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The weathering of a late Tertiary volcanic ash: Importance of organic solutes
Geochimica Ed Cosmochrmica ACM Vol. 47. pp. 623-629 6 Pergamon F’rex Ltd. 1983. Printed in U.S.A.
0016-7037/83/030623-07503.00/0
The weathering of a late Tertiary volcanic ash: importance of organic solutes RONALDC. ANTWEILER*and JAMESI. DREVER Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 8207 1 (Received August 17, 1982; accepted in revised form December
14, 1982)
Abstract-The weathering of a late Tertiary volcanic ash near Jackson, Wyoming, was studied by sampling water percolating through the ash with suction lysimeters, and by examination of the associated solid phases. Soluble organic compounds derived from vegetation control the release and transport of solutes by complexing Al and Fe, and by causing low pH values. The concentrations of Na, K, Ca, and Mg. as well as those of Al and Fe correlate with the dissolved organic carbon concentrations (DOC) and follow an annual cycle with a maximum in spring. DOC concentrations averaged 50 mg C per liter, and values as high as 260 mg/l were observed. Al and Fe concentrations ranged as high as 5 mg/l. The dissolved organic matter was largely in the form of humic acids, although minor amounts of oxalate, acetate, and formate were also present. The pH of the percolating waters ranged from 4.3 to 6.5 with a mean of 5.2. During laboratory weathering experiments with the same ash in the absence of dissolved organic compounds, pH values ranged from 7.3 to 9.5, dissolved Al and Fe concentrations were below the detection limit, and there was little resemblance between the compositions of the solutions and the compositions observed in the field. Any model attempting to describe weatheting in a comparable setting must incorporate biological mechanisms as the dominant controls.
INTRODUCTION ALTHOUGHthe geochemistry of weathering has been extensively studied in the past, the emphasis has been on the thermodynamics and kinetics of inorganic reactions, and the possible role of organic compounds in the weathering process has often been ignored. The reasons for this are that natural springs and streamsone end product of the weathering process-generally contain little organic matter, and that natural organic matter is difficult to characterize. Recently, however, GRAUSTEIN(1975, 1976, 1981) showed that organic anions (particularly oxalate) can attain high concentrations in soil water, and that the breakdown of primary minerals was strongly influenced by the presence of organic solutes. HOLDRENet al. (1977) and GRAUSTEINet al. (I 977) proposed that oxalate ion was responsible for the congruent dissolution of plagioclase by the complexation of aluminum. Organic solutes may have only a transient existence, being both produced and destroyed in the soil zone, but during their existence they appear to exert a major influence on the breakdown of primary minerals and hence on the compositions of ground and surface waters. In this study we investigated the role of organic solutes in the weathering of a volcanic ash by examining the chemistry of water passing through the soil zone.
at which snowpack permitted access) until mid November. The area is situated at an elevation of 1970 m, and has a mean annual precipitation of 40 cm/y. The mean ammal temperature of the area is 3’C, and moat of the precipitation occurs as snow, developing a large winter snowpack. Thestudysitewassituatadonatopographichighinan area that was CssentiaUy fret of human activities. The ash is rhyolitic in composition (Table 1) and contains no crystalline material with the exception of minor (less than 2%) amountsofquartzTheashisonrlainbyathinsoilzone (less than 0.2 m thick). This zone is gradational, the only identifiable minerals in it a quartz and smectite, and volcanic glass is abundant throughout. The composition of the
STUDY AREA An unconsolidated Pliocene volcanic ash of the Teewinot Formation (LOVE, 1957) in Jackson Hole, Wyoming (Fig. 1) was sampled in 1979 from mid May (the earliest time * Present address: Dept. of Mathematics. Colorado, Boulder, CO 80309.
University of
FIG. 1. Location of the study area. Stippled area is the National Elk Refuge. 623
,x(_-__-__
SiO-i
71 :
. .._...._
-
.-........_._.
._.
__..
I__
-.-- _
__ ., ,
Fe0
‘!totai :rji Al20? cao bO Na20 K2’3
11.3 0.59 0.15 2.j3 5.!5
p20= c9 11-o+ L SUM
:. 1’ :,:‘? 'E, 3d.l
soil indicates that it is derived from the ash below. The vegetation growing on the study site consists of grasses, aspen, rose and willow bushes. The locations of the sampling stations relative to the topography are shown in Pig. 2. Water passing through the zone of the weathering was sampled daily during the late spring and early summer with suction lysimeters (cf: PARIZEK and LANE, 1970). The lysimeters were installed by drilling holes into the ash between 0.5 and 2 m deep with a hand auger. Once a lysimeter was emplaced in the ground, it remained undisturbed for the duration of the study. During the winter a large snowpack, shown schematically on Fig. 2, developed on top of the study site. This snowpack lasted until early June and provided essentially all of the moisture in the ash; rainfall during the summer served to wet only the top few centimeters of the soil zone. ANALYTICAL METHODS The lysimeters were initially cleaned by successive nitric acid-distilled water rinses until the rinse water showed no significant amounts of the major aqueous species. A series of laboratqry experiments conlirmed that the contribution of solutes (particularly Al and Si) From dissolution of the ceramic cups was negligible compared to the concentrations measured in the field, and that adsorption by the cup was negligible except for the 6rst sample volume. After the lysimeters were installed, water was collected by suction through glass and PVC tubing. pH was measured imme-
Cross
section of the are0
FIG. 3. Volume produced per day as a function of time for hole 5 (X) and hole 1I (0). diately with a glass electrode (precision * 0. I pH units), and the samples were sealed and stored in pre-cleaned PVC bottles” Sodium, potassium, calcium. and magnesium were determined by atomic absorption spectrophotometry (precision ‘_ 5%). Aqueous silica was determined colorimetricaliy using the molybdate blue method (precision + 2%). Chloride, nitrate, sulfate, phosphate, oxalate, formate and acetate were determined with a Dionex 14 Ion Chromatograph (precision 2 10% for chloride, +5% for others). Oxalate identification was confirmed by a spot test (FEiGI 1954) and a semiquantitative calorimetric technique (BLMUELMARTI et al., 1953). Dissolved orgsmic ca&m was determined (as COz) by ignition following acidiheation and nitrogen purgation to remove carbonate species (precision It: 10%). Organic separations were performed on selected samples using the techniques of THURMANand MALCOLM ( 1982). Aluminum and iron were determined by both flame (+5%) and flameleas (* 10%) atomic absorption spectmphotometry. The chemistry of the ash was determined by atomic absorption following lithium metaborate fusion (BENNETT and OLIVER, 1976). RESULTS The complete results of the chemical analyses are reported in ANTWEILER ( 198 1), and only data from selected holes will be presented here. Chloride concentrations averaged l-2 ppm, sulfate about I ppm.
nitrate about 0.5 ppm, and phosphate about 0.1 ppm.
I 20
I
31 3
FIG. 2. Cross-section of the study site showing the positions of the lysimcten. Not all lysimeters are in exactly the same vertical plane.
20
Jo
60
0;s
100
I20
FIG. 4. Aqueous silica concentration as a fun&on for hole 5 (X) and hole I ! (0).
14”
60
of time
Weathering of volcanic ash
625
300s
0 l .
mo-
e ”
. .
g
-f .
E Pw-
Oo I
v-
20
,
. _
l
40
loa
60
120
140
I
160
G FIG. 5. Calcium (X). magnesium (+), sodium (O), and potassium (B) concentrations as a function of time for hole I I.
FIG. 7. DOC concentration as a function of time for holes 5 (X), 6 (+), 11 (O), and 12 (V).
The volume collected per day is plotted against time (t) in Fig. 3. Time is reported as the number of
by contact time alone. The rate of mass transfer for major cations is greatest by far in spring because water movement and concentration in solution are both at a maximum at that time. The pH values of the sample are variable and generally low, ranging between 4.3 and 6.5 with an average of about 5.2. The reason for the low values is the presence of organic acids, indicated by the high values for dissolved organic carbon (DOC), which ranged between 20 and 260 mg C per liter. On the basis 6f charge balance calculations we estimate that organic anions contributed at least 50% of the negative charge in solution. The pH shows a strong negative correlation with DOC (Fig. 6), as would be expected if organic acids were controlling pH. The variation of DOC with time (Fig. 7) is the same as that of the major cations: the concentration is high initially, decreases rapidly and then levels off and possibly increases slightly as the summer progresses. In order to charac%erize the DOC, the dissolved organic matter in selected samples was separated by means of a macroporous resin XAD-8 (see AIKEN et
days from the installation of the first lysimeter (May 16). The last snow from the winter snowpack melted at f = 2 1 days (June 6). If each lysimeter is inherently able to remove the same amount of water under the same conditions, then the volume per day is a measure of the moisture content of the ash in the vicinity of the lysimeter. Since that moisture was derived from the snowbank, it must have travelled to the lysimeter through the ash or the overlying soil. The distribution of points shown in Fig. 3 implies that moisture content decreased with increasing time. The concentration of aqueous silica (Fig. 4) shows a general increase with time, possibly levelling off at high concentrations. The concentrations of the major cations (Fig. 5) however, initially decrease with time, followed by a levelling off or a slight increase. Thus increasing contact time between ash and water results in increasing silica concentrations but decreasing concentrations of the major cations: The concentrations of the major cations thus cannot be controlled
. .
. 6
I, 5
.
x0 x
. ..*
0
4
1.0
15
.
.
2.0
.
.
.
l. 25
log DOC)
FIG. 6. pH and DOC values for holes 3 (0). 4 (0). 5 (XL 6 (+). 10 (D). 11 (0). 12 (A), and 16 (V).
FIG. 8. Dissolved aluminum (0) and iron (X) concentrations as a function of time for hole 11.
ii , 4ntweiler and J. I. Drevctr
h26
al., 1979) into hydrophobic and hydrophilic fractions (resultsare reportedin ANTWWE R, 1981). Foursamples were further separated into acid, base, and neutral fractions (Table 2). Basically, the hydrophilic Fraction consists of smail organic molecules and some humic/Etlvic acids with high molar solubiiity, whereas the hydrophobic Eaction consists entirely of humic substances (THURMAN et al., 1978). As can be seen from Table 2, acids make up the major part of both the hydrophobic and hydrophilic fractions. Based on ion chromatogmphic analyses, no more than about 20% of the hydrophilic fraction consists of simple organic acids (or anions), and so 80% of the hydrophilic acids and ahnost all of the hydrophobic E-action are humic/E&ic acids. The humic/Etlvic acids were not chara&&& further, but the simple organic acids were separated by ion chromatography; oxalate (up to 12 mg/l), acetate, and formate ions were definitely ptesent. Propionate, succinate, and malonate ions were also tenatively identiEed. The concentrations of dissolved aluminum and iron (Pii 8) follow the same pattern as the concentration of iXIC. Aluminum concentrations range up to 5 mgll which is on-lets of magnitude higher than the concentration pm&ted by the solubility of amorphous aluminum hydroxide at pH 4.5 in the absence of complexing ligands. The high concentrations are presumably due to complexation by dissolved or-
ganic matter. Although GRAUSTEIN (1976) stressed the importance of oxalate as a complexing agent for iron and aluminum in soils, humic and fulvic acids can serve the same function (for example fvkLcorJ4, 1972; MALCOLM et al., 1975; KERNDORFF and SCHNITZER, 1980; CRERAR et al., 1981). Because humic/fitlvic acids comprise 80-9096 of the total DGC in the ash water, it is probable that in our study they are largely responsible for the high concentrations of dissolved aluminum and iron. In summary, the concentrations of all solutes studied except silica cormlate closely with DOC and follow an annual cycle (Fig. 9) with a maximum in spring. The release of organic acids by the biota thus appears to be the immediate control on the rate of alteration of the volcanic ash. LABORATORY STUDY A series of laboratory experiments on the dissolution of ash from the field area were carried out to compare the behavior observed in the Eeld with the behavior under controlled conditions in the laboratory. In one series of experiments the carbon dioxide pressure was not controlled (the values in solution were near or somewhat below the atmospheric value of 10-3,5 atm), and in the second series the Pco, was maintained at 1O-’ atm by bubbling an air-COr mix-
0.3 -
so -
x - TDOC 0 -
AI
Fe - Ca 0 - Mg
01 l
0 3
60 -
B ii=i
s-
40
-
20
”
0
I
M
40
60
80
100
120
140
I60
Days FIG. 9. Normalized concentrations of DOC. by setting value for day IO equal to 100.
Al, Fe, Ca, and Mg for hole I 1. Values were normalized
Weathering of volcanic ash
627
Overall, there is little resemblance between the dissolution behavior observed in the laboratory and that observed in the field. This again points out the importance of the biota in the natural system.
. A
DISCUSSION
0
40
20
DEStot
60
120
MO
160
FIG. 10. Comparison between field (hole I I ) and lab pH values.
ture through the solutions. The details of the experiments arc reported in ANTWEILER ( 198 I), and only a few results relevant to the comparison with weathering in the field are presented here. The pH values in the laboratory experiments (Fig. 10) are above 7, in contrast to the field values, which average 5.2. The high values are consistent with buffering by the COJHCOS system (the decrease in pH in the first series is caused by the fact that in the early part of the experiment CO2 was consumed by reaction more rapidly than it could be replaced from the atmosphere). The low values in the natural system illustrate the effect of organic acids. The concentrations of the major cations increase with time in the experiments, whereas they generally decrease with time in the field (Fig. 11). Aluminum and iron were undetectable in the laboratory experiments, and only silica (Fig 12) behaves in a roughly similar manner in the field and in the experiments.
In the absence of dissolved organic matter, aluminosilicates dissolve incongruently to form clays chiefly because appreciable concentrations of aluminum and ferric iron cannot exist in solution. Mass transfer consists largely of the selective removal of the alkali and alkaline earth cations while the bulk of the material is left behind. When organic anions are present, aluminum and iron can be transported in solution and dissolution of primary aluminosilicates may be congruent (cJ HOLDREN and BERNER, 1979). Regardless of whether the rate of alteration is controlled by solid-state diffusion (WOLLAST, 1967; PA&S, 1973: BUSENBERG and CLEMENCY, 1976; WHITE and CLAASSEN, 1979) or by surface reaction (BERNER and HOLDREN, 1977, 1979; HOLDREN and BERNER, 1979). organic acids should increase the rate. For the solid-state diffusion model, dissolution of aluminum should inhibit the formation of a leached layer and the low pH should increase the concentration gradient for diffusion of H+. For the surface reaction model, both complexation of Al and low pH will increase the degree of undersaturation of the solution with respect to the primary mineral, and thus cause an increase in the rate of dissolution. HUANG and KELLER ( 197 1) demonstrated that the presence of organic acids causes a large increase in the rate of dissolution/alteration of clay minerals. The lifetime of organic matter in soil solution is probably fairly short (cl: GRAUSTEIN, 198 1). Bactetia use it for a source of energy, and it tends to be ad-
20 D
x - FIELD D-LAB
-12
xx
E g
t
s
clx 00
x x
0
x x
0
20
40
60
DBaDys
100
120
140
160
FIG. 11.Comparison between field (hole I I) and lab (unbuffered, waterxsh ratio 2.3: I ) calcium concentrations.
,’
i
.Antwelier
and J
I
Drcz,t-r
x
x - FIELD 0 - LAB
i t 0
20
40
60
so Days
loo
I20
140
I
160
FIG. 12. Comparison between field (hole I I) and lab (unbuffered, watecash ratio 2.3: I) dissolvedsilica concentrations.
sorbed by surfaces of the glass or secondary
clay minerals. The mobility of aluminum and iron is thus likely to be restricted to a zone whose extent depends on the relative rates at which organic matter is being both crcati and destroyed. In the soil and the top of the ash, where the rate of production of dissolved organic matter is a maximum, aluminum and iron will be carried away in solution and the rate of alteration of the glass will be a maximum. As the complexing ligands are destroyed or removed from solution, the iron and aluminum will precipitate out as hydroxides or clays, incorporating varying amounts of silica and other cations, depending on the overall composition of the groundwater solution. Water that leaves the ash to form part of the permanent groundwater system has probably lost most of its organic solutes. The chemistry of such water will have been determined by two processes: dissolution reactions in the upper part of the soil profile, and precipitation reactions at greater depths. SUMMARY Organic material provides the major control on the chemistry of near-surface waters in the ash by complexation of aluminum and iron and by lowering the pH. The majority of the mass transfer of the aqueous species (i.e. most of the weathering) occurs in the spring as a consequence of the yearly biological and hydrologic cycles. The extent of reaction cannot be explained simply by the time of contact between the water and the ash because the rates of reaction are dependent upon biologically derived compounds which vary independently of contact time. The concentrations of aluminum and iron are several orders of magnitude higher than would be possible in the
absence of organic complexing agents, and their variation with time mimics that of dissolved organic carbon. Any chemical model attempting to describe the weathering process cannot ignore the role of organic compounds. Acknowledgments-We thank the staff of the National Elk Refuee for permission to use the site and for assistance in the field. The organic sepnrations were performed in the laboratories of the US. Geological Survey in Denver. We thank R. L. Malcolm and E. M. Thurman for allowing us to use the labs and for much helpful advice. We also thank G. Moncure and S. Boese for their a&stance. Funding was provided by N.S.F. Grant EAR78-0440.
REFERENCES AIKEN G. R., THURMANE. M., MALCOLMR. L. and WALTONH. F. ( 1979)Comparison of XAD macroporous resins for the concentration of fulvic acid from aqueous solution. Anal. Chem. 51, 1799-1803. A~ILER R. C. (198 I) The chemistry of weathenng of a Pliocene volcanic ash: field and laboratory studies. Ph.D. Dissertation, Univ. of Wyomin& 155 p. BENNETTH. and OLIVERG. J. (1976) Development of fluxes for the analysis of ceramic materials by x-ray fluorescence spectrometry. Analyst 101. 803-807. BERNERR. A. and HOLDRENG. R. JR. (1977) Mechanism of feldspar weathering: some observational evidence. Geology 5, 369-372. BERNERR. A. and HOLDRENG. R. JR. ( 1979) Mechamsm of feldspar weathering: II. Observations of feldspars from soils. Geochim. Cosmochim. Acta 43, 1173-I 186. BIJRRIEL-MARTI F., RAMI=-MUNOZ J. and FERNANDEZCALDASE. (1953) Determination of oxalate ion and calcium ion by indirect calorimetry. ;Qnal. Chem. 25, 583585. BUSENBERG E. and CLEMENCY C. V. ( 1976) The dissolution kinetics of feldspars at 25°C and I atm CO2 partial pressure. Geochim. Cosmochim. Acta 40,41-49. CRERARD. A., MEANSJ. L., YURETICHR. F.. B~RCSIK M. P., AM!~T!ZR J. L., HASTINGSD. W., KNOX G. W..
Weathering of volcanic ash LYONK. E. and QUIZ R. F. (198 1) Hydrogeochemistry of the New Jetsey Coastal Plain, 2. Transport and deposition of iron and manganese. Chem. Geol. 33,23-44. &lOL F. (1954) Spar Tests. Trans. by R. E. Gesper, 4th. rev. English Edition, Elsevier, Amsterdam. GRAUSTEINW. C. (1975) Qn chemical weathering and forests. Geoi. Sot. Amer. Abst. with Programs 7, 1090-1091. GRAUSTEINW. C. (1976) Grganic complexes and the mobility of iron and aluminum in soil pro&s. Geol. Sot. Amer. Abst. with Programs 8, 891. GRWSTEIN W. C. (1981) The effects of forest vegetation on solute acquisition and chemical weathering: a study of the Tesuque watersheds near Santa Fe, New Mexico. Ph.D. Dissertation, Yale Univ., 645 p. GRAUSTEINW. C., CROMACKK. JR. and !XILIJNSP. (1977) Calcium oxahte: occummcein soils and effect on nutrient and geochemical cycles. Science 198, 1252-1254. HOLDRENG. R. JR. and BERNERR. A. (I 979) Mechanism of feldspar weathering: I. Experimental studies. Geochim. Cosmochim. Acza 43, 116 1- 1I 7 1. HOLDRENG. R. JR., GRAUSTEINW. C. and BERNERR. A. ( 1977) Chemical weathering in soils: evidence from surface compositions. Geol. Sot. Amer. Abst. with Programs 9, 1020-1021. HUANG W. H. and KELLER W. D. (1971) Dissolution of clay minerals in dilute organic acids at room tempemture. Amer. Mineral. 56, 1082-1095. KERNDORFFH. and !&HNINR M. (1980) Sorption of metals on humic acid. Geochim. Cosmochim. Acta 44, 1701-1708.
bX’E
629
J. D. (1957) Cretaceous and Tertiary sttatigmphy of the Jackson Hole area, northwestern Wyoming Wyo. Geol. Assn. Guiakok, 11th. Ann. Field. Co&, 76-94. MALCOLMR. L. ( 1972) Comparison of conditional stability constants of North Carolina humic and fulvic acids with Co(B) and Fe(III). Geo1. Sot. Amer. Memoir 133,79-83. MALCOLMR. L., LEENHEERJ. A. and WEED S. B. (1975) Dissolution of aquifer clay minerals during deep-well disposal of industrial organic wastes. Proc. Inf. Clay Conf.‘, Mexico Cuy 477-493. PACES T. (1973) Steady-state kinetic and equilibrium between ground water and gtanitic rock. Geochim. Cosmochim. Acta 37.2641-2663. PARIZEKR. R. and LANE B. E. (1970) soil-water sampling using pan and deep pressure-vacuum Iysimetees. J. Hp drofogy 11, l-2 1. THURMAN E. M. and MALCOLM R. L. (1982) Assay for aquatic humic substances. US. Goof. Survey Water Res. Inv. (in press). THURMANE. M., ELM R. L. and A~KENG. R. (1978) Prediction of capacity Iactors for aqueous organic solutes absorbed on a porous acrylic resin. Anal. Chem. 50,775779. WHITE A. F. and CLAASSENH. C. (1979) Dissolution kinetics of silicate rocks-application to solute modeling. Chemical Modeling in Aqueous Systems (ed. E. A. JENNE). Amer. Chem. Sot. Symp. Ser. 93,447-473. WOLUIST R. ( 1967) Kinetics of alteration of K-feldspar in buffered solutions at low temperatures. Geochim. Cosmochim. Acta 31,635-648.
0016-7037/83/030623-07503.00/0
The weathering of a late Tertiary volcanic ash: importance of organic solutes RONALDC. ANTWEILER*and JAMESI. DREVER Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 8207 1 (Received August 17, 1982; accepted in revised form December
14, 1982)
Abstract-The weathering of a late Tertiary volcanic ash near Jackson, Wyoming, was studied by sampling water percolating through the ash with suction lysimeters, and by examination of the associated solid phases. Soluble organic compounds derived from vegetation control the release and transport of solutes by complexing Al and Fe, and by causing low pH values. The concentrations of Na, K, Ca, and Mg. as well as those of Al and Fe correlate with the dissolved organic carbon concentrations (DOC) and follow an annual cycle with a maximum in spring. DOC concentrations averaged 50 mg C per liter, and values as high as 260 mg/l were observed. Al and Fe concentrations ranged as high as 5 mg/l. The dissolved organic matter was largely in the form of humic acids, although minor amounts of oxalate, acetate, and formate were also present. The pH of the percolating waters ranged from 4.3 to 6.5 with a mean of 5.2. During laboratory weathering experiments with the same ash in the absence of dissolved organic compounds, pH values ranged from 7.3 to 9.5, dissolved Al and Fe concentrations were below the detection limit, and there was little resemblance between the compositions of the solutions and the compositions observed in the field. Any model attempting to describe weatheting in a comparable setting must incorporate biological mechanisms as the dominant controls.
INTRODUCTION ALTHOUGHthe geochemistry of weathering has been extensively studied in the past, the emphasis has been on the thermodynamics and kinetics of inorganic reactions, and the possible role of organic compounds in the weathering process has often been ignored. The reasons for this are that natural springs and streamsone end product of the weathering process-generally contain little organic matter, and that natural organic matter is difficult to characterize. Recently, however, GRAUSTEIN(1975, 1976, 1981) showed that organic anions (particularly oxalate) can attain high concentrations in soil water, and that the breakdown of primary minerals was strongly influenced by the presence of organic solutes. HOLDRENet al. (1977) and GRAUSTEINet al. (I 977) proposed that oxalate ion was responsible for the congruent dissolution of plagioclase by the complexation of aluminum. Organic solutes may have only a transient existence, being both produced and destroyed in the soil zone, but during their existence they appear to exert a major influence on the breakdown of primary minerals and hence on the compositions of ground and surface waters. In this study we investigated the role of organic solutes in the weathering of a volcanic ash by examining the chemistry of water passing through the soil zone.
at which snowpack permitted access) until mid November. The area is situated at an elevation of 1970 m, and has a mean annual precipitation of 40 cm/y. The mean ammal temperature of the area is 3’C, and moat of the precipitation occurs as snow, developing a large winter snowpack. Thestudysitewassituatadonatopographichighinan area that was CssentiaUy fret of human activities. The ash is rhyolitic in composition (Table 1) and contains no crystalline material with the exception of minor (less than 2%) amountsofquartzTheashisonrlainbyathinsoilzone (less than 0.2 m thick). This zone is gradational, the only identifiable minerals in it a quartz and smectite, and volcanic glass is abundant throughout. The composition of the
STUDY AREA An unconsolidated Pliocene volcanic ash of the Teewinot Formation (LOVE, 1957) in Jackson Hole, Wyoming (Fig. 1) was sampled in 1979 from mid May (the earliest time * Present address: Dept. of Mathematics. Colorado, Boulder, CO 80309.
University of
FIG. 1. Location of the study area. Stippled area is the National Elk Refuge. 623
,x(_-__-__
SiO-i
71 :
. .._...._
-
.-........_._.
._.
__..
I__
-.-- _
__ ., ,
Fe0
‘!totai :rji Al20? cao bO Na20 K2’3
11.3 0.59 0.15 2.j3 5.!5
p20= c9 11-o+ L SUM
:. 1’ :,:‘? 'E, 3d.l
soil indicates that it is derived from the ash below. The vegetation growing on the study site consists of grasses, aspen, rose and willow bushes. The locations of the sampling stations relative to the topography are shown in Pig. 2. Water passing through the zone of the weathering was sampled daily during the late spring and early summer with suction lysimeters (cf: PARIZEK and LANE, 1970). The lysimeters were installed by drilling holes into the ash between 0.5 and 2 m deep with a hand auger. Once a lysimeter was emplaced in the ground, it remained undisturbed for the duration of the study. During the winter a large snowpack, shown schematically on Fig. 2, developed on top of the study site. This snowpack lasted until early June and provided essentially all of the moisture in the ash; rainfall during the summer served to wet only the top few centimeters of the soil zone. ANALYTICAL METHODS The lysimeters were initially cleaned by successive nitric acid-distilled water rinses until the rinse water showed no significant amounts of the major aqueous species. A series of laboratqry experiments conlirmed that the contribution of solutes (particularly Al and Si) From dissolution of the ceramic cups was negligible compared to the concentrations measured in the field, and that adsorption by the cup was negligible except for the 6rst sample volume. After the lysimeters were installed, water was collected by suction through glass and PVC tubing. pH was measured imme-
Cross
section of the are0
FIG. 3. Volume produced per day as a function of time for hole 5 (X) and hole 1I (0). diately with a glass electrode (precision * 0. I pH units), and the samples were sealed and stored in pre-cleaned PVC bottles” Sodium, potassium, calcium. and magnesium were determined by atomic absorption spectrophotometry (precision ‘_ 5%). Aqueous silica was determined colorimetricaliy using the molybdate blue method (precision + 2%). Chloride, nitrate, sulfate, phosphate, oxalate, formate and acetate were determined with a Dionex 14 Ion Chromatograph (precision 2 10% for chloride, +5% for others). Oxalate identification was confirmed by a spot test (FEiGI 1954) and a semiquantitative calorimetric technique (BLMUELMARTI et al., 1953). Dissolved orgsmic ca&m was determined (as COz) by ignition following acidiheation and nitrogen purgation to remove carbonate species (precision It: 10%). Organic separations were performed on selected samples using the techniques of THURMANand MALCOLM ( 1982). Aluminum and iron were determined by both flame (+5%) and flameleas (* 10%) atomic absorption spectmphotometry. The chemistry of the ash was determined by atomic absorption following lithium metaborate fusion (BENNETT and OLIVER, 1976). RESULTS The complete results of the chemical analyses are reported in ANTWEILER ( 198 1), and only data from selected holes will be presented here. Chloride concentrations averaged l-2 ppm, sulfate about I ppm.
nitrate about 0.5 ppm, and phosphate about 0.1 ppm.
I 20
I
31 3
FIG. 2. Cross-section of the study site showing the positions of the lysimcten. Not all lysimeters are in exactly the same vertical plane.
20
Jo
60
0;s
100
I20
FIG. 4. Aqueous silica concentration as a fun&on for hole 5 (X) and hole I ! (0).
14”
60
of time
Weathering of volcanic ash
625
300s
0 l .
mo-
e ”
. .
g
-f .
E Pw-
Oo I
v-
20
,
. _
l
40
loa
60
120
140
I
160
G FIG. 5. Calcium (X). magnesium (+), sodium (O), and potassium (B) concentrations as a function of time for hole I I.
FIG. 7. DOC concentration as a function of time for holes 5 (X), 6 (+), 11 (O), and 12 (V).
The volume collected per day is plotted against time (t) in Fig. 3. Time is reported as the number of
by contact time alone. The rate of mass transfer for major cations is greatest by far in spring because water movement and concentration in solution are both at a maximum at that time. The pH values of the sample are variable and generally low, ranging between 4.3 and 6.5 with an average of about 5.2. The reason for the low values is the presence of organic acids, indicated by the high values for dissolved organic carbon (DOC), which ranged between 20 and 260 mg C per liter. On the basis 6f charge balance calculations we estimate that organic anions contributed at least 50% of the negative charge in solution. The pH shows a strong negative correlation with DOC (Fig. 6), as would be expected if organic acids were controlling pH. The variation of DOC with time (Fig. 7) is the same as that of the major cations: the concentration is high initially, decreases rapidly and then levels off and possibly increases slightly as the summer progresses. In order to charac%erize the DOC, the dissolved organic matter in selected samples was separated by means of a macroporous resin XAD-8 (see AIKEN et
days from the installation of the first lysimeter (May 16). The last snow from the winter snowpack melted at f = 2 1 days (June 6). If each lysimeter is inherently able to remove the same amount of water under the same conditions, then the volume per day is a measure of the moisture content of the ash in the vicinity of the lysimeter. Since that moisture was derived from the snowbank, it must have travelled to the lysimeter through the ash or the overlying soil. The distribution of points shown in Fig. 3 implies that moisture content decreased with increasing time. The concentration of aqueous silica (Fig. 4) shows a general increase with time, possibly levelling off at high concentrations. The concentrations of the major cations (Fig. 5) however, initially decrease with time, followed by a levelling off or a slight increase. Thus increasing contact time between ash and water results in increasing silica concentrations but decreasing concentrations of the major cations: The concentrations of the major cations thus cannot be controlled
. .
. 6
I, 5
.
x0 x
. ..*
0
4
1.0
15
.
.
2.0
.
.
.
l. 25
log DOC)
FIG. 6. pH and DOC values for holes 3 (0). 4 (0). 5 (XL 6 (+). 10 (D). 11 (0). 12 (A), and 16 (V).
FIG. 8. Dissolved aluminum (0) and iron (X) concentrations as a function of time for hole 11.
ii , 4ntweiler and J. I. Drevctr
h26
al., 1979) into hydrophobic and hydrophilic fractions (resultsare reportedin ANTWWE R, 1981). Foursamples were further separated into acid, base, and neutral fractions (Table 2). Basically, the hydrophilic Fraction consists of smail organic molecules and some humic/Etlvic acids with high molar solubiiity, whereas the hydrophobic Eaction consists entirely of humic substances (THURMAN et al., 1978). As can be seen from Table 2, acids make up the major part of both the hydrophobic and hydrophilic fractions. Based on ion chromatogmphic analyses, no more than about 20% of the hydrophilic fraction consists of simple organic acids (or anions), and so 80% of the hydrophilic acids and ahnost all of the hydrophobic E-action are humic/E&ic acids. The humic/Etlvic acids were not chara&&& further, but the simple organic acids were separated by ion chromatography; oxalate (up to 12 mg/l), acetate, and formate ions were definitely ptesent. Propionate, succinate, and malonate ions were also tenatively identiEed. The concentrations of dissolved aluminum and iron (Pii 8) follow the same pattern as the concentration of iXIC. Aluminum concentrations range up to 5 mgll which is on-lets of magnitude higher than the concentration pm&ted by the solubility of amorphous aluminum hydroxide at pH 4.5 in the absence of complexing ligands. The high concentrations are presumably due to complexation by dissolved or-
ganic matter. Although GRAUSTEIN (1976) stressed the importance of oxalate as a complexing agent for iron and aluminum in soils, humic and fulvic acids can serve the same function (for example fvkLcorJ4, 1972; MALCOLM et al., 1975; KERNDORFF and SCHNITZER, 1980; CRERAR et al., 1981). Because humic/fitlvic acids comprise 80-9096 of the total DGC in the ash water, it is probable that in our study they are largely responsible for the high concentrations of dissolved aluminum and iron. In summary, the concentrations of all solutes studied except silica cormlate closely with DOC and follow an annual cycle (Fig. 9) with a maximum in spring. The release of organic acids by the biota thus appears to be the immediate control on the rate of alteration of the volcanic ash. LABORATORY STUDY A series of laboratory experiments on the dissolution of ash from the field area were carried out to compare the behavior observed in the Eeld with the behavior under controlled conditions in the laboratory. In one series of experiments the carbon dioxide pressure was not controlled (the values in solution were near or somewhat below the atmospheric value of 10-3,5 atm), and in the second series the Pco, was maintained at 1O-’ atm by bubbling an air-COr mix-
0.3 -
so -
x - TDOC 0 -
AI
Fe - Ca 0 - Mg
01 l
0 3
60 -
B ii=i
s-
40
-
20
”
0
I
M
40
60
80
100
120
140
I60
Days FIG. 9. Normalized concentrations of DOC. by setting value for day IO equal to 100.
Al, Fe, Ca, and Mg for hole I 1. Values were normalized
Weathering of volcanic ash
627
Overall, there is little resemblance between the dissolution behavior observed in the laboratory and that observed in the field. This again points out the importance of the biota in the natural system.
. A
DISCUSSION
0
40
20
DEStot
60
120
MO
160
FIG. 10. Comparison between field (hole I I ) and lab pH values.
ture through the solutions. The details of the experiments arc reported in ANTWEILER ( 198 I), and only a few results relevant to the comparison with weathering in the field are presented here. The pH values in the laboratory experiments (Fig. 10) are above 7, in contrast to the field values, which average 5.2. The high values are consistent with buffering by the COJHCOS system (the decrease in pH in the first series is caused by the fact that in the early part of the experiment CO2 was consumed by reaction more rapidly than it could be replaced from the atmosphere). The low values in the natural system illustrate the effect of organic acids. The concentrations of the major cations increase with time in the experiments, whereas they generally decrease with time in the field (Fig. 11). Aluminum and iron were undetectable in the laboratory experiments, and only silica (Fig 12) behaves in a roughly similar manner in the field and in the experiments.
In the absence of dissolved organic matter, aluminosilicates dissolve incongruently to form clays chiefly because appreciable concentrations of aluminum and ferric iron cannot exist in solution. Mass transfer consists largely of the selective removal of the alkali and alkaline earth cations while the bulk of the material is left behind. When organic anions are present, aluminum and iron can be transported in solution and dissolution of primary aluminosilicates may be congruent (cJ HOLDREN and BERNER, 1979). Regardless of whether the rate of alteration is controlled by solid-state diffusion (WOLLAST, 1967; PA&S, 1973: BUSENBERG and CLEMENCY, 1976; WHITE and CLAASSEN, 1979) or by surface reaction (BERNER and HOLDREN, 1977, 1979; HOLDREN and BERNER, 1979). organic acids should increase the rate. For the solid-state diffusion model, dissolution of aluminum should inhibit the formation of a leached layer and the low pH should increase the concentration gradient for diffusion of H+. For the surface reaction model, both complexation of Al and low pH will increase the degree of undersaturation of the solution with respect to the primary mineral, and thus cause an increase in the rate of dissolution. HUANG and KELLER ( 197 1) demonstrated that the presence of organic acids causes a large increase in the rate of dissolution/alteration of clay minerals. The lifetime of organic matter in soil solution is probably fairly short (cl: GRAUSTEIN, 198 1). Bactetia use it for a source of energy, and it tends to be ad-
20 D
x - FIELD D-LAB
-12
xx
E g
t
s
clx 00
x x
0
x x
0
20
40
60
DBaDys
100
120
140
160
FIG. 11.Comparison between field (hole I I) and lab (unbuffered, waterxsh ratio 2.3: I ) calcium concentrations.
,’
i
.Antwelier
and J
I
Drcz,t-r
x
x - FIELD 0 - LAB
i t 0
20
40
60
so Days
loo
I20
140
I
160
FIG. 12. Comparison between field (hole I I) and lab (unbuffered, watecash ratio 2.3: I) dissolvedsilica concentrations.
sorbed by surfaces of the glass or secondary
clay minerals. The mobility of aluminum and iron is thus likely to be restricted to a zone whose extent depends on the relative rates at which organic matter is being both crcati and destroyed. In the soil and the top of the ash, where the rate of production of dissolved organic matter is a maximum, aluminum and iron will be carried away in solution and the rate of alteration of the glass will be a maximum. As the complexing ligands are destroyed or removed from solution, the iron and aluminum will precipitate out as hydroxides or clays, incorporating varying amounts of silica and other cations, depending on the overall composition of the groundwater solution. Water that leaves the ash to form part of the permanent groundwater system has probably lost most of its organic solutes. The chemistry of such water will have been determined by two processes: dissolution reactions in the upper part of the soil profile, and precipitation reactions at greater depths. SUMMARY Organic material provides the major control on the chemistry of near-surface waters in the ash by complexation of aluminum and iron and by lowering the pH. The majority of the mass transfer of the aqueous species (i.e. most of the weathering) occurs in the spring as a consequence of the yearly biological and hydrologic cycles. The extent of reaction cannot be explained simply by the time of contact between the water and the ash because the rates of reaction are dependent upon biologically derived compounds which vary independently of contact time. The concentrations of aluminum and iron are several orders of magnitude higher than would be possible in the
absence of organic complexing agents, and their variation with time mimics that of dissolved organic carbon. Any chemical model attempting to describe the weathering process cannot ignore the role of organic compounds. Acknowledgments-We thank the staff of the National Elk Refuee for permission to use the site and for assistance in the field. The organic sepnrations were performed in the laboratories of the US. Geological Survey in Denver. We thank R. L. Malcolm and E. M. Thurman for allowing us to use the labs and for much helpful advice. We also thank G. Moncure and S. Boese for their a&stance. Funding was provided by N.S.F. Grant EAR78-0440.
REFERENCES AIKEN G. R., THURMANE. M., MALCOLMR. L. and WALTONH. F. ( 1979)Comparison of XAD macroporous resins for the concentration of fulvic acid from aqueous solution. Anal. Chem. 51, 1799-1803. A~ILER R. C. (198 I) The chemistry of weathenng of a Pliocene volcanic ash: field and laboratory studies. Ph.D. Dissertation, Univ. of Wyomin& 155 p. BENNETTH. and OLIVERG. J. (1976) Development of fluxes for the analysis of ceramic materials by x-ray fluorescence spectrometry. Analyst 101. 803-807. BERNERR. A. and HOLDRENG. R. JR. (1977) Mechanism of feldspar weathering: some observational evidence. Geology 5, 369-372. BERNERR. A. and HOLDRENG. R. JR. ( 1979) Mechamsm of feldspar weathering: II. Observations of feldspars from soils. Geochim. Cosmochim. Acta 43, 1173-I 186. BIJRRIEL-MARTI F., RAMI=-MUNOZ J. and FERNANDEZCALDASE. (1953) Determination of oxalate ion and calcium ion by indirect calorimetry. ;Qnal. Chem. 25, 583585. BUSENBERG E. and CLEMENCY C. V. ( 1976) The dissolution kinetics of feldspars at 25°C and I atm CO2 partial pressure. Geochim. Cosmochim. Acta 40,41-49. CRERARD. A., MEANSJ. L., YURETICHR. F.. B~RCSIK M. P., AM!~T!ZR J. L., HASTINGSD. W., KNOX G. W..
Weathering of volcanic ash LYONK. E. and QUIZ R. F. (198 1) Hydrogeochemistry of the New Jetsey Coastal Plain, 2. Transport and deposition of iron and manganese. Chem. Geol. 33,23-44. &lOL F. (1954) Spar Tests. Trans. by R. E. Gesper, 4th. rev. English Edition, Elsevier, Amsterdam. GRAUSTEINW. C. (1975) Qn chemical weathering and forests. Geoi. Sot. Amer. Abst. with Programs 7, 1090-1091. GRAUSTEINW. C. (1976) Grganic complexes and the mobility of iron and aluminum in soil pro&s. Geol. Sot. Amer. Abst. with Programs 8, 891. GRWSTEIN W. C. (1981) The effects of forest vegetation on solute acquisition and chemical weathering: a study of the Tesuque watersheds near Santa Fe, New Mexico. Ph.D. Dissertation, Yale Univ., 645 p. GRAUSTEINW. C., CROMACKK. JR. and !XILIJNSP. (1977) Calcium oxahte: occummcein soils and effect on nutrient and geochemical cycles. Science 198, 1252-1254. HOLDRENG. R. JR. and BERNERR. A. (I 979) Mechanism of feldspar weathering: I. Experimental studies. Geochim. Cosmochim. Acza 43, 116 1- 1I 7 1. HOLDRENG. R. JR., GRAUSTEINW. C. and BERNERR. A. ( 1977) Chemical weathering in soils: evidence from surface compositions. Geol. Sot. Amer. Abst. with Programs 9, 1020-1021. HUANG W. H. and KELLER W. D. (1971) Dissolution of clay minerals in dilute organic acids at room tempemture. Amer. Mineral. 56, 1082-1095. KERNDORFFH. and !&HNINR M. (1980) Sorption of metals on humic acid. Geochim. Cosmochim. Acta 44, 1701-1708.
bX’E
629
J. D. (1957) Cretaceous and Tertiary sttatigmphy of the Jackson Hole area, northwestern Wyoming Wyo. Geol. Assn. Guiakok, 11th. Ann. Field. Co&, 76-94. MALCOLMR. L. ( 1972) Comparison of conditional stability constants of North Carolina humic and fulvic acids with Co(B) and Fe(III). Geo1. Sot. Amer. Memoir 133,79-83. MALCOLMR. L., LEENHEERJ. A. and WEED S. B. (1975) Dissolution of aquifer clay minerals during deep-well disposal of industrial organic wastes. Proc. Inf. Clay Conf.‘, Mexico Cuy 477-493. PACES T. (1973) Steady-state kinetic and equilibrium between ground water and gtanitic rock. Geochim. Cosmochim. Acta 37.2641-2663. PARIZEKR. R. and LANE B. E. (1970) soil-water sampling using pan and deep pressure-vacuum Iysimetees. J. Hp drofogy 11, l-2 1. THURMAN E. M. and MALCOLM R. L. (1982) Assay for aquatic humic substances. US. Goof. Survey Water Res. Inv. (in press). THURMANE. M., ELM R. L. and A~KENG. R. (1978) Prediction of capacity Iactors for aqueous organic solutes absorbed on a porous acrylic resin. Anal. Chem. 50,775779. WHITE A. F. and CLAASSENH. C. (1979) Dissolution kinetics of silicate rocks-application to solute modeling. Chemical Modeling in Aqueous Systems (ed. E. A. JENNE). Amer. Chem. Sot. Symp. Ser. 93,447-473. WOLUIST R. ( 1967) Kinetics of alteration of K-feldspar in buffered solutions at low temperatures. Geochim. Cosmochim. Acta 31,635-648.