environmental tritium oxidation in surface soil

4
excess of 150 OF will probably break the emulsion with no further addition of agent required. Emulsions of the water- in-oil type are best treated at the oily waste treatment plant using recommended procedures or other even more effective methods (8). Literature Cited (1) Naval Facilities Engineering Command, “Scenario for the Op- eration of a System for the Collection and Treatment of Ship’s Bilge Waste”, circa 1975. (2) Jefferson, T. H., Boulware, S. B., “Surfactants and Their Effects on Filter Separators”, Rep. 2066, MERDC, Fort Belvoir, Va., June 1973. (3) Becher, P., “Emulsions: Theory and Practice”, 2nd ed., Reinhold, New York, N.Y., 1965. (4) Sumner, G. G., “Clayton’s Theory of Emulsions and Their Technical Treatment”, 5th ed., Blakiston, New York, N.Y., 1954. (5) Little, R. C., Patterson, R. L., “Breaking Emulsions in Navy Donut Oilmater Separators”, Quarterly Status Rep. No. 3 to NAVFAC, 17 Feb. 1976. (6) Little, R. C., “Breaking Emulsions in Navy Donut Oilmater Separators”, Rep. No. 1 to NAVFAC, 3 June 1975. (7) Cutler, R. A., Drobeck, H. P., “Toxicology of Cationic Surfac- tants”, in “Cationic Surfactants”, E. Jungermann, Ed., p 527, Surfactant Science Series, Dekker, New York, N.Y., 1970. (8) Little, R. C., Fuel, 53,246 (1974). Received for review April 7,1977. Accepted November 21,1977. Environmental Tritium Oxidation in Surface Soil James C. McFarlane”, Robert D. Rogers, and Donald V. Bradley, Jr. Monitoring Systems Research and Development Division, Environmental Monitoring and Support Laboratory, P.O. Box 15027, Las Vegas, Nev. 891 14 The site, rate, and method of oxidation of elemental tritium (T2 or HT) to tritiated water (HTO) were determined. Ex- posures of leaves (attached or detached), sterilized clay loam, and various extractable nonliving soil components to H T re- sulted in less than 4% conversion to HTO after 48 h. However, exposure of natural (unsterilized) clay loam or of sterilized soil inoculated with a water extract from the former yielded over 97% conversion. This reaction occurred primarily near the soil surface. Microbial isolations from the soil yielded bacteria that were able to reproduce this reaction in solution. This reaction is considered important due to the expectation of increasingly large atmosphereic tritium discharges from nuclear fuel re- processing plants, which may result in significant contami- nation of food and water with HTO. In the production of electricity by nuclear reactors, ap- proximately 50 mCi of tritium are produced for each mega- watt-day of energy generated (1). Most of the tritium pro- duced is retained within the fuel element and released to the environment during reprocessing. Estimates vary between 25 and 90% as to the amount of tritium that will be released in the elemental form during reprocessing (1-3). Because of its low solubility and the relative stability of hydrogen gas, the importance of gaseous tritium (HT or T2) as a biological hazard is not great compared to that of tritiated water (HTO). Consequently, the maximum permissible concentrations for occupational exposure to elemental tritium (4) are higher than for exposures to tritiated water vapor. Maximum permissible concentrations of tritium gas and tritiated water vapor are 400 and 5 picocuries per cubic centimeter (pCi cm-3), respectively. Eakins and Hutchinson (5) determined the conversion times of H T to HTO in the presence of various catalyzing surfaces and reported that the reaction half-times varied from 4 years in the presence of platinum to 1151 years for glass. On the basis of these findings, they suggested that elemental tritium was of little consequence as a local contaminant and proposed that exposure guidelines for H T be further relaxed. However, when lettuce plants in a growth chamber were exposed to Tz, rapid tritium accumulation occurred in both the water and organic components of plants (3). This rapid rate of contamination was contrary to what had been expected from the data in the available literature and suggested that elemental tritium in the environment should be considered not as an innocuous pollutant but as one with a significant potential for contaminating food and water. Based on those data, it was postulated that the reaction converting HT to HTO existed in either the plant leaves or in the soil. Whether it was a catalytic reaction or one which involved metabolism by plants or soil microorganisms was not known at that time. In expectation of large releases of gaseous tritium into the atmosphere, knowledge of the nature, site, and kinetics of this oxidation or exchange reaction becomes important. To answer these questions, a series of experiments was conducted to determine whether the conversion of HT to HTO was due to oxidation or exchange and to identify the site or sites of the reaction. Methods Leaf exposures were conducted in a Plexiglas chamber (14 X 25 X 7 cm). The leaf was supported in the center of the chamber on a nylon monofilament grid. The petiole entered through the side of the chamber and was sealed with an oil base clay. The leaf chamber was supported on a ring stand and positioned to allow leaf enclosure without disturbing the plant. Exposures were conducted within an environmentally con- trolled chamber in which the temperature was maintained at 25 f 1 “C and the photosynthetic photon flux density (PPFD) of 32.5 nanoEinsteins per square centimeter per second (nE cm-2 s-l) was produced by a combination of fluorescent and incandescent lamps. Air was circulated in Teflon tubing from the leaf chamber through a temperature-controlled water bath, dew point sensor, infrared COz analyzer, and back to the chamber. The concentration of COz was maintained at 350 ppm by adjusting the rate of COZ injection. This was done by altering the speed on a syringe pump (syringe filled with 100% COP) until the COz concentration continued unchanged. At this point the COz injection rate equaled the COz assimilation rate, and this value was used as an indication of net photo- synthesis. The COz assimilation rate for bean plants was 0.008 cc cm-2 h-1. The corresponding transpiration rate was 15 mg ern+ h-1. These rates are typical of actively growing, healthy leaves and assured us that the stomates were open and normal physiological processes were occurring. Other leaf exposures were conducted without trying to control COz concentration. During some of these trials, the COz concentration was mon- itored and found to decrease rapidly to the compensation point in 0.5-1 h, depending on plant species and leaf size. It remained within this range during the light periods but in- 590 Environmental Science & Technology This article not subject to U.S. Copyright. Published 1978 American Chemical Society

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excess of 150 O F will probably break the emulsion with no further addition of agent required. Emulsions of the water- in-oil type are best treated at the oily waste treatment plant using recommended procedures or other even more effective methods (8).

Literature Cited (1) Naval Facilities Engineering Command, “Scenario for the Op-

eration of a System for the Collection and Treatment of Ship’s Bilge Waste”, circa 1975.

(2) Jefferson, T. H., Boulware, S. B., “Surfactants and Their Effects on Filter Separators”, Rep. 2066, MERDC, Fort Belvoir, Va., June 1973.

(3) Becher, P., “Emulsions: Theory and Practice”, 2nd ed., Reinhold, New York, N.Y., 1965.

(4) Sumner, G. G., “Clayton’s Theory of Emulsions and Their Technical Treatment”, 5th ed., Blakiston, New York, N.Y., 1954.

(5) Little, R. C., Patterson, R. L., “Breaking Emulsions in Navy Donut Oilmater Separators”, Quarterly Status Rep. No. 3 to NAVFAC, 17 Feb. 1976.

(6) Little, R. C., “Breaking Emulsions in Navy Donut Oi lmater Separators”, Rep. No. 1 to NAVFAC, 3 June 1975.

(7) Cutler, R. A., Drobeck, H. P., “Toxicology of Cationic Surfac- tants”, in “Cationic Surfactants”, E. Jungermann, Ed., p 527, Surfactant Science Series, Dekker, New York, N.Y., 1970.

(8) Little, R. C., Fuel, 53,246 (1974).

Received for review April 7,1977. Accepted November 21,1977.

Environmental Tritium Oxidation in Surface Soil

James C. McFarlane”, Robert D. Rogers, and Donald V. Bradley, Jr. Monitoring Systems Research and Development Division, Environmental Monitoring and Support Laboratory, P.O. Box 15027, Las Vegas, Nev. 891 14

The site, rate, and method of oxidation of elemental tritium (T2 or HT) to tritiated water (HTO) were determined. Ex- posures of leaves (attached or detached), sterilized clay loam, and various extractable nonliving soil components to H T re- sulted in less than 4% conversion to HTO after 48 h. However, exposure of natural (unsterilized) clay loam or of sterilized soil inoculated with a water extract from the former yielded over 97% conversion. This reaction occurred primarily near the soil surface. Microbial isolations from the soil yielded bacteria that were able to reproduce this reaction in solution. This reaction is considered important due to the expectation of increasingly large atmosphereic tritium discharges from nuclear fuel re- processing plants, which may result in significant contami- nation of food and water with HTO.

In the production of electricity by nuclear reactors, ap- proximately 50 mCi of tritium are produced for each mega- watt-day of energy generated ( 1 ) . Most of the tritium pro- duced is retained within the fuel element and released to the environment during reprocessing. Estimates vary between 25 and 90% as to the amount of tritium that will be released in the elemental form during reprocessing (1-3). Because of its low solubility and the relative stability of hydrogen gas, the importance of gaseous tritium (HT or T2) as a biological hazard is not great compared to that of tritiated water (HTO). Consequently, the maximum permissible concentrations for occupational exposure to elemental tritium ( 4 ) are higher than for exposures to tritiated water vapor. Maximum permissible concentrations of tritium gas and tritiated water vapor are 400 and 5 picocuries per cubic centimeter (pCi cm-3), respectively. Eakins and Hutchinson ( 5 ) determined the conversion times of H T to HTO in the presence of various catalyzing surfaces and reported that the reaction half-times varied from 4 years in the presence of platinum to 1151 years for glass. On the basis of these findings, they suggested that elemental tritium was of little consequence as a local contaminant and proposed that exposure guidelines for H T be further relaxed.

However, when lettuce plants in a growth chamber were exposed to Tz, rapid tritium accumulation occurred in both the water and organic components of plants (3 ) . This rapid rate of contamination was contrary to what had been expected from the data in the available literature and suggested that elemental tritium in the environment should be considered not as an innocuous pollutant but as one with a significant

potential for contaminating food and water. Based on those data, it was postulated that the reaction converting H T to HTO existed in either the plant leaves or in the soil. Whether it was a catalytic reaction or one which involved metabolism by plants or soil microorganisms was not known at that time. In expectation of large releases of gaseous tritium into the atmosphere, knowledge of the nature, site, and kinetics of this oxidation or exchange reaction becomes important. To answer these questions, a series of experiments was conducted to determine whether the conversion of H T to HTO was due to oxidation or exchange and to identify the site or sites of the reaction.

Methods Leaf exposures were conducted in a Plexiglas chamber (14

X 25 X 7 cm). The leaf was supported in the center of the chamber on a nylon monofilament grid. The petiole entered through the side of the chamber and was sealed with an oil base clay. The leaf chamber was supported on a ring stand and positioned to allow leaf enclosure without disturbing the plant. Exposures were conducted within an environmentally con- trolled chamber in which the temperature was maintained at 25 f 1 “C and the photosynthetic photon flux density (PPFD) of 32.5 nanoEinsteins per square centimeter per second (nE cm-2 s-l) was produced by a combination of fluorescent and incandescent lamps. Air was circulated in Teflon tubing from the leaf chamber through a temperature-controlled water bath, dew point sensor, infrared COz analyzer, and back to the chamber. The concentration of COz was maintained at 350 ppm by adjusting the rate of COZ injection. This was done by altering the speed on a syringe pump (syringe filled with 100% COP) until the COz concentration continued unchanged. At this point the COz injection rate equaled the COz assimilation rate, and this value was used as an indication of net photo- synthesis.

The COz assimilation rate for bean plants was 0.008 cc cm-2 h-1. The corresponding transpiration rate was 15 mg ern+ h-1. These rates are typical of actively growing, healthy leaves and assured us that the stomates were open and normal physiological processes were occurring. Other leaf exposures were conducted without trying to control COz concentration. During some of these trials, the COz concentration was mon- itored and found to decrease rapidly to the compensation point in 0.5-1 h, depending on plant species and leaf size. It remained within this range during the light periods but in-

590 Environmental Science & Technology This article not subject to U.S. Copyright. Published 1978 American Chemical Society

creased in the dark due to respiration. Detached leaves were sealed in the chamber without monitoring transpiration or assimilation. These different exposure conditions were studied to explore the possibilities that various plant species, catalytic surfaces, or physiological conditions might hinder or induce hydrogen oxidation. At the end of the exposure, water was extracted from the leaves, a r d the tritium content was de- termined.

The extent and rate of the H T - HTO reaction in soil were studied by adding a known amount of elemental tritium (1500 nCi) and recovering tritiated water after various increments of time. In this study, soil (20 g) and water (15 mL) were added to a 1-L round-bottomed flask. The resultant soil paste was spread over the internal surface by shaking the flask. The flask was closed with a rubber stopper, and 5 mL of gaseous tritium were injected through the stopper into the flask with a syringe. Elemental tritium was obtained from a 1-L gas bottle pres- surized to 1500 psi with nitrogen. The resultant tritium con- centration a t standard atmospheric pressure and temperature was 300 nCi cmW3. At the termination of the exposure period, benzene (50 mL) was added to the flask, and the water was azeotropically distilled from the soil (6). The tritium content of the water was determined by liquid scintillation (7).

In the previous studies ( 3 ) lettuce, corn, and beans were grown in different potting media. In every case, regardless of the species of plant used, HTO was found to occur in the potting media. For this reason the location of the H T - HTO reaction site was determined in three different potting media. These included vermiculite, a mixture of peat and vermiculite (Jiffy Mix), and a silty clay loam soil. The soil was of the Ov- erton series, a member of the fine montmorillonitic, calcareous thermic family of Mollic haplaquepts . The soil contained 14.7% sand, 34.5% clay, and 3.4% organic carbon. The pH of a saturated soil paste was 7.8. This soil was also used in other segments of this study and is referred to simply as soil or clay loam.

Three replicates of each potting material were contained in carefully prepared soil columns and were exposed to ele- mental tritium in environmental simulation chambers. Their incubation temperature was maintained at 25 & 1 "C, and the relative humidity was controlled a t 70 & 5%. The tritium concentration was maintained a t 5 nCi L-' by incorporating a tritium analyzer as a feedback controller to a pulsed injection

Table 1. Formation of Tritiated Water in Various Solutions Exposed to 1500 nCi of Elemental Tritium for 48 h

Sample type

Corn leaf (segment) Tobacco leaf (detached) Bean leaf (attached)a Bean leaf (detached) H20 H20 shaking H20 on filter paper HCI 0.4 N NaOH 0.5 M Humic material

% of trltium recovered as HTO

1.12 f 0.2oc 1.09 f 0.38 0.86 f 0.06 1.06 f 0.10 1.98 f 1.24 1.16 f 0.51 1.08 f 0.31 3.32 f 0.30 3.05 & 0.12 2.30 f 0.10

Exposure to KT was 8 h. Extracted from soil in 0.5 M NaOH. f 1 standard deviation.

Table 11. Conversion of Elemental Tritium to Tritiated Water in Presence of Soil

Sample

Sterilized clay loam

% of tritium recovered as HTO after 90 h exposure

3.4 f 0.7 100.0 f 1.2 Natural clay loam

system. The details of this system were presented by McFarlane (3). Samples were taken by slicing a 1-cm segment from the top of the column and cutting a 4-cm diameter sample from the center. This was facilitated by using a round cutting tool and guide. This procedure was repeated to the depth desired.

Other tests conducted included oxidation rate in sterilized soil; sterilized soil amended with aqueous extracts of natural soil; serial dilutions of aqueous extracts from natural soil; and various dispersed and agitated water solutions of abiological soil components.

Resul ts and Discussion The results of the leaf fumigations are shown in Table I.

Under all conditions the amount of elemental tritium recov- ered as HTO was very low. This indicated that although plant leaves became quickly contaminated when whole potted plants were exposed, the reaction site was definitely not fol- iar.

Concern about the water-gas contact area and the possible effect of nonliving soil components led us to test the HTO formation rates in the presence of various soil extractants with water dispersed on filter paper and with vigorously agitated water. The results of these tests are also shown in Table I. As with plants, all values indicate that very little conversion of H T to HTO occurred and suggest that the responsible reaction was not facilitated by a catalytic surface on the leaves, nor was it an abiological reaction involving some extractable soil component.

The fact that increased contact between water and the gaseous phase, as provided by filter paper, leaf surface and solution agitation, had no effect on the rate of the conversion of H T to HTO suggests that exchange reactions with water are not responsible for the contamination of water with tri- tium. Thus, biological oxidation is implied.

Examination of the soil was originally directed toward finding the reaction site. The first experiments were designed to compare sterilized and natural soils. The data in Table I1 show the results of these studies. I t is obvious from these data that the oxidation of elemental tritium occurred in the soil and that it was mediated by some soil microorganisms that were inactivated by steam autoclaving.

Our next task in verifying that this reaction was mediated by microbial activity was to reestablish the reaction in sterile soils. This was done by preparing water extract of a natural soil and adding these to soils sterilized by steam autoclaving. The results of this study yielded tritium recoveries of 2.1,97, and 1OOOh for the sterile, natural, and reinoculated sterile soils, respectively. This experiment was repeated in triplicate, and the coefficients of variation were less than 5% in all cases. Three dilutions were also made of this and they were incu- bated with H T to determine the reaction rate in the absence of soil. These trials yielded HTO formations that were pro- portional to the total numbers of microorganisms, but the yields were very low compared to those observed when the same solution was added to sterilized soil. Microbial cultures were subsequently established and tested for this reaction on various nutrient media. Isolations of hydrogen-oxidizing mi- croorganisms from the clay loam soil used in these studies have finally yielded bacteria that can reproduce the H T - HTO oxidation in solution a t rates comparable to those observed in the nonsterilized soil. The identification and character- ization of these organisms are currently being investigated and will be presented in a subsequent report.

Much of our earlier experimentation ( 3 ) was done with plants growing in artificial soils. I t was evident from those studies that tritium was oxidized even in the absence of soil, which therefore implies that hydrogen-oxidizing organisms existed in the artificial mixes as well as in soil. Comparisons

Volume 12, Number 5, May 1978 591

7 r

-CLAY LOAM ---JIFFY MIX’ . * .VERMICULITE

5 10 15 20 25 30

COLUMN DEPTH Ism1

Figure 1. Tritiated water (HTO) concentrations at various depths in materials after exposing surface for 24 h to 5 nCi L-’ of elemental tri- tium (HT)

10

0 1 ( 1 1 ’ 1 ’ ) 6 ; I l ‘ I ; l l ’ l /

0 50 100 150 200

HOURS AFTER INJECTION

Figure 2. Tritium conversion (HT - HTO) in presence of unsterilized clay loam Reaction conducted in sealed I-L-round flask at 25 O C

of reactions in soil and a mixture of peat and vermiculite showed that after 92 h, 100% of the H T had been converted to HTO in the presence of both growth media. Again, steam autoclaving reduced the extent of the reaction to less than 2% in both potting mixtures.

Three potting mixtures were exposed a t the surface for 24 h to H T ( 5 nCi/L) in a controlled environment chamber. Af- terward, they were segmented, and the HTO concentrations a t various depths were determined. The data shown in Figure 1 represent the contamination profile of these potting media. An approximation of the reaction rates was obtained by in- tegrating the areas under the curves and multiplying by the water content. These values were 6, 3, and 1 nCi of tritium oxidized per cm2/h for the soil, peat mixture, and vermiculite, respectively. I t is obvious that these are oversimplified esti- mates since they do not take into account the loss of water from the columns by evaporation. Since evaporation occurs from the surface and oxidation occurs near the same locus, the estimated conversion rates are thought to be lower than actual. Nevertheless, from these data it is apparent that the reaction was most rapid in soil and the conversion values corresponded to the expected relative abundances of microorganisms in the three media tested.

The lack of deep penetration of HTO in any of the columns suggest that the oxidation reaction occurs near the surface. The low HTO concentrations a t lower levels were probably due to diffusive exchange with uncontaminated water and not due to elemental tritium penetration and subsequent oxida- tion. This conclusion is based on the lack of significant tritium concentrations a t lower depths in the more porous synthetic soil mixes, and the similarity of HTO movement after applying it to the surface as a liquid (8).

The time course of the H T - HTO reaction was deter- mined using the clay-loam soil (Figure 2 ) . This curve expo- nentially approaches an asymptote which represents 100% conversion. The reaction half-time was approximately 8 h in this configuration, and the changing slope represents changes in the concentration of the reaction substrate (HT). Since these experiments were set up with ambient H2 as the initial substrate and HT as a tracer, the slope is thought to represent conversion of H2 from approximately 0.5 ppm (9) to nearly zero. In an ambient accidental exposure, the tritium concen- tration in air would be determined by the release character- istics and meteorological conditions. The reaction rate would therefore be best approximated by the original linear portion of the curve in Figure 2. In terms of percent, this is estimated as being approximately 20% conversion per hour.

The rate of water turnover in a soil-plant system is deter- mined by a complex array of meteorological, physiological, and edaphic factors. It is therefore not possible with the available data to accurately predict the actual short- or long-term contamination of food and water by chronic or acute releases of elemental tritium. Nevertheless, the potential for elemental tritium conversion into the more critical molecule (HTO) has been shown to exist. The location of this oxidation within the top layer of the soil suggests the possibility that vegetation and water may be critical sinks for this form of the pollutant. The reaction rates observed in the laboratory were sufficiently rapid to suggest that Hr can represent a significant source of tritiated water in the envircnment. Further investigations to evaluate the importance of this reaction with respect to in- dustrial release standards for elemental tritium are clearly needed. In particular, the effective mixing and contact of the air column with soil needs evaluation. Field studies to evaluate the importance of this reaction in the conversion of the rela- tively innocuous tritium molecule into its much more im- portant form, water, should be conducted.

Summary The oxidation of elemental tritium, previously shown to be

responsible for the contaminating of plants grown in a growth chamber ( 3 ) , was demonstrated to occur in the soil. This was determined by eliminating other possible sites and finally by showing that the oxidation reaction, H T t l/zOz - HTO oc- curred only in nonsterile soils. The rapid rate of this reaction suggests that environmental levels of elemental tritium pose the same problem as tritiated water vapor. Field studies based on the awareness of this oxidation reaction are needed to re- solve the importance of releasing large amounts of elemental tritium.

Acknowledgment The technical assistance of Harry Hop and Allan Batterman

has been valuable in accomplishing this research. Their pre- cision in making observations and dedication to excellence are greatly appreciated.

Literature Cited (1) Weaver, C. L., Harward, D., Peterson, H. T., Jr., Public Health

( 2 ) Cochran. J. A,. Griffin, W. R.. Troianello, E. J., “Observation of Rep. , 84 (4), 363-71 (1969).

Airborne Tritium Waste Discharge from a Nuclear Fuel Repro- cessing Plant”, Office of Radiation Programs, U.S. Environmental Protection Agency EPA/ORP 73-1,1973.

(3) McFarlane, J. C., “Tritium Accumulation in Lettuce Fumigated with Elemental Tritium”, Ecological Research Ser. EPA-600/3- 76-006, 1976; Enuiron. Enp. Bot., in press (1978).

(4) International Commission on Radiological Protection, Rep. of Committee I1 on Permissible Dose for Internal Radiation, ICRP Publ. 2, Pergamon, Oxford, London, England, 1959.

(5) Eakins, J. D., Hutchinson, W. P., “The Radiological Hazard from the Conversion of Tritium to Tritiated Water in Air by Metal Catalysts”, “Tritium”, A. A. Moghissi and M. W. Carter, Eds., pp

592 Environmental Science & Technology

392-9, Messenger Graphics, Phoenix, Ariz., Las Vegas, Nev., 1973. 1976.

(6) Moghissi, A. A., Bretthauer, E. W., Compton, E. H., AnaLChern., 45,1565-6 (1973).

(7) Lieberman, R., Moghissi, A. A., J. Appl. Rad. Isotopes, 21,3A-327 (1970).

( 8 ) McFarlane, J. C., Beckert, W. F., Brown, K. W., “Tritium in

Plants and Soil”, Ecological Research Ser. EPA-600/3-76-052,

(9) Ehhalt, D. H., Heidt, L. E., Lueb, R. H., Roper, N., “Vertical Profiles of CH4, Hz, CO, NzO, and COz in the Stratosphere”, Third Conf. on CIAP, US. Dept. of Transportation, Feb. 1974.

Receiued for reuieu, June 3,1977. Accepted December 5, 1977.

NOTES

Lead Accumulation in a Northern Hardwood Forest

Thomas G. Slccama’ and William H. Smith School of Forestry and Environmental Studies, Yale University, New Haven, Conn. 0651 1

Lead from the atmosphere is accumulating in the soil of a remote northern hardwood forest ecosystem at a rate of 305 g ha-’ yr-l. At the current input rate the doubling time of the lead concentration in the humus is approximately 50 years.

Lead is naturally present in small amounts in biota, soil, rocks, surface waters, and the atmosphere. Since the beginning of the industrial revolution and especially since its use as a gasoline additive, lead has been introduced into the atmo- sphere in ever increasing amounts (1,2). The behavior of this potentially toxic metal in biogeochemical cycles of terrestrial ecosystems has long-term implications with respect to eco- system functions such as productivity, decomposition, nu- trient cycling, and insect and microbial population dynamics. Lead deposited from the atmosphere has a mean residence time of 5000 years (3 ) in the surface organic soil horizons, and long-term concentration increases can be predicted as long as inputs exceed outputs.

The deposition of lead on the forests of New England sev- eral hundred kilometers from major population centers is relatively higher than on many other rural regions of North America due to patterns of air movement and location of sources ( 4 , 5 ) .

We have determined the lead budget for a forested water- shed ecosystem at the Hubbard Brook Experimental Forest in central New Hampshire. The Experimental Forest ranges in altitude from 229 to 1006 m and covers 3076 ha. It is typified by an unbroken canopy of second-growth northern hardwoods with patches of spruce-fir, particularly a t higher elevations. Major species of overstory trees are Acer saccharum (sugar maple), Fugus grandifolia (beech), Betula alleghaniensis (yellow birch), Picea rubens (red spruce), Betula papyri fera (white birch), and Abies balsamea (balsam fir).

A number of small watersheds within the experimental forest are under intensive hydrologic study by the U.S. Forest Service. Six of these have been under intensive biogeochemical study since 1962. The lead budget for watershed six (WS-6) was determined for 1975. Watershed 6 is the control area for study of the biota and flow of nutrients and energy through an undisturbed forested watershed ecosystem. Watershed 6 has an area of 13.23 ha and ranges in altitude from 546 to 791 m above sea level. Slope inclination averages about 12-13’, and aspect is generally toward the southeast.

The watershed is covered by a mantle of bouldery glacial till with occasional outcrops of gneissic bedrock. The pre- dominant soil is a sandy loam podzol of the Hermon series with a thick H layer and a discontinuous but often well-de- veloped A2 horizon. Locally the soil surface has been disturbed by windthrows, and pits and mounds are extensive. Large

rocks are frequent. The soil surface is very permeable; how- ever, a compact hardpan occurs a t about 60 cm. Overland flow of water is minimal (6-8).

Exper imen ta l Precipitation was collected in forest openings in 1-L plastic

bottles connected by vapor-lock loop-tubes to 27-cm-diameter plastic funnels mounted on wooden frames. Samples were collected monthly. Two collectors were used, and the annual lead input was calculated as the sum of the monthly mean concentration times monthly total precipitation. Winter snow collectors (two) were open 120-L plastic barrels. Stream water was similarly collected monthly in 1-L plastic bottles ac- cording to procedures developed and described by Likens et al. (8). All water samples were acidified with 1 mL of Ultrex nitric acid and sent to the Environmental Trace Substances Research Center a t the University of Missouri for analysis.

Analyses of plant materials (including leaves, twigs, branches, bark and wood, but no roots) were done on all major woody species. Analyses were done independently a t both Yale and Missouri.

Sixty 15 X 15 cm samples of the forest floor from WS-6 were obtained extending down to the mineral soil. Samples were ground in a Wiley mill to pass a 20 mesh sieve. A 2-g aliquot was ashed at 500 “C in a muffle furnace, and the ash was eluted with 6 N nitric acid. Lead was determined by atomic absorption spectrophotometry.

Resul ts and Discussion The mean and SE of the monthly lead precipitation input

was 0.264 f 0.026 pg cm-2, Weighted lead concentration in bulk precipitation averaged 23.0 pg L-l, with a total lead input to the ecosystem of 317 g ha-l yr-l. This estimate of annual lead input is consistent with those obtained by others (9 , lO). A distinctive seasonal pattern occurred with summer (June, July, August) input (42 g ha-l mo-l) approximately double winter (December, January, February) input, (19 g ha-’ mo-l). Since precipitation is evenly distributed throughout the year, this difference was due to increased concentration of lead in the summer rain. The seasonal pattern is probably due to the combination of summer air masses coming from the more urban regions southwest of the study area relative to north- west winter winds from Canada and to increased regional motor vehicle use during the summer.

Concentration of lead in stream water draining from the ecosystem was approximately 1 pg L-l (mean and SE of 1 2 monthly determinations was 1.21 f 0.57). In addition to the 11.4 g ha-’ yr-l leaving in the dissolved form, 0.7 g ha-l yr-l left as coarse particulate matter. Use of the term dissolued does not imply that the lead in the stream water samples was

0013-936X/78/0912-0593$01 .OO/O 0 1978 American Chemical Society Volume 12, Number 5, May 1978 593