Parameterizing water fluxes in the geosphere-biosphere interface zone : For use in biosphere modelling as part of the long-term safety assessment

• The post-closure safety assessment of the disposal system for high-level nuclear waste should reflect a sufficient understanding of the natural environment surrounding the engineered barriers and future effects of releases of contaminants on human health and the environment. A critical part of radiological models for the biosphere is due to the transport of deep groundwater and subsequent mixing with surface waters when it emerges instreams and wetlands. These processes are highly influenced by the so-called hyporheic flow; the small-scale flow field in the sediments below the stream caused by local pressure gradients at the streambed interface, which contributes to a fragmentation of the upwelling groundwater. The present work addresses the effects of catchment-scale upwelling groundwater on the hyporheic fluxes over a wide range of spatial scales in boreal, glacial landscapes. In such landscapes, the groundwater surface generally follows the topography and soil layers are relatively thin. A model framework was developed to account for both the effects of the regional (groundwater) and locally (streambed) induced flow fields on the hyporheic exchange, specifically accounting for the hydrostatic and dynamic head fluctuations induced by the streambed topography. We show that the hyporheic flow field substantially affected the distribution of deep groundwater discharge points in streams and thus increased the fragmentation of the upwelling, here defined as the size of coherent up- or downwelling areas at the streambed interface. Due to the inverse relationship between the rate coefficient and the groundwater discharge areas, this fragmentation implies a considerable increase in both the flow velocities and rate coefficients through the upper part of the GBI, which are crucial components in dose assessments. Furthermore, the groundwater-surface water interactions were studied in several sub-watersheds to provide a basis for understanding the importance of different topographic and geographic factors and for statistical derivation of general relationships for mass transfer rate coefficients to be used to parameterize water fluxes in radiological dose models for the biosphere. It was found that site specific conditions were difficult to generalize in the form of proxy factors with high confidence, but the most essential factors were found to be stream-order, landscape slope, thickness of Quaternary deposits and indices representing the fractal nature of the landscape topography.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Oceanografi, hydrologi och vattenresurser (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Oceanography, Hydrology and Water Resources (hsv//eng)

Nyckelord

Biospherics

Catchments

Discharge (fluid mechanics)

Flow fields

Mass transfer

Radioactive wastes

Radioactivity

Rivers

Safety engineering

Soils

Stream flow

Topography

Waste disposal

Biosphere modelling

Engineered barriers

Ground water-surface water interactions

Groundwater discharge area

High level nuclear wastes

Inverse relationship

Natural environments

Quaternary deposits

Groundwater

Publikations- och innehållstyp

ref (ämneskategori)

kon (ämneskategori)