Assessing Element Speciation, Bioavailability, and Cycling using Diffusive Gradients in Thin Films (DGT)
Seeps at base of TP1, Elizabeth Copper Mine Superfund Site Vermont, USA. [Large version of seep photo]
Project work & strategy: Our work involves understanding how elements cycle through the environment and determining how chemical speciation affects the availability and toxicity of elements to biota. Specifically, we determine and model critical physical and biogeochemical processes, such as hydrologic transport, precipitation of mineral phases, and adsorption onto particles, that affect the concentration of elements in aquatic systems influenced by mining activities and evaluate chemical speciation using thermodynamic calculations and a new in-situ technique called Diffusive Gradients in Thin Films (DGT) (http://www.dgtresearch.com). Relationships among dissolved element concentrations, speciation, and toxicity are evaluated using the Biotic Ligand Model (http://www.hydroqual.com/wr_blm.html). We accomplish this work through a series of integrated laboratory, field, and modeling studies.
Background: Bioaccumulation and toxicity of elements in the environment are not related to total or dissolved concentrations, but rather to the concentration of specific chemical species of the elements. Speciation of dissolved elements can be calculated using thermodynamic data, although the necessary information to do such calculations is not always reliable or available (e.g., equilibrium constants that define interactions between natural organic matter and metals or the complete chemical characterization of water samples). An alternative approach is to use an in-situ technique called Diffusive Gradients in Thin Films (DGT) to assess chemical speciation.
The DGT technique determines dissolved labile inorganic and organic elements in-situ by trapping species in binding agents (Chelex resin or Fe-oxide) after diffusion through a polyacrylamide gel (or hydrogel). The gels are >90% water and are manufactured to have different pore sizes (i.e., open and restricted hydrogels). The gels discriminate between ions based on size and kinetic lability. DGT samplers containing gels of different pore sizes and with different binding agents are placed in aquatic systems for time periods of hours to days. The samplers are retrieved and the gel containing the binding agent is eluted with acid to recover and analyze the elements of interest. The mass of elements trapped on the binding agents and other measurable parameters (diffusion coefficients, surface area of sampler, thickness of gels, and time of exposure) are used to calculate the concentration of labile species using Fick's First Law. In addition, solutions in which DGT samplers are deployed are filtered and complete analyses (pH, alkalinity, and major and minor ions) are done.
USGS scientist tests new method of collecting Diffusion Gradients in Thin Films (DGT) to assess and monitor water quality in areas with dissolved metal concentrations. [Large version of sampling photo., Large version of DGT film photo.]
Laboratory, field, and modeling studies: Our initial experiments characterized and calibrated the hydrogels in the laboratory. We assessed background concentrations of target elements in the hydrogels, determined elution efficiencies of the Chelex resin impregnated gel, validated the published diffusion coefficients for Cd, Cu, Ni, Pb, and Zn in open pore gels, and determined diffusion coefficients for Ni, Pb, and Zn in restricted gels.
Our next studies involved laboratory and field experiments in two mine drainage systems. The experiments were designed to examine dynamic processes in-situ and compare metal uptake by the DGT units with thermodynamic predictions of metal speciation and toxicity. The DGT results and thermodynamic predictions were then compared with previously conducted toxicity studies.
In the first system, open and restricted pore DGT units were deployed in the mixing and reaction zone of the Ompompanoosuc River (OMPR) downstream of an acidic tributary (Copperas Brook) that drains the Elizabeth Copper Mine Superfund site in Vermont (http://yosemite.epa.gov/r1/npl_pad.nsf/f52fa5c31fa8f5c885256adc0050b631/2281487131782426852569E400719BBE?OpenDocument). The composition of the river water was determined and the DGT units were analyzed for Cd, Cu, Ni, Pb, and Zn. In a companion laboratory study, solutions from an upstream river site and Copperas Brook were mixed in various proportions. DGT units were placed in the mixtures to assess labile metal concentrations.
The major conclusions of this work were presented at the Geological Society of America Annual Meeting in October 2005 (http://gsa.confex.com/gsa/2005AM/finalprogram/abstract_92453.htm), are published in Applied Geochemistry and are as follows:
- Various processes occur that result in downstream attenuation of acid and dissolved metal concentrations below the confluence of the OMPR and Copperas Brook. Mixing and dilution affect the concentrations of all elements in the mixing and reaction zone of the river and are the dominant processes controlling dissolved concentrations of Ca, K, Li, Mn, and SO4. Dissolved Fe and Al concentrations also decrease throughout the reach due to precipitation of Fe and Al hydroxide and Fe hydroxysulfate phases. Only mixing and dilution affect dissolved concentrations of Cd, Co, Cu, Ni, and Zn for pH < 4.5-5.6. For higher pH values, newly formed Fe precipitates adsorb the metals and decrease dissolved metal concentrations.
- Results from open and restricted DGT units for pH > 4.9 indicate that almost all Cd, Cu, Ni, and Zn species are labile and that labile metal concentrations are generally equal to total dissolved concentrations in the mixing and reaction zone of the river.
- Results of The Biotic Ligand Model calculations indicate that LC50 concentrations for Cd and Zn interactions with fathead minnows and water fleas do not exceed observed dissolved and labile concentrations. In contrast, dissolved and labile concentrations of Cu at sites closest to the confluence (~8-30 m downstream) exceed LC50 Cu concentrations for minnows and water fleas. These results are in good agreement with toxicity tests done by U.S. EPA that indicate minimal to no survival of fathead minnows and water fleas at a site 16 m downstream of the confluence.
In another field experiment, open DGT units were deployed in rivers that flow through The Bunker Hill Mining and Metallurgical Complex Superfund Site in Idaho (http://yosemite.epa.gov/r10/cleanup.nsf/sites/cda). The rivers receive near neutral mine drainage from the world-class Coeur d'Alene Mining District. Previous work indicated that the diversity, abundance, and metal contents of fish depend on dissolved Zn concentrations. The focus of the study was 1) assessing dissolved Cd, Cu, Pb, and Zn speciation using various thermodynamic speciation models for metal-organic matter complexation (WHAM VI, NICA-Donnan, and Stockholm Humic Model), 2) comparing the calculated thermodynamic speciation with DGT results, and 3) evaluating how the choice of speciation model for metal-organic matter complexation influences the binding or loading of metal on a biotic ligand, such as fish gills. A paper on the results is being written and will be in review by October 2007.
Balistrieri, L.S., Seal, R.R., II, Piatak, N.M., and Paul, B., 2007, Assessing the concentration, speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine, Vermont, U.S.A.: Applied Geochemistry, vol. 22, pp. 930-952.
Abstracts, Presentations, and Posters for Professional Technical Meetings
Balistrieri, L.S., Seal, R., II, and Piatak, N., 2005, Processes affecting dissolved, labile, and toxic metal concentrations during mixing of acid-mine drainage and ambient surface water downstream of the Elizabeth Copper Mine Superfund Site, Vermont [abs.]: Geological Society of America 2005 Annual Meeting, Salt Lake City, Utah, October 16-19, 2005, Abstracts with Programs, v. 37, no. 7, p. 179. (Abstract and presentation) Available online at http://gsa.confex.com/gsa/2005AM/finalprogram/abstract_92453.htm.
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