COMERN Project Description
PROJECT DESCRIPTION
1. Identification
Project Manager: Dr. Peter Dillon
Environmental and resources studies, Trent
University, Peterborough, Ont.
Research Title: The role of DOC on mercury chemistry
Research Theme: Mercury dynamics in aquatic ecosystems
Theme Leader: Dr. Peter Dillon
Collaborators:
Dr. Doug R. Evans
Environmental and resources studies, Trent
University, Peterborough, Ont.
Dr. Holger Hintelmann Environmental and resources studies, Trent
University, Peterborough, Ont.
Dr. Marc Amyot
Université du Québec, INRS-Eau, Québec, Qc
Dr. David Lean
Department of Biology, University of Ottawa,
Ottawa, Ont.
Dr Greg Mierle
Dorset Environmental Science Center, Ontario
Ministry of the Environment
2. Project Summary
The transport of mercury (Hg) through terrestrial catchments into the aquatic components
of ecosystems, as well as its concentration and speciation in streams and lakes, is strongly
influenced by the presence of dissolved organic carbon (DOC), particularly the fulvic and
humic fractions of DOC. By forming soluble complexes with Hg, DOC increases the
concentration of dissolved Hg in soil water, wetlands and streams, mobilizes Hg from
watersheds, and thereby increases the flux of metal into downstream lakes. Although
there is some evidence that the DOC-bound Hg is less biologically available than other
fractions, the validity of this in natural systems is largely unknown. Useful
thermodynamic data for modeling are lacking because measurements at environmentally
realistic concentrations were not possible until recently. Furthermore, the importance of
photodegradation of DOC as a principal pathway for DOC loss in lakes is now becoming
recognized, as is the realization that this may result in re-release of bound Hg in more
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COMERN Project Description
available forms. In addition, the variation in DOC composition among lakes and streams
may necessitate development of different equilibrium constants for particular types of
water bodies. Recent analytical advances, however, should make it possible to assess the
toxicological significance of changing DOC composition and concentrations in lakes and
streams on Hg chemistry and bioavailability.
3. Research Objectives
•
Determine binding constants between MeHg and humic and fulvic acids, the role in
this binding of the sulphur functional groups in the dissolved organic matter, the
effects of competing cations, pH and DOC age (which is related to structure
because of the photochemical breakdown.
•
Obtain thermodynamic data for Hg-DOC complexes at levels found in the natural
environment for use in speciation models; relate thermodynamic data to DOC
characteristics including functional groups, molecular weight, etc., so that the
predominant DOC fractions responsible for Hg binding can be identified.
•
Apply data collected (above) to speciation, transport and bioaccumulation models.
•
Determine bioavailability of DOC-bound Hg and Me-Hg to several classes of
organisms, and the role that DOC structure has in Hg bioavailability.
•
Develop a geographical information system (GIS)-based mass balance model for
regional-scale DOC fluxes, link the DOC model to a Hg mass balance model, and
evaluate on the basis of regional measurements of DOC in lakes and Hg in fish.
Details
1. MeHg binding to DOC
Recent evidence suggests that many of the sulphur groups of DOC are unavailable for
methyl-Hg binding. This study demonstrated that binding of MeHg to extracted fulvic
acids ranged between 6.65 x 10-11 moles/mg DOC and 2.37 x 10-10 moles/mg DOC, or
23.9 ng Hg/mg carbon. Average binding capacity for extracted humic acids was 137 ng
Hg/mg carbon. These values are much lower than what is commonly assumed in MeHghumic binding models. When used as input parameters in the Regional Hg Cycling
Model very different outputs are obtained compared to the default input parameters. Use
of binding constants obtained experimentally predicts approximately 2 and 4 times as
much total Hg in predatory fish (5 years old), compared to the default parameters.
This project will build to determine variation in binding capacity of humic and fulvic
acids due to the effects of competing cations, pH, and solar radiation. The type of
functional sites involved in the binding of Hg species to humic substances will be
determined. Methodological advances will be required to accomplish this including
further development of on-line combinations of separation and detection methods (e.g.
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COMERN Project Description
high pressure liquid chromatography coupled with inductively coupled plasma mass
spectrometry HPLC/ICP-MS).
2. Speciation methods and models
Mercury distribution models rely on estimated of dissolved Hg that are most frequently
calculated from speciation models; most of these are based on equilibrium distribution
calculations. A large percentage of the Hg present in any natural system will be bound to
DOC compounds. Thus, modeling the speciation of Hg requires the use of equilibrium
constants for DOC-Hg complexes. However, the available thermodynamic data are
extremely limited in applicability. Largely, this is because the techniques used to
generate them required use of high Hg concentrations and because relatively few workers
have studied natural DOC assemblages rather than model compounds.
Techniques have been developed in our labs to obtain thermodynamic data suitable for
use in speciation models using concentrations typical of natural environments. We use
the separation power of capillary electrophoresis (CE) and/or size exclusion liquid
chromatography (SELC) coupled to ICP-MS and ESI-MS-MS. Using these techniques, it
is possible to measure the fraction of free versus DOC-bounded Hg in natural waters.
Thus, relevant thermodynamic data can be obtained. Moreover, the methods provide
information on variation in binding as a function of molecular size or functionality of the
DOC molecules. The importance of these methods is that they can be used at
environmentally realistic concentrations of Hg and DOC.
Recent work has shown clear differences in molecular weight distribution of DOC during
the course of a year. We will correlate these observed differences to the amount of
dissolved Hg and the availability of Hg for transformation and accumulation reactions.
Ultimately, we will be able to identify the predominant DOC fractions responsible for Hg
binding. Concurrently, we will undertake characterization of the organic matter by
MS/Time on Flight/MS techniques to determine functional groups capable of Hg binding.
The ultimate goal is to provide the binding constants necessary for speciation models
along with a mechanistic understanding of the qualitative differences in DOC molecules
that determine the fate of Hg.
3. Bioavailability of DOC-bound Hg
The extent to which Hg species bound to DOC are available to aquatic organisms and to
microbes for methylation may be a function of both the structure of the DOC and the
physiology and life history of the organism. For example, Hg bound to small molecular
weight organics may pass membrane barriers whereas larger molecular weight
compounds may not. If this is the case, then the size spectrum of DOC will be important.
Organisms, which pass large quantities of water through their guts, may obtain higher
amounts of DOC-bound Hg than those which do not, as a result of pH changes in the gut.
Thus it is important for us to be able to characterize the availability of DOC bound metals
for different groups of organisms and bacteria. Utilizing the stable tracer techniques that
have been developed at Trent, we will determine the availability of Hg bound to DOC.
Classes of organisms will be selected, based on their physiology, following which their
ability to incorporate Hg and MeHg bound to different size classes of DOC will be
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COMERN Project Description
determined. The organisms and DOC fractions will be collected from study lakes used
by other network projects.
4. Prediction of Spatial and Temporal Variability in Hg and DOC Concentrations in
Lakes Using a GIS-Based Mass Balance Model
The concentrations of substances in lakes and streams are maintained by a dynamic
balance of inputs and outputs. Although meteorological and hydrologic variables are
always important, fluxes of most substances from catchments into lakes are markedly
affected by a variety of geologic, edaphic, and landscape features. Among the latter,
wetlands are an exceptionally important feature because, in most lake-catchment systems,
they act as the principal source of DOC which, in turn, serves as the transport mechanism
for substances associated with or bound to it. Although the initial development of mass
balance models requires a considerable amount of research into the factors which
influence the inputs and outputs of a substance, once these factors are understood and the
appropriate information collected for a particular lake, the calculation of an average
substance concentration from mass balance considerations is, in most instances, relatively
straight forward. Expansion of a single lake model into one for a series of interconnected
lakes is largely a matter of linking the output of one to the input of another in a
mathematical model. As a result, the likely effect of watershed/lake disturbances such as
changes in land use or remediation/prevention options can be tested by computer
simulation before actual implementation or occurrence. This forecasting capability can
be a powerful tool in formulating rational decisions for the management of lakes and
watersheds.
The mass balance models we will use require spatial data including lake and watershed
areas, land use characteristics including wetland area, and geologic characteristics. Using
geographical information systems (GIS), we will obtain these data from digital maps and
digital aerial photos. In combination with a database management system, GIS will
provide a means to extend the application of mass balance modeling to large spatial
areas.
We will carry out this study in southern Ontario, where long-term DOC concentration
data are available for about 30 lakes, while data based on surveys (one-time
measurement) are available for about 700 lakes. Mass balance models will be
constructed for DOC and Hg. The spatial data required for construction of mass balance
models for these lakes will be generated using standard GIS techniques. Along with
export and retention coefficients for DOC and Hg, the data will be used to calculate
expected mean concentrations of DOC and Hg in the lakes. The results of the predictions
will be compared to known mean concentrations of DOC and measured concentrations of
Hg, and the differences will be used to quantify the effectiveness of the models.
The lakes in the above test are primarily headwater lakes. Inputs to these lakes come only
from precipitation and runoff. Most lakes, however, have inputs from upstream lakes as
well. Mass balance models can predict this input, but if the prediction is biased, the bias
will add to the prediction for the receiving lake. In principle, the bias could continue to
grow through a chain of lakes. To test for bias arising from mass balance predictions in
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COMERN Project Description
this circumstance the spatial data for two, interconnecting chains of lakes (two adjacent
quaternary watersheds) in south central Ontario will be generated using the above GIS
techniques. The data, along with known export and retention coefficients for DOC, and
coefficients for Hg (determined in other projects of this proposal) will be used to predict
concentrations of DOC and Hg in these lakes. The actual concentrations will be
determined by averaging measured monthly, volume weighted concentrations in these
lakes from a one-year sampling period.
Another test will target sediments. The Geological Survey of Canada (GSC) has sampled
the sediments of over 10,000 lakes in Ontario for a suite of metals as well as ancillary
parameters such as organic carbon. Due to the phenomenon of sediment focusing the
concentration of a substance in the sediments usually varies markedly across the lake
bottom. While mass balance models can predict an average, aerial concentration of a
substance in the sediments, they cannot predict the value at a particular point. If,
however, a substance is strongly associated with organic carbon, the substances are well
mixed once in the lake, and no fractionation occurs once in the sediments, the ratio of a
substance to organic carbon is predictable. To test this possibility, a subset of headwater
lakes from the GSC sediment survey will be chosen. The spatial data required for mass
balance models of these lakes will be assembled using the GIS techniques described
above, and along with the export and retention coefficients for DOC and Hg, predicted
ratios of Hg/DOC will be calculated. These ratios will be compared to ratios calculated
from the GSC sediment data.
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