In Situ Long-Term Chloride Transport through a Layered, Nonsaturated Subsoil. 2. Effect of Layering on Solute Transport Processes

fects on solute dispersion. Studying the abrupt passage from a coarse to a finer-textured layer, Koch and Flühler We analyze observed Cl transport during a large-scale in situ (1994) showed that streamlines tend to diverge at such unsaturated infiltration experiment in terms of the observation scale of the transport process and the physical stratification of the Tertiary textural boundaries. At steady state, the water content sedimentary flow domain. By comparing piston flow and local velocity is higher in the finer-textured layer, which decreases the profiles, we show that velocity variations cannot be explained by water pore velocity and promotes lateral mixing. If streamlines content changes alone. We also demonstrate that by comparing a layare diverging horizontally, apparent vertical longitudiered convection–dispersion (CD) model with the observations, the nal dispersion will be smaller. In contrast, this process dispersivity profile cannot be explained with the model, which prois reversed when flow occurs from a fine-textured layer duced unrealistic local dispersivity values. These discrepancies were to a coarse layer, and the apparent longitudinal disperpartially explained by nonrepresentative sampling using porous cup sivity increases. This process generally should be flow solution samplers (PCS). We hypothesize that fingering flow or convergence phenomena below sand–clay interfaces leads to nonrepresentarate–dependent and may change at the lower flow rates. tive artificially high dispersivity values. Velocity and dispersivity valAlso, structural discontinuities between layers may inues immediately above the clay layers, however, seem more reliable terrupt pore-scale transport processes. Therefore, prefdue to convergence and more lateral mixing induced by a larger water erential flow may be disrupted at such boundaries. Laycontent. Following the criteria derived from the Chuoke equation, we ering may additionally generate a local saturated zone show that the subsoil can be subjected to fingering flow. We found above the interfaces due to textural or capillary barriers. that this process likely persisted for some 17 yr. Therefore, we conclude It is well known that the water content may have an that the fine-textured clayey layers regulated the flow rate through effect on the dispersion by changing the tortuosity and time, approaching a quasi-steady-state flow condition. Overall, solute continuity of the flow paths. The effect of moisture contransport processes in the unsaturated layered subsoil appeared to be very strongly influenced by stratification of the flow domain. An tent on dispersivity, however, is not unique. Some auapparent highly variable flow field was induced by the clay layers thors observed an increase in the dispersivity with moisinterbedded in a sandy deposit, while the local PCS sampling devices ture content and flow rate (e.g., Vanderborght et al., were likely too small to properly assess the mixing regime at this 2001) and explained this by the fact that macropores large scale. are activated when moisture content at higher flow rates. Other studies showed that water content decreases the apparent dispersivity (Maraqa et al., 1997; Nützmann A horizontal layering is a common feature et al., 2002) and explained this by the flow paths becomof most soils and subsoils, solute transport through ing less tortuous when the moisture content increases. unsaturated stratified natural porous media is still poorly Finally, wetting front instability may be induced just understood. Transport in stratified media is affected by below a structural interface. It has been observed for a series of processes at the interfaces of the different many years that vertical flow from a fine to a coarselayers, thereby influencing the continuity of the transtextured layer is a condition that promotes fingering and port process. As shown by Flühler et al. (1996) and Forwetting front instability. Observations in laboratories rer et al. (1999), layering may have a considerable effect under various conditions have been reported by several on solute dispersion. They found that soil stratification authors (Hill and Parlange, 1972; Glass et al., 1989a; induces twoor three-dimensional transport processes, Baker and Hillel, 1990; Wang et al., 1998b; Sililo and Telwhich then modify the degree of lateral mixing. This lam, 2000). Using a simple descriptive approach, Hillel may occur for several reasons. First, layering is often and Baker (1988) introduced the concept of water entry related to a significant textural change across the intersuction e as the maximum suction (in absolute value) face. At steady state, this sharp change in effective pothat will allow water to enter an initially dry porous rosity forces streamlines to converge or diverge to ensure matrix. If the potential flow rate at e in the lower layer flux continuity. Little is known about the connectivity exceeds the actual discharge through the upper layer, of the liquid phase between two different porous media then the flow paths tend to constrict and concentrate with different volumetric water contents and their efinto individual streams (Hillel and Baker, 1988). This was further confirmed experimentally by Wang et al. M. Javaux* and M. Vanclooster, Department of Environmental Sciences and Land Use Planning, Université Catholique de Louvain, (1998b) and Miller and Gardner (1962). Furthermore, Croix du Sud, 2 Bte. 2, B-1348 Louvain-la-Neuve, Belgium; M. Javaux, Glass et al. (1989b) showed that hysteresis in the water currently, Agrosphere Inst., ICG-IV, Forschungszentrum GmbH, retention curve (WRC) promotes stability of the nonD-52425 Juelich, Germany. Received 30 Sept. 2003. Original Research Paper. *Corresponding author ([email protected]). Abbreviations: BC, boundary condition; BTC, breakthrough curve; Published in Vadose Zone Journal 3:1331–1339 (2004). © Soil Science Society of America CD, convective–dispersive; CDE, convection–dispersion equation; PCS, porous cup samplers; WRC, water retention curve. 677 S. Segoe Rd., Madison, WI 53711 USA