Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

Using a Combined OIHPT Probe to Evaluate the Distribution and Efficiency of a Remediation Agent with an Added Dye Tracer

Background/Objectives:

High-resolution site characterization (HRSC) tools make up an important element of most remediation projects. In addition to their valuable contribution in the pre-remediation investigation stage, which usually helps in terms of forming an understanding of the extent and characteristics of the contamination, these tools can be used further for the evaluation of the remediation process during and following treatment. The use of these tools with suitable tracers allows for the mapping of the spread and migration of the injected remediation agent in the aquifer, as well as for the evaluation of the impacts of the agent. Various dye tracers can be used to mark the affected depth horizons of the aquifer. The most important aspect in this respect concerns the stability of the dye tracer in the contaminated aquifer and its compatibility with the selected remediation agent. The data obtained provides important input for the decision-making process surrounding reselecting or modifying e.g. the injection technique, the injection density, the remediation agent and the amount thereof required for the successful conclusion of the remediation process.

Approach/Activities:

A combined Optical Image Profiler probe with a Hydraulic Profiling Tool (OIHPT, Geoprobe, USA) usually used for non-aqueous phase liquids (NAPLs) can also be used to detect various dye tracers. In combination with a newly-developed software tool, it provides for the clear visibility of the affected horizons under short-wave UV light. The data obtained from the application of the Hydraulic Profiling Tool (HPT) is important in terms of capturing changes in the permeability of the aquifer.

Several on-site pilot scale tests were performed in 2022 and 2023 at the site of a dump that was primarily used for the landfilling of residual petrol waste produced by a refinery that produced mineral oils. The contamination (in addition to petroleum residuals) comprised vertically-stratified highly-mineralized acidic water, which negatively affected the functioning of the underground sealing barrier. Previous remediation attempts made at the site produced unsatisfactory results and one of tasks of the project was to investigate the reasons for this. The pilot tests followed previous remediation attempts; however, in this case HRSC tools were employed. The tests consisted of the modified direct-push injection of a Ca(OH)₂ solution enriched with a sodium fluoresceine tracer aimed at increasing the pH in the aquifer and at initiating the precipitation of the minerals present (gypsum, ettringite, etc.)

Results/Lessons Learned:

The OIHPT investigation clearly revealed the affected horizons based on the fluorescence graphical output. The investigation confirmed that the marking of the affected horizons with fluoresceine had two important aspects, the first of which concerned the evaluation of the distribution and preferential migration of the agent in the aquifer, and the second the stability of the affected horizons with respect to the transition back to acidic conditions. The OIP data revealed only the limited migration of the remediation agent, as well as the significant rebound effect back to acidic conditions. Only around 10% of the aquifer was neutralized between 30 to 100 days following injection. Moreover, the HPT data revealed only minor changes in the permeability of the aquifer, which was confirmed by the soil analysis which revealed that the precipitation of newly-formed crystalline mineral phases was lower than expected. The OIHPT tool combined with the fluoresceine tracer proved to be a valuable tool for the evaluation of the distribution of the remediation agent in the aquifer, its stability and its impact on the remediation process. This detailed approach allowed for the clarification of the causes of the previous failed attempts at remediation.

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Tomáš Cajthaml