Environ. Sci. Technol. 1998, 32, 2371-2376
Lead Isotopes in Tree Rings: Chronology of Pollution in Bayou Trepagnier, Louisiana F R A N C O M A R C A N T O N I O , * ,†,‡ G E O R G E F L O W E R S , †,‡ L E O N A R D T H I E N , ‡,§ A N D E R I K E L L G A A R D ‡,§ Department of Geology and Department of Cell and Molecular Biology, Tulane University, New Orleans, Louisiana 70118
We have measured the Pb isotopic composition of tree rings from seven trees in both highly contaminated and relatively noncontaminated regions of Bayou Trepagnier, a bayou in southern Louisiana that has had oil refinery effluent discharged into it over the past 70 years. To our knowledge, this is the first time that Pb isotope treering records have been used to assess the sources and extent of heavy-metal contamination of the environment through time. When tree ring 206Pb/208Pb and 206Pb/207Pb isotope ratios are plotted against one another, a straight line is defined by four of the most contaminated trees. This linear correlation suggests mixing between two sources of Pb. One of the sources is derived from the highly polluted dredge spoils on the banks of the bayou and the other from the natural environment. The nature of the contaminant Pb is unique in that it is, isotopically, relatively homogeneous and extremely radiogenic, similar to ores of the Mississippi Valley (i.e., 206Pb/207Pb ) 1.28). This singular pollutant isotope signature has enabled us to determine the extent of Pb contamination in each cypress wood sample. The isotope results indicate that Pb uptake by the tree is dominated by local-scale root processes and is, therefore, hydrologically and chemically controlled. In addition, we propose that the mobility and bioavailability of Pb in the environment depends on its chemical speciation.
Introduction Lead in the environment is derived from both natural and anthropogenic sources. Airborne transport of Pb is the most common manner in which lead moves in the environment (1). It is removed from the atmosphere and transferred to surfaces through wet or dry deposition. Transport of Pb in dissolved form through the hydrosphere is rare because Pb is extremely particle reactive, binding most often with organics. The fate of Pb in natural waters is governed, therefore, by its sorption behavior on sediments and particulates (2). Anthropogenic Pb derives from massive ore bodies that occur mainly in the U.S., Canada, the former USSR, and Australia. * Corresponding author. E-mail:
[email protected]; phone: 504-862-3195; fax: 504-865-5199. † Department of Geology. ‡ Tulane-Xavier Center for Bioenvironmental Research, New Orleans, LA 70112. § Department of Cell and Molecular Biology. S0013-936X(98)00109-6 CCC: $15.00 Published on Web 07/03/1998
1998 American Chemical Society
Pb has four naturally occurring, long-lived isotopes, only one of which is stable, 204Pb. 206Pb, 207Pb, and 208Pb are the radiogenic daughter products of 238U, 235U, and 232Th, respectively. The 206Pb/207Pb ratio, the most precisely measured ratio of the Pb isotopes, is used most frequently to compare isotope compositions of anthropogenic and natural lead. Differences in the 206Pb/207Pb ratio between anthropogenic and natural Pb sources result from differences in age and U/Pb concentration ratios between ore bodies and crustal rocks from which the soil or sediment is derived. In general, Pb derived from ore bodies and used in industrial processes has a lower 206Pb/207Pb isotopic ratio (e.g., 1.15) than natural Pb (e.g., 1.21) in sediments and soils. An important exception to this rule is in the U.S., where the Pb currently used in industry is derived from ore bodies in Missouri (Mississippi Valley ores) that have radiogenic 206Pb/ 207Pb isotopic ratios between 1.28 and 1.33 (3). The shift to the use of lead from these Mississippi Valley ore bodies began in the early 1970s (4-7). Given the uniqueness of the anthropogenic Pb isotope signature, there have been several recent attempts in the literature to identify the different sources of pollution within contaminated biotic and abiotic environments (8-13). In this study, we attempt to analyze tree rings as biotic archives of Pb pollution. We make the assumption, therefore, that the chemical composition of tree rings is indicative of the chemistry of the environment within which the tree grew (e.g., ref 14). Tree rings are good indicators of past climate conditions (e.g., ref 15) and are known to provide a record of the effects of heavy-metal pollution. Indeed, tree rings have been used as a tool for studying temporal and spatial variations in anthropogenic chemical uptake (16-27). Many of these studies have investigated the concentration of heavy metals, including Pb. Although there is at least one study in which the Pb isotope composition of tree wood has been measured (8), to our knowledge, no study has yet used the source-identifying characteristics of Pb isotopes as a pollution tracer in dated tree rings. As a result of petroleum and other chemical industrial activities along the lower Mississippi River, heavy-metal pollution of southern Louisiana ecosystems is a widespread phenomenon. During the past 80 years, Bayou Trepagnier, Louisiana, 35 kilometers west of New Orleans, has been greatly affected by the discharge of heavy metals (lead, zinc, chromium, and copper), crude oil, and refining intermediates from the Norco Manufacturing Complex. In this study, we identify the sources of Pb and the method of its uptake in more than 80 cross-dated baldcypress tree-ring samples.
Methodology Study Site. Bayou Trepagnier is currently the site of a DOEfunded research program managed by the Tulane-Xavier Center for Bioenvironmental Research. The bayou is located approximately 35 km west of metropolitan New Orleans, adjacent to the lower guide levee of the Bonnet Carre´ Spillway (Figure 1). It flows northeast approximately 5 km from the northern boundary of the Shell Norco Manufacturing Complex and then joins Bayou LaBranche before flowing into Lake Pontchartrain. Shell Petroleum Company acquired the New Orleans Refining Company (NORCO) in 1929. Dredging of the bayou took place from the 1930s to the early 1950s. The dredge spoil was piled mainly on the spillway side of the bayou. The spoil bank sediments are extremely contaminated by heavy metals, especially Pb (28). The contamination is most likely due to the refinery’s use of alkyllead in the preparation of gasoline. Occasionally, Pb concentrations VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Location map of Bayou Trepagnier. Numbered markers (0-160) along the bayou are used to help locate each baldcypress tree studied here. See Table 1 for exact location of each tree within the bayou ecosystem. reach values in excess of 1000 times the concentration of Pb in average crustal rocks (28). Tree-Ring Samples: Dendrochronology and Pb Concentrations. Latimer et al. (26) have reconstructed a history of pollutant Pb additions to baldcypress trees in the Bayou Trepagnier ecosystem using state-of-the-art dendrochronological analysis. We have measured the Pb isotopic composition of tree ring composites for seven of the same trees analyzed by Latimer et al. (26). Baldcypress trees were sampled at a height of about 1.5 m using a Teflon-coated corer. To prevent extraneous Pb contamination, the cores were transferred to plastic tubes before dendrochronological analysis. On account of the small-diameter corer and the low concentrations of Pb in the wood, we used composite tree-ring samples which integrated either five or 10 years of growth (26). Five of the trees investigated are within 30 m of bayou’s southeastern bank (locations given in Table 1). One tree stands approximately 210 m from the southeastern bank (sample 4A-TR). The selected trees span the entire length of the bayou, from the headwaters at the Shell Refinery to the area where Bayou Trepagnier meets Bayou LaBranche (Figure 1). Tree rings for one tree from Stinking Bayou, a control site located 35 km NNE of Bayou Trepagnier (26), were also studied. Pb Isotope Analysis. The chemical extraction of Pb from tree ring samples was carried out under Class 100 “ultraclean” conditions at Tulane University. Tree-ring composite samples were digested using concentrated nitric acid in PFA Teflon beakers (Savillex brand). The samples were dried and treated with aqua regia twice in order to fully oxidize the remaining organic-rich residue. After drying, each tree ring composite was taken up in 0.5 N HBr in order to prepare it for Pb extraction. The separation of Pb from the resulting solution was carried out in 40 µL Teflon columns filled with AG1-X8 anionexchange resin (100-200 mesh) using the procedure outlined in Manhe`s et al. (29). Purified, quartz-distilled acid reagents were purchased from Seastar Inc. Exchange chromatography was performed twice in order to ensure a thorough separation and purification of the Pb. Isotopic analysis of samples was performed on a Finnigan Mat 262 thermal ionization mass spectrometer in the laboratory of Alan Zindler at Florida State University. The purified Pb was dried down, redissolved in 2372
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1 µL of H20 and loaded onto single rhenium filaments with 1 µL of 0.15 N H2PO4 and 1 µL of 2 µg/µL solution of silica gel. Samples were run in static multiple collector mode and were corrected for an apparent fractionation of 0.12%/amu (based on multiple analyses of standard, NIST-981). The error on each isotope ratio analysis is essentially determined by the reproducibility of multiple analyses of NIST-981 and is equal to 0.05%/amu (2σ). Total procedural Pb blanks, measured by isotope dilution analysis using a 204Pb spike, were less than 20 pg. Hence, blank corrections were negligible. Pb isotope results are listed in Table 1.
Results and Discussion The baldcypress is a long-lived deciduous gymnosperm capable of growing in flooded or dry soils. It makes up a significant biotic component of the freshwater swamp ecosystems of the southeastern U.S. In Louisiana’s Bayou Trepagnier and Stinking Bayou, Latimer et al. (26) have measured the Pb concentrations of tree rings analyzed in this study for Pb isotopes. The highest concentrations of Pb (average of 4.5 µg/g, n ) 25) occur in trees within 2 km of the refinery (26), close to the headwaters of Bayou Trepagnier (see Figure 1). Three to five kilometers downstream, closer to the mouth of the bayou, the tree rings have an average concentration that is lower by a factor of 2 [i.e., approximately 2.2 µg/g, n ) 25 (26)]. Rings from trees located adjacent to the banks of Stinking Bayou have average Pb concentrations of about 1 µg/g [n ) 5, (26)]. Latimer et al. (26) have shown that, for trees within 2 km of the Norco refinery, higher-than-background concentrations of Pb appear in the tree-ring record at the time of the plant’s opening (ca. 1920). The highest Pb concentrations occur within rings that formed in the late 1940s and early 1950s, indicating a correlation with the time of extensive dredging of the bayou. Latimer et al. (26) interpret the broad correlations between Pb pollution events and the Pb concentrations within the tree rings as evidence for relatively minor translocation of Pb along the xylem rays in cypress trees. Other studies have also demonstrated that Pb is relatively immobile within tree rings [e.g., balsam fir (20),
TABLE 1. Baldcypress Pb Isotope Ratios sample
avg year
208Pb/204Pb
207Pb/204Pb
206Pb/204Pb
206Pb/207Pb
206Pb/208Pb
1.2449 1.2255 1.2665 1.2458 1.2419 1.2250 1.2333 1.2229 1.2459 1.2350 1.2420
0.5014 0.4967 0.5073 0.5015 0.5012 0.4965 0.4985 0.4960 0.5019 0.4993 0.5009
7-10A (0-10, 10 m)a
1990 1986 1982 1978 1974 1970 1966 1962 1958 1954 1950
41-0-A (40-50, 20 m)
1990 1986 1982 1978 1974 1970 1966 1962
39.297 38.699 39.470 39.392 39.373 39.404 39.224 39.140
15.746 15.701 15.760 15.750 15.751 15.750 15.741 15.719
19.925 19.169 20.157 20.072 20.024 20.086 19.811 19.724
1.2654 1.2209 1.2790 1.2744 1.2713 1.2752 1.2585 1.2547
0.5070 0.4954 0.5107 0.5096 0.5086 0.5097 0.5051 0.5039
2-0-2A (0-10, 30 m)
1992 1987 1982 1977 1972 1967 1957 1952 1947 1942 1937 1932 1927
38.562 38.581 38.742 38.742 38.743 38.739 38.731 39.353 39.059 39.144 39.200 38.882 39.128
15.632 15.619 15.671 15.684 15.660 15.670 15.700 15.743 15.700 15.708 15.724 15.648 15.701
19.100 19.203 19.300 19.264 19.355 19.287 19.197 20.030 19.680 19.784 19.836 19.547 19.727
1.2218 1.2294 1.2316 1.2283 1.2360 1.2309 1.2227 1.2723 1.2535 1.2595 1.2615 1.2493 1.2564
0.4953 0.4977 0.4982 0.4973 0.4996 0.4979 0.4956 0.5090 0.5039 0.5054 0.5060 0.5027 0.5042
4A-TR (20-30, 210 m)
1990 1980 1972 1967 1960 1950 1940 1932 1927 1917 1907
38.615 38.607 38.941 38.654 38.592 38.556 38.668 38.538 38.524 38.322 38.580
15.698 15.698 15.734 15.697 15.664 15.685 15.698 15.670 15.669 15.624 15.689
19.011 18.988 19.582 19.106 19.107 18.979 19.129 18.955 18.972 18.801 18.990
1.2110 1.2095 1.2446 1.2171 1.2198 1.2100 1.2186 1.2096 1.2108 1.2034 1.2104
0.4923 0.4918 0.5029 0.4943 0.4951 0.4922 0.4947 0.4918 0.4925 0.4906 0.4922
7-12B (80-90, 17 m)
1990 1986 1982 1978 1974 1970 1966 1962 1958 1954 1946 1942 1938
38.576 38.510 38.565 38.563 38.560 38.674 38.593 38.567 38.563 38.559 38.677 38.564 38.549
15.685 15.671 15.686 15.683 15.683 15.727 15.695 15.688 15.687 15.683 15.725 15.686 15.680
18.930 18.952 18.981 18.987 18.974 19.040 18.986 18.979 18.981 18.983 19.028 18.978 18.975
1.2069 1.2093 1.2101 1.2107 1.2098 1.2104 1.2097 1.2098 1.2100 1.2104 1.2100 1.2099 1.2103
0.4907 0.4921 0.4922 0.4924 0.4921 0.4923 0.4920 0.4921 0.4922 0.4923 0.4920 0.4921 0.4922
159-2B (150-160, 20 m)
1990 1986 1982 1978 1974 1970 1962 1958
38.582 38.578 38.551 38.538 38.566 38.705 38.524 38.556
15.692 15.690 15.681 15.678 15.686 15.724 15.673 15.681
18.982 18.985 18.980 18.977 18.989 19.034 18.972 18.987
1.2097 1.2100 1.2103 1.2104 1.2105 1.2105 1.2105 1.2108
0.4920 0.4921 0.4923 0.4924 0.4924 0.4918 0.4925 0.4924
SB
1990 1986 1982
38.632 38.574 38.556
15.700 15.685 15.683
19.038 19.004 18.987
1.2126 1.2116 1.2107
0.4928 0.4927 0.4924
7-12B
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TABLE 1. (Continued) sample
avg year
208Pb/204Pb
207Pb/204Pb
206Pb/204Pb
206Pb/207Pb
206Pb/208Pb
1978 1974 1970 1966 1962 1958 1954 1950 1942 1938 1934 1930
38.581 38.559 38.563 38.556 38.582 38.576 38.612 38.631 38.572 38.557 38.603 38.564
15.690 15.684 15.682 15.682 15.692 15.691 15.701 15.695 15.685 15.684 15.694 15.680
19.005 18.988 19.032 19.002 18.988 18.977 19.000 19.158 19.008 18.981 19.008 19.007
1.2112 1.2107 1.2137 1.2118 1.2101 1.2094 1.2101 1.2207 1.2118 1.2102 1.2111 1.2122
0.4926 0.4924 0.4936 0.4929 0.4921 0.4919 0.4921 0.4959 0.4928 0.4923 0.4924 0.4929
a First two numbers in brackets are the marker numbers along Bayou Trepagnier between which the tree is located, the third number is the distance the tree resides southeast of Bayou Trepagnier (see Figure 1).
FIGURE 2. 206Pb/208Pb versus 206Pb/207Pb isotope ratios for all tree ring samples. The straight lines (solid) defined by the four contaminated trees (solid symbols) have identical slopes (m) within 1 standard deviation. Tree 2-0-2A, m ) 0.257 ( 0.005; tree 7-10A, m ) 0.266 ( 0.003; tree 41-0-A, m ) 0.263 ( 0.004; 4A-TR, m ) 0.306 ( 0.006. eastern red cedar (23), and oak (30)]. Minimal Pb translocation in the rings of baldcypress trees along this freshwater bayou is also suggested by research performed Yanosky et al. (25). They found no evidence of chloride mobility in the rings of baldcypress trees growing along freshwater rivers. This is noteworthy since one might expect Pb concentrations to mimic those of chloride through the formation of Pb chloride complexes. In our analysis of tree rings from seven baldcypress trees (Table 1 and Figure 2), six from Bayou Trepagnier and one from Stinking Bayou, we find spreads in the 206Pb/207Pb and 206Pb/208Pb ratios of 6 and 4%, respectively. In Figure 2, the tree ring samples are divided into two groups according to their 206Pb/207Pb ratios. The solid symbols represent tree rings from four trees in which the spread in 206Pb/207Pb ratios is between 2 and 6% in each tree. The open symbols, on the other hand, represent tree rings from three trees with a spread of less than 1% in the 206Pb/207Pb ratios. The 206Pb/207Pb ratios are much higher than the highest ratios observed for Pb atmospheric emissions (∼1.22) in the United States (4, 5). The highest 206Pb/207Pb ratios (up to 1.28) are extremely radiogenic and similar to the 206Pb/207Pb ratios observed in ores from the Mississippi Valley (3). Indeed, the unusual Pb isotopic signature of the ores and the fact that we find similar signatures in the bayou ecosystem make the ores the most likely ultimate source of contaminant Pb in the region. In 2374
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addition, the most radiogenic ratios observed in the baldcypress trees are identical to those observed in soil and sediment samples from the dredged spoil banks approximately 30 m west of the bayou (Marcantonio et al., in preparation). These spoil banks (see Figure 1) constitute the most contaminated region within the bayou and yield concentrations of Pb that range in the hundreds to the tens of thousands of micrograms per gram (28). The most contaminated spoil banks are located within 1 km of the Norco refining facility (Figure 1). Further downstream, near the confluence of Bayou Trepagnier and Bayou LaBranche (Figure 1), the spoil bank concentrations of Pb are only slightly elevated over background, in the tens of micrograms per gram (28). The average 206Pb/207Pb ratio for the most contaminated spoil banks is approximately 1.28 (Marcantonio et al., in preparation). This indicates that the contaminant Pb derives from Mississippi Valley ores. The unique isotope ratio characteristic of the pollutant Pb suggests that, prior to the 1950s, the Norco refinery used MississippiValley-type Pb in the production of its alkyllead. By contrast, the majority of U.S. industry did not begin using Pb derived from Mississippi-Valley-type ores until the mid-1970s (4). In a plot of 206Pb/207Pb versus 206Pb/208Pb isotope ratios (Figure 2), all of the tree ring data appear to follow a single linear trend. The r2 linear correlation coefficients are high, ranging from 0.996 to 0.998 for the four trees that have the greatest spread in the 206Pb/207Pb ratio. A linear regression analysis of each of these trees reveals identical slopes at the 1σ confidence interval (see caption to Figure 2). The linear trend implies that Pb in the tree rings comes from two distinct sources. These sources may be defined by their Pb isotope signatures. One of the sources yields 206Pb/207Pb and 206Pb/ 208Pb ratios of about 1.28 and 0.51, respectively. These match the ratios measured in soils collected from the spoil banks (Marcantonio et al., in preparation). The other source yields 206Pb/207Pb and 206Pb/208Pb ratios of about 1.21 and 0.492, respectively. These ratios represent natural Pb found in the uncontaminated soils of the bayou ecosystem (Marcantonio et al., in preparation). In a more detailed 206Pb/207Pb-206Pb/208Pb plot (Figure 3), individual linear regressions indicate that the trees with the larger spread in 206Pb/207Pb ratios produce slopes (solid lines, Figure 3) that are significantly different from those of trees with the smaller spread (dashed lines, Figure 3). We interpret the two sets of slopes (solid versus dashed lines, Figure 3) in the following way. The steeply sloped dashed lines are caused by the variability in the natural Pb endmember. Since this variability is so small, the natural Pb endmember forms a cluster in the broader context of Figure 2. The variability in Pb isotope composition of the natural Pb endmember is virtually negligible when compared to the total observed variation. The solid lines most likely represent mixing
FIGURE 3. 206Pb/208Pb versus 206Pb/207Pb isotope ratios for the uncontaminated trees (open symbols). The straight lines (dashed) defined by the three uncontaminated trees (including a tree from Stinking Bayou, a control site) have identical slopes (m) within 1 standard deviation. These slopes are outside of the error calculated for those of the contaminated trees. Tree 7-12B, m ) 0.36 ( 0.20; 159-2B, m ) 0.44 ( 0.03; Stinking Bayou (SB), m ) 0.36 ( 0.01.
FIGURE 4. 206Pb/207Pb of composite tree ring samples plotted against corresponding average age. The contaminated trees (solid symbols) show diverse contaminant histories. Uncontaminated trees (open symbols) have invariant Pb isotope ratios through time. between average natural Pb and contaminant Pb derived from the spoil banks. Tree rings can be recorders of past environmental conditions because they enable temporal resolution. Latimer et al. (26) have shown that Pb concentrations in cypress tree rings may be used to produce a historical record of pollution. In Figure 4, the 206Pb/207Pb ratio is plotted against the average age of the rings from which the Pb was derived. Two trees within the Bayou Trepagnier region show little variation in the 206Pb/207Pb ratio through time. This pattern is identical to the one observed for the tree from Stinking Bayou, a control site located 35 km to the northwest. The uncontaminated trees from Bayou Trepagnier are 3-5 km downstream from the most contaminated spoil banks, currently the main source of Pb pollution in the region. We interpret the small variability in 206Pb/207Pb ratios of uncontaminated trees as an indication
FIGURE 5. Estimated fraction of pollution Pb within each composite tree ring sample versus average age. Fractions were calculated using the following average parameters for natural (uncontaminated trees) and anthropogenic (average for dredged spoil banks) Pb: natural 206Pb/207Pb and 206Pb/208Pb ratios are 1.21 and 0.492, anthropogenic 206Pb/207Pb and 206Pb/208Pb ratios are 1.28 and 0.510. of the uptake of Pb through the roots rather than through the leaves of the baldcypress. If leaf uptake were important, we would expect there to be variations in the 206Pb/207Pb ratio at the control site due to the known 206Pb/207Pb variation in Pb fallout from atmospheric emissions through time (4, 6). If, as we believe, root uptake is the controlling factor in the transport of Pb into the wood, then one would expect the 206Pb/207Pb ratio to match the signature of either the groundwater or soil within which the roots reside. The degree to which each tree ring sample is contaminated with Pb through time depends on the baldcypress root system and the local hydrology. The depth to which the baldcypress root biomass extends in the Trepagnier region has not yet been measured. However, in a similar cypress stand in Florida, greater than 80% of the baldcypress root biomass was measured within approximately 20 cm of the ground surface (31). In the uncontaminated regions of Bayou Trepagnier (downstream from the Norco refinery and close to Bayou LaBranche, Figure 1), one would expect the natural Pb isotopic signature of the groundwater and soil within the top 20 cm to be similar and invariable through time. (The groundwater table is essentially within the top meter of this swamp ecosystem.) The soil contains natural Pb released upon the breakdown of minerals in the homogeneous clays and silts deposited in the Mississippi River delta. We may predict that the groundwater is in isotopic equilibrium with the soil. In contrast, within the contaminated areas at the headwaters of the bayou, the soil and groundwater will contain a mixture of contaminant and natural Pb. Consequently, the trees in this region are more likely to have a variable Pb isotope signature. In Figure 5, we calculate the fraction of pollution Pb within the tree wood and plot it against the average age of each composite tree ring sample. Only the rings from the four contaminated trees in the headwaters region are represented in Figure 5. In estimating the fraction of pollution Pb within the wood, we assume a constant isotopic composition for natural Pb and pollutant spoil-bank Pb through time (see caption to Figure 5). This assumption is reasonable given that uptake through the cypress roots is a near-surface phenomenon. The fraction of pollutant Pb uptake is variable through time for each contaminated tree within the bayou VOL. 32, NO. 16, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ecosystem. This fraction varies from virtually zero in tree 4A/TR to almost 90% in trees 2-0-2A and 41-0-A. The trees that contain the highest fraction of pollutant Pb (trees 2-02A, 41-0-A, and 7-10A) are all within 20-40 m of the most highly contaminated spoil banks in the Bayou Trepagnier ecosystem. By contrast, tree 4A/TR, the farthest from the high-concentration spoil banks (220 m distance), has the lowest fraction of pollution Pb among the contaminated trees. The variable contaminant history of each tree suggests that Pb uptake through the roots into a cypress tree must be dominated by local processes. Specifically, local hydrological and chemical processes may determine the amount of contaminant Pb metabolized by the tree roots. Physical transportation of the Pb-containing particles probably occurs by water currents during floods or storms. The most likely carriers of Pb are the fine-grained iron- and manganeseoxyhydroxides, sulfides, or colloidal organic particulates (2). Pb bound to such large molecules is unlikely to be metabolized through the root system. Under certain chemical conditions, however, complexed or adsorbed Pb may be released and mobilized in ionic form making it available for metabolization through the root system. In the case of the fine-grained sulfide or colloidal organolead complexes, oxidation reactions might release ionic Pb, which may enter root cells. If, on the other hand, Pb is predominantly held by iron and manganese-oxyhydroxides, then reducing conditions would be expected to release ionic Pb into the environment. Ionic Pb is less likely to be rescavenged and made immobile when local pH conditions are low [
