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

Anders G Christensen,Markus Reischer,Knud Dideriksen,Fabian De Weirdt,Stefan Bruns

doi:10.1016/j.jconhyd.2020.103636

Anders G Christensen, Markus Reischer + Show 3 more

Open Access

https://doi.org/10.1016/j.jconhyd.2020.103636

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Abstract

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.

Highlights

Detailed hydrogeological characterization is crucial for understanding groundwater flow and contaminant transport
We will denote the light sources employed as Optical Imaging Profiler (OIP)-source A/source B (Table 1), e.g., OIP-UV/VIS refers to the use of a UV light source for inducing fluorescence and a broad spectrum, visible light for general imaging
To probe the performance of the OIP for inducing and detecting fluorescence, laboratory experiments were performed with two different light sources for inducing fluorescence and four fluorophores in the absence or presence of a sand matrix

Summary

Introduction

Detailed hydrogeological characterization is crucial for understanding groundwater flow and contaminant transport. Characterization of a given site consists of soil coring accompanied by well installation for piezometric measurements, water sampling and groundwater tracer testing. These methods allow only limited insight into the flow conditions and the transport of the constituents the fluid carries. Other applications include logging with EC and laser induced fluorescence (LIF) for detection of groundwater tracers (Vienken et al, 2017; Suthersan et al, 2014), in-situ colorimetric measurements for characterizing stratigraphical variations using the SCOST-system (Hausmann et al, 2016, 2017) and assessment of microbial activity (Schurig et al, 2014). This means that such NAPL's can be fluorescently tagged through fluorophore injection and detected indirectly with LIF (Horst et al, 2018)

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