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Research project 1: The role of DOC on mercury chemistry Ongoing research, 2003-2004 |
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Ongoing research, 2003-2004 |
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Past research, 2001-2003 |
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Ongoing research, 2003-2004 |
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In this project, we are focusing on four aspects related to the role of DOM in Hg chemistry: Hg binding to DOM, Hg and DOM speciation methods and models, the bioavailability of DOM-bound Hg, and spatial/temporal modeling of Hg and DOM concentrations in lakes using GIS-based mass balance models. The first two topics are closely related and the discussion of them is combined below. Through other ongoing projects that focus on aspects of DOM chemistry and that we have integrated into our COMERN work, we have learned so far a number of important things that relates directly to our Hg studies; these are very briefly summarized below. There have been no significant changes in direction during the first two years of this work, and none are anticipated in year 3. As expected in any research program that focuses heavily on method development, some approaches have been more successful than others. As a result, some have been extended significantly, while other less successful approaches have been investigated then either discarded or reduced in importance in the project. 1. Factors affecting DOM character and metal binding ability: The major sink for DOM in all waters that we have studied is photodegradation, resulting in production of dissolved inorganic carbon that is lost as CO2. This is supported by the finding that long-term patterns in DOC in lakes are inversely correlated with incident solar radiation. In addition, photolysis results in production of small molecular-size DOM with different chelating properties than the parent compounds. Photodegradation is strongly pH-dependent, relates, in part, on availability of Fe (for the photo-Fenton reaction), and is affected by nitrate levels. In whole-catchment studies, new DOM produced in the upper part of the catchments has a large proportion of high molecular-weight DOM; the total DOM is reduced as the water moves downstream, and the average molecular size of the remaining DOM decreases. 2. Hg binding to DOM: To date, we have focused on three methods of separating DOM fractions and evaluating the Hg binding capability of these fractions. Immobilized metal ion affinity chromatography (IMAC) is a relatively new technique that we used to show that only a very limited portion of the DOM comprising a standard fulvic acid was able to bind Hg, and that this binding was relatively weak compared with Cu, Ni, Co, and Cd. We will pursue this method with naturally occurring DOM with a greater portion of high molecular weight humic substances. The second method that we have used extensively is high performance liquid chromatography. We have combined this separation technique with 3-D emission-excitation fluorescence detection, a new characterization method that we are also using in kinetic studies of metal binding. This measurement technique has been exceptionally useful in these studies. We showed that as DOM is photo degraded, binding drops by up to 10-fold. We also showed that the kinetics of Hg binding are very slow compared with other metals. We studied the pH dependence of Hg binding, and found that there is none below pH 4.5. We also found size exclusion chromatography (SEC-HPLC) in conjunction with these other techniques to be a very effective means of fractionating DOM. 3. Bioavailability of DOM-bound Hg: Our preliminary studies focused on the effects of changes in DOM structure resulting from photolytic degradation on Cu and Cd bioavailability to an invertebrate test species. We have begun work on Hg availability, in this case using an algal species as the test organism. The organism is in culture, all methods have been worked out and experiments have been initiated but no results are available at this time. We also have begun work with Hg and an invertebrate species; these studies have been designed and were initiated in April 2003. Test organisms have been selected, and field and lab methods worked out. 4. Prediction of spatial and temporal variability in Hg and DOC concentrations in lakes using a GIS-based mass balance model: We have combined a DOC catchment flux model and a DOC lake mass balance model that includes transformation to DIC and loss to sediments as major sinks with physiographic data collected using the Ontario Base Maps (1:10,000) for the Muskoka River tertiary catchment (4,200 km2; 859 lakes). Measured DOC in ca. 200 of the lakes was significantly higher than DOC predicted using these models. We know that the reason is that DOC sources are underestimated (wetland areas are substantially underestimated on the OBM’s), and have developed several new methods of improving the estimates. These new methods yield much better predictions; we are currently evaluating three different approaches to making these estimates to determine which is best. We have picked the method of using these data to estimate Hg flux from the catchments and have gathered all appropriate data on Hg input via atmospheric deposition. We have also gathered all available data on Hg in sports fish in lakes in this catchment (>150 lakes) for testing the relationships between Hg flux and levels in biota. In addition, we have started a new collaboration with a group studying macro invertebrates in this region, and will utilize biological material collected in their sampling programme as another means of evaluating our mass balance-Hg accumulation relationships. We are providing basic information on how Hg interacts with DOM in the environment; DOM is ubiquitous in aquatic systems and has a major role in Hg transport, speciation, loss, bioaccumulation and toxicology. Because of this the thermodynamic and other information on Hg-DOM interactions that we generate will be useful in any mechanistic Hg modeling. In fact, all of our work is aimed toward development and /or refinement of models that relate aspects of DOM and Hg chemistry to levels in biota, particularly fish. Since the models of interest are dynamic, they can be used to predict ecosystem response to changing inputs, i.e. changing emissions or deposition. We expect one of our outputs will be a set of simple relationships for the estimation of Hg levels in fish based on watershed characteristics and simple DOC models. This will be useful to those responsible for managing resources. It may also be possible to use data collected as part of applied research or monitoring programmes (e.g. Hg levels in fish used for consumption) to test models that are developed and to extend their range geographically. Finally, the results of this project should help us predict responses of ecosystems to Hg inputs and to understand better the role of DOM in governing Hg dynamics, including bioavailability. |
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COMERN Head Office: Université du Québec à Montréal, President-Kennedy Bldg – Suite PK-7150 C.P.8888, Downtown STA (Qc) H3C 3P8. Phone: (514) 987-3601, Fax : (514) 987-3635 www.unites.uqam.ca/comern -- email: comern@uqam.ca |
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