Ground Monitoring using Resistivity Measurements in Glaciated Terrains

The most common method of monitoring and mapping groundwater contaminants is to extract and analyse a number of groundwater samples from wells in the investigation area. However, there are a number of limitations with this type of point-wise investigation, as it is hard to acquire an adequate picture of a heterogeneous and anisotropic subsurface using a few points. To overcome the limitations of point investigations and to improve ground monitoring investigations in a cost-effective way, support can be provided by direct current resistivity measurements, which give a characterisation of the electrical properties of a ground volume. The main objective with this work was to investigate the usability of the resistivity method as a support in monitoring groundwater contaminants in glaciated terrains and under different seasons, both in long-term monitoring programmes and in tracer tests. The work comprised field investigations at several different sanitary landfills and four tracer tests in different geological environments, around the Stockholm region. The main investigations have been done at Högbytorp, Stockholm which has been used for long-term investigations of the resistivity variation, together with a field set up for monitoring and measurements on seasonal variation in soil moisture, ground temperature and precipitation. It can be concluded that the use of resistivity measurements supplies valuable information in the case of mapping and monitoring conductive groundwater contaminants and furthermore: The variation in resistivity (in shallow investigations < 1 m) can be extensive between different seasons (around 30 % compared to a mean value in till and clay soils) and should be considered, so that anthropogenic affects can be separated from natural resistivity variation. For deeper investigations (> 5 m) the seasonal resistivity variation was more moderate (around 15% compared to a mean value in till and clay soils). Soil moisture variation shows a strong relationship to resistivity variation in the investigated clay and till soils. Together with temperature correction 47 to 65% of the variation has been explained. Three types of monitoring systems can be applied: Permanently installed, partly installed and fully mobile systems. For the actual measurements, all three types can use either high-density techniques such as CVES (Continuous Vertical Electrical Sounding) or low-density measuring with one or some different electrode spacings. The suggested evaluation tool for monitoring programmes showed that it was possible to detect a decrease of 15 % in the mean value at a specific site using Modified Double Mass calculations between resistivity time series and time series at a reference site with a comparable seasonal variation. Resistivity measurements may be used as a valuable complement to groundwater sampling in tracer tests. A decrease in resistivity, a minimum and a recovery phase reflect the passage of a NaCl-solution, which can be used to estimate flow velocity and flow patterns of the investigated aquifer. The achieved recovery of NaCl in the tracer tests carried out was estimated to 20 to 70 %. The measurement system for long-term monitoring or tracer tests, which should be chosen with regard to layout and frequency, depends on the purpose of measurement and on site-specific conditions and therefore no standard solution can be proposed.