|Open-File Report 01-0429: World Trade Center USGS Leachate|
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A subset of the loose dust samples and samples of material coating girders collected from around the World Trade Center was subjected to chemical leach tests to examine potential release of metals from the dusts and beam coatings. The USGS leach test is a modification of a test described in detail in Hageman and Briggs (2000). This leach test and the one developed by Hageman and Briggs (2000) are modifications of the US EPA 1312 (Synthetic Precipitation Leaching Procedure, or SPLP) method. The USGS tests were originally designed as a screening method to quickly assess potential metal release from mine wastes.
As applied to the materials from the World Trade Center, these leach tests can be used to infer the potential for release of various metals, anions, and cations from the dusts as a result of rainfall or interactions with water used in fire fighting or street washing. The test also provides an indication of metals that might be bioavailable should the dusts be inhaled, ingested, or discharged into ecosystems.
For this leach test, deionized (DI) water (pH ~5.5) is used as the extractant. Dust samples were leached at a 1:20 ratio (2.5 grams dust / 50 milliliters DI water). A representative subsample of each dust sample was weighed on a balance. Each dust sample was then placed in a 125 milliliter (ml) high-density polyethylene (HDPE) bottle to which 50.0 ml DI water was added. Each sample was then shaken for 5 minutes. Following shaking, the solution was allowed to settle for 5 minutes. Leachate solutions were then filtered using plastic syringes and 0.45 micrometer pore size nitrocellulose membrane filters. Sub-samples of the leachate were collected and preserved for further analysis.
The procedure uses deionized water as the extractant solution rather than the synthetic acid rain used in the EPA 1312 method. It also uses a 5 minute agitation rather than an 18-hour agitation. Hence, it is possible that the concentrations of soluble metals measured with this test would be less than those measured using EPA 1312 method. However, it is likely that the leach procedure used in this study successfully reveals the metals likely to be mobilized from the dusts and girder coatings. A comparative study between the EPA Method 1312 (SPLP) procedure and this simplified leach can be found in Hageman and Briggs (2000).
The leachate samples were analyzed for major cations and trace metals by Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS) following the protocols outlined by Lamothe et al. (1999) and for anions by ion chromatography. The elements measured by the chemical analyses are those routinely measured by the USGS for studies of rocks, sediments, soils, and environmental samples.
Leachate solutions for a subset of the dust samples were also analyzed for dissolved mercury concentrations. For these samples, an aliquot was filtered using a syringe and disposable 0.45 micrometer pore size nitrocellulose filter, and then acidified and preserved by the addition of a 1 percent sodium dichromate/concentrated nitric acid solution in a ratio of 1:19 (one part sodium dichromate/nitric acid solution to 19 parts water sample). Leachate samples were stored in nitric acid-washed, flint glass bottles with Teflon lined lids. Samples were then analyzed for mercury using a Lachat QuikChem Mercury Analyzer with fluorescence detector. This method has a lower reporting limit of 5 part per trillion (ng/L).
Quality assurance-quality control information for the process and the chemical analyses are available upon request.
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Leach Figure 1. Plots showing the ranges (blue boxes) and means (horizontal white bars) in pH and specific conductance (upper plot), major cations and anions (middle plot), and trace elements (lower plot) in leachate solutions from WTC dusts and girder coatings. Elements for which one or more samples were below the analytical detection limits are indicated by arrows extending downward from the detection limit concentration. Abbreviations: mg/L = milligrams per liter (approximately the same as parts per million); µg/L = micrograms per liter (approximately the same as parts per billion); mS/cm = milliSiemens per centimeter. For comparison, 1 milligram per liter equals 1000 micrograms per liter, and 1 part per million equals 1000 parts per billion. Also, one mS/cm in specific conductance is approximately equal to 1000 milligrams per liter dissolved solids.
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Leach Figure 2. Map of lower Manhattan showing variations in pH (blue columns) and specific conductance (yellow columns) of leachate solutions from the various dusts and girder coating samples. Dust samples collected indoors are indicated by the single hatch pattern and girder coating samples by the cross-hatch pattern; all others are dust samples collected outdoors.
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Leach Figure 3. Map of lower Manhattan showing variations (as stacked bar charts) in major cation and anion concentrations of leachate solutions derived from the various dusts and girder coating samples. Dust samples collected indoors are indicated by the single hatch pattern and girder coating samples by the cross-hatch pattern; all others are dust samples collected outdoors.
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Leach Figure 4. Map of lower Manhattan showing variations (as stacked bar charts) of metals and anions present in intermediate concentrations in leachate solutions derived from the various dusts and girder coating samples. Dust samples collected indoors are indicated by the single hatch pattern and girder coating samples by the cross-hatch pattern; all others are dust samples collected outdoors. Note changes in scale of the concentration axis of the plots between this figure and leach figures 2-6.
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Leach Figure 5. Map of lower Manhattan showing variations (as stacked bar charts) in concentrations of predominant trace metals and metalloids for leachate solutions derived from the various dusts and girder coating samples. Dust samples collected indoors are indicated by the single hatch pattern and girder coating samples by the cross-hatch pattern; all others are dust samples collected outdoors. Note changes in scale of the concentration axis of the plots between this figure and leach figures 2-6.
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Leach Figure 6. Map of downtown Manhattan showing variations (as stacked bar charts) in concentrations of less abundant trace metals and metalloids for leachate solutions derived from the various dusts and girder coating samples. Dust samples collected indoors are indicated by the single hatch pattern and girder coating samples by the cross-hatch pattern; all others are dust samples collected outdoors. Note changes in scale of the concentration axis of the plots between this figure and leach figures 2-5.
Results of the leach tests are summarized in Leach Table 1, and in Leach Figures 1-6. The metal concentrations summarized in Leach Table 1 may not represent truly dissolved material, because the nitrocellulose filter (0.45 micrometer pore size) used to filter the leachate fluids prior to analysis will not filter out metals present in very small particles or colloids.
In general, the leachate solutions developed moderately alkaline to alkaline pH values (8.2 - 11.8), and high specific conductances (1.31 - 3.41 milliSiemens/cm, indicating high dissolved solids). Alkalinities of the leachate solutions were not measured due to insufficient sample volume, but are by inference from the pH and specific conductances, likely to be quite high. The leachate solutions are composed primarily of sulfate, bicarbonate, carbonate, and calcium, with lesser concentrations of the major cations sodium, potassium, and magnesium.
The alkaline pH of the leach solutions, coupled with the high concentrations of calcium, carbonate, and sulfate, are consistent with an origin resulting primarily from the dissolution of concrete, glass fibers, gypsum, and other material in the dusts. The leach fluids with the highest pH and highest specific conductance are from dust samples collected indoors (including WTC01-20, collected indoors from the gymnasium across West Street from the World Trade Center, and WTC01-36, which was collected in a 30th-floor apartment in a building southwest of the WTC). The higher specific conductances and pH values of indoor dust samples indicate that the outdoor samples have already experienced some leaching by rainfall and water used for fighting fires and street cleaning between September 11 and the time that the samples were collected. Leach solutions from the indoor dust samples also contain slightly less sulfate, but greater calcium, than leach solutions from several outdoor dust samples (Leach Figure 2). This suggests that dissolution of concrete or glass fibers is greater in the indoor dusts than in the outdoor dusts, and is another indication that the outdoor dusts have already undergone some leaching by rainfall or wash waters.
Heavy metals and metalloids are present in low to quite high concentrations in many of the leach solutions Leach Table 1, Leach Figure 1). Mercury is present in generally low concentrations in the leachate solutions from outdoor dust samples (near 10 nanograms per liter, or parts per trillion). Mercury concentrations in leachate solutions from indoor dust samples (as high as 130 nanograms per liter), although low compared to concentrations of other metals in the leachate solutions, are relatively high compared to mercury concentrations measured in many types of environmental water samples. Arsenic, cobalt, cadmium, thorium, and uranium are present in relatively low concentrations in the leachate solutions (maximum concentrations of 3.3, 3.2, 1.6, 0.5, and 0.5 micrograms per liter, µg/L, respectively). Lead, selenium, and vanadium are present in moderate concentrations (maximum concentrations of 11.5, 10.5, and 16.1 µg/L, respectively). Metals or metalloids present in relatively high concentrations in the leachate solutions include (maximum concentrations listed in parentheses): aluminum (702 µg/L), chromium (403 µg/L), antimony (74 µg/L), molybdenum (140 µg/L), barium (62 µg/L), manganese (35 µg/L), copper (39 µg/L), and zinc (62 µg/L).
Of the various major and trace elements, aluminum is leached in greatest amounts from the indoor dust samples relative to outdoor dust samples. This indicates that the indoor dusts, in addition to having a greater proportion of reactive concrete, also contain some sort of reactive aluminum-bearing material. This material has presumably been partly to largely leached from the outdoor dusts by rain water and wash water.
Leachate solution from one of the beam coating samples (WTC01-09) contain unusually high amounts of chromium (408 µg/L). As noted in the SEM section, the mineralogy of this sample is generally similar to those of the dust samples in overall mineralogy. However, the source of the leachable chromium in the material is currently unknown.
The source of the metals and metalloids in the leach solutions is unclear, but many components of the dust and debris are possible sources, including particles from concrete, aggregate, gypsum wallboard, glass fibers, construction steel, wiring, computer equipment, etc.
The results of the leach tests also show that metals are not leached from the dusts and beam coating samples in proportion to their total concentrations in the samples (compare concentrations in Leach Table 1 with those in Chemistry Table 1 from the previous section). For example, chromium, molybdenum, and antimony are leached in relatively high amounts from the samples, but occur in relatively low total concentrations in the samples. In part, these trace elements are likely being leached in greater proportions from the samples due to their enhanced solubilities in alkaline solutions. It is also possible that they are being leached more aggressively because they may occur in materials that are more readily dissolved in alkaline solutions. In contrast, iron, zinc and lead, which are relatively more abundant in the samples (Chemistry Table 1), are leached in proportionally quite low amounts from the samples (Leach Table 1). These metals are generally less mobile in alkaline solutions, and may also occur in materials that are not readily soluble in alkaline solutions.
Results of the leach tests indicate that the dusts released from the WTC collapse, when exposed rainwater or wash water, likely produce slightly alkaline to quite alkaline, calcium-sodium-potassium-sulfate-bicarbonate-carbonate solutions. At least some heavy metals and metalloids may be readily leached from the dusts: aluminum, chromium, antimony, molybdenum, and barium are generally leached in the greatest amounts, but other metals such as zinc, copper, manganese, titanium, vanadium, lead and mercury are also leached in measurable quantities. It is unclear if these heavy metals and metalloids may be leached in sufficient quantities to be of environmental or health concern. These results indicate that continued EPA monitoring of runoff water quality is warranted. Continued rainfall will likely continue to decrease the amounts of metals and alkalinity that can be released from the outdoor dusts.
The results of the leach tests also indicate that cleanup of dusts should be done with appropriate respiratory protection to prevent possible inhalation of alkaline material with potentially bioavailable heavy metals and metalloids. This is especially true for cleanup of dusts from indoor localities that have not been exposed to rainfall.
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For further information on the leach test procedures, contact:
Philip L. Hageman
For further information on interpretation of the leach test results,
Geoffrey S. Plumlee
For further information on analytical chemistry procedures, contact:
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