Maryland Department of Natural Resources

Reports

Baltimore Harbor Multi-Dimensional Sedimentary Metals Study


2017, Gillmor, A., Ortt, R.A., Jr., Connallon, C., Nicholson, B., Van Ryswick, S., and Sylvia E.R.

File Reports, Coastal and Estuarine Geology, File Report 17-05


Executive Summary

Recent and renewed consideration of potential innovative and beneficial uses of dredged material has prompted inquiry into the comparability of various analytical methods used to characterize sediments by environmental evaluators, particularly when considering the potential application of sediments on land. In order to support these efforts, a methods comparison study was performed by the Maryland Geological Survey (MGS) utilizing four commonly used analytical methods for metals; the most commonly used total digestion used widely in the environmental community (2-acid), a vigorous total digestion used in the environmental and mining community (4-acid), the toxicity characteristic leaching potential procedure (TCLP) used for landfill screening, and the synthetic precipitation leaching potential procedure (SPLP) sometimes used in evaluating planned land application of materials. To a first approximation, a leachate concentration may be estimated from a total sediment concentration as 5%, due to the way the leachate sample is prepared.

In the fall of 2016, a total of 65 sediment samples were collected from the Baltimore Harbor and environs, as well as a subset of semi-dried sediment from a dredged material containment facility. These samples were analyzed for a suite of 25 metals commonly evaluated in environment investigations, as well as characterized by their sedimentological composition. The objectives of this study were:

  1. To determine comparability of sedimentary metals concentrations as determined by 4-acid total digestion, 2-acid total digestion, and two commonly used leaching potential tests.
  2. Where intra-metal comparability exists, determine and quantify correlations between the tests.
  3. Relate correlation to the ability to predict sediments most likely to produce high leachate concentrations of metals.
  4. Display all four data-types spatially in order to identify areas of greatest concentration and/or greatest leaching potential within the study area.

The major findings of this study are:

  1. The concentrations of most (22 of 25) metals demonstrated correlations between the vigorous 4-acid digestion and the commonly-used total 2-acid digestion. Most of these correlations are of strong or medium strength of association. The commonly-used 2-acid digestion in most cases produced a concentration result which was somewhat less than to generally comparable with the result of the 4-acid digestion. Exceptions were noted for some lithophilic elements (the commonly-used 2-acid digestion does not dissolve the mineral backbone) and also exceptions were noted for some metalloids (the commonly-used 2-acid digestion may recover these elements more faithfully than does the vigorous 4-acid digestion, which can induce minor volatilization).
  2. The concentrations of about half of the TCLP leachates demonstrated a proportionality with either parent total concentration. Depending on which statistical measure of correlation is used, between 8 and 11 (of 25) of the TCLP leachate concentrations demonstrated a relationship. Most of these have a medium or marginal strength of association. In every case (except Tl), the leachate concentration was less than the 5% of the total concentration (sometimes referred to as the “Rule of 20”) which would be expected as a maximum. In most cases, the TCLP leachate concentration was an additional order of magnitude less than expected, even when a moisture correction was applied.
  3. The concentrations of about one third of the SPLP leachates demonstrated a proportionality with parent total concentration. Depending on which statistical measure of correlation is used, between 2 and 9 (of 25) of the SPLP leachate concentrations demonstrated a relationship. Most of these have marginal strength of association. In every case (except Ag), the leachate concentration was less than the 5% of the total concentration which would be expected as a maximum. In most cases, the SPLP leachate was an additional order of magnitude less than expected, even when a moisture correction was applied. One third of the metals were not detected at all in SPLP leachates.
  4. The “Rule of 20” proportionality in which the leachate concentration would be expected to be 5% of the total is conservative, and in almost every case the observed leachate concentrations were much lower than 5% of the initial total concentration.
  5. For metals which demonstrated some correlation, predictive equations have been prepared for estimating a leachate concentration from a total concentration. Adjusted R2 values for these equations, which are a measure of how much of the variability may be explained by the model, range from ~4% to 80%. Users are reminded that predictions with higher R2 values are more reliable. TCLP leachate concentrations for Sb, Co, Fe, Mn and Ag could be modeled with greatest confidence (adjusted R2 above 0.5). No SPLP leachate concentrations had modeling equations with an adjusted R2 above 0.5.
  6. The pH of the parent sediment exerts a marked influence over the composition of the SPLP leachate. The pH of the parent sediment does not have the same strong influence over the composition of the TCLP leachate. TCLP leachate fluids are buffered, whereas SPLP leachate fluids are not. TCLP leachate fluids are buffered at approximately pH 5, a pH relevant for comparisons to groundwaters and at which many metals are mobile. The SPLP leachate fluids in this study adopted an estuarine pH of approximately pH 8, a pH less relevant for comparisons to groundwaters and at which some metals have decreased aqueous solubility. The difference in aqueous solubility as a function of pH may account for some of the difference in the results between the two leachate tests. Although the TCLP is narrowly prescribed to screen for toxicity relative to landfill placement, environmental evaluators may consider the additional reproducibility benefit of this test. Additionally, environmental evaluators may consider that the pH stability of a sediment will influence its SPLP result.
  7. None of the surficial grab samples collected from the Baltimore Harbor exhibited a TCLP characteristic of toxicity for As, Ba, Cd, Cr, Pb, Hg, Se and Ag, as specified in the 40th Code of Federal Regulations (40th CFR § 261.24). None of the semi-dried sediment samples collected the Cox Creek DMCF exhibited a TCLP characteristic of toxicity for TCLP for As, Ba, Cd, Cr, Pb, Hg, Se and Ag.

Since the major findings of this study include that of the 25 metals evaluated, one third to one half demonstrated a relationship between the initial sediment and leachate concentrations, evaluators may wish to select additional testing for higher degrees of certainty when screening dredged material. We found that the alkaline pH of the estuarine sediments was adopted by the synthetic precipitation leachate, which may limit extrapolations relative to soil and groundwater environments for which an alkaline pH would not be representative. We found that neither the semi-dried sediments collected from the containment facility nor the fresh sediments collected from the Baltimore Harbor produced a leachate with concentrations above toxicity regulatory thresholds for landfill application specified in the 40th Code of Federal Regulations (40th CFR § 261.24).

Downloads and Data

File Report 17-05 (pdf, 35 MB)