Trace Inorganics In Water - ACS Publications - American Chemical

6 Mineral-Water Interaction During the

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Chemical Weathering of Silicates O W E N P. B R I C K E R and A N D R E W E . G O D F R E Y 1

2

The Johns Hopkins University, Baltimore, M d . E M E R Y T. C L E A V E S Maryland Geological Survey, Baltimore, M d .

The geochemical balance of a 103 acre watershed underlain by silicate bedrock was investigated. Base flow composition of the stream water was essentially constant, but flood flows showed a decrease in concentration of silica, bicarbonate, and sodium and an increase in sulfate, magnesium, calcium, and potassium. Laboratory experiments indicate that fresh rock or soil reacts rapidly with distilled water and achieves a composition similar to the stream water, suggesting control of water composition by reaction with the silicate minerals. The alumino-silicate minerals react with CO charged water to form kaolinite, releasing cations and silica to solution. The products of weathering are removed as particulate matter (0.28 metric tons per year) and dissolved material (1.5 metric tons per year). 2

Silicate minerals constitute approximately 9 5 % of the earth's crust. Most natural waters, at some time during the hydrologie cycle, con tact and react with these minerals, altering them, forming new minerals, and reflecting the interaction by changes i n their own composition. One of the most obvious and widespread examples of this process is the weathering of silicate rocks at the surface of the earth. Water is a pri mary agent of rock weathering and conversely the chemical behavior of silicate minerals i n the earth's surface environment plays an important role i n governing the composition of natural waters (1). 1 2

Geology Department Isaiah Bowman Department of Geography

128

Baker; Trace Inorganics In Water Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

6.

BRICKER E T A L .

Mineral-Water Interaction

129


T o assess the behavior of some common silicate minerals i n the weathering environment and the effect of water-silicate reactions on both the minerals and the resulting solution composition, we investigated the geochemical balance of a small wooded watershed underlain by silicate bedrock.

Cockeytville Quad

L study area

Figure I.

Baltimore Co

Location of Batsman Run and Pond Branch

Location and Description of the Watershed Pond Branch of Baisman R u n is located in the Piedmont Province of Maryland about eight miles north of Baltimore C i t y (Figure 1). This area was chosen for study because: (1) Pond Branch is a perennial stream that drains a small (103 acre) watershed, (2) all waters draining


130

TRACE INORGANICS I N W A T E R


from the basin originate as precipitation on the watershed, (3) the water shed is entirely underlain by one rock type, the Lower Pelitic Schist of the Wissahickon Formation (11), (4) the watershed is entirely forested except for a narrow transverse band of grasses, and (5) there is a pond at the mouth of the stream that was surveyed for sediment accumulation.

500 ft

Figure 2. Block diagram showing topography of the watershed

The upper portion of the watershed is a broad, open swale that changes into a narrow T ' shaped gorge toward the lower end (Figure 2 ) . Total relief i n the upper part of the valley is about 40 feet; i n the gorge the valley has a relief of over 100 feet. The long profile of the stream bed and two cross-sections of the watershed are shown i n Figure 3. The maximum and minimum average annual temperatures of the area are 65° and 45 ° F . respectively. Water temperature of the stream varied between 34° and 5 0 ° F . during period of study. Average annual precipita tion is 40 inches. Precipitation for the year 1966 was below average and amounted to 33.7 inches (Maryland State Climatologist, per. comm.).



6.

Β RICHER E T A L .

Figure 3.


131

Long profile of Pond Branch and two cross profiles of watershed

Stream run-off of Pond Branch has been measured on a continuously recording stream gage since M a r c h 1, 1966. Despite the severe drought in the northeastern United States during the summer of 1966, the base flow remained constant at two liters per second (0.07 cu. ft./sec.). The maximum flow observed, 17 liters per second (0.6 cu. ft./sec.) occurred on M a r c h 7, 1967. Six floods were recorded during the one year record of stream level. From the data at hand we estimate that these floods amounted to less than 1 % of the total annual discharge. N o overland flow of any kind was observed even during the heaviest storms. Calcu lations based on yearly input from rain and snow indicate that stream run-off removes about 20% of the average annual precipitation. The remaining 80% must leave as evapo-transpiration, percolation through the floodplain of the stream, or through deep circulation. Bedrock and Soil Mineralogy The watershed is underlain by the Lower Pelitic Schist of the Wissahickon Formation. The minerals constituting the rock, i n order of abundance, consist of quartz, muscovite, oligoclase ( A n 4 A b e ) , biotite, and staurohte with minor amounts of garnet, tourmaline, zircon, apatite, chlorite, and pyrite. The bulk chemical composition of the unweathered rock calculated from the modes is shown in Table I. The mantle of weathered rock and soil consists primarily of quartz and muscovite i n the sand sized fraction and kaolinite and illite in the clay sized fraction. Gibbsite was observed in samples from the tops of the drainage divides. 2

7


132


Minor amounts of vermiculite and mixed layer illite-vermiculite were found i n several samples and appear to be intermediate products i n the weathering sequence of muscovite and biotite to kaolinite. Table I.

Chemical Analysis of Fresh R o c k " Wt %


Oxide Si0

64.3

2

Al O 2

19.8

a

Fe 0 2

2.8

3

FeO

3.9

MgO

2.6

CaO

0.4

Na 0

0.9

κ ο

3.6

2

2

P

2

O

Zr0

5

0.1

2

0.1 1.5

H 0 2

100.0 β

Calculated from modal analysis.

The weathering sequence for oligoclase and biotite has been estab lished as follows: oligoclase —» kaolinite gibbsite (on drainage divides) oligoclase —> kaolinite (side slopes, valley bottom) biotite —> vermiculite —» kaolinite

The minerals in the unweathered rock were identified with a pétrographie microscope, and their weathering products by x-ray diffraction. Material to establish these sequences was obtained from exposures of bedrock, from pits dug on the side slope i n the gorge area, and from hand auger holes along the ridge and i n the swale. Pseudomorphs after oligoclase and biotite were picked from the soil samples. Although garnet, staurolite, and muscovite alter to new mineral phases under the influence of this environment, they are much less sus ceptible to weathering than are oligoclase and biotite. Quartz appears to be relatively inert to weathering reactions. The overall weathering relations i n this system may be summarized by the general equation: Primary alumino-silicates + H 0 + C 0 —» Aluminum rich solids + alkali cations 4- alkaline earth cations + H C 0 " (aq) + H S i 0 (aq) 2

2

3

4

4


6.

Β RICHER E T A L .

133



Water Composition The compositions of rain and stream waters were monitored for a one year period and a limited number of samples of soil water were ob tained. The stream was sampled on the average of twice a month with the exception of the summer months (see Figure 4 ) . T o minimize con tamination the samples were collected i n polyethylene bottles. Stream samples were collected by immersing the bottles i n the stream just above the gage. Rain samples were collected at the site of the stream gage by inserting a polyethylene funnel into a polyethylene bottle and mount ing this apparatus about one foot above ground level. Rain sampling was limited to the fall, winter, and spring seasons, because of the un usually dry summer. A total of seven samples were collected. O n the basis of this and other rainfall data (2), we believe that the average composition of these samples approximates the yearly average rainfall composition. Table II. Constituent silica aluminum potassium sodium calcium magnesium iron sulfate chloride bicarbonate pH

A n a l y t i c a l Methods Method

Reference

colorimetric as reduced silicomolybdate complex (7) colorimetric as colored "lake" of aurine tricarboxylate (8) flame spectrophotometric (9) flame spectrophotometric (9) E D T A compleximetric with spectrophotometric (JO) end point E D T A compleximetric with spectrophotometric (JO) end point colorimetric as bipyridine complex (10) thorin method (JO) indirect colorimetric (5) (JO) acidimétrie titration p H end point (JO) potentiometric

Soil water samples were collected by inserting a poly (vinyl chloride) pipe into an auger hole. One end of the pipe was covered by cementing a Millipore filter (0.45/*) sandwiched between two supporting screens of nylon. The other end was fitted with a rubber stopper and hose ar rangement that allowed a vacuum to be drawn on the tube. Three samples were collected i n A p r i l , M a y , and June of 1966. The waters were analyzed for S i 0 ( a q ) , aluminum, iron, mag nesium, calcium, sodium, potassium, bicarbonate, chloride, and sulfate (Table I I ) . The p H was measured i n the field using a glass electrode. The average compositions of these waters are shown i n Table III. The stream composition represents the average of 18 samples taken at base 2


134


Table III.

Average Chemical Analyses of Rain, Stream, and Soil Water at Pond Branch 0

Si0 Al Fe Mg Ca Na Κ HC0 so CI pH


2

8

4

0 b

Rain

Stream

Soil

0 0 0 1.52 2.07 1.30 1.27 0 4.58 2.91 4.63

15.46 Τ Τ 4.08 3.89 7.80 2.26 10.88 5.08 6.48 6.56

16.3 Τ Τ 3.29 4.15 5.45 4.86 »

5.45

Molal concentrations X HP. Not determined.

flow; two flood samples were not included. The rain composition repre sents the average of the seven rain samples. The soil water composition represents the average of three samples. Except during periods of flood, the composition of the stream water remained surprisingly constant throughout the year. Flood waters d i d not simply reflect dilution but were characterized by a decrease i n the concentration of silica, bicarbonate, and sodium, a depression of p H , and an increase i n sulfate, magnesium, calcium, and potassium (Figure 4). Almost all of the sulfate and a large portion of chloride are derived from rain. N o aluminum was found i n rain and the concentration of aluminum in the stream never exceeded 0.05 p.p.m. The maximum A l / S i 0 (aq) ratio i n the stream water is less than 5 X 10' , thus aluminum appears to be relatively immobile i n this system. As an approximation we w i l l as sume that aluminum is conserved in the solid phases. W i t h this stipula tion, stability diagrams for some of the common silicate minerals can be constructed. Figure 5 is a stability diagram showing relations among some minerals in the system N a 0 - H 0 - A l 0 3 - S i 0 . A similar diagram for the system K 0 - H 0 - A l 0 2 - S i 0 is shown i n Figure 6. The average compositions of soil and stream waters from Pond Branch watershed are plotted on the diagrams. The points indicate that the stable silicate phase in contact with these waters is kaolinite. F i e l d data suggest that kaolinite is the stable product of weathering throughout the watershed except at the crests of drainage divides where a kaolinitegibbsite assemblage is present. N o soil water samples are available from the drainage divides but we would predict that their composition would 2

3

2

2

2

2

2

2

2

2


6.


2 0

135


BRICKER E T A L .

H

I

M 60

I

I

I

.

•

I

I

1

•

'

'

'

A

M

J

J

A

S

0

Ν

0

J 67

F

M

L

A

Τ»·· 0» MOAffct) Figure 4.

Temporal variation of seven ions in Pond Branch

Twofloodsare represented, one on 12/66 and the other on 3167

fall somewhere along the gibbsite-kaolinite boundary with respect to silica concentration. The soil water samples collected from the side slopes have essentially the same composition as the stream water and appear to be i n equilibrium with kaolinite.



136


ANALCIME

I

2

°

J

J

-6

-5

Al Si 0 (OH) 2

L5 -4

J -3

2

5

4

ι -2

log [ S i 0 ] 2

Figure 5. Stability rehtions of some phases in the system NaoO-Al O -Si0 -H 0 at 25°C. and one atmosphere, as functions of [Να ]/[/Γ] and [SiO ] (3) 2

s

2

2

+

g(aa)

Circle represents analysis of Pond Branch water, square represents analysis of soil water Reactivity of Silicate Minerals The small magnitude and short duration of the compositional vari ation of the stream i n flood stage and the constancy of base flow com position suggest that the water-silicate reactions are rapid. The rapidity with which water equilibrates with the fresh and weathered bedrock was tested i n the laboratory ( term project of Thomas Dunne and Terrance Smith). T w o columns of soil were collected from the side slopes of the watershed. Plexiglass tubes 6 inches inside diameter by 3 % feet long were forced into the soil, then dug up. Distilled water was continuously added to the first tube and allowed to percolate through it. The effluent was sampled and then discarded. Distilled water was initially allowed to percolate through the second


6.

BBICKER E T A L .


137

10

Ô


8 LOGJK]]_

7

M 6

5 4 3

2

-5

-4

-3

-2

LOG [SiOj Figure 6. Stability relations of some phases in the system ΚιΟ-ΑΙβ,SiO-H 0 at 25°C. and one atmosphere, as a function of [K*] /[//*] and [ S i O ] (3) 2

n e o J

Circle represents analysis of Pond Branch water, square represents analysis of soil water tube, the effluent was sampled and then continuously recirculated through the tube, no more distilled water being added during the course of the experiment. In a matter of days the effluent from both tubes achieved a composition similar to that of the stream water (Table I V ) . The effluent from the tubes maintained this composition for a period of 19 weeks during which time a volume of water estimated to be equivalent to six years of rainfall was passed through the tubes. This phase of the study was then terminated and the soil columns were dried. A solution containing 50 p.p.m. S i 0 (aq) was prepared and circulated through 2


138



tube two. Concentration of silica i n the effluent stabilized i n a matter of hours at the same value observed i n the leaching experiments (•— 9 p.p.m.). Distilled water was passed through tube one and the effluent again achieved a stable silica concentration of 9 p.p.m. i n a matter of hours. Other dissolved species were not determined i n this experiment. Table IV.

Composition of Waters from Soil Column Experiments

Ion Si0

n

2 ( a q )

Tube 1

Tube 2

15.50

15.58

Na

4.35

5.00

Κ

5.08

4.60

Ca

5.62

4.68

β

Molal concentration Χ 10·\

In another experiment several grams of crushed unweathered rock from the watershed were placed in distilled water i n a polyethylene bot tle. In a second bottle, several grams of fresh rock were placed i n stream water. The bottles were then placed on a shaker table, and the waters were analyzed periodically. Within a week the distilled water had achieved a composition similar to the stream (less rain water contribu tion) and the stream water had not changed (Table V ) . W e conclude from the experiments that the water reacts rapidly with the silicate minerals and the final concentrations of the dissolved constituents i n the water are controlled by the silicate minerals. (The results of these ex periments and data for a group of individual minerals w i l l be presented in detail i n a separate publication. ) Table V .

Composition of Waters from Crushed Rock Experiments* Ion

Si0

rt

Crushed Rock in Distilled Water

Crushed Rock in Stream Water

15.13

15.49

Na

5.60

7.46

Κ

0.93

2.45

Ca

1.73

3.97

2 ( a q )

Molal concentration X 10 \ r

Calculation of Weathering Reactions The contribution of minerals i n the fresh rock to the dissolved load of the stream can be assessed by calculating the weathering reactions i n



6.

BRICKER E T A L .


139

reverse; i.e., by reconstituting the original minerals by back reaction of the stream water with the products of weathering ( 6 ) . The results of a balance of this type for the Pond Branch watershed is shown i n Figure 7. The contribution of rain to the stream water composition is subtracted and leaves that portion of the dissolved load that is supplied by rock weathering. Goethite, an abundant iron oxide i n the weathered mantle, was used to back react a l l of the sulfate, forming pyrite and liberating some bicarbonate. The sodium and calcium i n the stream is supplied by the breakdown of oligoclase. Reaction of all of the sodium and calcium with kaolinite and gibbsite to reconstitute oligoclase accounts for most of the dissolved silica. Note that the composition of the reconstituted oligo clase is almost the same as the composition of the oligoclase i n the fresh rock, which suggests that the assumption concerning the source of sodium and calcium is correct. A t this point a small amount of silica and bicarbonate and a l l of the potassium, magnesium, and chloride remain. Biotite, an alumino-silicate containing magnesium, iron, and potassium, is the most abundant magnesian mineral i n the rock. Reconstituting biotite of the composition found i n the fresh rock (4) by the reaction of the potassium, bicarbonate, silica, and magnesium i n the water with kaolinite and goethite leaves a residual of chloride, a slight deficit of bicarbonate and some magnesium. The small imbalance i n charge of the residual ions is a reflection of the initial small error of the stream water analysis. Although this balance is rather crude, it shows that most of the dis solved constituents i n the stream water can be accounted for by the breakdown of oligoclase. The weathering of biotite probably contributes the rest of the silica, the potassium and a portion of the magnesium. These two minerals, however, constitute only about 20% by weight of the original rock. This implies that the rate of weathering of the other minerals i n the rock is slow with respect to that of oligoclase and biotite. The excess of chloride ( and the balancing cations ) is hard to explain in terms of rock weathering as none of the minerals constituting the fresh rock are known to contain chloride. Gambell and Fisher (2) observed a chloride excess i n a comparison of rain and river compositions i n North Carolina and Virginia. They attribute the excess chloride i n the streams to rock weathering. Therefore there is either a minéralogie source of chloride that has been overlooked or our rain samples are not representa tive. It is also possible that there is a contribution to the watershed from fallout of airborne dust. W e cannot evaluate the importance of this source at the present time.


140


Moles of Material i n Si0

Ca

15.46 15.46 15.46 12.68 1.26 0 0

3.89 1.82 1.82 1.60 0 0 0


2

stream rain geothite —» py gibbsite —» olig. kaolinite —> olig. kaolinite —> bio residual

Na

2+

K*

+

2.26 0.99 0.99 0.99 0.99 0 0

7.80 6.50 6.50 5.72 0 0 0

4.08 2.56 2.56 2.56 2.56 0.91 0.91

Reactions (1) Geothite -> py: 0.25FeOOH + 0.5SO ' + C 0 = 0.25FeS + I.OHCO3 + 0.94O + 0.38H O 4

2

2

2

2

2

(2) Gibbsite - » olig: 0.61Al O · 3 H 0 + 0.78Na + 0.22Ca + 2.78Si0 = N a o . C a o . A l S i . 0 + 1.22H* + 1.22H 0 2

2

78

3

+

2

22

122

2

78

2+

8

2

Figure 7. Mass balance calculation for Fond Branch Watershed. The balance weathering to reconstitute the original mineral constituents of the rock (see text Weathering Model O n the basis of mineral distribution and water compositions a simple model of weathering can be deduced. Rain, low in cations and devoid of silica, falling on the drainage divides contacts a kaohnite-gibbsite as semblage. It reacts with kaolinite to form gibbsite, releasing silica to the solution until the silica concentration of the gibbsite-kaolinite equilibrium is achieved. As the water percolates downward into less weathered mate rial it encounters and reacts with more siliceous minerals than kaolinite. The silica concentration of the solution increases. Gibbsite is no longer stable, and kaolinite alone becomes the stable phase. It is this water that emerges as the base flow of the stream. Transport of Material The products of weathering are removed from the watershed as dis solved and particulate material. Baseflow of the stream for the year of record removed 1.5 metric tons of dissolved material, or 0.015 metric tons per acre. This figure is a minimum value, and would be increased by material leaving the basin by deeper circulation and by flood flow. Mechanical transport of solids by the stream, as suspended or bed load, removes additional material. Direct measurement of suspended load yields a figure of 0.28 metric tons, or 0.0027 metric tons per acre for the


6.

141


BRICKER E T A L .

10 Liters of Water 5


SOf 0.02 -2.33

5.08 0.50

-2.33 -1.11 0 0 0

0 0 0 0 0

HCOf

ci-

10.88 10.88

6.48 3.57

11.88 11.88 4.07 -0.22 -0.22

3.57 3.57 3.57 3.57 3.57

Amount of material pro duced by the back reaction of 10"' 1. stream water with weathered rock 0.25 mole FeS 1 mole olig. 7.32 mole olig. 1.18 mole biotite 2

(3) Kaolinite -> olig: 4 . 4 7 A l S i 0 ( O H ) + 5.72Na + 1.60Ca 11.42Si0 + 7.8IHCO3- = 7.32Na C a A l ! 22812 0 12.29H 0 + 1.11H + 7.81C0 2

2

5

+

4

2

0 7 8

+

2

+ +

2+

0 2 2

7 8

8

2

(4) Kaolinite -> bio: 0 . 9 6 A l S i O ( O H ) + 1.25Si0 + 4.29HC0 - + 0.99K + 1.65Mg + 1.59FeOOH= 1.18K M g i F e ! Ali.e3Si .eo0 (OH) + 7.97H 0 + 4.29C0 + 0.71O 2

+

2

6

4

2

2+

2

10

3

0 8 4

2

2

2

4 0

35

2

is based on the reaction of 10 liters of Pond Branch water with products of for discussion) 5

year of record. Flood flows carried about the same concentration of suspended load as baseflow. The sediment trapped in the pond at the lower end of Pond Branch provides a nine year record of suspended and bedload. The pond was surveyed in 1957 and again in 1966. Comparison of the two surveys shows a slight deficit in accumulation. Thus, if there was any accumulation it was less than the errors in measurements. The ratio of material removed as dissolved load to that transported as clastic load is 5.5 on the basis of figures cited above. This ratio may be somewhat high because of the limited number of very high flows. C a l culations suggest that it would take a stream similar to Pond Branch 3.5 million years to remove the amount of material necessary to form the gorge and swale areas of the present watershed. Conclusions This work, although a modest beginning in our effort to understand the geochemical balance of a small watershed, permits us to draw some tentative conclusions: ( 1 ) The major portion of the dissolved load of Pond Branch is de rived from rock weathering. (2) C 0 charged rain reacts rapidly with the silicate minerals in the fresh rock and soil producing solid phases higher in aluminum than 2



142


the original silicates; the solution is enriched in dissolved silica, cations, and bicarbonate ion. (3) Aluminum and iron are conserved in the solid phases. (4) The reactions of water with silicates are so rapid that the waters remain in near-equilibrium with one or more solid phases at all times. (5) The major portion of the dissolved silica in the stream is de rived from the breakdown of oligoclase feldspar even though quartz is the major silica mineral in the rock. (6) The rate of removal of material as dissolved load is about five times greater than the rate of mechanical removal for this small wooded watershed. Acknowledgements W e would like to thank M . A . Wolman and B. F . Jones and C . B. Hunt for reading the manuscript and for offering many useful sugges tions. Terrance Smith and Thomas Dunne were most helpful in various aspects of both the field and laboratory work. This work was supported by grants from the Petroleum Research F u n d of the American Chemical Society (665-G2) and National Science Foundation ( G P 2660). Literature Cited (1) Bricker, O. P., Garrels, R. M., "Principles and Applications of Water Chemistry," p. 449, S. D . Faust and J. V . Hunter, eds., John Wiley & Sons, New York, New York, 1967. (2) Gambell, W . Α., Fisher, D . W., U. S. Geol. Surv. Water Supply Papers 1535-K (1966). (3) Hess, P. C., Am. J. Sci. 264, 289 (1966). (4) Hopson, C. Α., "The Geology of Howard and Montgomery Counties," Maryland Geol. Surv., 1964. (5) Iwasaki, Iwaji, Utsumi, S., Ozawa, T., Bull. Chem. Soc. Japan 25, 226 (1952). (6) Mackenzie, F. T., Garrels, R. M., Am. J. Sci. 264, 507 (1966). (7) Mullin, J. B., Riley, J. P., Anal. Chim. Acta 12, 162 (1955). (8) Packham, R. F., Proc. Soc. Water Treat. Exam. 7, 102 (1958). (9) Pinta, M., "Recherche et Dosage des Elements Traces," Dunod, Paris, 1962. (10) Rainwater, F. H., Thatcher, L . L., U. S. Geol. Surv. Water Supply Papers 1454 (1960). (11) Southwick, D . L., Fisher, G . W., "Revision of Stratigraphic Nomenclature in the Glenarm Series, Appalachian Piedmont," Maryland Geol. Surv., (report in preparation). R E C E I V E D May

11,

1967.


Trace Inorganics In Water - ACS Publications - American Chemical

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