Geochemical Indicators of Groundwater Stability
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*HRFKHPLFDO,QGLFDWRUVRI*URXQGZDWHU6WDELOLW\ Adrian Bath1 and Bo Strömberg2 1 Intellisci Ltd, Loughborough LE12 6SZ, U.K. DEDWK#LQWHOOLVFLFRXN 2 Swedish Nuclear Power Inspectorate (SKI), Stockholm S-10658, Sweden. ERVWURPEHUJ#VNLVH $%675$&7 The interpretation of geochemical and isotopic data as indicators of past hydrodynamic stability of groundwaters in fractured rock is discussed. Knowledge of the conditions and timing of past groundwater stability supports scenarios for safety assessments. Stratification of water masses according to densities is an important factor affecting stability. Chloride variations and concentration gradients are therefore diagnostic of mixing and the duration of stable conditions. Further evidence of timescales is provided by carbon-14 and tritium data although sampling difficulties and other uncertainties may allow only qualitative interpretations. Data from Äspö and other Swedish sites, Sellafield (UK) and Olkiluoto (Finland) are used to illustrate the approach. The palaeohydrogeology of such sites has been strongly influenced by distributions of saline waters. Scenarios for groundwater stability should consider these as well as glacial melt waters. ,1752'8&7,21 One of the desirable features of a deep rock repository is that the groundwater conditions in the vicinity should be stable over a long period of time. There are two principal reasons: firstly that groundwater stability provides a known environment in which the performance of the engineered barrier system can be optimised, and secondly that it improves confidence in the predictability of groundwater pathways that would be the routes for radionuclides to move through the geosphere into the biosphere. This paper discusses geochemical indicators with respect to the stability of groundwater movements. Evidence for stability of chemical conditions in groundwaters is outside the scope of this paper. The hydrodynamic evolution and chemical stability of groundwaters at proposed deep repository sites in Sweden and Finland has recently been reviewed by SKB [1]. Conceptual models for Äspö, including density-driven groundwater mixing and evolution, have already been discussed in [2]. This paper takes the discussion further by comparing various data sets and by considering also the possible significance of 14C data. Chemical and isotopic compositions of groundwaters from published studies in a number of typical fractured hard rock environments in Sweden, UK and Finland are considered as indicators of groundwater stability. The significance of these data is identified, even though it may be only qualitative. 6$/,1,7>1, where 'H is the effective bulk diffusion
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coefficient and / is a characteristic length which represents the scale of interest [3]. At length scales and/or rates of water movement where Y/'H is 800 m) are saline mixtures of isotopically-light cold-climate water (with an uncertain age range that may be up to 2 million years old) and isotopically-heavy saline waters probably of ‘ancient’ origin, similar to those wit
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