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FRINATEK-Fri mat.,naturv.,tek

Combining multi-compartment sampler and geophysical techniques for monitoring contaminant transport in soils

Awarded: NOK 3.3 mill.

Pollution of soils is a widespread problem and is an important part of the still to be implement Soil Directive (EU). The impact of agriculture, industry, airport activities on soil and water quality is strongly influenced by soil heterogeneity. To improve risk assessment, monitoring, and treatment strategies, we require a better understanding of the effect of soil heterogeneity on contaminant movement and better methods for monitoring heterogeneous contaminated transport. Monitoring water and contaminant transport in the unsaturated zone is highly challenging due to spatial variability and difficult access to soil water. The project focused on the situation at Oslo airport, where every winter large amounts of de-icing chemicals are used for removal of snow and ice for airplanes and runways. The impact of annual infiltration of large quantities of de-icing chemicals at all airports and de-icing salt along roads with winter frost represents a common challenge. For airport management it is important to have control of contaminant transport and degradation within the unsaturated zone. Especially the snowmelt period is important when the contaminants are present in the infiltrating water. In this project innovative integration of geophysical and multi-compartment sampler (MCS) techniques, and modelling, has been developed and used to improve our process understanding and techniques for monitoring contaminant transport. Geophysical techniques have become more widespread to monitor hydrogeological processes. To improve time-lapse electrical resistivity (ERT) to monitor contaminant transport in the unsaturated zone there is need to quantify the effect of infiltrating water and contaminant movement on the bulk resistivity. The interpretation of ERT images can be improved by a combination with classical measurements of soil physical properties, soil suction, contaminant concentration and temperatures. Solute monitoring is often limited to observations of resident concentrations, while flux concentrations govern the movement of solutes in soils. To improve techniques for monitoring contaminant transport an innovative integration of ERT and multi-compartment sampler (MCS) techniques has been made, which makes it possible to measure solute fluxes. The newly developed multi-compartment sampler (MCS) is divided into 100 separate compartments of 31 by 31 mm with at each corner an electrode for ERT measurements (total 121). The instrument is capable of measuring downward water fluxes at a high spatio-temporal resolution under natural conditions. Tracer leaching can be monitored by ERT and sampling of the collected leachate. The new instrument has been used successfully for two laboratory experiments. Snow has been applied on top of the sand column to observe the effects of snowmelt on the temporal and spatial variation of water content / flow pattern through the column. For the second experiment a rainfall simulator was constructed, with which we studied the breakthrough of an inactive tracer through the column. Field experiments have been performed at Oslo airport, Gardermoen, where we applied a degradable de-icing chemical and an inactive tracer to the snow cover prior to snowmelt. Time-lapse cross borehole electrical resistivity tomography (ERT) measurements were conducted and at the same time soil water samples were extracted at multiple depths with suction cups, as well as with one MCS at 51 cm depth. By combining these measurement techniques we are able to quantify water content and solute concentration contributions to electrical resistivity measurements, which can help interpret and understand the relative importance of the various contributions to the bulk electrical resistivity. The combined results of tensiometer data (saturation) and suction cup data (EC) explain most of the relative change in the ERT results. The results of MCS data (saturation and EC) also explain the relative change in the ERT profiles at one depth, with high spatial detail. An optimal monitoring and clean-up strategy depends on the following: i) A correct conceptual understanding of the processes which determine the flow and transport of the contaminants. ii) Sufficiently proven and appropriate scientific knowledge, i.e. good methods exists for mapping and monitoring surface and subsurface heterogeneities. In this project we have made a step towards overcoming present challenges of monitoring flow and transport processes in a heterogeneous subsurface. A combination of destructive and non-invasive techniques are recommended in combination with modelling. Hence, the Combitech project with the development of simultaneous resistivity measurements and water sampling, as well as examples of its application, has contributed towards establishing a fully integrated monitoring system, which has the potential for upscaling for use at contaminated sites.

Pollution of soils is a widespread problem and is an important part of the still to be implement Soil Directive (EU). Solute transport is strongly affected by heterogeneity. This effect needs to be understood to improve risk assessment, monitoring, and tr eatment strategies for natural attenuation in an optimal way both environmentally and cost effectively. This project aims at development of integrated technologies and modelling tools for soil contamination assessment and site characterisation at the scal e of management decisions (field scale). Multi-compartment samplers (MCS) measure downward fluxes with high spatio-temporal resolution and show the effect of subsurface heterogeneities. At the field scale geophysical techniques have become more widespread to monitor hydrogeological processes. These techniques are promising regarding cost efficiency and spatial versatility, but require ground truth. Geophysical time-lapse measurements in combination with novel ground truth methods (like MCS) give the possi bility to determine absolute contamination levels, spatial spreading. Better monitoring of contaminant behavior is a prerequisite for improved measures and reduced concentrations of contaminants. Never before has the high spatio-temporal resolution data of contaminant transport provided by the MCS at field scale been combined with different geophysical methods. In this project the MCS will be modified to meet the needs for more flexible use and be combined with 2D geophysical techniques. The MCS will be equipped with instrumentation for geophysical measurements, and MCS water samples will provide ground truth for the geophysical methods. In this proposal innovative integration of MCS and geophysical techniques, and modelling, will be used to improve our process understanding of contaminant transport and for optimising monitoring strategy. This integration of techniques and tools is then used to develop instruments for better decision making by the stakeholders.

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FRINATEK-Fri mat.,naturv.,tek