Formatted Title

Sustainable Remediation: An Approach to Reach and Completely Destroy Contaminant Mass in Low-Permeability Storage Zones with High-Resolution Data

Background/Objectives

One of the primary challenges encountered in the remediation of contaminated sites involves effectively addressing persistent contaminant masses present in low-permeability storage layers (Guilbeaut, 2005; Brewer, 2016; US EPA, 2019). Despite their small size, these layers can contain over 90% of the contamination within a given area (Guilbeaut, 2005; Souto et al. 2021). Unfortunately, storage layers are frequently inadequately characterized and underrepresented in conceptual site models (CSMs) (Souto et al., 2020; Suthersan, 2015), emphasizing the necessity of employing high-resolution characterization techniques to accurately evaluate the factors governing the fate and transport of contaminants within a contaminated site.

Successful remedial plans necessitate a robust CSM that encompasses comprehensive information enabling the identification of pertinent compound characteristics and the hydrogeological conditions that influence their subsurface presence.

This study outlines a characterization approach, remedial design, and the implementation of tailored remedial actions to address storage zones impacted by chlorinated compounds. In situ chemical oxidation utilizing direct-push technology and controlled pressure was selected as the method for mass destruction. The applicability and design of this approach were guided by the CSM constructed through high-resolution (HRSC) data collection, in conjunction with the TRIAD approach's applicability, and proved that a mass destruction rate higher than 95% is achievable even under challenging, low permeability conditions.

Approach/Activities

The study site is situated in the Sao Paulo Metro area and was formerly an industrial site where trichloroethene (TCE) was used as a solvent in the process. The primary source of contamination was undocumented or unknown, but a few areas around the warehouses were identified as potential sources of TCE infiltration into the subsurface.

Two campaigns using the Membrane Interface Probe - Hydraulic Profiling Tool (MiHPT) system were conducted to locate the source areas and assess the hydrogeological conditions at the site. The hotspots and contamination mass were identified, and remedial actions were planned and implemented in the main source of contamination.

The contamination mass was predominantly located above the groundwater level in layers with overall low permeability (<10-5 cm/s). However, the area's geology is characterized by soils derived from metamorphic formations, with shale as the predominant lithotype. The fine structural layers of the shale were still present in the soil structure and improved the probability of successful remedial actions at the site.

Given that the main compound of interest was TCE, but also considering limitations presented at the site under study, in situ chemical oxidation (ISCO) was the selected remediation method, even considering the permeability challenges at the site. The remedial design considered the HPT data to determine the optimal volume and pressure parameters that the soil could accommodate. Data from undisturbed soil samples were also utilized in combination with the HPT to finely assess the probable flow dynamics that could affect the delivery of the oxidants.

The selected oxidants were sodium persulfate and sodium percarbonate in the proportion 1:1. This combination allows for soil pH correction, enhancing oxidation efficiency while reducing the natural oxygen demand of the medium.

The injection process was conducted via direct-push and utilized a specially designed Finkler injection probe with controlled pressure and volume, and multiple injection ports. An automated unit was employed to prepare and mix the oxidants, reducing operator exposure and increasing the safety and efficiency of the process. A total of 3 tons of oxidants were injected in a volume of 20 thousand liters of water at a 15% concentration. Injection intervals were set at 0.5 meters, and a projected influence radius of 1.5 to 2 meters was expected based on the injection volume. The injection points were evenly spaced in a triangular arrangement around the center of mass identified during the HRSC investigation campaign. The pressure, flow, and volume of oxidants were controlled minute by minute to guarantee the saturation of the layers that contained the most contaminant mass. Flow rates were kept below 1000 liters/hour to avoid fracturing and the creation of preferential path flows that would not allow the delivery of oxidants to the contaminant mass.

Post-injection, additional MiHPT logs were conducted to compare the XSD signals, and soil samples were collected to assess the efficacy of the pilot study's mass destruction.

Results/Lessons Learned

The mass reduction achieved at the site was around 97% on average in the source zone, eliminating the need for further remedial actions. Most of the points had a reduction of 100% presenting concentrations below legal standards.  Only one injection campaign was necessary to achieve this rate of destruction. The remaining residual mass of contaminants is small enough to be managed in the long term without causing any risk to human health or the environment.

The application of remediation processes is complex and heavily relies on the hydrogeological properties of a given site. Storage zones, which contain the majority of the contaminant mass and pose greater challenges in accessibility and treatment, necessitate high-resolution characterization and the integration of multiple lines of evidence to design remedial efforts with a higher likelihood of success.

This case study demonstrates that combining MiHPT data with thorough interpretation of the geological conditions at the site enables the selection and design of appropriate remedial processes.

Despite the challenging permeability conditions observed at the site, the ISCO methodology proved to be efficient in mass removal. This efficiency can be attributed to the comprehensive understanding of site conditions and the precise delivery of oxidants to the layers harboring the highest concentration of contaminants, accomplished through controlled application.

The control of the injection process was achieved by utilizing a specially designed pressure injection probe and an automated injection system, ensuring consistent flow and pressure parameters throughout the injection campaigns. This controlled environment guaranteed optimal contact between the oxidants and contaminants, leading to complete destruction of the source area.

In conclusion, the success of remediation design relies on a densely detailed conceptual site model (CSM), while the effective implementation of remediation methods in low permeability zones demands a good understanding of the site’s conditions, equipment design, and meticulous control over the applied processes. These conditions significantly enhance the chances of success in remediating low permeability zones, which typically harbor the highest contaminant mass.