PARAMETERIZING WATER FLUXES IN THE GEOSPHERE-BIOSPHERE INTERFACE ZONE: FOR USE IN BIOSPHERE MODELLING AS PART OF THE LONG-TERM SAFETY ASSESSMENT

摘要

The post-closure safety assessment of the disposalsystem for high-level nuclear waste should reflect asufficient understanding of the natural environmentsurrounding the engineered barriers and future effects ofreleases of contaminants on human health and theenvironment. A critical part of radiological models for thebiosphere is due to the transport of deep groundwater andsubsequent mixing with surface waters when it emergesinstreams and wetlands. These processes are highlyinfluenced by the so-called hyporheic flow; the small-scaleflow field in the sediments below the stream caused by localpressure gradients at the streambed interface, whichcontributes to a fragmentation of the upwellinggroundwater. The present work addresses the effects ofcatchment-scale upwelling groundwater on the hyporheicfluxes over a wide range of spatial scales in boreal, glaciallandscapes. In such landscapes, the groundwater surfacegenerally follows the topography and soil layers arerelatively thin. A model framework was developed toaccount for both the effects of the regional (groundwater)and locally (streambed) induced flow fields on thehyporheic exchange, specifically accounting for thehydrostatic and dynamic head fluctuations induced by thestreambed topography. We show that the hyporheic flowfield substantially affected the distribution of deepgroundwater discharge points in streams and thusincreased the fragmentation of the upwelling, here definedas the size of coherent up- or downwelling areas at thestreambed interface. Due to the inverse relationshipbetween the rate coefficient and the groundwaterdischarge areas, this fragmentation implies a considerableincrease in both the flow velocities and rate coefficientsthrough the upper part of the GBI, which are crucialcomponents in dose assessments. Furthermore, thegroundwater-surface water interactions were studied inseveral sub-watersheds to provide a basis forunderstanding the importance of different topographic andgeographic factors and for statistical derivation of generalrelationships for mass transfer rate coefficients to be usedto parameterize water fluxes in radiological dose modelsfor the biosphere. It was found that site specific conditionswere difficult to generalize in the form of proxy factors withhigh confidence, but the most essential factors were foundto be stream-order, landscape slope, thickness ofQuaternary deposits and indices representing the fractalnature of the landscape topography.