Towards understanding resistivity signals measured with time-lapse electrical resistivity during contaminated snowmelt infiltration

To improve risk assessment, control, and treatment strategies of contaminated sites, we require accurate methods for monitoring solute transport and infiltration in the unsaturated zone. Highly spatio-temporal heterogeneous infiltration during snowmelt increases the risk of contaminating the groundwater in areas where de-icing chemicals are required for winter maintenance of roads and runways. The objective of this study is to quantify how the different processes occurring during snowmelt infiltration of contaminated meltwater affects bulk electrical resistivity. Field experiments conducted at Moreppen experimental lysimeter trench are combined with heterogeneous unsaturated soil modelling. The experimental site is located next to Oslo airport, Gardermoen, Norway, where large amounts of de-icing chemicals are used to remove snow and ice every winter. Bromide, an inactive tracer, and the de-icing chemical propylene glycol were applied to the snow cover prior to the onset of snowmelt and their percolation through the unsaturated zone was monitored with water sampling from 37 suction cups. At the same time, cross-borehole time-lapse electrical resistivity measurements were recorded along with measurements of soil water tension and temperature. Images of 2D bulk resistivity profiles were determined and were temperature-corrected, to compensate for the change in soil temperature throughout the melting period. By using fitted parameters of petrophysical relations for the Moreppen soil, the tensiometer data gave insight into the contribution of water saturation on the changes in bulk resistivity, while water samples provided the contribution to the bulk resistivity from salt concentrations. The experimental data were compared with a numerical simulation of the same experimental conditions in a heterogeneous unsaturated soil and used to quantify the uncertainty caused by the non-consistent resolutions of the different methods, and to increase our understanding of the resistivity signal measured with time-lapse electrical resistivity. The work clearly illustrates the importance of ground truthing in multiple locations to obtain an accurate description of the contaminant transport

Thermal properties of treatment wetlands operated under freezing conditions

ABSTRACT: The use of treatment wetlands (TWs) presents particular challenges in regions with sub-zero winter temperatures, due to reduced biological activity and risk of pipe breakage or clogging due to freezing. We studied the vertical temperature distribution in four pilot-scale TWs exposed to winter temperatures in order to determine the impact of operational system parameters and the role of insulation on heat conservation inside the filtering bed. The overall temperature pattern was similar in all wetlands, with a trend of increasing temperature from the surface toward the bottom during the cold season. No freezing was detected in the wetlands despite average daily temperatures as low as −20 °C. Influent water temperature and hydraulic loading had a stronger influence on TW temperatures in winter than air temperature. The vertical distribution of temperatures in TWs is more sensitive to hydraulic loading variation in the percolating operating condition than in the saturated flow with forced aeration configuration. Our results suggest that TW systems can remain operational under cold winter conditions provided the surface is properly insulated by vegetation, mulch and/or snow

Projet ROSEEV : ROle du Sol dans les zonEs de rEjet Végétalisées. Etude de l'application de différentes charges hydrauliques sur la plateforme lysimétrique de Mionnay (69)

Ce rapport est consacr

Projet ROSEEV : ROle du Sol dans les zonEs de rEjet Végétalisées : Résultats des six mois de suivi suite à la remise en fonctionnement de la plateforme lysimétrique de Mionnay (69)

[Departement_IRSTEA]Ecotechnologies [ADD1_IRSTEA]Valoriser les effluents et déchets organiquesCe rapport est consacr

Pore-scale observation of deposit within the gravel matrix of a vertical flow constructed wetland

International audienceLifespan and well-operation of French vertical flow constructed wetlands (VFCW) depend on how the organic deposit forms and evolves within filter media. This study aimed to demonstrate the feasibility of thin section methods application to VFCW. Unfortunately, constructed wetland scientists are currently missing tools to observe how deposit physically occupies pore space. Thin sections allow a direct and undisturbed observation of filter media and deposit. Undisturbed samples were taken from the surface of an experimental VFCW. Water was exchanged with a solvent before resin impregnation to preserve the sample structure. Several thin sections were successfully produced. Results highlight that deposit significantly reduces pore space. It forms a structured media crossed by large channels which can participate to fast gravity-driven flow and media oxygenation. The deposit structure seemed also made of a large bundle of small pores less than 100 µm in radius. They can effectively store water by capillarity and provide a large surface for potential pollutant adsorption. An image analysis of thin sections provided hints at understanding the structuration of the porous media linked to organic matter deposition

Ecoulements préférentiels dans les filtres plantés de roseaux

International audiencePreferential flows (PFs) in soil are long-known phenomena and are well documented and classified although challenges remain in the understanding of their driving phenomena and in their modelling. In Treatment Wetlands (TW) literature, such classification does not exist and the terminology "preferential flows" is indistinctively used to describe various phenomena. Based on the experience accumulated at Irstea over the last ten years and on the existing literature, we first aimed at providing a better description of what are PFs in TW. For example, a distinction must be made between hydraulic shortcuts, where flow is significantly faster in an area of the TW creating dead zone with limited contribution to treatment and preferential flows through macropores that exchange water and solute with a much slower matrix flow. Then, we described experimental methodologies to characterize PF based on their assumed nature and the type of TW. Single or multiple tracers experiments are the most common approaches but innovative and complementary methods exists ranging from a macroscopic description and quantification of macropores using thin section or X-ray tomography to full-scale imaging of hydraulic shortcut using electrical resistivity tomography. Finally, we introduce models used to predict PF, their domain of validity, their advantages and drawbacks. Continuous Flow Stirred Tanks Reactors (CSTR) in parallel with Plug Flow Reactors (PFR) can efficiently model hydraulic shortcuts but their applicability is limited to the wetland on which the model has been calibrated. On the other hand, dual-porosity and dual-permeability could be used to predict preferential flows in a TW when design it but their calibration remain challenging. Knowing the nature and mechanisms of PF is a key to understand current limitations of existing TW. If used appropriately, numerical models may be a powerful tool to improve TW design in order to prevent PFs

Simple deterministic model of the hydraulic buffer effect in septic tanks

International audienceSeptic tanks are widely used in on-site wastewater treatment systems. In addition to anaerobic pretreatment, hydraulic buffering is one of the roles attributed to septic tanks. However, there is still no tool for assessing it, especially in dynamic conditions. For gravity fed-system, it could help both researchers and system designers. This technical note reports a simple mechanistic model based on the assumption of flow transition between the septic tank and the outflow pipe. The only parameter of this model was calibrated using computational fluid dynamics (CFD) modeling for a wide range of discharge rates. The resulting model highlights that a septic tank plays a hydraulic buffer role when faced with sudden and large discharge flow, but this role tends to disappear when input hydrographs are smoother. In those cases, there is an observable lag between the input hydrograph and outflow hydrograph

Contrôle des installations d’assainissement non collectif : l’apport déterminant des outils de diagnostic non destructifs

Depuis juillet 2012, un nouveau cadre juridique s’applique aux installations d’assainissement non collectif, visant à améliorer leur contrôle et leur rénovation progressive. Cette note nous présente une méthodologie de contrôle simplifié des installations élaborée grâce l’apport déterminant des outils de diagnostic non destructifs