Regional channelized transport in fractured media with matrix diffusion and linear sorption

Abstract

[1] A regional-scale solute transport model with long-range flow channeling is used to study the effect of matrix diffusion and linear sorption on channelized transport. We start from a fracture-network-based block model to build up a large-scale flow and transport model with regional flow channeling, and then incorporate the processes of matrix diffusion and linear sorption. Regional-scale solute transport is then studied by applying the model to the fracture data set from Sellafield, England. The results demonstrate the significant impact that matrix diffusion has on regional-scale solute travel times for different degrees of long-range channeling. With no channeling, a relatively sharp and significantly delayed arrival can be observed, which is the well-known retardation effect of matrix diffusion and linear sorption. However, with increasing regional channeling the delay becomes much smaller while the spread of transit times becomes much larger. The solute breakthrough curves obtained are analyzed with both the traditional advectiondispersion equation (ADE) and a Continuous Time Random Walk (CTRW) method developed for non-Fickian transport. The low b values obtained from the CTRW model indicate an extremely non-Fickian transport, which is also confirmed by the low Peclet numbers (much less than 1) required for the best fit to the ADE model. In particular, the times for the first arrival of solute are much earlier when regional channeling occurs. In other words, the degree of large-scale channeling is a crucial parameter for determining the first arrival of particles, and it becomes even more important when matrix diffusion and linear sorption are included in the model.

The impact of medium architecture of alluvial settings on non-Fickian transport

“…15(c) and (h)), due to the concurrent channeling flow that stretches the plume front downstream. The enhanced spread of the plume is similar to the numerical result in Odén et al [71], who found that matrix diffusion in channelized transport causes an increased spread in regional-scale fractured media. The persistent non-Fickian transport due to the long-term competition between the strong channeling and sequestration is reflected by the non-zero skewness and kurtosis ( Fig.…”

“…The mechanism of transport in either the alluvial settings or the fracture/matrix system (simplified by a dual-porosity model) can be described by random-walking particles whose waiting time distribution is controlled by the non-flowing portions [16]. It is therefore expected that the above methods, such as the TSM and the CTRW framework, may capture complex transport in fractured media at various scales, as demonstrated in [19,71,103]. In alluvial settings, the particle waiting time distribution is affected by the molecular diffusion coefficient and the thickness of the host floodplain layer.…”

Fractional dynamics of tracer transport in fractured media from local to regional scales

“…For example, analyses show that the effective matrix diffusion coefficient may be scale dependent [40], permeability and porosity changes with support scale and spatial resolution [41], and apparent dispersivity in fractures varies with the scale of observation (in time and space) [7]. The extensive upscaling experiments conducted by Öhman et al [42] and Odén et al [43] further reveal that the standard continuum model cannot capture the dynamics of tracer transport through regional-scale, nonstationary fractured media.…”

A study of flow‐wetted surface area in a single fracture as a function of its hydraulic conductivity distribution

“…Characterization and modeling of groundwater flow and solute transport in fractured rocks has long been a challenging scientific task because of the strongly heterogeneous nature of the medium. Recent reviews of the approaches used are given, for example, by Öhman et al [2005], Oden et al [2008], and Neuman [2005]. Flow in heterogeneous systems gets channelized into pathways of least resistance.…”

“… Tsang and Neretnieks [1998] summarized results from a number of experimental and modeling studies that demonstrated the occurrence of flow channeling in fractured rocks. The typically anomalous breakthrough curves, characterized by fast early peak arrivals and long tails, can in many cases be explained by the occurrence of fast‐flowing channels interacting with the slow‐flowing/stagnant domains and the rock matrix [e.g., see Carrera et al , 1998; Moreno and Neretnieks , 1993; Shapiro , 2001; Oden et al , 2008]. The interaction can take place through several interfaces: the channel‐rock matrix interface, the channel‐stagnant zones interface, and stagnant zones‐rock matrix interface [ Bodin et al , 2003].…”