Environ. Sci. Techno/. 1995, 29,135-139
Evidence of Comalete Retention of Atmospheric b a d in the Soils of Northern Hardwood Forested Ecosystems EDWARD X. WANG,* F. HERBERT BORMANN, A N D GABOURY BENOIT School of Forestry and Environmental Studies, Yale University, 370 Prospect Street, New Haven, Connecticut 0651 1
Lead is a highly toxic contaminant that has been delivered in large quantities to forested ecosystems worldwide via atmospheric deposition. In the past, it has been well-documented that high concentrations of lead are found in surface soils of even remote ecosystems. Recent long-term measurements have shown a significant decline in lead inventories in the forest floor, and there is the danger that this lead is being released to streams, rivers, or groundwater. However, understanding lead's behavior has been complicated by the extreme difficulty of measuring it at the low levels that occur in natural waters (soil water, groundwater, streams). This study conducted at the Hubbard Brook Experimental Forest (HBEF) provides some of the first reliable data on lead concentrations in streams and surface waters using state-of-the-art clean techniques at all stages of sample collection, processing, and analysis. Results show that dissolved lead in streams and seeps at HBEF is surprisingly low (mostly under 10 parts per trillion), though a significant amount of lead is found to be leached out of the forest floor layer (approximately 5 parts per billion in soil solutions beneath 0 horizons). Our study also shows that lead concentrations in streams and seeps are at least 10-100 times lower than those measured previously at HBEF without clean techniques. W e conclude that mineral soil horizons act as the net sink for atmospheric lead (approximately 0.8 ppb in recent bulk precipitation) as it passes through forested ecosystems. The ecosystem appears to be an excellent "filter" that completely retains industrial contaminant lead in its soil profile. In estimates of total Pb outflow in streams, over 80% is found t o be associated with particulate matter. Since a large portion of the particulate matter may be derived directly from surface soil debris, the stream Pb contributed by soil percolates is virtually nil. Undisturbed forested ecosystems control soil erosion and particulate matter output, so leakage of Pb from the ecosystem is minimal.
0013-936x/95/0929-0735$09.00/0
0 1995 American Chemical Society
Introduction For years, lead was used as an antiknock additive in gasoline, resulting in its global dissemination (1, 2). In the United States, legislation restricting the sale of leaded gasolinehas resulted in dramatic decreases in leaded gasoline consumption and lead in air (3).As a result, some investigators have inferred that the decrease in atmospheric Pb would be reflected in the historical patterns of Pb accumulation in the sediments of lakes and rivers (4-6). However, this assumes that Pb in streamwater responds directly to the decline in atmospheric input, since Pb in sediments is not contributed only by direct precipitation (7, 8). Atmospheric lead deposited in forested ecosystems has been found to be preferentially accumulated in the surface soil layer (9-12). It has been suggested that Pb is strongly bound to soil organic matter and would remain there for a period of centuries (11, 13, 14). Most recently, based on long-term Pb mass balance data for Hubbard Brook, it has been estimated that fully 29% of the lead present in the forest floor was lost by leaching in the period from 1977 to 1987 (30). The question remains whether Pb leached from the forest floor eventually reaches streams, and whether the mobilized Pb poses a threat to downstream water quality. However, previous measurements of Pb in streams at HBEF are questionable due to a failure to use clean techniques. Failure to follow appropriate clean protocols calls into question much of the previous research on metal cycling in freshwaters (15-1 7),just as virtually all marine trace metal data from before about 1975are now considered invalid. Therefore, the evidence linking Pb in streams to that in precipitation and soils is unclear.
Methods This study was conducted at Hubbard Brook Experimental Forest (HBEF),New Hampshire. Rigorous clean techniques were used, and a sequence of streamwaterand groundwater samples were collected from Bear Brook, west of the HBEF reference watershed W6. Seeps provide access to subsurface waters without soil disturbance, which can introduce serious contamination. Zero-tension lysimeters at high, middle, and low elevations of the watershed were used to collect soil solutions. These lysimeters were installed in 1983 and 1984 and are considered to be well equilibrated with soil conditions over the years (18). Samples of precipitation, streams, seeps, and soil solutions were collected in 1993 on a monthly basis over a 1-year period. Since lead concentrations in streams and seeps were extremely low (parts per trillion level), extraordinary precautions were taken to prevent contamination of water samples. Detailed clean technique protocols are discussed elsewhere (19). Stream and seep samples were collected by peristaltic pumping through acid-cleaned Teflon tubing into acid-cleanedlow-densitypolyethylene bottles. Waters were filtered during collection by passage through acidcleaned 0.45-ym Millipore durapore filters contained in acid-cleaned 47-mm diameter Teflon filter holders. In addition to the filter-retained and filtrate fractions, awhole water sample was also collected by pumping directly into a bottle without filtration. This allows a mass balance check to be performed to monitor quality control. Procedural blanks were analyzed routinely on each field trip. Blank
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2500 A 1886 Stream Data
Precipitation
seeps
I
Thir Study
L
Soil Oa Soil Bh
7-
Soil Bs
E
-
1500
-
1000
-
500
-
11
a
Seeps
e
Streams
0
1
2
3
4
5
6
Pb (PPb)
FIGURE 1. Lead in bulk precipitation, soil 0.. Bh, and Bs horizons, seeps, and streams at Hubbard Brook Experimental Forest, New Hampshire. Nota that sample measurements are dissolved Pb concentrations(passing0.4!bpn filters)except for bulk precipitation. Uncertainities (error bars) reflect sample heterogeneity between sampling dates rather than analytical variability.
samples consisting of distilled water were “collected” in the field using the standard protocol. After water samples were returned to the laboratory, they were acidified using 2 mL of ultrapure HN03/L of sample. All sample handling for trace element analysis was carried out in a class-100 positive-pressure clean laboratory at all times. Construction and installation of lysimeters were described in ref 18. Lysimeter solutions were pumped through Teflon tubing directly into a 2-L polyethylene bottle. At each of the three sites, water from three lysimeters was composited. During the summer months, when soil moisture was low, and during winter periods of sub-zero temperatures, the lysimeter collection bottles were often empty. Occasionally, individual lysimeters collected no water at all, and this highlights the sporadic and episodic nature of subsurface flow at this location. Precipitation was collected by the HBEF staff and measured by us using clean techniques. For streamwaters and seep waters, lead was preconcentrated by evaporating 100-200-mL samples acidified with ultrapure HN03 in a Teflon beaker at 80 “C. The National Research Council of Canada standard reference river water SLRS-1 was used to monitor the recovery of the preconcentration method. Lead was determined by graphite furnace atomic absorption spectrophotometry (GFAAS), using a Perkin-Elmer 5000 atomic absorption spectrophotometer equipped with an HGA-600 graphite furnace atomizer and an AS40 autosampler.
Results and Discussion Figure 1 shows the Pb concentration gradient startingwith bulk precipitation and moving through soil O,, Bh, and B, horizons to seeps and streams. Clearly, as atmospheric Pb passes through the soil profile its concentration is augmented by Pb leached out of the forest floor (0, horizon). The dissolved Pb (passing a 0.45-pm filter) is decreased by a factor of 1000 in passing from 0, horizons to streams (from about 5 ppb to about 5 ppt). Clean techniques and sample preconcentration were crucial for the measurement of Pb at the parts per trillion level in streams and seeps. 736 s ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL.
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850 8 0 0 7 5 0 7 0 0 6 5 0 6 0 0 5 5 0 500 450 400 350 ELEVATION ( m )
FlGURE2. Elevationalvariationof lead in streams and seeps (sampled during April-November 1993) and comparison to previous Pb data measured in 1986 (adopted from ref 18)- The stream data are bulk stream samples without being filtered. The seep data are dissolved Pb concentrations (passing 0.45-pm filters). The 1986 data from below 790 m are averages that include some values listed as “0” (under the detection limit) in the original data. In this case, we used the computer program UNCENSOR 3.0 to calculate estimated means based on the left censored data.
Figure 2 shows the elevational patterns of Pb along Bear Brook and the comparison to previous Pb data measured in 1986 (18). Without clean techniques, the 1986 Pb measurements are 10-100 times higher than those of the current study at comparable elevations. Since the trend of declining atmospheric Pb inputs flattened after 1985 at HBEF, the extremely low level of stream Pb we measured does not appear to be a result of declines in response to decreasing atmospheric inputs. Furthermore, direct precipitation into stream channels is considered to contribute only a small portion of total Pb in streamwater. It is possible that the large change in Pb concentrations in streams over time is real. However, it i s more likely that this disparity can be explained by contamination artifacts in the previous data. We conducted some test measurements of parallel samples with and without clean techniques. Samples that were collected without using our standard protocols (19) were found to have an approximated one-third chance of contamination (one out of three samples). Although the comparison was preliminary and it is not clear exactlywhich part of the procedure contributed to the contamination, this result suggests that clean techniques are crucial for studying Pb at the parts per trillion level. There are only small variations in Pb concentration in the stream at different elevations (Figure 2), except that relatively higher values occurred in samples collected at the top ofthe watershed (above700 m). The stream samples collected at the top ofwatershed may be more closelyrelated to precipitation, since shallow soil profiles there do not effectively filter the rainwater. The seasonal variation of Pb in streams and seeps over a 1-yr period is also small (Figure 3). One exception, occurring in the spring, is
60
0 Seepa
e stream4
50
Precipitation
40
a
9
30
Soil Oa
20
Bh
e
7
10
Bs 0
Apr.
June
July
Aug
Sept
Oct
”-
Stream
FIGURE 3. Temporal variation of dissolved lead in streams and seeps over a 1-yr period (1993) at HBEF.
elevated Pb, which derives almost directly from melting snow. Pb in seeps is consistently higher than that in streams. There are two possible explanations for the relatively higher Pb in seeps: (a) Seep water has a different flow path through soils compared to deep soil waters that supply the stream. (b) Pb derived from deep soil waters and seeps is retained on streambed sediments. Further studies on Pb scavengingby streambed sediments and the determination of C horizon soil waters are needed to compare the two possibilities. To correct for dilution or evaporation effects, we converted the Pb concentration gradient to a mass flux gradient. Estimated Pb fluxes include bulk precipitation inputs, soil solution fluxes from the O,,Bh, and B, horizons, and stream outflow. Estimates of Pb bulk precipitation inputs and stream outputs were based on measured 1993 water fluxes and monthly average Pb concentrations. Pb concentrations in bulk precipitation were used to compute total atmospheric input. This input may be slightly underestimated because it does not include dry deposition. The total input of dissolved and suspended particulate Pb in streams is calculated using the Pb concentrations in bulk streamwater. Bedload particles, however, are not included in this estimate of Pb output. Soil water fluxes were computed based on a hydrologic simulation model for Hubbard Brook (20) with input data of daily precipitation, daily maximum and minimum temperatures, and daily solar radiation. The simulateddaily soil water flux was summed for each month. Pb fluxes were calculated by multiplying the average concentration of dissolved Pb in each month by the monthly water flux. Fluxes of particulate Pb in soil water could not be determined based only on lysimeter samples. Beier et al. (21)have reported that significant differences exist among lysimeter types for the collection of large organic macromolecular particles in the size range from 1 to 10 pm. To avoid this uncertainty, we filtered soil waters before Pb was determined. Therefore, the estimates of soil water fluxes are lower limits, since it has long been observed that some quantity of particle-associated pollutants migrates downward in the soil profile along with particles (22-24). Although our estimates are only approximate, the mass flux gradient shows a pattern similar to that for Pb concentrations in bulk precipitation, soil solutions, and streams (Figure 4). This study shows that a significantfraction of Pb is being transported out of the forest floor layer. After correcting for evaporation effects, the Pb flux of 0, soil solutions is still much higher than that in precipitation inputs. This
I
Pb Fluxes ( g/ha/yr )
FIGURE 4. Lead fluxes in bulk precipitation, soil 0,. BI,, and B, horizon pore waters, seeps, and streams at HBEF.
confirms the long-term Pb mass balance data indicating a substantial recent net loss of Pb from the forest floor (12). Estimation of Pb outflow via streams is a key component of this study. The estimated value (Figure 4) is the total output of both dissolved and suspended particulate Pb under base flow conditions. (The influence of storm events is discussed below.) Compared with about 21 g ha-’ yr-’ Pb escapingfromthe forest floor layer, Pb outflow in streams is very low (less than 0.2 g ha-’ y-l). In order to determine the distribution between dissolved and suspended particulate Pb, we measured the Pb partition coefficient, %, in Bear Brook. Kd is defined as the ratio of metal concentration on filter-retained particles (> 0.45pm) to that dissolved in solution: Kd = (MplSPM)/Ma. k f d and Mpare, respectively, concentrations of dissolved and suspended particulate Pb normalized to solution volume (ng/L). SPM is suspended particulate matter (units, kglL). Calculated Kd values (units, Llkg) are inversely correlated with the concentrations of suspended particulate matter (Figure 5). We believe this to be the first field study documenting the “particle concentration effect” (25)at SPM less than 1mglL in streams. Regression shows that log(%) = 6.7-0.7 log(SPM) ($ = 0.88). Surprisingly, the equation is in perfect agreement with the results from other northeastern US. freshwaters where background chemical and physical parameters are very different from those in Bear Brook (31). & is predictable and can be used to calculate the relative contribution of particulate Pb to total Pb carried in the streams of HBEF. From the & definition, we derive Md/(Md
+ Mp)= I / ( & x SPM + 1)
Therefore kfd/
(Md
+ Mp)= 1/
X
+
SPM0.3 1)
(1)
Based on eq 1, the percentage of particulate Pb is plotted as a function of suspended particulate matter (Figure 6). As SPM increases, the proportion of Pb associated with particulate matter increases dramatically. When SPM is more than 1mglL, over 80% of total Pb exists in particulate form. Particle-associated Pb approaches 100% at higher VOL. 29, NO. 3, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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5l
0 Hubbard Brook
3 10-2
lo-‘
100
10’
102
SPM (mg/L) FIGURE 5. Correlation between suspended particulate matter (SPM) and paltitioning coefficient 4. (The data for other northeastern freshwaters are adopted from ref 31.)
n
a P
a 0 4 d r.
:
s 2 a
u 0 0
Y d
4
g0 I4
a
SPM (mg/L) FIGURE 6. Fraction of total Pb occurring as suspended particulate Pb in streams.
SPM. The effect is even greater if the bedload particulate matter is added into the total Pb mass. Smith and Siccama (11)reported that bedload matter (> 1mm) made up about 16% of the total Pb output from W6. Our finding is particularly important because both suspended and bedload particulate matter are directly and exponentiallyrelated to stream discharge (26, 27). When stream discharge is high, particulate Pb dominates total Pb output. Therefore, output of Pb in streams is stronglyrelated to the occurrence of random storms and the capacity of the ecosystem to damp and buffer the discharge rates. Suspended particulate matter has been found to be almost 10 times higher in a deforested watershed (W2) than in one that is undisturbed (W6) (26). In an undisturbed and mature forest ecosystem, the loss of particulate matter is reduced by two factors. First, discharge rates are lowered due to interception and transpiration. Second, the forest structure reduces soil erosion. Biotic regulation of the forested ecosystem plays 738 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 3, 1995
an important role in retaining Pb by controlling the loss of both suspended load and bedload particulate matter. This study has shown that stream dissolved Pb derived from soil percolates is negligible. As suggested by Smith and Siccama (111,the ecosystem appears to be an excellent “filter” that effectively accumulates atmospheric lead in the forest floor. The ecosystem also completely retains Pb leached from the forest floor in mineral subsoil. The extremely low Pb values we measured in the streams of HBEF are comparable to those observed by Patterson’s group studying the Sierra Nevada Mountains in California (10, 28). Since the soil profile acts as a sink by effectively scavenging Pb released from surface soil horizons, Pb levels in streams do not appear to respond directlyto atmospheric inputs. Instead, they respond to the characteristics of an ecosystem (its biotic and abiotic regulations). Thus, the use of downstream lacustrine sediment records for estimating historical atmospheric deposition of Pb may be inaccurate and sometimes inappropriate. Based on the annual output of dissolved and suspended particulate Pb in the stream (0.2g ha-ly-l), the estimated current turnover time (pool sizeloutput) of exchangeable Pb in the mineral soil (29) at HBEF is approximately 20 000 yr. (We use “turnover time” instead of “residence time” because we do not wish to imply steady-state conditions for Pb dynamics in HBEF soils.) Although the Pb transported via bedload particles may hasten the Pb movement out of the ecosystem, the residence time is substantially higher than estimates for the forest floor alone (11-14). Our Pb speciation studies of soil solutions (32)indicate that Pb is mobilized from 0, horizon primarily through (a) association with mobilized colloids and (b) partitioning of Pb2+between aqueous and solid phases. Pb complexes with truly dissolved organic and inorganic ligands are insignificant. Both Pb2+ and colloidal Pb from the 0, horizon are effectivelyadsorbed down the soil profile, albeit independently of each other. Although continuing Pb accumulation in mineral soil may increase Pb levels in B h and B, soil solutions through Pb2+ partitioning between aqueous and solid phases, such increases are most likely to be gradual and slight. Unless there are drastic environmental changes (e.g.,pH and dissolved organic colloids) affecting the metal adsorption of thse horizons in the future, it is expected that streams at HBEF will remain at extremely low Pb levels as long as particulate material is held at current low levels in the stream and streambed. Therefore, it is concluded that no imminent threat of lead contamination is found for streams of undisturbed forested watersheds.
Acknowledgments We thank E. Morency, C. Lin for help with sampling, C. Driscoll for allowing us to access his lysimeters, and W. Smith and T. G. Siccama for their interest and encouragement. The study is financially supported for a grant of the A. W. Mellon Foundation to F.H.B., by a G. Evelyn Hutchinson Fellowship to E.X.W. from the Yale Institute for Biosphere Studies, and by the Yale School of Forestry and Environmental Studies support of G.B. This is a contribution of the Hubbard Brook Ecosystem Study. The HBEF is owned and operated by the U.S. Department of Agriculture Forest Service, Radnor, PA.
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Murozumi, M.; Chow, T. J.; Patterson, C. C. Geochim. Cosmochim. Acta 1969,33, 1271-1294. Nriagu, J. 0. Sci. Total Enuiron. 1990, 92, 13-28. Galloway, J. N.; Likens, G. E. Lumnol. Oceanogr. 1979,24,427. Heit, M.; Tan, Y.; Klusek, C.; Burke, J. C. Water Air Soil Pollut. 1981, 15,441. Hanson, D. W.; Norton, S.A.;Williams,J. S. WaterAirSoilPoElut. 1982, 18,227. Cornett, R. J.; Chant, L.; Link, D. Water Pollut. Res. J. Chem. 1984, 19, 97-109. Benoit, G.; Hemond, H. F. Geochim. Cosmochim. Acta 1987,51, 1445-1456. Page, A. L.; Ganje, T. J. Environ. Sci. Technol. 1970,4, 140-142. Erel, Y.; et al. Chem. Geol. 1990, 85, 383-392. Smith, W. H.; Siccama,T. G.J.Environ. Qual. 1981,10,323-333. Friedland, A. J.; et al. Ambio 1992, 21, 400-403. Lewis, D. M. Geochim. Cosmochim. Acta 1977, 40, 164-167. Turner, R. S.; Johnson, A. H.; Wang, D. 1.Enuiron. Qual. 1985, 14, 305-314. Coale, K. H.; Flegal, A. R. Sci. Total Enuiron. 1989, 87/88,297304. Ahlers, W. W.; et al. Aust. 1.Mar. Freshwater Res. 1990,41,713720. Windom, H. L.; et al. Environ. Sci. Technol. 1991,25,1137-1142. Driscoll, C. T.; Fuller, R. D.; Simone, D. M. J. Environ. Qual. 1988, 17,101-107. Benoit, G. Environ. Sci. Technol. 1994, 28, 1987-1991.
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Received for review July 6, 1994. Revised manuscript received November 21, 1994. Accepted November 22, 1994.@
ES9404173 @Abstractpublished in Advance ACS Abstracts, January 1, 1995.
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