Interpretative modelling of palaeohydrogeological data : final report

The objective of Work Package (WP) 4 of the PADAMOT project has been to test and improve our understanding of palaeohydrogeological information obtained from ‘proxy’ data (geochemical, mineralogical, isotopic, etc) by using numerical interpretative models. The potential value of palaeohydrogeology in Performance Assessment (PA) is in understanding better the time-varying changes of the groundwater flux and flow direction, chemical environment, and other scenarios that are related to climate or other external environmental changes. Proxy data are interpreted using expert judgement and quantitative modelling to extract information that is relevant to PA. Interpretative models have been developed with particular focus on the processes that are relevant to data that has been acquired in WP2 from the sites in Spain (Los Ratones), UK (Sellafield) and Czech Republic (Melechov). Essentially, the interpretative models for process-understanding can be calibrated using palaeohydrological information and thus provide interfaces between palaeohydrogeological information and FEPs (features, events and processes) for scenario development in PA. For Los Ratones, the objective has been to investigate the ways that climate changes might be propagated into groundwater recharge and thence into changes in groundwater compositions and ultimately into the geochemical and mineralogical proxies. This has involved the integration of models for surface water mass balance, groundwater flow and reactive transport of solutes. The methodology allows different climatic and hydrological proxies to be interfaced with a palaeohydrogeological model which supports the construction and calibration of a groundwater model for PA with boundary conditions appropriate for future changing climate. This has been achieved in the model for Los Ratones by using the VISUAL-BALAN code to estimate timedependent changes of recharge rate constrained by palaeoclimate information from measurements of microfaunal, pollen, isotopic and organic geochemical proxies. The evolution of groundwater compositions and secondary minerals for different boundary conditions has then been simulated with the CORE2D code. For Sellafield, the objective has been to mimic the geochemical reactions that might have accounted for precipitation of late-stage calcites in fractured rock groundwater systems and to understand the significance of variations of Fe and Mn contents of secondary calcite with respect to past redox conditions in groundwater systems. Equilibrium modelling has been carried out with the PHREEQC2 code for a range of batch mixing and reaction conditions and this has been supplemented by some coupled transport and reaction modelling with the PRECIP code. The conceptual geochemical models are not unique and involve assumptions about the reactions, water mixing and pre-existing solid phases that control dissolved Fe and Mn, and about how Fe and Mn are distributed between calcite and water. For Melechov, the objective is to understand groundwater conditions in the western part of the granite massif, using the results of a hydrogeological survey with numerical modelling of the spatial distribution of hydraulic potential, groundwater flows and travel times. This is the initial stage in the development of a site investigation methodology that is appropriate for fractured granitic rocks in the terrain and climate of the Bohemian massif. The MODFLOW code was used to construct a two-dimensional spatial model of groundwater flow. Knowledge of the hydrogeological properties from borehole testing is limited to the shallow part of the system, so modelling has tested the sensitivity of flows and travel times at greater depths to uncertainties in hydrogeological properties and infiltration. Modelling in WP4 has made progress towards the integration of independent models of biosphere/climate, groundwater and geochemistry. Integration requires information to be transferred between the various models, e.g. recharge data from the biosphere model to the groundwater model, water flux data from the groundwater model to the geochemical model. Data for palaeoclimate, geochemical, isotopic and mineralogical proxies are needed to calibrate the models. Integration also involves expert judgement and understanding of uncertainties especially with respect to temporal and spatial variability. The possibilities and realities of integration with PA groups, especially with regard to the identification and quantification of FEPs and scenarios, are considered in the report for WP5.