Solute transport in coupled inland – coastal water systems : General conceptualisation and application to Forsmark

• We formulate a general theoretical conceptualisation of solute transport from inland sources to downstream recipients, considering main recipient load contributions from all different nutrient and pollutant sources that may exist within any catchment. Since the conceptualisation is model-independent, its main hydrological factors and mass delivery factors can be quantified on the basis of inputs to and outputs from any considered analytical or numerical model. Some of the conceptually considered source contribution and transport pathway combinations are however commonly neglected in catchment-scale solute transport and attenuation modelling, in particular those related to subsurface sources, diffuse sources at the land surface and direct groundwater transport into the recipient. The conceptual framework provides a possible tool for clarification of underlying and often implicit model assumptions, which can be useful for e.g. inter-model comparisons in SKB’s site investigation or safety assessment programmes. In order to further clarify and explain research questions that may be of particular importance for transport pathways from deep groundwater surrounding a repository, we concretise and interpret some selected transport scenarios for model conditions in the Forsmark area. Possible uncertainties in coastal discharge predictions (that underpin all transport results), related to uncertain spatial variation of evapotranspiration within the catchment, were shown to be small for the relatively large, focused surface water discharges from land to sea, because local differences were averaged out along the length of the main water flow paths. In contrast, local flux values within the diffuse groundwater flow field from land to sea are more uncertain, although estimates of mean values and total sums of submarine groundwater discharge (SGD) along some considerable coastline length may be robust. The present results show that 80% to 90% of the total coastal discharge of Forsmark occurred through focused flows in visible streams, whereas the remaining 10% to 20% was diffuse and occurring through submarine groundwater discharge (SGD), small transient streams and/ or coastal wetlands. Regarding transport quantifications, hydrogeochemical characteristics and pollution source loads may generally differ between larger, monitored catchments and smaller unmonitored coastal catchments. Since national hydrological monitoring data systematically exclude smaller, coastal catchments, they may not be representative for conditions in Forsmark (or Simpevarp). This emphasises the importance of extending in time the recently started hydrological and hydrogeochemical data series in the Forsmark and Simpevarp coastal catchment areas, since they are in effect unmonitored from a hydrological viewpoint, due to the lack of extended discharge time series. In the performed initial demonstration analysis of solute transport pathways from deep groundwater to recipients at the surface, we considered the main scenarios: (I) transport in the quaternary deposits/bedrock interface zone only (assuming that the deep groundwater transport pathway to the coast excludes the inland surface water system), and (II) transport in the coupled groundwater-surface water system. Considering mean travel times from each model cell to the coast, and disregarding travel times in the deep bedrock domain itself (which may be added to the here presented values), results show that travel times in scenario (II) were less than 4 years in 90% of the considered model area (i.e., the Forsmark catchment area). Travel times were longer in scenario (I) with values higher than 10 years in 40% of the catchment area. These results are based on the assumption that the pathways do not go through zones of near-stagnant groundwater (found e.g. below Lake Bolundsfjärden, Lake Eckarfjärden and Lake Gällsboträsket in Forsmark). If they would do so (and the above assumption is violated), results show that travel times can be considerably longer, for instance exceeding 400 years in half of the model area in scenario (I). Considering possible solute attenuation (caused by e.g. biogeochemical reactions or decay) along the hydrological transport pathways to inland surface waters and to the coast, we estimate solute mass delivery factors, representing the fraction of mass released in a cell that reaches the considered recipient. Results showed that average delivery factors, representing the whole catchment and equalling expected delivery factors in the probabilistic case, can exhibit considerable differences between transport pathway scenarios (I) and (II). However, the magnitude of the differences in average delivery factors (between transport pathway scenarios as well as between considered release points) depends on the actual attenuation rates (i.e., l-values). This is because for low l (for Forsmark: l<0.01 year-1), practically all mass reaches the coast regardless of release point and scenario, and for high l (for Forsmark: l>10 year-1) only a small fraction of the mass reaches the coast regardless of release point and scenario. The above results imply that, in general, mass delivery factors to recipients are sensitive to both pathways and entrance points or areas in the quaternary deposits of Forsmark, with for instance a remaining key question being to which extent the deep groundwater transport pathway to the coast includes the surface water system and /or quaternary deposits/bedrock interface zone. However, given more specific sub-catchment areas (e.g., of biosphere objects of interest) and possible ranges of attenuation rates (of compounds of interest) from parallel studies, the present analyses also show that robust predictions regarding e.g. mass delivery can in some cases be obtained despite considerable pathway and entrance point uncertainties. Because such cases then can be excluded from further investigation, it appears that specific transport analyses that considers relevant combinations of possible release points, transport pathway scenarios and attenuation rates can be used for delimiting specific priority regions, where remaining uncertainties are high and further experimental investigations and/or monitoring hence may be needed to reduce the uncertainties.

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