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Compacted Na-bentonite clay barriers, widely used in the isolation of solid-waste landfills and other contaminated sites, have been proposed for a similar use in the disposal of high-level radioactive waste. Molecular diffusion through the pore space in these barriers plays a key role in their performance, thus motivating recent measurements of the apparent diffusion coefficient tensor of water tracers in compacted, water-saturated Na-bentonites. In the present study, we introduce a conceptual model in which the pore space of water-saturated bentonite is divided into ‘macropore’ and ‘interlayer nanopore’ compartments. With this model we determine quantitatively the relative contributions of pore-network geometry (expressed as a geometric factor) and of the diffusive behavior of water molecules near montmorillonite basal surfaces (expressed as a constrictivity factor) to the apparent diffusion coefficient tensor. Our model predicts, in agreement with experiment, that the mean principal value of the apparent diffusion coefficient tensor follows a single relationship when plotted against the partial montmorillonite dry density (mass of montmorillonite per combined volume of montmorillonite and pore space). Using a single fitted parameter, the mean principal geometric factor, our model successfully describes this relationship for a broad range of bentonite-water systems, from dilute gel to highly-compacted bentonite with 80% of its pore water in interlayer nanopores.

Biogeochemical processes occurring during infiltration of surface water from the Lot River into an alluvial aquifer are described using chloride as a natural tracer of water mixing in a well field where a C1--rich aquifer water is recharged with a C1--poor river water. Near the river bank a slightly reduced zone (depleted in 0 2 , DOC, Nos, Na, and K and enriched in Mn, Ca, Mg, bicarbonate, and silica) is observed. Sulfate behaves conservatively. Nearest to the infiltration zone some of the pH-regulating processes are not at equilibrium. These phenomena can all be explained by bacterial degradation of organic matter in the river bank Sediments and weathering of minerals along the infiltration path. In some cases (degradation of DOC and dissolution of calcium and magnesium carbonates) a semiquantitative confirmation of the stoichiometry of the reactions is given. Zinc is efficiently filtered after the first 10-15 m of the bank sediment-alluvion system. Some chemical changes occurring in the reduced zones are reversible (depletion of dissolved oxygen, dissolution of Mn). Others are not. IntroductionBecause of easy access and high productivity, alluvial aquifers supply a very significant amount of drinking water in many countries (1-3). A large fraction (up to 90%) of the water pumped from such aquifers can come directly from a neighboring river by infiltration through its bottom and bank sediments. By pumping in the aquifer rather than directly in the river, it is hoped that the porous geological medium (river bank sediments followed by aquifer solids) acta as a natural physical, biological, and chemical filter by retention of suspended solids and microorganisms and adsorption, precipitation, or degradation of river-borne chemical pollutants (3-5). It is therefore crucial to have good knowledge of the filtering capacity of the river-alluvial aquifer system, and the expected benefits must be evaluated in terms of the overall biogeochemical processes occurring in the system. Biological activity in the sediments can have detrimental effects on the water quality (5-11). It is not uncommon, for example, to find Fe and Mn in the water pumped, due to dissolution of manganese and iron oxides (1). While these two metals are not highly toxic, they do lead to very serious quality problems (taste, odor, color, staining, corrosion).The most investigated field site with respect to the processes described above is located on the Glatt River, Switzerland, where river water infiltrates through the bank sediments to become the upper layer of the local aquifer (8). The most significant chemical changes were found to occur within the first few meters of infiltration and were related to microbial degradation of organic matter (7-9). Further changes along the flow path were attributed to solubility adjustments controlled by calco-carbonic equilibria and to mixing with a deeper aquifer of different chemical composition (IO).Here we describe the results of another field investigation of hydrogeochemical processes occurring during the infi...