Work Package 2 – Linking geological/geochemical/contaminant properties and geo-physical signal (WP leader: Anders V. Christiansen, AU)
This WP aims to develop improved parameter relationships for the quantitative interpretation of geophysical DCIP measurements so they can be used in the field to map the extent of groundwater contamination. It seeks to better describe the relationship between the geological, geochemical and contaminant properties and the induced polarization signal. This is a key outcome of GEOCON and is essential if the developed technologies are to be useful for authori-ties and consultants. For this purpose, specific zones of the pre-investigated landfills will be selected, where both a leachate plume and a discharge area are present.
An understanding of the relationship between geology, contamination and DCIP signal will be obtained by combining surface 3D DCIP measurements, in situ measurements with the El-log drilling method, and insight obtained through laboratory experiments. When interpreting IP signals, a model of the polarization is used, for example the four-parameter Cole-Cole model (Pelton et al. 1978). Inverse methods are used to determine the 2-D or 3-D variation of the geophysical model parameters in space (Fiandaca et al. 2013), and the resulting parameter fields are then interpreted to map geology and contamination (e.g. Gazoty et al. 2012). To achieve this, it is necessary to be able to interpret the model parameters such as chargeability in terms of contaminant concentration and geology. Currently geophysicists use routinely qual-itative analysis of the geophysical model parameter fields to determine contaminant concentra-tions. Two new quantitative approaches will be developed in this project based on i) direct cal-ibration, as recently shown to be feasible at contaminated sites (Flores Orozco et al. 2012), and ii) joint inverse methods.
For direct calibration, the contaminant concentration observed in monitoring wells will be com-pared with the imaged Cole-Cole parameters and an empirical functional relationship describing how the parameters vary with concentration fitted. Since these functional relationships are likely to be site specific, the work package will investigate whether it is possible to use data from a relatively small number of groundwater monitoring wells and in situ measured IP signals to obtain the site specific Cole-Cole parameter-concentration functional relationships needed to interpret results and map the corresponding concentration field. That such functional relationships can be established has previously been demonstrated (Chen et al. 2012).
For the joint inverse approach, the method of Herckenrath et al. (2013) will be explored. They employed a joint inversion algorithm to interpret head and salinity measurements, geology and time domain electromagnetic geophysical signals for the salt water intrusion problem. Here a similar approach will be used for DCIP, geology, head and contaminant concentration.
Finally, 3D geological models will be built based upon DCIP data, boreholes with in situ meas-urements, and geological logs. The resulting geologic model will be used to build 3-D numerical flow and transport models describing the measured flow, transport and geophysical data.
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