This report assesses the concept of 'solute residence time' and considers whether the definition of\ud 'reference' hydrogeological environments (RHEs) can be used to aid the geochemical estimation of\ud solute residence times in groundwaters.\ud In general, previous studies of techniques for estimating groundwater 'ages' have emphasised\ud determinations of the times since groundwaters were recharged. However, there have been\ud relatively few assessments of the time for which solutes have been resident in the aqueous phase\ud within a particular 'rock reservoir' (defined as a volume of rock, including all the matrix and\ud fracture porosity which occurs within it). Such assessments need to be made during investigations\ud of potential deep sites for the geological disposal of radioactive wastes.\ud For groundwaters, solute residence times are normally defined for steady state conditions as\ud follows (Stumm and Morgan, 1981):\ud 'tE = [ElV\ud [E]inq\ud where [E] and [Elin are the total concentration of solute in the system and the inflow respectively,\ud V is the total volume of the groundwater, and q is the rate of groundwater infiltration. The\ud 'residence time' in this case is simply the time that is required for the quantity of a solute that is\ud present in the system at any instant, to be replaced completely by a fresh quantity of that solute.\ud However, operational difficulties arise when this definition is applied to solutes in groundwater\ud systems. This is because compositional gradients commonly arise along a groundwater flow path\ud (even when such a system is at steady state), and also because samples are typically taken from·\ud point sources. Therefore, the total quantity of a solute that is present in a reservoir of interest is\ud difficult to assess and a range of residence times will be estimated from waters sampled in different\ud places along a groundwater flow-path. A more appropriate definition of solute residence time is the\ud maximum estimated time for which a solute has been present within a rock reservoir of interest (or\ud alternatively, the minimum time that would be taken to flush all the solute of a given type from the\ud reservoir of interest).\ud There are three main types of technique that can be used in solute residence time studies:\ud 1. Techniques that, under ideal circumstances, may. yield quantitative residence time\ud estimations;\ud 2. Techniques that, under ideal circumstances, may yield quantitative, but limiting (maximum\ud or minimum) residence time estimates;\ud 3. Techniques that, under ideal circumstances, may yield only relative residence time\ud indications.\ud The actual type of residence time estimate that may be made using a particular technique will\ud depend partly upon the hydrogeological setting of the rock reservoir of interest. A particular\ud technique that may yield quantitative residence time estimates under some circumstances, may yield\ud only limiting residence time estimates under others.\ud If non-steady state conditions prevail, then the estimation of residence times for solutes in any rock\ud reservoir becomes much more difficult or even impossible. In general, techniques that are\ud potentially quantitative under steady state conditions will give only limiting residence times under\ud non-steady state conditions. Techniques that may yield limiting or relative residence times under\ud steady state conditions will also tend to yield limiting or relative residence times under non-steady\ud state conditions.\ud Any hydrogeological environment can be considered to contain a number of different groundwater\ud reservoirs (e.g. rock matrix, faults, an aquifer, an aquiclude, etc.), and a given solute will have a\ud different residence time within each reservoir. Additionally, the 'ages' that are obtained may be an\ud integration of the residence times of the water and/or solutes within a series of reservoirs, if the\ud boundaries of the reservoirs are poorly defined. A set of rules is proposed for evaluating what type\ud of residence time indication (quantitative, limiting or relative) might be obtained from a particular\ud technique in any rock reservoir of interest. These rules are then applied to different rock reservoirs\ud within each of seven different reference geological environments. These environments were chosen\ud in order to represent a spectrum of hydrogeological environments within which potential\ud radioactive waste repositories might be constructed:\ud 1 . Crystalline Shield Type\ud 2. Argillaceous Rock Beneath Low-lying Plain Type\ud 3. Coastal, Basin Margin Type\ud 4. Salt Dome Beneath Low-lying Plain Type\ud 5. Basin and Range Type\ud 6. Argillaceous Rock in Foreland Basin Type\ud 7. Sedimentary Basin at a Destructive Plate Margin Type\ud For each environment a table is presented which indicates what type of information might be\ud obtained from each of the major techniques for estimating solute residence times. The aim is to present a tool which might be used to design strategies for evaluating residence times of solutes in\ud groundwaters.\ud It is possible to use such reference environments in order to establish priorities for the application\ud of techniques. However, it is nec:essary for these techniques to be applied in conjunction with\ud groundwater head data and geochemical data for the origins of the waters and their salinities. Such\ud data enable the boundary conditions of rock reservoirs to be defined, and the flow paths of\ud groundwaters to be constrained. A knowledge of groundwater flow paths is essential in order to\ud make a thorough assessment of the applicability of a technique for groundwater dating
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