta are evident. First, all the long-term averages fallin the kaolinite stability field. This reflects the fact that these streamsare fed by water from the saprolite, which invariably contains at leastsome kaolinite at depth (above; in prep.) Second, watersheds underlainby the two different rock types appear as two distinct, non-overlappinggroups. The effect of parent lithology, although subtle, is evident. Note(Figure 2) that the "less weatherable" Coweeta Group rocks haveconsumed less hydrogen ions and produced less alkalis and silica thanTallulah Falls rocks at comparable flushing rates, precisely as would beexpected.

Finally, within each of the "rock groups," there is a trend towardsincreasing alkali or alkaline earth to hydrogen ion ratio, and increasingsilica activity, with decreasing discharge. Recall (from 14) that a dilutesolution acquiring solutes by the weathering of primary silicate mineralswould evolve toward thermodynamic equilibrium by picking up alkalis,alkaline earths and silica, and losing hydrogen. On an activity-activitydiagram, this evolution toward equilibrium would manifest itself as apath from the lower left (dilute, acidic) to the upper right (cation-rich,acid-depleted). The degree of evolution towards equilibrium is reflectedin the plots of Coweeta streamwater chemistry; at high discharge, lesshydrogen ion has been consumed by weathering reactions, and lessalkalis, alkaline earths, and silica have been produced. As the flushingrate decreases, a given parcel of water acquires more solutes, because itremains in contact with the minerals longer before being flushed out ofthe system. In other words, more slowly-flushed systems are able toevolve to a state closer to thermodynamic equilibrium than rapidly-flushed systems. This illustrates the importance of flushing rate,relative to reaction rate, in determining clay mineralogy andgroundwater and streamwater chemistry (1,2).

Simple Mass-Balance Considerations

Tardy (34) presented a simple method for estimating the ratio ofsilica to alumina retained in the solid weathering products, from thedissolved concentration of major alkalis, alkaline earths, and silica being

ROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 241

TABLE 2: Re FOR COWEETA CONTROL WATERSHEDS

Watershed Re

2 1.6518 1.3627 1.6336 1.41

loved from the weathering systems in streams. Several assumptionsinvolved, the most important of which are: a) that the weatherable

lerals all have feldspar or biotite stoichiometries (effectively limitingmethod's applicability to granitic and similar crystalline rocks, for

ich Tardy considers quartz and muscovite unweatherable), and b) thatminerals dissolve stoichiometrically. Given these assumptions,

SiO- 3K+ + 3Na+ + 2Ca2+ - SiO-_ r 2

*•_ _ .

• A1_O, -residue "*• „+ KI + __ 2+2 3 K + Na + 2Ca -'streams

Re = 0, the weathering products are aluminous (e.g., gibbsite).athering systems exhibiting this character are said to be allitic.ere Re = 2, there is a 1:1 atom ratio of silica to aluminum, whichresponds to the 1:1 clay, kaolinite. These weathering products ared to be monosiallitic. If Re = 3 or more, enough silica has beenained in the weathering profile to form 2:1 clays (e.g., smectite), and'. profile is said to be bisiallltic. Intermediate cases are also tractable.• instance, equal molar proportions of gibbsite and kaolinite would•respond to a Re of 1.33. Tardy's Re has proven to be a usefulilitative indicator of the characteristic weathering products inlinage basins of various size on crystalline rocks (e.g., 34,43).

Table 2 shows the Re values for Coweeta control watersheds forich -sufficient data are available. Values between zero and twoicate the both gibbsite and kaolinite form in Coweeta weathering>files; more specifically, approximately subequal molar proportions ofibsite and kaolinite are suggested with kaolinite dominant.

The "Re" approach is not without its conceptual difficulties, but,the moment, it is sufficient that the "Re" approach is at least

ilitatively in accord with the observed clay mineralogy and with theationship of clay mineralogy and hydrology (i.e., the fact that streams; fed by saprolitic water, which explains its "kaolinitic" character).

242 M. A. VELBEL

SUMMARY

The relationships between hillslope hydrology, clay mineralogy ofplagioclase weathering products and aqueous geochemistry aresummarized schematically in Figure 3. Feldspar dissolvesstoichiometrically (37,38,40,41) releasing all of its calcium, sodium,silica, and aluminum to solution, where they join the alkalis, alkalineearths, silica, and aluminum already in solution. Aluminum isprecipitated as gibbsite from this solution in the upper parts of theprofile (Figure 3), via the reaction:

A1(OH)+ + H90,n ->• Al(OH), + H+

(aq) u; ^(gibbsite) (Reaction 1)

because rapid flushing keeps silica concentrations low, preventing theattainment of kaolinite saturation. (In some schistose saprolites, thealuminum is consumed as fixed-hydroxy-interlayers and neither gibbsitenor kaolinite form.) Where no aluminum sinks exist (i.e., in the absenceof biotite), and where there is prolonged contact between water andweatherable primary minerals, waters deep in the saprolite can acquireenough aluminum and silica to reach kaolinite saturation, and kaoliniteprecipitates (Figure 3) via the reaction:

2A1(OH)^ + 2SiO_ + H-O/.v -»• Al_Si-,O.(OH), + 2H+

2(aq) 2(aq) 2 (1) 2 2 5 *(Reaction 2)

Upon reaching the rock-saprolite interface, most of the water isshunted laterally down the slope, ultimately emerging to feed streams.A small fraction of the water enters the system of grain-boundary andgrain-traversing fractures in the rock below the rock-saprolite interface(Figure 3), where it continues to attack primary minerals until it has a)transformed biotite to expandable clay, and b) reached saturation withsmectite, which is then precipitated as pseudomorphous void-fillings inetched feldspar, and as fracture-fillings. In this manner, the rapidlyflushed waters effect the mineral transformations observedpetrographically (35-42), produce the observed distribution of clayminerals as summarized Figure 3, and give rise to the character ofaqueous solutions.

It is most important to note that, although weathering"microsystems" which form expandable clays exist, the hydrology of theweathering profile limits the amount of water which passes through the"smectitic," subsaprolite portion to an insignificant fraction of the waterwhich passes through the saprolite. The geochemical character ofsubsurface waters, of the streams they feed, and the stream dissolvedeffluxes from these profiles, are therefore determined almostexclusively by weathering reactions taking place above the rock-saprolite interface. (This is also suggested by the stability field relationsand Tardy's Re.) From a quantitive point of view, rock-waterinteractions taking place below the rock-saprolite interface do notcontribute to elemental budgets, and the complex microfracture

JROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 243

So/7

--—

S

a

P

r

o

I

i

t

e

R

o

c

k SMECTITE

Figure 3 .-Schematic Diagram ofWatershed 2 7 Hillslope Hydrology

and Weathering Profile

244 M. A. VELBEL

hydrology and stoichiometry of reactions therein can be safely ignored inconstructing geochemical mass-balances for the watersheds. Furtherrefinement of geochemical mass-balances in such deeply weatheredlandscapes will require more emphasis on weathering reactions in soiland saprolite, the hydrogeochemically active portions of the weatheringprofile.

ACKNOWLEDGEMENTS

This research was supported by U.S. National Science FoundationGrant EAR 80-07815 to R. A. Berner, was performed under acooperative agreement with the U.S. Department of Agriculture ForestService, Southeast Forest Experiment Station, and is based on a portionof the author's Ph.D. dissertation at Yale Univeristy. I am grateful toProf. Berner, E. T. Cleaves, K. K. Turekian, D. Neary, W. T. Swank, B.Cunningham, R. Beale, and J. Douglass for their assistance.

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(*) Clayton, J. L. 1979, "Nutrient Supply to Soil by Rock Weathering,"in "Impact of Intensive Harvesting on Forest Nutrient Cycling,"Environmental Sci. and Forestry, SUNY, College of Env. Sci. andFor., Syracuse, NY, pp. 75-96.

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(ROGEOCHEMICAL CONSTRAINTS ON MASS BALANCES IN FORESTED WATERSHEDS 245

) Hatcher, R. D., Jr. 1976, "Introduction to the Geology of theEastern Blue Ridge of the Carolinas and Nearby Georgia," CarolinaGeol. Soc. Field Trip Guidebook, 53 pp.

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