Environ. Sci. Technol. 1988, 22, 1293-1299
(16) Yin, C.; Hassett, J. P. Enuiron. Sci. Technol. 1986, 20, 1213-1217. (17) Danckwerts, P. V. Gas Liquid Reactions; McGraw-Hill: New York, 1970; 276 pp.
U.S.Environmental Protection Agency: Washington, DC, 1982.
Emmert, R. E.; Pigford, R. L. Chem. Eng. Prog. 1954,50, 78-95.
Hassett, J. P.; Milicic, E. Environ. Sci. Technol. 1985, 19, Received for review November 20,1987. Accepted April 11,1988.
638-643.
Aluminum Chemistry Downstream of a Whole-Tree-Harvested Watershed Gregory 6. Lawrence*'$and Charles T. Driscoll*
Department of Plant and Soil Sciences, University of Maine, Orono, Maine 04469, and Department of Civil Engineering, Syracuse University, Syracuse, New York 13244-1 190
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From fall 1983 through spring 1984, watershed 5 at the Hubbard Brook Experimental Forest in New Hampshire was commercially whole-tree harvested. Stream chemistry was monitored within the cut watershed, in an adjacent reference watershed (watershed 6), and below the confluence of these two streams for a period of 15 months prior to completion of the cut and 12 months following the cut. Whole-tree harvesting acidified watershed 5, due to increased soil nitrification, resulting in stream acidification within the disturbed watershed and for a distance downstream that encompassed a drainage area approximately 5 times that of watershed 5. Concentrations of NOs-, inorganic Al, and basic cations (Ca2+,Mg2+,Na+, K+) increased following the cut, whereas SO?- concentrations decreased. Concentrations of inorganic A1 below the confluence of watersheds 5 and 6 exceeded values toxic to fish, but were temporally variable. Inorganic A1 concentrations downstream of watershed 5 appeared to be controlled by dilution following the cut. There was no indication that hydrolysis was induced by mixing of streamwater from the acidic experimental watershed and undisturbed adjacent watershed. Absence of hydrolysis was due to low acid-neutralizing capacity of streamwater from the undisturbed drainage.
Introduction Impacts of forest removal on stream chemistry have recently been evaluated following the whole-tree harvest of watershed 5, at the Hubbard Brook Experimental Forest (HBEF) ( I ) . Stream concentrations of NO, and inorganic A1 increased within the harvested reach, whereas pH and SO?- concentrations decreased during the first year following the cut. Loss of vegetation increased soil moisture and temperature. These effects, coupled with a large pool of available organic nitrogen, stimulated nitrification and accelerated loss of NO, from the watershed. Acidity, generated by nitrification, mobilized inorganic A1 within the soil. This led to AI concentrations in streamwater that exceeded values found to be toxic to fish and other aquatic life (2, 3). Although stream chemistry changed substantially within watershed 5, impacts downstream of the disturbed watershed have not been previously reported. Changes in water chemistry and associated effects on aquatic life would be expected as water with higher acid-neutralizing capacity (ANC) from adjacent drainage areas mixed with the acidic stream. Formation of A1 hydrolysis products was observed (4) below the confluence of a stream acidified by pyrite oxidation and a stream with more neutral pH 'This is a contribution of the Hubbard Brook Ecosystem Study. Research Associate, University of Maine. 8 Professor, Syracuse University. 0013-936X18810922-1293$01.50/0
containing high concentrations of basic cations (Ca2+, Mg2+,Na+, and K+). The stream bed below this confluence was coated by aluminum hydroxide precipitates, which adversely affected periphyton and the benthic community (5). Nordstrom and Ball (6) also investigated the mixing of acidic drainage waters with nonacidic streams. These workers sampled streamwater in a basin receiving acid mine drainage with maximum A1 concentrations that exceeded 20 mM. Chemical speciation of Al in these samples suggested that below pH 4.6 A1 concentrations were conservative with respect to pH and that concentrations were controlled by dilution. Above pH 4.6, A1 hydrolysis occurred leading to the formation of an aluminum trihydroxide mineral. Decreases in pH and shifts in A1 speciation have also been observed in streams impacted by acidic deposition (7,8). These changes are generally more episodic in nature, but also result in deleterious impacts on aquatic life. Mortality of spawning Atlantic salmon (Salmo salar) was observed in a Norwegian river during an acidification episode when pH decreased to between 5.17 and 5.54 and inorganic A1 concentrations ranged from 4.04 to 4.93 pM (9). Aluminum has been found to be most toxic to fish in oversaturated concentrations (10). Experimental acidification coupled with addition of A1 to a stream at the HBEF has also been observed to disrupt the aquatic community, causing increased invertebrate drift (3). To more completely assess the effects of whole-tree harvesting of watershed 5, streamwater was analyzed at locations along a longitudinal gradient, downslope of the cut watershed. Our specific goals were to evaluate changes in A1 chemistry as acidic water draining clear-cut watershed 5 mixed with water from undisturbed drainages and to assess the downstream impacts of these changes.
Methods The streams sampled in this study all drain south facing watersheds with similar geology and soils. Vegetation is predominantly mixed northern hardwoods, with small stands of coniferous vegetation, primarily at the uppermost elevations. Prior to the current experiment, this region of the HBEF had not been disturbed since 1910-1919, when it was extensively logged. Detailed descriptions of these watersheds are given elsewhere (1, 11-13). Commercial whole-tree harvesting of watershed 5 began in the fall of 1983 and continued through the spring of 1984. Data collection for this study began in March 1983 and extended to May 1985, providing 15 months of precut information and 12 months of evaluation following the cut. Sampling that occurred during the cut was included with the precut period, since the first changes in stream chemistry were not observed until after completion of the cut. Streamwater samples were collected monthly along ele-
@ 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988
1293
Flgure 1. Stream sampling locations and drainage areas of watersheds 5 (W5; stream sampling sites 5-1 through 5-6) and 6 (We; stream sampling sites 6-1 through 6 4 ,Bear Brook (stream sampling sites BE1 through BE3), and Hubbard Brook (stream sampllng sites H E 1 and HB-2). Elevations expressed in meters are indicated by dashed Ilnes.
vational gradients in the clear-cut watershed (watershed 5; 22.5 ha) and adjacent reference watershed (watershed 6; 13.1 ha), in addition to the samples collected below the confluence of the streams draining these watersheds (Figure 1). The streams draining watershed 5 and watershed 6 will be referred to as W5 and W6, respectively. Samples collected below the confluence included three sites in Bear Brook (BB-1through BB-3), one site just upstream of the point where Bear Brook enters Hubbard Brook (HB-l), and one site in Hubbard Brook 50 m below the entry of Bear Brook (HB-2; Fibure 1). Sampling locations in watersheds 5 and 6 corresponded to the sites sampled in previous investigations of water chemistry in these watersheds (1, 12,13). A detailed description of stream chemistry within watershed 5, following the whole-tree harvest, is given elsewhere (I). Stream-gauging weirs located at the base of watersheds 5 and 6 provided continuous measurement of flow. Flow at sampling locations in Bear Brook was estimated from flow measured at the weirs and the respective proportion of the basin drained by each site. Flow at sites HB-1 and HB-2 was not estimated due to the large area drained by these sites (-2500 ha) relative to the area of the gauged watersheds (35.6 ha). A correction factor, F, was developed watershed 6 precut growing season flow FR6 = watershed 6 postcut growing season flow watershed 5 postcut growing season flow FR5 = watershed 5 precut growing season flow growing season F = FR6 X FR5 = 1.513 nongrowing season F = 1.007 to adjust postcut flow estimates for any changes in the relationship between flow and drainage area resulting from vegetation removal in watershed 5. Discharges used for this calculation were obtained by summing the daily flow measurements during both growing 1294
Envlron. Sci. Technol., Vol. 22, No. 11, 1988
seasons (May 15 to October 15; 11) and dormant seasons (October 16 to May 14), over a 4-year period. The period when harvesting was in progress was not included in these calculations. Since F equaled 1.0 during the dormant season, but not during the growing season, observed differences in dischargedrainage area relationships, following the cut, were attributed to elimination of transpiration due to removal of the trees. Streamwater solutions were analyzed for all major solutes (>1pM) by procedures previously described (13). Labile monomeric A1 was estimated by the difference of total monomeric A1 and nonlabile monomeric Al. This and fraction includes aquo A1 (A13+)as well as F, S042-, OH- complexes and will be referred to as inorganic Al. The development and application of the A1 speciation procedure is discussed in detail elsewhere (14). Dissolved inorganic carbon (DIC) was measured on selected samples with a total carbon analyzer (15). Sample concentrations of DIC were generally close to equilibrium with the atmospheric C02and did not contribute significantly to total anion equivalence due to the low pH of most samples (4.2-6.0). Acid-neutralizing capacity (ANC) was also determined, using strong-acid titration and Gran plot analysis (16). Organic anion concentration was estimated by charge discrepancy obtained from measured solute concentrations (17).Chemical speciation of inorganic A1 was calculated by the chemical equilibrium model ALCHEMI from measured values of ionic strength, temperature, pH, and the concentrations of labile monomeric Al, total F, S042-, and dissolved Si (18). ALCHEMI also calculates mineral saturation indices (log [ion activity product/solubility product]) for synthetic gibbsite, natural gibbsite, microcrystalline gibbsite, amorphous aluminum trihydroxide, kaolinite, halloysite, jurbanite, and alunite. To evaluate controls of A1 chemistry, ALCHEMI was used to predict inorganic A1 concentration and saturation indices immediately followingmixing of W5 and W6. This was done by averaging (using flow as a weighting factor)
6ot 50
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Flgure 2. Annual mean streamwater concentrations of NO, and : O S in watershed 5 (triangles), watershed 6 (circles), and the downstream reaches of Bear Brook and Hubbard Brook (solld squares), following whole-tree harvesting of watershed 5. Open squares represent annual mean precut stream chemistry downstream of watersheds 5 and 6; standard errors are represented by vertical bars.
the measured concentrations of W5 and W6 needed as model inputs, prior to running the model.
Results Changes in the chemistry of Bear Brook following the whole-tree harvest were similar to the responses observed in W5, but were smaller in magnitude. Annual mean concentrations of NO3-, inorganic Al, and basic cations were generally higher following the cut than concentrations averaged over a 1-year period prior to completion of the cut (Figures 2 and 3). Also similar to W5, annual mean SO-: concentrations, following the cut, were less in Bear Brook than annual mean precut concentrations (Figure 2). With the exception of inorganic Al, postcut concentrations in Bear Brook were intermediate to concentrations in W5 and undisturbed W6 (Figures 2 and 3). No other measured solute exhibited statistically significant (P > 0.05) differences between annual mean precut and postcut concentrations at any of the sites below the confluence of these streams. Temporal responses of stream chemistry below the confluence of W5 and W6 were also similar to trends observed in W5, but solute concentrations exhibited considerable variation during the postcut period, particularly in the case of NO3- and inorganic A1 (Figures 4 and 5 ) . Nitrate and basic cation concentrations began to increase and Sod2-concentrations began to decrease during midsummer of 1984 in both watershed 5 and Bear Brook (Figures 4 and 5). The trend of basic cation concentrations in Bear Brook was somewhat obscured by high concen-
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660
560
1
460
360
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Figure 3. Annual mean streamwater concentrations of inorganlc AI and basic cations (CB;the sum of Ca2+, Mg2+, Na', and K+ in hequiv L-') in watershed 5 (triangles), watershed 6 (circles), and the downstream reaches of Bear Brook and Hubbard Brook (solid squares), following whole-tree harvesting of watershed 5. Open squares represent annual mean precut stream chemistry downstream of watersheds 5 and 6; standard errors are represented by vertical bars.
trations measured during an extremely dry period in summer and early fall of 1983. An increase of inorganic A1 concentrations was first evident in the fall of 1984 in the streams of these watersheds (Figure 5 ) . The delay in this response was due to the ability of watershed 5 to initially buffer stream chemistry by the release of basic cations (1). In each case, effects on stream chemistry below the confluence of W5 and W6 decreased moving down the reach (Figure 4 and 5 ) , with only NO3- concentrations affected at site HB-2 (Figure 1). To investigate inorganic A1 chemistry in more detail, early winter samples were compared before (precut; December 17,1983) and after (postcut; January 4,1985) the whole-tree harvest. Sampling dates were chosen when inorganic A1 concentrations were high (Figure 5) and seasonal variation due to biological activity was minimal. Transport of A1 on these dates was evaluated by multiplying concentrations by flow to obtain flux values. A positive slope yielded by plotting solute flux as a function of elevation would indicate that the solute is being added throughout the reach, whereas a negative slope would indicate that the solute is being removed from solution as it travels through the reach. A slope of zero would indicate that the solute has been added as a point source and is reacting conservatively throughout the reach; that is, any removal of solute from solution is balanced by an equal mass entering the stream solution. In both precut and postcut samples, flux of inorganic A1 increased between W5 and W6, and site BB-1, but did Environ. Sci. Technol., Vol. 22, No. 11, 1988
1295
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Figure 4. Stream concentrations of NO3- and SO4*- downslope of watersheds 5 and 6 (stream sampling sites BB-1 through BB-3 in Bear
Brook and site HB-2 in Hubbard Brook) from March 1983 through May 1985. The approximate completion date of whole-tree harvesting in watershed 5 is indicated by the dashed vertical line.
not change substantially through the remainder of the reach (Figure 6). The combined flux of W5 and W6 were somewhat less than that observed at site BB-1, in each case, due to the additional contribution of the remainder of the drainage area (Figure 1). The relative importance of the nonwatershed 5 contribution (watershed 6 plus additional drainage outside the boundaries of watersheds 5 and 6; Figure 1))however, was much less in the postcut samples. In addition, a substantially greater flux occurred in the postcut samples, even though flow on this date was less than half that of the precut date (Figure 6). The speciation of inorganic A1 also differed between the precut and postcut samples (Figure 7 ) . Fluoride-bound A1 was the dominant fraction in the precut sample, exceeding 30% of total inorganic A1 at all sites below the confluence of W5 and W6. In the postcut sample, F-bound A1 was the major species at sites BB-2 and BB-3, whereas A13+ was the dominant fraction at site BB-1 and A1(OH)2+ was the dominant fraction at site HB-2. In addition, postcut concentrations of A13+exceeded 20% of total A13+ at sites BB-1 through BB-3, but were less than 15% of total A1 at these sites in the precut sample. Comparison of saturation indices (SI) indicated that dilution rather than hydrolysis/precipitation controlled A1 concentrations following mixing. Stream solutions of W5 and W6 were close to saturation with natural gibbsite, ,oth before (5-6 SI = 0.23; 6-5 SI = 0.30) and after mixing (SI = 0.30). Mixed solution concentrations of inorganic A1 species predicted by ALCHEMI were nearly identical with those calculated by mass balance relationships. Hydrolysis/precipitation did occur (SI = l.l), however, when neutral streamwater chemically similar to HB-1 1296
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Environ. Sci. Technol., Vol. 22, No. 11, 1988
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Figure 5. Stream concentrations of inorganic AI and basic cations (CB; the sum of Ca2+, Mg2+,Na', and K+) downslope of watersheds 5 and 6 (stream sampling sites BB-1 through BB-3 in Bear Brook and site HB-2 in Hubbard Brook) from March 1983 through May 1985. The approximate completion date of whole-tree harvesting in watershed 5 is indicated by the dashed vertical line.
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and January 4, 1985 (postcut).
(Figure 1)on this date (pH 7.04; ANC 20 pM) was mixed with W5. Flows of the postcut sampling date were also assumed for these calculations. Allowing equilibration of this solution with natural gibbsite resulted in precipitation of -20% of the total inorganic Al. By use of the change balance approach of Driscoll and Newton (17), neutralization processes were also investi-
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gated before and after the whole-tree harvest (Figure 8). Little neutralization of acidity occurred below the confluence of W5 and W6 on the precut sampling date. Sulfate was the dominant acidic anion at all sites, followed by organic anions, and C1-, respectively. Streamwater at all sites was very dilute on this date with an ionic strength less than 240 pequiv L-l. Charge balances of the postcut sampling date exhibited a more pronounced elevational trend below the confluence of W5 and W6. Ionic strength at all sites was substantially higher than in the precut samples, ranging from 370 pequiv L-l at site BB-1 to 260 Hequiv L-l at site HB-2. A more pronounced increase in pH also occurred over the reach than on the precut sampling date. Basic cation concentrations, however, exhibited a decrease from site BB-1 through site HB-2.
Discussion Acidification of watershed 5, resulting from the wholetree harvest, influenced stream chemistry along the entire sampled reach. Observed changes downstream of watershed 5, however, were more subtle than the responses observed within the cut watershed. Superimposed upon the effects of the clear-cut were the ambient controls of stream chemistry throughout the undisturbed drainage basin. Watershed 5 comprised only 24% of the area drained by site BB-1 and 19% of the area drained by site BB-3 (Figure 1). Site HB-2 drained the majority of the Hubbard Brook basin, with watershed 5 comprising less than 5% of this site's drainage. Previous work in W6 indicated that stream chemistry was closely tied to hydrologic conditions, which vary substantially throughout the year (I I). Temporal variations in stream chemistry below the confluence of W5 and W6 were, therefore, a function of hydrologic conditions in the
undisturbed drainage areas as well as processes occurring within watershed 5. Due to these factors, the major impact in stream chemistry was on an episodic rather than a sustained basis. The potential effect of episodic changes in stream chemistry was illustrated by the flux of inorganic AI on the postcut sampling date (Figure 6). Not only was downstream transport of inorganic A1 increased, but differences in speciation also occurred relative to the precut date in Bear Brook. A much larger fraction of total inorganic Al was transported as A13+on the postcut date than on the precut date. This shift in speciation resulted from large increases in A1 concentrations on the postcut date relative to increases in concentrations of complexing ligands. In Bear Brook, total F concentrations were approximately one-third of total inorganic A1 concentrations (Figures 7 and 8) and pH was below 5.25, limiting available hydroxide ligands. On the precut date, total F concentrations were approximately half that of total inorganic A1 concentrations and pH was somewhat higher, resulting in a smaller fraction of A1 being transported as A P . Hydrolysis did not occur following the mixing of W5 and W6 in postcut solutions, however, due to the low ANC of W6. Acid-neutralizing capacity of site 6-5 equaled -0.4 pequiv L-' on the postcut date and ranged between -12 and +18.0 pequiv L-l during the study period. Aluminum concentrations were sufficiently reduced by dilution to prevent hydrolysis, due to minimal neutralization of SOlution acidity. Mixing streamwater from W5 with water chemically similar to that found lower in the drainage, however, did result in moderate hydrolysis/precipitation of Al. A more extensive cut would have extended elevated A1 concentrations further downstream, increasing the potential for A1 hydrolysis/precipitation to occur. Downstream transport of A1 has been previously investigated by Nordstrom and Ball (6) who also reported evidence that A1 concentrations were controlled by dilution, suggesting that below pH 4.6 Al was conservative with respect to H+ concentration. In Bear Brook, however, pH was above 4.95 on the precut and postcut sampling dates (Figure 8) and A1 was not conservative with respect to pH. A slope of 2.68 (SE = 0.20) was obtained by plotting the negative logarithm of A13+activity as a function of pH for all samples collected below the confluence of W5 and W6. This value approximates a slope of 3.0, which would suggest that A1 concentrations were being controlled by an Aluminum trihydroxide mineral (19). Analysis of solution charge balances indicates that although basic cation concentrations increased slightly through the reach in the precut samples and decreased in the postcut samples, more neutralization of acidity actually occurred in Bear Brook on the postcut sampling date (Figure 8). Neutralization of the postcut sample occurred because basic cation concentrations were diluted less than NO,- concentrations due to the influence of the undisturbed drainage. The small increase in precut basic cation concentrations through the reach did little to increase pH because Sod2concentrations remained fairly stable.
Conclusion Changes in downstream chemistry showed that impacts of the whole-tree harvest were not confined within the boundaries of the disturbed watershed. The substantial effects of the cut, relative to the extent of the upstream disturbance, indicated the potential for substantial impacts on water quality from this form of timber harvesting. The majority of the Hubbard Brook basin was logged 65-75 years ago (11). If changes in water chemistry were similar to those observed following the cutting of watershed 5, the Environ. Sci. Technol., Vol. 22, No. 11, 1988
1297
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,
-re 8. The distribution of ionic solutes on December 17. 1983 (precut) and January 4, 1985 (postcut) in streams drahhg watersheds 5 and 6 (sites 58 and 6 5 . respectively), Bear Bmok (sites B E 1 throw B W ) , and Hubbard Brook (site HE2). The sum of bask catkms (Ca". Mg", Na+. and K+) b repesented by G:A r + repesem~the total charge of inagsnk AI; R C ~is-an &mate of unmeasued orgenk anbn equivaknce obtained by discrepency in electroneutralky; DOC refers to dissolved organic carbon (micromolar C); TF refers to total fluoride (micromolar): I refers to ionic strength. Note the alternate scale used for postcut sample 5 6
impact on downstream chemistry would have been severe. The effects of the whole-tree harvest on downstream chemistry have important implications for aquatic life. The absence of hydrolysis, following mixing, prevented disruption of the benthic community and reduced nutrient concentrations due to coprecipitation with aluminum hydroxides. Control of AI concentrations through dilution, however, extended the downstream transport of inorganic AI. The net result of these factors may have been a less severe impact occurring over a larger distance of the reach. The observed episodic increases of inorganic AI concentrations were similar to responses that have been observed during high flow in watersheds impacted by acidic deposition (8, 13). Neutralization of acidic pulses from headwater streams depends upon the characteristics of the downstream reach. If drainages immediately downstream do not have the ability to increase basic cation concentrations, relative to acidic anions, the flux of acidic streamwater will be extended through the reach. Furthermore, temporal variations related to changes in stream 1298 Environ. Sci. Technol.. Voi. 22. NO. 11, 1988
flow mav cause a uarticular section of the stream channel to be a zone of Alhydrolysis/precipitation when solutions are oversaturated with resoed to mineral Dhase solubilitv and a source of dissolved inorganic AI under acidic conditions that are undersaturated with respect to mineral phase solubility. Considerable effort has been spent investigating the processes causing stream acidification; however, less consideration has been given to the processes involved in the neutralization of acidic streamwater. Additional work is needed to integrate watershed acidification/neut.ralization processes with mechanisms that operate within the stream channel. Acknowledgments We are very grateful for the field assistance of C. W. Martin and D. C. Buso and the stream flow data provided by J. W. Hornbeck and the Northeastern Forest Experiment Station, U.S. Department of Agriculture, Broomall,
Environ. Sci. Technol. 1988, 22, 1299-1304
PA. Aluminum speciation was made possible through the modeling efforts of W. D. Schecher. The advice and assistance of R. D. Fuller was also greatly appreciated. Registry No. Al, 7429-90-5; H+, 12408-02-5;Ca, 7440-70-2; Mg, 7439-95-4; Na, 7440-23-5; K, 7440-09-7.
Literature Cited Lawrence, G. B.; Fuller, R. D.; Driscoll, C. T. J.Environ. Qual. 1987.16.383-390.
Schofield, C. L:; Trojnar, J. R. In Polluted Rain; Toribara, T. Y., Miller, M. W., Morrow, P. E., Eds.; Plenum: New York, 1980; pp 347-366. Hall, R. J.; Driscoll, C. T.; Likens, G. E.; Pratt, J. M. Limnol. Oceanogr. 1985,30, 212-220.
Theobald, P. K., Jr.; Lakin, H. W.; Hawkins, D. B. Geochim. Cosmochim. Acta 1963,27, 121-132.
Mcknight, D. M.; Feder, G. L. Hydrobiologia 1984, 119, 129-138.
Nordstrom, D. K.; Ball, J. W. Science (Washington,DE.) 1986,232, 54-56.
Driscoll, C. T.; Schafran, G. C. Nature (London) 1984,310, 308-310.
Sullivan, T. J.; Christophersen,N.; Muniz, I. P.; Seip, H. H.; Sullivan, P. D. Nature (London) 1986,323, 324-327.
(9) Henriksen,A.; Skogheim, 0. K.; Rosseland, B. 0. Vatten 1984, 40, 255-260. (10) Baker, J. D.; Schofield, C. L. Water,Air, Soil Pollut. 1982, 18, 289-309. (11) Likens, G. E.; Bormann, F. H.; Pierce, R. S.; Eaton, J. S.; Johnson, N. M. Biogeochemistry of a Forested Ecosystem; Springer-Verlag: New York, 1977; p 146. (12) Lawrence, G. B.; Fuller, R. D.; Driscoll, C. T. Biogeochemistry 1986, 2, 115-135. Lawrence,G. B.; Driscoll, C. T.; Fuller, R. D. Water Resour. Res., in press. Driscoll, C. T. Znt. J. Environ. Anal. Chem. 1984, 16, 267-284. Dohrmann, Xertex Corp., Santa Clara, CA, 1984. Gran, G. Znt. Congr. Anal. Chem. 1952, 77,661-671. Driscoll, C. T.; Newton, R. M. Environ. Sci. Technol. 1985, 19,1018-1024. Schecher, W. D.; Driscoll, C. T. Water Resour. Res. 1987, 23, 525-534. (19) Driscoll, C. T.; Baker, J. P.; Bisogni, J. J.; Schofield, C. L. Nature (London) 1980, 284, 161-164.
Received for review July 7,1987. Accepted April 26,1988. This work was supported by the National Science Foundation (Grant BSR-8406634).
Aqueous Reaction of Fly Ash and Ca(OH), To Produce Calcium Silicate Absorbent for Flue Gas Desulfurization Joseph R. Peterson and Gary T. Rochelle" The University of Texas at Austin, Austin, Texas 78712
Fly ash was slurried with Ca(OHIzat 85 O C to produce reactive solids for use in dry processes for flue gas desulfurization. Reacting slurries of fly ash and Ca(OH)z were monitored for dissolved metal concentrations. The solids produced were dried and tested for reactivity toward SOz in a packed-bed reactor at bag filter conditions. The dissolved calcium concentration in the slurry was a very important parameter for solids reactivity. The solids formed in slurries containing 10-100 ppm dissolved calcium were up to 40% more reactive than the solids formed in slurries containing less than 10 ppm or more than 100 ppm dissolved calcium. The dissolved calcium concentration was affected by the NaOH concentration, the ratio of fly ash to Ca(OH)z,the slurry temperature, the fly ash type, and the presence of calcium sulfite.
Introduction Two important dry processes for flue gas desulfurization (FGD) utilize spray drying of Ca(OH)zand the injection of dry, calcium-based sorbent into humidified gas at 60-100 "C (I). In both of these technologies, gas/solids reaction makes a major contribution to SOz removal both in the ductwork and in the particulate removal device, especially if a bag filter is used. The reaction of fly ash with Ca(OH)2 to produce reactive solids for use in these processes has been investigated up to the pilot-plant scale (2-6). The reaction of fly ash with Ca(OHIz is called a pozzolanic reaction and has primary importance in the chemistry of cement (7). The use of fly ash as a reagent material for the production of reactive solids for FGD is very attractive, both economically and environmentally, because fly ash is a waste product from all coal-fired power plants. Jozewicz (2)studied the reaction of fly ash with Ca(OH), to produce reactive solids for use in FGD. In bench-scale experiments using a packed-bed reactor, Jozewicz found 0013-936X/88/0922-1299$01.50/0
that the solids produced by slurrying fly ash with Ca(OH)z were much more reactive toward SOz than was Ca(OH)z. He found that increasing the fly ash loading from 0.5 to 20 g of fly ash/g of Ca(OH)z increased the Ca(OH)zutilization from 17 to 78%. Jozewicz also found that silica was the most reactive component of the fly ash and that the solids reactivity increased with slurry time and slurry temperature. Jozewicz postulated that the rate-limiting step of the reaction of fly ash with Ca(OHI2was the dissolution of silica from the fly ash. In order to increase the reaction rate between fly ash and Ca(OH)2,several researchers have tested additives to the fly ash-Ca(OH)z slurries in an attempt to increase the dissolution rate of silica from the fly ash ( 3 , 4 ) . Since fly ash is primarily a glassy substance, and since it is wellknown that NaOH etches glass, Chu (3)tested the addition of NaOH to the fly ash-Ca(OH)z slurries in order to enhance the reaction of fly ash with Ca(OH),. The addition of 0.08 M NaOH to the fly ash-Ca(OH)z slurry increased the reactivity of the product solids toward SOz from 18 to 65 mol of SOz removed/100 mol of Ca(OH),. The addition of 0.08 M NaOH to the slurry also increased the reactivity of the product solids toward NO, from 1to 4 mol of NO, removed/100 mol of Ca(OH),. Phosphoric acid and ammonium phosphate have also been tested as additives for fly ash dissolution (4))and similar increases in the product solids reactivity were reported. An alternative way to increase the reaction of silica with Ca(OH), is to use a more reactive form of silica. Jozewicz and Chang ( 4 ) have tested diatomaceous earths and clays as sources of silica for the production of reactive solids for use in dry FGD systems. These naturally occurring substances are composed of essentially pure amorphous silica. Although diatomaceous earth was shown to be much more reactive than fly ash, the most likely source for silica comes from the power plant fly ashes, since the cost of the al-
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988
1299
