Capabilities of an optical direct push probe for 2D-subsurface imaging

Characterization of hydrological conditions at polluted sites is critical for understanding of contaminant distribution and transport. Standard techniques for site characterization, such as soil coring together with well installation for piezometric measurements and water sampling, allow only some insights into subsurface properties and processes. To obtain additional data, direct-push techniques are often used in soils and unconsolidated formations. The various available techniques provide high resolution information on cm to mm scale. Recently, the Optical Imaging Profiler (OIP) was developed for detection of fluorescent contaminants. Here, we have investigated the applicability of the OIP for groundwater tracing using fluorophores. Our laboratory experiments show that it is possible to qualitatively trace various fluorophores meaning that light emitted by the fluorophores can be detected by a standard digital camera sensor. The measured fluorescence depends on the number of fluorophore molecules present in the pore space adjacent to the OIP and decreases with smaller pore size as well as fluorophore concentration. In a field trial, an injected eosin Y solution could be very clearly detected after the injection within a radius of 0.5 m around the injection point. When the OIP is equipped with a second light source emitting visible light, images of the soil texture and color can be captured. Sediment color can act as a proxy for various soil properties. Tests at a second field site, indicate that detected variation in soil color depend on water saturation and redox processes. Hence, the OIP is a flexible, cost effective and multifunctional tool for characterization of contaminated sites.