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Element transport in aquatic ecosystems – Modelling general and element-specific mechanisms
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Radionuclides are widely used in energy production and medical, military and industrial applications. Thus, understanding the behaviour of radionuclides which have been or may be released into ecosystems is important for human and environmental risk assessment. Modelling of radionuclides or their stable element analogues is the only tool that can predict the consequences of accidental release.

In this thesis, two dynamic stochastic compartment models for radionuclide/element transfer in a marine coastal ecosystem and a freshwater lake were developed and implemented (Paper I and III), in order to model a hypothetical future release of multiple radionuclides from a nuclear waste disposal site. Element-specific mechanisms such as element uptake via diet and adsorption of elements to organic surfaces were connected to ecosystem carbon models. Element transport in two specific coastal and lake ecosystems were simulated for 26 and 13 elements, respectively (Papers I and III). Using the models, the concentration ratios (CR: the ratio of the element or radionuclide concentration in an organism to the concentration in water) were estimated for different groups of aquatic organisms. The coastal model was also compared with a 3D hydrodynamic spatial model (Paper II) for Cs, Ni and Th, and estimated confidence limits for their modelled CRs. In the absence of site-specific CR data, being able to estimate a range of CR values with such models is an alternative to relying on literature CR values that are not always relevant to the site of interest.

Water chemistry was also found to influence uptake of contaminants by aquatic organisms. Empirical inverse relationships were derived between CRs of fish for stable Sr (CRSr) and Cs (CRCs) and water concentrations of their biochemical analogues Ca and K, respectively (Paper IV), illustrating how such relationships could be used in the prediction of more site-specific CRCs and CRSr in fish simply from water chemistry measurements. 

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2014. , 34 p.
Keyword [en]
radionuclides, elements, concentration ratio, bioaccumulation, biomagnification, fish, modelling, aquatic food web, ecosystem, Cs, Sr, environmental risk assessment
National Category
Environmental Sciences
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-110064ISBN: 978-91-7649-026-6 (print)OAI: oai:DiVA.org:su-110064DiVA: diva2:768965
Public defence
2015-01-21, föreläsningssalen, Institutionen för ekologi, miljö och botanik, Lilla Frescativägen 5, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2014-12-28 Created: 2014-12-05 Last updated: 2016-11-02Bibliographically approved
List of papers
1. Radionuclide transfer in marine coastal ecosystems, a modelling study using metabolic processes and site data
Open this publication in new window or tab >>Radionuclide transfer in marine coastal ecosystems, a modelling study using metabolic processes and site data
2014 (English)In: Journal of Environmental Radioactivity, ISSN 0265-931X, E-ISSN 1879-1700, Vol. 133, 48-59 p.Article in journal (Refereed) Published
Abstract [en]

This study implements new site-specific data and improved process-based transport model for 26 elements (Ac, Ag, Am, Ca, Cl, Cm, Cs, Ho, I, Nb, Ni, Np, Pa, Pb, Pd, Po, Pu, Ra, Se, Sm, Sn, Sr, Tc, Th, U, Zr), and validates model predictions with site measurements and literature data. The model was applied in the safety assessment of a planned nuclear waste repository in Forsmark, Oregrundsgrepen (Baltic Sea). Radionuclide transport models are central in radiological risk assessments to predict radionuclide concentrations in biota and doses to humans. Usually concentration ratios (CRs), the ratio of the measured radionuclide concentration in an organism to the concentration in water, drive such models. However, CRs vary with space and time and CR estimates for many organisms are lacking. In the model used in this study, radionuclides were assumed to follow the circulation of organic matter in the ecosystem and regulated by radionuclide-specific mechanisms and metabolic rates of the organisms. Most input parameters were represented by log-normally distributed probability density functions (PDFs) to account for parameter uncertainty. Generally, modelled CRs for grazers, benthos, zooplankton and fish for the 26 elements were in good agreement with site-specific measurements. The uncertainty was reduced when the model was parameterized with site data, and modelled CRs were most similar to measured values for particle reactive elements and for primary consumers. This study clearly demonstrated that it is necessary to validate models with more than just a few elements (e.g. Cs, Sr) in order to make them robust. The use of PDFs as input parameters, rather than averages or best estimates, enabled the estimation of the probable range of modelled CR values for the organism groups, an improvement over models that only estimate means. Using a mechanistic model that is constrained by ecological processes enables (i) the evaluation of the relative importance of food and water uptake pathways and processes such as assimilation and excretion, (ii) the possibility to extrapolate within element groups (a common requirement in many risk assessments when initial model parameters are scarce) and (iii) predictions of radionuclide uptake in the ecosystem after changes in ecosystem structure or environmental conditions. These features are important for the longterm (>1000 year) risk assessments that need to be considered for a deep nuclear waste repository.

Keyword
Probabilistic simulations, Concentration ratio, Baltic Sea, Bioaccumulation, Risk assessment
National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-106194 (URN)10.1016/j.jenvrad.2013.05.003 (DOI)000337555300009 ()
Note

AuthorCount:4;

Available from: 2014-07-31 Created: 2014-07-28 Last updated: 2017-12-05Bibliographically approved
2. Radionuclide Transport and Uptake in Coastal Aquatic Ecosystems: A Comparison of a 3D Dynamic Model and a Compartment Model
Open this publication in new window or tab >>Radionuclide Transport and Uptake in Coastal Aquatic Ecosystems: A Comparison of a 3D Dynamic Model and a Compartment Model
Show others...
2013 (English)In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 42, no 4, 464-475 p.Article in journal (Refereed) Published
Abstract [en]

In safety assessments of underground radioactive waste repositories, understanding radionuclide fate in ecosystems is necessary to determine the impacts of potential releases. Here, the reliability of two mechanistic models (the compartmental K-model and the 3D dynamic D-model) in describing the fate of radionuclides released into a Baltic Sea bay is tested. Both are based on ecosystem models that simulate the cycling of organic matter (carbon). Radionuclide transfer is linked to adsorption and flows of carbon in food chains. Accumulation of Th-230, Cs-135, and Ni-59 in biological compartments was comparable between the models and site measurements despite differences in temporal resolution, biological state variables, and partition coefficients. Both models provided confidence limits for their modeled concentration ratios, an improvement over models that only estimate means. The D-model enables estimates at high spatio-temporal resolution. The K-model, being coarser but faster, allows estimates centuries ahead. Future developments could integrate the two models to take advantage of their respective strengths.

Keyword
Steady state, Biosphere, Process modeling, Bioaccumulation, Point source
National Category
Environmental Sciences
Identifiers
urn:nbn:se:su:diva-90772 (URN)10.1007/s13280-013-0398-2 (DOI)000318285100010 ()
Note

AuthorCount:7;

Available from: 2013-06-14 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
3. Transfer of 13 elements in a lake using a process-based ecosystem model
Open this publication in new window or tab >>Transfer of 13 elements in a lake using a process-based ecosystem model
(English)Manuscript (preprint) (Other academic) [Artistic work]
National Category
Environmental Sciences
Research subject
Radiology
Identifiers
urn:nbn:se:su:diva-110054 (URN)
Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2014-12-09
4. Evaluation of factors influencing accumulation of stable Sr and Cs in lake and coastal fish
Open this publication in new window or tab >>Evaluation of factors influencing accumulation of stable Sr and Cs in lake and coastal fish
Show others...
2016 (English)In: Journal of Environmental Radioactivity, ISSN 0265-931X, E-ISSN 1879-1700, Vol. 160, 64-79 p.Article in journal (Refereed) Published
Abstract [en]

As a result of nuclear accidents and weapons tests, the radionuclides Cs-137 and Sr-90 are common contaminants in aquatic ecosystems. Concentration ratios (CR) based on concentrations of stable Cs and Sr in biota and media are used for the estimation of transfer of their radioisotopes for radiation dose calculations in environmental and human safety assessments. Available element-specific CRs vary by over an order of magnitude for similar organisms, thus affecting the dose estimates proportionally. The variation could be reduced if they were based on a better understanding of the influence of the underlying data and how that affects accumulation and potential biomagnification of stable Cs and Sr in aquatic organisms. For fish, relationships have been identified between water concentrations of K and CR of Cs-137, and between water concentrations of Ca and CR of Sr-90. This has not been confirmed for stable Cs and Sr in European waters. In this study, we analysed an existing dataset for stable Cs and Sr, as well as K and Ca, in four Swedish lakes and three Baltic Sea coastal areas, in order to understand the behaviour of these elements and their radioisotopes in these ecosystems. We found significant seasonal variations in the water concentrations of Cs, Sr, K and Ca, and in electrical conductivity (EC), especially in the lakes. CR values based on measurements taken at single or few time points may, therefore, be inaccurate or introduce unnecessarily large variation into risk assessments. Instead, we recommend incorporating information about the underlying variation in water concentrations into the CR calculations, for example by using the variation of the mean. The inverse relationships between fish CRCs -[K](water) and fish CRSr-[Ca](water), confirmed that stable Cs and Sr follow the same trends as their radioisotopes. Thus, they can be used as proxies when radioisotope data are lacking. EC was also strongly correlated with K and Ca concentrations in the water and could potentially be used as a quick and cost-effective method to estimate water chemistry to obtain less variable CR. We also recommend some simple improvements to data collection that would greatly enhance our ability to understand Cs and Sr uptake by fish.

Keyword
Caesium, Strontium, Bioaccumulation, Biomagnification, Fish, Concentration ratio
National Category
Earth and Related Environmental Sciences Chemical Sciences
Research subject
Environmental Chemistry
Identifiers
urn:nbn:se:su:diva-132934 (URN)10.1016/j.jenvrad.2016.04.022 (DOI)000378465500008 ()27153476 (PubMedID)
Available from: 2016-09-01 Created: 2016-08-26 Last updated: 2017-11-21Bibliographically approved

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