Imaging groundwater infiltration dynamics in the karst vadose zone with long-term ERT monitoring

Water infiltration and recharge processes in karst systems are complex and difficult to measure with conventional hydrological methods. In particular, temporarily saturated groundwater reservoirs hosted in the vadose zone can play a buffering role in water infiltration. This results from the pronounced porosity and permeability contrasts created by local karstification processes of carbonate rocks. Analyses of time-lapse 2-D geoelectrical imaging over a period of 3 years at the Rochefort Cave Laboratory (RCL) site in south Belgium highlight variable hydrodynamics in a karst vadose zone. This represents the first long-term and permanently installed electrical resistivity tomography (ERT) monitoring in a karst landscape. The collected data were compared to conventional hydrological measurements (drip discharge monitoring, soil moisture and water conductivity data sets) and a detailed structural analysis of the local geological structures providing a thorough understanding of the groundwater infiltration. Seasonal changes affect all the imaged areas leading to increases in resistivity in spring and summer attributed to enhanced evapotranspiration, whereas winter is characterised by a general decrease in resistivity associated with a groundwater recharge of the vadose zone. Three types of hydrological dynamics, corresponding to areas with distinct lithological and structural features, could be identified via changes in resistivity: (D1) upper conductive layers, associated with clay-rich soil and epikarst, showing the highest variability related to weather conditions; (D2) deeper and more resistive limestone areas, characterised by variable degrees of porosity and clay contents, hence showing more diffuse seasonal variations; and (D3) a conductive fractured zone associated with damped seasonal dynamics, while showing a great variability similar to that of the upper layers in response to rainfall events. This study provides detailed images of the sources of drip discharge spots traditionally monitored in caves and aims to support modelling approaches of karst hydrological processes

A comparison of tube model predictions of the linear viscoelastic behavior of symmetric star polymer melts

We present a modified version of the time-marching algorithm (TMA) [van Ruymbeke et al., 2005, 2010] for predicting the linear viscoelasticity of monodisperse symmetric star polymer melts. Several new elements were added to the original TMA model. In particular, the remaining fraction of the initial tube segments is considered as a function of time, while taking into account the past relaxation history of each molecular segment. We validate the TMA model and, for the first time, compare it with the so-called BoB model [Das et al., 2006]. The predictions obtained with the two models are compared to a large set of experimental data, which cover a broad range of arm molecular weights (from 1.5 to 55 entanglements per arm) of different chemistries (polystyrene, polybutadiene, and polyisoprene). We indicate the significant differences between the two approaches, which mainly affect the choice of material parameters of the models. We then point out a systematic deviation of the TMA model in accurately predicting the intermediate regime of relatively short star chains, while the BoB model fails in correctly describing the plateau modulus of these short chains. From our point of view, both disparities have the same origin and, in the case of the TMA, can be solved by removing the high-frequency mode contribution to the contour length fluctuation of the primitive path. An excellent agreement between data and theory is then obtained for all molecular weights of the arms, which indicates the capability of the TMA to provide a quantitative prediction of the rheology of monodisperse star-shaped polymers. A very good agreement is also obtained with the BoB model, despite the expected discrepancy observed with short star chains. This proves the fact that current tube-based models are successful in quantitative predictions of linear rheology of monodisperse star-shaped polymers

Comparative analysis of different tube models for linear rheology of monodisperse linear entangled polymers

\u3cp\u3eThe aim of the present paper is to analyse the differences between tube-based models which are widely used for predicting the linear viscoelasticity of monodisperse linear polymers, in comparison to a large set of experimental data. The following models are examined: Milner-McLeish, Likhtman-McLeish, the Hierarchical model proposed by the group of Larson, the BoB model of Das and Read, and the TMA model proposed by the group of van Ruymbeke. This comparison allows us to highlight and discuss important questions related to the relaxation of entangled polymers, such as the importance of the contour-length fluctuations (CLF) process and how it affects the reptation mechanism, or the contribution of the constraint release (CR) process on the motion of the chains. In particular, it allows us to point out important approximations, inherent in some models, which result in an overestimation of the effect of CLF on the reptation time. On the contrary, by validating the TMA model against experimental data, we show that this effect is underestimated in TMA. Therefore, in order to obtain accurate predictions, a novel modification to the TMA model is proposed. Our current work is a continuation of earlier research (Shchetnikava et al., 2014), where a similar analysis is performed on well-defined star polymers.\u3c/p\u3