Superfund Research Program
Chemical Transport, Transformation and Human Exposure on the Aberjona
Project Leader: Harold F. Hemond
Grant Number: P42ES004675
Funding Period: 1995 - 2000
Toxic Metals at Wells G and H; Pathways and Modeling
Previously, researchers demonstrated that by using electron microprobe analysis (EMPA) and analytical SEM techniques, the fine details of the textural relationships between plant-soil fragments within these sediments could be preserved. A preliminary look at mineral and root plaque compositions in the sediments using SEM-EDS indicates that metal sequestration may be controlled mainly by mineral precipitation and plant alteration of wetland sediments. Under the reducing conditions in the wetland, the researchers expected to find arsenic in either sulfide minerals or adsorbed to iron hydroxide plaques in the oxidized plant root zone. Using scanning electron microscopy with x-ray analysis (SEM-EDX), project investigators discovered that the outer cells of some root tips contain metals in the atomic ratios of either pyrite (FeS2) or sphalerite (ZnFeS). By identifying the solid phases scientists will be able to better predict the solubilities of toxic metals and determine how effectively they are sequestered within the peatland sediments.
Toxic Metal Regeneration in Surface Waters
The Upper Mystic Lake (UML), a eutrophic, dimictic lake on the Aberjona watershed, received large inputs of toxic metals due to upstream industrial activity over the past century. Every year, hypolimnetic anoxia results in the remobilization of "trapped" arsenic and iron from the sediments. Previous UML studies had suggested that the majority of remobilized arsenic existed in the oxidized form, arsenate, throughout stratification, despite highly reducing conditions in the hypolimnion. To quantify the processes giving rise to this apparent thermodynamic disequilibrium condition, an in-situ cascade filtration system was developed to maintain low flow-rates and anoxic conditions while collecting filtrates for determining size classes of particles and associated trace metals. Nearly 90% of the arsenic in the near bottom waters was found to be arsenate associated with particles of deff >50nm (and believed to be iron oxides).
In order to better estimate the redox conditions in the lake, nitrate and Fe(II)/Fe(III) measurements were made. High nitrate levels were found to persist into late fall, and field measurements of Fe(II) indicated that the majority of remobilized iron existed as Fe(III) throughout stratification. Recent studies in the literature report direct evidence of oxidation of Fe(II) by nitrate in enrichment cultures from sediments and in activated sewage sludge. Nitrate may thus play a central role in controlling the remobilization and subsequent transport of iron, and therefore arsenic and other toxic metals in contaminated lakes. The implications for management of toxic metals are clear, since many current lake cleanup strategies target nutrients such as nitrate; this may not be appropriate until metals sources are fully under control.
MIT investigator's previous work has shown remarkably high concentrations of total mercury and, especially, the extremely toxic methylmercury in water just below the picnocline of the Lower Mystic Lake. The speciation of mercury in sulfidic waters is of prime concern; inorganic mercury speciation may control the amount of methylation that occurs. A recent paper by Paquette and Helz demonstrated the importance of considering polysulfides in mercury speciation at mM levels of sulfide; an increase in soluble mercury was observed in the presence of S(0). One mercury-polysulfide species, HgSxHS-, was proposed to dominate solubility from pH 6 to 10 at sulfide concentrations above 0.1 mM. However, this species does not fit the pH trend observed by combining the Paquette and Helz data set with the MIT investigation. Further, it cannot account for the increase in soluble mercury seen at higher pH. MIT investigators find that the deprotonated form of the complex, HgSxS (2-), results in a greatly improved fit. Kow experiments have confirmed that the mercury-polysulfide complexes are indeed largely charged complexes. In sulfate-reducing medium pre-equilibrated with (So), no increase in methylation was observed, consistent with the finding of the charged nature of the complexes.
Reconstructing Historical Contamination
Reconstructing a history of ambient levels of metals in a local environment using tree ring chemistry is controversial, but important to understanding public health trends. Investigators show that this controversy can be resolved in part through the use of selective microanalysis of individual stem wood cells. Using secondary ion mass spectroscopy, researchers observed a large degree of inhomogeneity in the abundance of toxic metals (As, Cr, Pb, and Cd) within annual growth rings of Quercus rubra (red oak) and characterized individual xylem members responsible for introducing micro-scale gradients in toxic metal abundances. These gradients may be the key to disentangling the complex dendrochemical records stored within oaks.
Fingerprinting Lead Sources in an Urban Watershed
Researchers are developing a rapid assessment method to fingerprint historical and contemporary Pb sources in an urban watershed by combining X-ray fluorescence analyses with ion microprobe mass spectrometry. This is believed to be the first attempt at combining these techniques in wetland sediments and soil samples. The benefits of this approach include: a) with little sample preparation it is possible to determine both the bulk chemistry and the Pb isotopic abundances on the same material; b) for wetland sediments containing 100 g/gm of Pb is possible to determine 208Pb/206Pb and 207Pb/206Pb ratios with enough precision to enable differentiation of a variety of typical Pb pollution sources; c) because of the reduced cost it becomes feasible to undertake characterization of Pb isotopes, and hence possible source attribution, in a complex urban setting over a region as large as a watershed.
Human cell mutagenicity of chlorine disinfection byproducts
Studies were completed this year to determine whether chlorine disinfection byproducts of water from the wells G and H wetland are mutagenic to human cells expressing mixed function oxidase enzyme activity. Samples of both untreated and chlorine-treated water were prepared and tested for mutagenicity. It was found that the water samples were substantially less mutagenic to human cells than was expected based on studies reported in the literature involving bacterial assays. It was generally found that chlorination increased the mutagenicity of water samples. The powerful bacterial mutagen MX was also tested for mutagenicity in human cells; however, it was only weakly active (about 1000-fold less active than benzo[a]pyrene, the positive control for the assay).