A laboratory method for the experimental determination of air-water

A laboratory method for the experimental determination of air-water Henry's law constants for ... Ekaterina L. Ratkova , David S. Palmer , and Maxim V...
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Environ. Sci. Technol. 1988, 22, 1289-1293

(28) Iliceto, A. Gazz. Chim. Ztal. 1954, 84, 536-552. (29) Pocker, Y.; Meany, J. E.; Nist, B. J.; Zadorojny, C. J. Phys. Chem. 1969, 73, 2879-2882. (30) Green, L. R.; Hine, J. J.Org. Chem. 1974,39,3896-3901. (31) Perrin, D. D.; Dempsey, B.; Serjeant,E. P. p K , Prediction for Organic Acids and Buses; Chapman and Hall: London, 1981. (32) Olson, T. M.; Hoffmann, M. R.Atmos. Environ., in press.

(33) Johnstone, H. F.; Leppla, P. W. J. Am. Chem. SOC.1934, 56, 2233-2238. Received for review February 9, 1988. Accepted April 26, 1988. W e gratefully acknowledge the Electric Power Research Institute (RP1630-47), the U.S. Environmental Protection Agency (R811496-01-l), and t h e U.S. Public H e a l t h Service (ES04635-01) for their financial support.

A Laboratory Method for the Experimental Determination of Air-Water Henry’s Law Constants for Several Pesticides Nicholas J. Fendlnger and Dwlght E. Glotfelty* Agricultural Research Service, Environmental Chemistry Laboratory, U S . Department of Agriculture, Beltsville, Maryland 20705

rn A wetted-wall column (WWC) was used to determine experimentally the air-water Henry’s law constants (HLCs) for lindane (yBHC), alachlor, and diazinon. With this type of apparatus, a thin film of water flows (1.4-2.0 mL/min) down the inside of a vertical column (2.2 cm i.d. X 57 cm) and is equilibrated with a concurrent flow of air (37-220 mL/min). Lindane, diazinon, and alachlor were introduced into the WWC as aqueous solutes. In a separate experiment lindane was introduced as a vapor. Dimensionless HLCs for lindane, diazinon, and alachlor present as single aqueous solutes were determined to be (8.2 f 0.9) X (4.6 f 2.8) X lo4, and (3.4 f 0.5) X lo-’, respectively. HLCs determined with an aqueous solution containing a mixture of all three pesticides were statistically identical with the HLC obtained in single-solute solutions. HLCs were statistically the same at the 95% confidence level when lindane was introduced into the WWC as either an aqueous solute or a vapor. Equilibrium was confirmed for lindane by varying airlwater contact time in the WWC. Introduction Environmental scientists and engineers use Henry’s law constants (HLCs) or air-water partitioning coefficients to predict the direction and rate of vapor exchange between the atmosphere and natural waters. The problem faced by many scientists is the lack of accurate HLCs for many organic compounds routinely introduced or measured in the environment. Given the importance of HLCs, a number of methods have been developed to calculate or experimentally determine the HLCs for environmentally sensitive compounds. Theoretically, the HLC for compounds that have low water solubilities is equal to the vapor pressure of a compound divided by its aqueous solubility. This method is frequently used because there are few direct measurements. Vapor pressure and solubility data have been used by several investigators to estimate the HLCs for polychlorinated biphenyls ( I ) , selected pesticides (2, 3 ) , and various hydrocarbons (2). The overall variance of HLCs determined in this manner is the sum of variances in vapor pressure and solubility determinations (2). Accuracy of vapor pressure and solubility determinations made with generator column techniques are well established (4-7). However, the accuracy of other solubility and vapor pressure data is often difficult to assess because of the inherent problems in making such measurements. It is not unusual for literature values of vapor pressure and solubility for a particular compound to range over several

orders of magnitude. For example, Suntio et al. (3) reported solubility and vapor pressure values for lindane obtained from the literature that ranged from 0.15 to 10 mg/L and from 0.00125 to 4 Pa, respectively. Therefore, the need exists to verify many published HLCs experimentally. Experimental methods used to measure HLCs include bulk equilibration, dynamic equilibration techniques, and relative volatilization. The bulk equilibration method is based on equilibrium air/water partitioning of a solute between a volume of water and a headspace in a sealed container (8-11). HLCs are determined directly by measuring the vapor concentration and aqueous concentration of the analyte or from gas headspace concentration ratios from pairs of sealed bottles containing different liquid volumes (11). Generally, the bulk equilibration technique is only sensitive enough for compounds with relatively high HLCs (H > 0.1) because of the limited size of the headspace and water sample. The dynamic equilibration techniques or gas purging experiments involve equilibration of an aqueous solute with a known volume of purge gas (12). The HLC determination is based on either the change in analyte concentration in the water phase or the analyte concentration in the purge gas. The dynamic equilibration technique is generally more accurate than bulk equilibration because the measurement is based on a relative concentration change in one phase, and as a result, it is also capable of measuring much lower HLCs. Potential problems associated with the dynamic equilibration technique are incomplete equilibration, preferential association of the analyte with either the surfaces of the rising bubbles or with the purge bottle, and entrainment of solution aerosol into the vapor trap of the apparatus. The relative volatilization technique proposed by Mackay et al. (13)is based on the codistillation of a slightly volatile solute in aqueous solution. This method requires extrapolation of data to environmental conditions and has not been fully evaluated. The approach we chose to measure HLCs of several pesticides is the wetted-wall column (WWC). Like bulk equilibration it is a direct measurement technique but has the enhanced sensitivity of the purge method. The apparatus used is based on a design originally proposed by Emmert and Pigford (14) for the study of gas absorption in a falling liquid film. Experimental Section HLC determinations for lindane, diazinon, and alachlor made with the WWC were obtained by equilibrating the

Not subject to U S . Copyright. Published 1988 by the American Chemical Society

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Figure 1. Schematic diagram (not drawn to scale) of the wetted-wall column system. System components are (A) valveless metering pumps, (6)impinger to saturate incoming air with water, (C) Chromosorb 102 vapor trap, (D) air outlet to bubble flow meter, (E) Sep Pak cartridge, (F) optional pesticide vapor source, and (G) wetted-wall column. For water-to-air HLC determinations, pesticides-amended water was introduced at “H,O in”; for air-to-water HLC determinations, pesticide was introduced in the vapor phase at F. pesticide solute between a thin film of water that flows down the inside of a vertical column and a concurrent flow of air. Determinations were made by introducing pesticide either as a vapor from a generator column (air-to-water HLC) or as an aqueous solute (water-to-air HLC). Both modes of operation are illustrated in Figure 1. The WWC system was used in a controlled temperature environment (23 “C). I Reagents. The organic-free water used in this investigation was produced by the Hydro (Rockville, MD) Model 4C2-18 ultrapure water system with a critical applications column. Chromosorb 102 (60/80 mesh; Manville Products Corp., Denver, CO) was extracted with 1:l hexane:acetone for 32 h in a Soxhlet apparatus before use. Pesticides in the water phas3 were adsorbed on C-18 Waters (Milford, MA) Sep Paks. These were cleaned prior to use by eluting with 5 mL of MeOH, followed by 5 mL of water. Pesticides [y-hexachlorocyclohexane(lindane), diazinon, alachlor] were obtained from the U.S. Environmental Protection Agency’s Pesticide and Chemical Repository (Las Vegas, NV) and had reported purities of greater then 99.4%. The purities of the pesticides were confirmed by gas chromatographic analysis. Constant Pesticide Sources. Pesticide-spiked water samples were prepared by applying 100-5000 pg of pesticide dissolved in hexane to the walls of individual 4-L amber glass bottles. The hexane was evaporated, organic-free water added, the solution stirred for 24 h, and the solution allowed to equilibrate for at least 24 h before use. The resulting pesticide concentrations were less than 5% of the published aqueous solubility for each compound (3). Constant pesticide-vapor density sources were produced by using a generator column. The generator columns were constructed from 15 cm X 0.75 cm i.d. glass tubing with a 3 cm x 0.25 cm i.d. constriction in the middle of the column. The vapor was produced by passing water-saturated breathing quality air through the pure compound (lindane) contained in the column constriction with glass wool plugs. Wetted-Wall Column. The wetted-wall column consisted of three components: a reservoir/pulse dampener, column, and collector (Figure 2). The reservoir/pulse dampener and collector were constructed of machined Teflon. U-shaped pieces of no. 16 copper wire were placed at the top of the column to disrupt the surface tension of the water in the reservoir and establish a uniform film. The glass columns of 10-57 cm in length (2.2 cm i.d.) were mechanically etched to maintain the uniform liquid film 1290

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.-Air out

Flgure 2. Schematic diagram of the wetted-wall column. Column lengths of 10-57 cm were used for HLC determinations. estimated to be C0.1 mm in thickness. Water or water spiked with pesticide was pumped through the apparatus at flow rates from 1.4 to 2.0 mL/min by two valveless metering pumps (Fluid Metering, Inc., Oyster Bay, NY; Model RP, SY), one each for the inlet and outlet of the column. Pumping rates were adjusted so that the flow rate into the column equaled the flow rate pumped from the collector. The amount of water applied to and collected from the wetted-wall column was determined gravimetrically. Either all or part of the water eluting from the column was passed through a waters C-18 Sep Pak cartridge for extraction of pesticide. Water applied to the column was analyzed at the beginning and end of each run by the same Sep Pak extraction technique. For the concurrent airflow, we used compressed breathing quality air that was saturated with water vapor. Airflow rates ranged from 50 to over 200 mL/min and were determined at 5or 10-min intervals with a soap-bubble flow meter. Pesticides in the air phase were trapped by 10 cm X 0.6 cm i.d. columns packed with Chromosorb 102 resin. Pesticide Analysis. Sep Pak cartridges were extracted with 10 mL of hexane (lindane) or 1:l hexane:acetone (alachlor and diazinon). Chromosorb 102 columns were eluted with 10 mL of hexane. Eluants from the Sep Pak and Chromosorb 102 columns were either diluted or concentrated on a steam bath with a micro-Snyder column. Reductions to volumes less than 1 mL were done under a stream of nitrogen gas at room temperature. Lindane and alachlor were analyzed with a Tracor (Austin, TX) Model 560 gas chromatograph equipped with a Ni-63 electron capture detector (ECD) and 1.8 m X 4 mm i.d. glass column packed with 1.5% OV-17 and 1.9% OV-210 on Chromosorb W-HP 100/120. The chromatographic conditions were as follows: injector temperature, 225 “C; column temperatures, 210 “C; detector temperature, 350 “C; and carrier flow rate (Ar with 5% CHI), 60 mL/min. For HLC determinations of the pesticide mixtures a HP-1 5 m X 0.53 mm x 2.6 pm film thickness column was used with He as the carrier gas and Ar with 5% CH, as the makeup gas with the same gas chromatograph and under similar chromatographic conditions. Diazinon was analyzed by a Hewlett-Packard (Palo Alto, CA) Model 5890A gas chromatograph equipped with a

Table I. Dimensionless Henry's Law Constants for Lindane, Diazinon, and Alachlor Determined at 23 'C from Water-to-Air Equilibration with Pesticides Present as Single Aqueous Solutes or Mixtures and Air-to-Water HLCs for Lindane; Selected Literature Values for the Same Pesticides Are Also Listed compound lindane diazinon alachlor

water-to-air HLC

air-to-water HLC

water-to-aira HLC mixtures

0.9) x 10-5 (n = 5) (4.6 i 2.8) X lo* (n = 5) (3.4 f 0.5) X lo-' (n = 5)

(9.5 i 1.2) x 10-6 (n = 3) NDb

(7.8 i 1.5) X (n = 7) (3.4 i 1.6) X lo* (n = 6) (2.5 i 1.1) X lo-' (n = 5)

(8.2

ND

literature values HLC T, 'C

ref

3.2 X 10"' 5.2 x 10-5 2.7 x 10-5

25 20 20

2 3 3

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20

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aTotal pesticide concentration, 1660 pg/L. bND indicates no data.

nitrogen/phosphorus detector (NPD) and 30 m X 0.32 mm X 0.25 km film thickness fused silica DB-1 (J&W Scientific, Rancho Cordova, CA) capillary column. The chromatographic conditions were as follows: injection, split (181); carrier gas, UPC-grade He at 6 mL/min (41 cm/s linear gas velocity at 120 kPa inlet pressure); detector gases, UPC-grade HP, 3.4 mL/min, air 120 mL/min; injector temperature, 250 "C; initial column temperature, 160 "C for 1.0 min; temperature ramp, 5 "C/min to 210 "C; and detector temperature, 250 "C. The gas chromatographs were calibrated daily; additional calibration standards were injected after each duplicate sample injection. HLC Determination. Each WWC run consisted of one to eight HLC determinations. The WWC was allowed to operate at least 30 min before the first Chromosorb 102 and C-18 Sep Pak extraction columns were fitted to the assembly. The (3-18 Sep Pak and Chromosorb 102 extraction columns were replaced at intervals from 20 min to 6 h depending on the HLC and the aqueous- and vapor-phase concentrations of the compound of interest. Individual HLCs were calculated from the water and gas-phase concentrations determined by the Chromosorb and Sep Pak analyses and the known water and air volumes. Recovery of pesticide from the WWC was calculated from the mass balance between the amount of pesticide in the water applied to the WWC and the sum of the pesticide eluted from the column in the aqueous and vapor phases. Recovery of pesticides from the WWC was generally greater than 90%.

Results and Discussion HLC Measurements. Dimensionless HLCs for lindane, diazinon, and alachlor determined from water-to-air equilibration with the WWC both as single solutes and as mixtures are listed in Table I. For lindane, the air-towater equilibration HLC is also listed in Table I. Each determination for the water-to-air HLC represents at least five determinations, while the air-to-water HLCs were run in triplicate. The HLC expressed in kPa m3/mol can be obtained from the dimensionless HLCs listed in Table I by multiplying the values given by RT where R is the gas constant (8.314 X KPa m3/mol 2') and T i s the temperature (296 K). Evaluation of WWC System for Determining HLCs. The HLC determination is affected by uncertainties involved with the GC analyses of pesticides and volumetric measurement of air and water eluted from the WWC. The random errors associated with each measurement are GC/ECD analysis .of C-18 extract, *5%; GC/ECD analysis of Chromosorb 102 extract, &5%; volumetric measurement of air, f3%; and volumetric measurement of water, f l % . Quadratic addition of these random errors yields a minimum uncertainty of *8% for the HLC determination for lindane. This value is comparable to the RSD of f l l % calculated for replicate HLC determination made for lin-

dane. The RSD calculated for alachlor was slightly larger ( & E % ) and probably reflects the greater difficulty in determining the lower vapor densities associated with the HLC determination for alachlor. The RSD calculated for diazinon was f61%. A part of the large uncertainty is related to the slightly greater uncertainty in the GC/NPD analysis (f7%) compared to the GC/ECD analysis of the other compounds. However, this does not account for the much larger uncertainty in the diazinon measurement. Other possible sources of error may include loss of diazinon during volume reduction of the sample and/or degradation during sample handlng. Accuracy of the HLCs reported may be affected by back-pressure in the WWC caused by air-flow resistance from the Chromosorb 102 column. The back-pressure was determined to be only 0.1 mmHg greater than ambient atmospheric pressure. Because this error was considered small compared to other possible sources of error, no correction was made to the reported HLCs. Other possible sources of error in the measurement of the HLC include the efficiency of C-18 Sep Pak columns for extracting pesticide from water and the efficiency of Chromosorb 102 in trapping volatilized pesticide from air. The efficiency of C-18 for extracting lindane, diazinon, and alachlor from aqueous solution was 92 f 5%, 100 f 7%, and 97 f 4%, respectively. The efficiency of Chromosorb 102 for trapping volatilized lindane, diazinon, and alachlor from air was 97 f 4%, 111 z t 5%, and 96 f 2%, respectively. Because recoveries of pesticides from both air and water approached loo%, no corrections were applied to the reported HLCs. The standard deviations calculated for the extraction efficiencies were similar to the RSDs calculated for the GC analysis (*5-7%). We concluded that the error contributed by differences in sample-tosample extraction efficiency was very small. Therefore, uncertainty from sample extraction was not included in the HLC error analysis. ' Vapor/ Solution Equilibrium. An important factor in the accuracy of the HLC determination with the WWC system is whether pesticide vapor/solution equilibrium is achieved between the liquid film and concurrent flow of air. HLC determinations made by other investigators (12, 15, 16) using the gas purge technique have shown that equilibrium is dependent on the contact time between the gas and liquid phase in the purge bottle. In the WWC, contact between the gas and liquid phases is dependent on the water flow and airflow rates and column length. Water flow rates influence the fluid characteristics of the liquid film and residence time within the column. With the water flow rates (1.4-2.0 mL/min) used for all HLC determinations, liquid flow is predicted to be laminar with residence times of 30-45 s in a 57-cm column (17). Residence time of the gas phase in the column is inversely proportional to the airflow rate. Column length influences both the travel time of the Environ. Sci. Technol., Vol. 22, No. 11, 1988

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aqueous film in the column and residence time of the gas phase. HLCs reported for lindane in Table I were obtained with airflow rates of 72-74 mL/min and a 57-cm column. In order to determine if the air/water contact time in the WWC was sufficient to establish equilibrium between the aqueous- and vapor-phase pesticide, additional HLC determinations were made for lindane with different airflow rates and column lengths. Air/water contact time in the WWC was approximated from the phase with the shortest residence time. With a column length of 57 cm and water flow rates of 0.6-0.9 mL/min, air/water contact time in the WWC will be determined by the airflow rate. The average HLC for lindane calculated from eight determinations made with airflow rates that ranged from 37.3 to 223.46 mL/min, which yields an air residence time of 0.98 -5.71 s, was (8.4 f 1.2) X lob5(RSD = f14%), the same HLC as determined from measurements made with constant airflow rate. Additional HLC measurements were made for lindane with column lengths of 10-57 cm and airflow rates of 84-157 mL/min. These measurements were not made in the controlled temperature environment and yielded slightly different HLCs. However, the short-column HLC was the same as the long-column HLC, and the RSD for the HLC average over the range of conditions was less than f 7 %. From these airflow rate and column length studies, it is apparent that the conditions used to obtain the data in Table I (48-128 mL/min airflow, 1.4-2.0 mL/min water flow, and 57 cm column length) gave air-/water-phase equilibrium. Final verification that equilibrium was reached in the WWC was obtained from the two methods employed to determine HLCs for lindane. Lindane was introduced into the WWC system in an aqueous solution (water-to-air HLC) or as a vapor (air-to-water HLC). If equilibrium between the air and water phases is reached, then the same HLC should be obtained regardless of the phase in which the lindane is introduced into the WWC. The difference between the water-to-air HLC and air-to-water HLC in Application of the null Table I for lindane is 1.3 X hypothesis at the 95% confidence interval shows that there is no statistical difference between the values obtained from the two different techniques used to determine the HLC for lindane. HLCs for Pesticide Mixtures. No differences could be established between the HLCs for lindane, diazinon, and alachlor determined separately as single aqueous solutes and HLCs determined from pesticide mixtures (Table I). The sum of the concentrations of the individual pesticides (lindane, 12 Hg/L; diazinon, 1200 kg/L; alachlor, 450 pg/L) in the pesticide mixture was approximately 1660 HUg/L. Comparison of Experimental HLCs to Literature Values. The experimentally determined HLCs for lindane agreed reasonably well with those of Suntio et al. (3). The HLC for lindane (5.2 x reported by Suntio et al. (3) was calculated from vapor pressure and solubility data “selected from a number of literature sources. The actual vapor pressure and solubility data covered a considerable range, as did previously reported values for lindane’s HLC. References cited in Suntio et al. (3)for previous literature values of lindane HLC ranged from 2.0 X to 3.0 X 10“. The HLCs for diazinon and alachlor measured with the WWC were at least an order of magnitude smaller than the “suggested”value reported by Suntio et al. (3). Again, there was a range of vapor pressures and solubilities for each compound. The discrepancies between the experimentally determined HLCs and literature values calculated 1292

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from published vapor pressure and solubility data illustrate the need for experimentally verified HLCs and the advantage of HLCs with known accuracy determined with the WWC apparatus.

Conclusions The WWC proved to be an effective techique for the determination of HLCs for several pesticides in the range of to loT5.The apparatus is probably capable of determining HLCs of organic compounds as low as depending on the analytical technique used to measure analyte concentrations. For example, the technique could be readily adapted and the HLC range extended with the use of radiolabeled compounds. The minimum uncertainty estimated for the technique was 8%,which was due primarily to gas chromatographic errors. Equilibrium was verified by HLC measurements made for lindane with different column lengths and airflow rates. Agreement between water-to-air and air-to-water measurements of HLCs for lindane provided additional verification of the technique. The fugacity or partial pressure exerted by the pesticides was the same in mixtures as in pure solutions, Le., there were no interactions. Other investigators using this technique should select a “calibration” compound to test new apparatus. Lindane proved to be an ideal compound for such purposes because of its relatively high HLC and its excellent responses with the electron capture detector. We also suggest that potential users perform both air-to-water and water-to-air HLC measurements with the calibration compound for the operating conditions selected. Acknowledgments The assistance of Ann Lucas in the laboratory is greatly appreciated. The WWC apparatus was constructed by Paul E. Balsley, Research Equipment Unit, USDA-ARS, Beltsville, MD. Registry No. Lindane, 58-89-9; diazinon, 333-41-5; alachlor, 15972-60-8.

Literature Cited Mackay, D.; Shiu, W. Y.; Billington, J.; Huang, G. L. In Physical Behavior of PCBs in the Great Lakes; Mackay, D., Paterson, S., Eisenreich, S. J., Simmons, M. S., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 59-69. Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10, 1175-1199. Suntio, L. R.; Shiu, W. Y.; Mackay, D.; Sieber, J. N.; Glotfelty, D. Rev. Environ. Contam. Toxicol. 1987, 103, 1-59. May, W. E.; Wasik, S. P.; Freeman, D. H. Anal. Chem. 1978, 50, 175-179. Spencer, W. F.; Cliath, M. M. Environ. Sci. Technol. 1969, 3, 670-674. Spencer, W. F.; Cliath, M. M. J. Agr. Food Chem. 1970, 18, 529-530. Sonnenfeld, W. J.; Zoller, W. H.; May, W. E. Anal. Chem. 1983, 55, 175-280. Murphy, T. J.; Mullin, M. D.; Meyer, J. A. Environ. Sci. Technol. 1987,21, 155-162. Burkhard, L. P.; Andren, A. W.; Armstrong, D. E. Environ. Sci. Technol. 1985, 19, 500-507. Murphy, T. J.; Pokojowczyk, J. C.; Mullin, M. D. In Physical Behavior of PCBs in the Great Lakes; Mackay, D., Paterson, s.,Eisenreich, s. J., Simmons, M. s., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 49-58. Gossett, J. M. Environ. Sci. Technol. 1987, 21, 202-208. Mackay, D.; Shiu, W. Y.; Sutherland, R. P. Enuiron. Sci. Technol. 1979,13, 333-337. Mackay, D.; Shiu, W. Y.; Bobra, A.; Billington, J.; Chau, E.; Yeun, A.; Ng, C.; Szeto, F. Report EPA-600/3-82-019;

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(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

rn

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

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