Progress 08/01/07 to 07/31/08
Outputs
OUTPUTS: Increasing concern over the long term consequences of exposure to low levels of arsenic and lead has spurred interest in the fate of arsenical pesticides applied to orchard lands during the early 1900's. To address this research, we have collected soil samples from a former orchard in southern New Hampshire where lead arsenate was applied, and channel surface sediment from the adjacent stream. Concentrations and phases of arsenic and lead in both the soil and the stream sediment were determined using total digestions, selective sequential extractions, and synchrotron X-ray fluorescence techniques, and isotopes were used to trace specific Pb isotope sources. In the former orchard soils, the total mass of Pb is ~3.9 times that of As, consistent with the mass ratio in the applied pesticide. However, in the stream sediment this mass ratio was reversed, with the total mass of As ~4 times that of Pb. The transfer of As and Pb was observed during several storms in overland flow. There is a corresponding change in Pb isotopes that indicates that Pb sources change from pesticides to natural sources (and, to some extent, atmospheric Pb emissions) as the concentration changes in overland flow). This mass ratio reversal reflects the differential mobilization of As-bearing phases relative to Pb-bearing mineral phases. Sequential extractions and X-ray microprobe analysis of soil and sediment grains reveal that As is primarily adsorbed onto amorphous and crystalline iron oxides while Pb adsorbs to amorphous iron oxides, manganese oxides, and in some cases forms pyromorphite. Given that erosion appears to control the extent of As transport in this system, the preferential long-distance transport of As-bearing iron oxides suggests that these phases are more easily mobilized by erosion than lead-bearing Mn oxides. PARTICIPANTS: Benjamin Bostick (PD). Oversaw research and graduate students, worked with graduate students on field work and synchrotron analyses. Carl Renshaw (CoPD): Oversaw graduate student research, assisted with field research Brian Jackson (CoPD): performed isotopic analyses with graduate and undergraduate students on the project. Owen Cadwalader (graduate student). Supported to examine transport in overland flow and establish physical mechanisms of erosion. This work forms the basis for his recently completed MS thesis. Jing Sun (grasdaute student): supported to examine colloidal transport using field flow fractionation and chromatography, perform laboratory experiments and perform spectroscopy. Samantha Saalfield (graduate student): performed research on the possible solute release of arsenic under oxidizing conditions. This was included in her PhD thesis and is a submitted manuscript. Elizabeth Asher (Undergraduate Student) : Examined downstream transport of Pb and As as a part of her thesis work. Brenna Gibbons (Women in Science Program intern): developed data on the mobilization of As and Pb from contaminated soils as part of her internship. Brian Dade (collaborator): worked with Owen Cadwalader on model development (in progress) to describe physical transport. Sam Webb (SSRL, collaborator): helped with instrument development for the concurrent analysis of As and Pb by X-ray microprobe. TARGET AUDIENCES: Target Audiences. This work is directly applicable to the many residents of the region of study, much of which is former orchard land under development. It is more broadly applicable to the sensistive areas surrounding urban areas throughout the country, where more than 30,000 square km of farmlands, many of which are impacted by As and/or Pb pesticides, have been developed over the last 15-20 y. Efforts. Pedagogy is a major component of this work; a number of students (undergraduate and graduate) have been supported by this work to date. We have three theses completed using research performed as part of this study, including two by women (one PhD and one BS). We have incorporated this work into teaching soil chemistry and soil science courses at Dartmouth. All of these students have formally presented their research findings in national meetings. PROJECT MODIFICATIONS: The initial phase of site selection and sampling is largely completed, although those sampling efforts have resulted in the collection of numerous aqueous and sediment samples from several storm events and locations. Our analysis of water samples has established with reasonable certainty that transport is physically-mediated rather than chemical transport (possibly during transient reducing conditions), so most of our continuing efforts will center on the analysis of nanoparticles and colloids implicated in the transfer of As and/or Pb during storm events. This grant will be transferred from Dartmouth College to Columbia University in the coming months; until the completion of this grant, we will continue to address our original research objectives and hypotheses through the following research: Additional Analyses. As a part of our sampling, we have sampled suspended sediments from a number of storm events. While our data above is compelling and shows that physical transport is important in the system, more samples are needed to show that the processes studied in such a detailed sampling of a specific event are more broadly applicable to the study of the environment. To address this need, we have examined the soils and suspended sediments from the contaminated orchards, from numerous control sites, and McQuade Brook during several storm events, and want to compare our data from these additional sampling efforts with the detailed investigations of the site presented above. As part of our continued sampling efforts, we have collected several soil and sediment samples, some of which have elevated Pb and/or As levels, to analyze using spectroscopy (to determine Pb and As speciation). We will be continuing to analyze these samples in the coming year as synchrotron time is available. A focus of remaining work revolves around using isotopes for the study of Pb sources (to differentiate natural, "geogenic" Pb from gasoline Pb or pesticide Pb. Such studies help show that the Pb is derived from pesticides, but they are also useful in that they can be used to establish more conclusively that pesticides are being redistributed (or are not being redistributed) from their points of origin. Modeling. Modeling is very useful to examine the posited relationship between mineralogy and sediment transport. We will be developing a model based on the geochemical information that we have collected for Pb and As in sediments in overland flow, to quantify the effect of mineralogy (i.e., mineral size and/or density) on contaminant transport.
Impacts
Lead arsenate pesticides have contaminated much of the agricultural lands in the United States. This is a concern both for the toxicity to the local environment and for potential toxicity to residents of developed farmland. While contamination of soil where lead arsenate was applied has been studied in detail, a better understanding of potential transport pathways and mechanisms for redistributing As and Pb from the contaminated soil is necessary to assess the impact of these contaminants on the local ecology. This study examines the transport of As and Pb from these contaminated soils by observing intense storm events that erode soil and produce overland flow from an area contaminated by lead arsenate. Analysis of these storm events takes place in two stages. In our initial work to date, we establish soil erosion and subsequent overland flow from the contaminated site as the transport mechanism of As and Pb. Additionally we find that As travels a greater distance relative to Pb, and that differences in physical transport are responsible for their variable transport properties. There are several significant conclusions that arise from these analyses. 1. Land disturbance promotes the mobilization of significant fractions of As and Pb from lead arsenate pesticide contaminated soil through soil erosion and overland flow. 2. The particles bearing As and Pb in these overland flow events are 1-10 microns in diameter; an order of magnitude larger than the colloid size range that commonly transports metals in the environment. 3. In the soil and suspended sediment As is found adsorbed to Fe oxides, while Pb is found adsorbed to Fe and Mn oxides as well as bound in the dense mineral pyromorphite. The difference in solid phase speciation controls the relative mobility of As and Pb. Due to rapid settling of dense pyromorphite grains in overland flow, the fraction of Pb bound in pyromorphite is less mobile than the fraction bound to Fe oxides. These conclusions establish a mechanism for the solid phase differential transport of As and Pb, and provide a greater understanding of the extent and processes of watershed-scale contamination from lead arsenate pesticides.
Publications
- Cadwalader, G. O.; C. E. Renshaw; B. C. Bostick. 2009. Differential Mobility of Arsenical-Pesticide Derived Lead and Arsenic. Environ. Sci. Technol., Submitted.
- Saalfield, S., B. C. Bostick. 2009. Effect of calcium and magnesium carbonates on arsenate release from ferrihydrite in flowing and static systems. Environ. Sci. Technol. Submitted.
- Cadwalader, G. O.; C. E. Renshaw; B. E> Jackson. B. C. Bostick. 2009. Characterization of arsenic and lead speciation in soils contamined by lead arsenate pesticides . For submission to Soil Sci. Soc. Am. J.
- Cadwalader, G. O.; C. E. Renshaw; B. E. Jackson; B. C. Bostick. 2009. Transport of silt-sized particles bearing arsenic and lead in overland flow from soils contaminated with arsenical pesticides. For submission to Geology.
- Cadwalader, G. O.; C. E. Renshaw; B.E. Jackson; W.B. Dade and B. C. Bostick. 2009. Mineralogical controls on the physical transport of arsenic and lead contamination in overland flow. For submission to Water Res. Res.
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