Hydraulic conditions at the source zone and their impact on plume behavior
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on Environmental engineering procedures such as remediation of contaminated sites rely on precise prediction of flow and transport processes in the subsurface, which are controlled by the subsurface’s chemical and physical Received: 26 July 2012 / Accepted: 10 February 2013 Published online: 15 March 2013 * Springer-Verlag Berlin Heidelberg 2013 N. Gueting ()) : A. Englert Hydrogeology Department, Ruhr University Bochum, Universitätsstraße 150, 44801 Bochum, Germany e-mail: [email protected] Tel.: +49-234-3223335 Fax: +49-234-3214120 Hydrogeology Journal (2013) 21: 829–844
heterogeneity. Among the various chemical and physical properties, the hydraulic conductivity is generally recognized as the primary control on groundwater flow and solute migration (e.g. Dagan 1989; Koltermann and Gorelick 1996). The spatial distribution of the hydraulic conductivity in aquifers has been extensively studied (e.g. Sudicky 1986; Hess 1989; Boggs et al. 1992; Vereecken et al. 2000). These studies showed that the local hydraulic conductivity in aquifers can vary over several orders of magnitude, with typical correlation lengths ranging from decimeters to meters. The spatial variability of the local hydraulic conductivity causes convergence and divergence of streamlines, which locally focuses and defocuses groundwater flow. The impact of heterogeneous flow fields on solute spreading and plume behavior is, as such, relatively well understood. Among many others, Silliman and Simpson (1987) investigated the influence of heterogeneity on solute transport and showed that the presence of heterogeneities significantly increases solute spreading and causes dispersion to scale with travel distance. Levy and Berkowitz (2003) demonstrated that heterogeneous structures strongly affect plume spreading and lead to irregularly shaped solute plumes. Werth et al. (2006) and Rolle et al. (2009) investigated the effect of ‘flow focusing’ through high-permeability inclusions and found that flow focusing enhances transverse mixing and therefore controls the lateral spreading of a plume and the length of a mixing-controlled reactive plume. Although plume behavior in heterogeneous conductivity fields has been extensively studied, only a few studies investigated the influence of the hydraulic conditions at the source zone. Since the hydraulic conductivity in aquifers changes in space, the hydraulic conditions at a localized solute source can be very different at different locations within the aquifer, because they depend on the local hydraulic conductivity at the source zone relative to the surrounding conductivity field. De Barros and Nowak (2010) could show analytically and numerically that flow focusing/defocusing is of particular importance for plume development when it occurs directly at the source zone. The authors found the volumetric water flux passing through the source zone to control the ensemble characteristics of downgradient plumes. Their results are in alignment with the findings of Nowak et al. (2010), who identified the flow field at the source zone to be a
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