Two and three-dimensional resistivity surveys in a landfill leachate investigation

Geophysical methods are established in contaminated land and groundwater investigations (Meju, 2000); their importance is underpinned by the fact that (1) groundwater distribution is controlled by geological factors that can be mapped, (2) groundwater quality is controlled by geochemical factors, and (3) rock and soil resistivity is inherently related to the porosity, fluid content and chemistry. The electrical conductivity of the subsurface is highly influenced by dissolved solids in groundwater making electrical and electromagnetic (including GPR) methods indispensable in groundwater quality studies. There is an enormous variety of contamination sources, one of which is our past waste disposal practise. It has resulted in increased salinity of groundwater and a large variation in different kinds of non-aqueous phase liquids (NAPL), both dense and light (compared to water). For electric methods, the increased salinity of the groundwater due to the spills is the key factor. For low concentrations of NAPL’s, borehole samples must be analysed, yet correlations with increased salinity may be found. For heterogeneous subsurface lithology, the salinity distribution is also heterogeneous and hence so is the NAPL distribution. From line data in an area with cross-line heterogeneities, at the scale of a survey, inaccurate model reconstructions will be obtained. Therefore, we have investigated the difference in model reconstruction results near a former landfill in the Netherlands, called Banisveld. It is shown that well positioned 3D surveys at small scales do provide better model reconstructions than those based on line data as it eliminates side effects. The flow direction of the leachate plume can be obtained from 3D surveys, or from a suitable combination of different 2D surveys.