Theme: 10. Characterization, Fate and Transport

Topic Area: 10h. HRSC Suites of Tools to Improve CSMs

Formatted Title:

High-Resolution Investigation to Improve CSM, Determinate Hidroestratigraphic Model and Mass Flux to External Areas

Background/Objectives:

The study area comprises a former industry with a total area of 26,000 m², which is located in São Paulo, Brazil, and has residential and commercial neighbors. Local geology is related to tertiary sediments of the São Paulo formation, with sandy and clayey layers, in a complex context of intercalations, whose layers were drilled over 30 years of studies. Groundwater level ranges between 1.00 and 6.00 m bgs. The area, contaminated mainly by PCE, has been investigated since 2003 and underwent different remediation techniques along the years, without success. The contamination has already exceeded the limits of the area and there is no significant formation of degradation byproducts. The buildings were demolished in 2018, making it impossible to observe structures that could be characterized as ancient primary sources of contamination. In this context, considering the importance of stopping the advancement of contamination and the need to remedy and give a new use to the area, the work was developed, seeking a conceptual site model capable of allowing decision making about the most viable remediation approaches.

Approach/Activities:

The investigation started with five exploratory surveys up to 30 m bgs to characterize soil properties in which 41 samples were collected for metals and SVOC, 46 for VOC, 65 samples for granulometric analysis and 49 samples for fraction of organic carbon and organic matter. Eight infiltration tests were also performed, and 10 samples were collected with Shelby sampler for determination of soil physical properties. Concurrently with the exploratory drilling, 52 boreholes were performed using MIP-HPT which allowed to understand the distribution of contamination and hydraulic properties of soil layers. These surveys based the location of 46 boreholes for description of local lithology and collection of soil samples for characterization of the contaminant retained phase, totaling in this step 532 samples for determination of VOC, 193 for metals and 198 for SVOC. Fifty-two monitoring wells were then installed for better characterization of the dissolved phase (30 of them in the region of the border wall with the external area, considering the main flow zones), and all installed wells were submitted to hydraulic conductivity tests. The 52 monitoring wells installed, and 61 existing monitoring wells were sampled.

Results/Lessons Learned:

Layers descriptions and analyzes allowed to differentiate 12 main lithological groups. The horizontal hydraulic conductivity varied between 10^-3 and 10^-6 cm/s and vertical hydraulic conductivity ranged between 10^-4 and 10^-7 cm/s. The MiHPT surveys indicated contaminants associated with the former solvent washing area and a landfill of batteries was also found. The surveys carried out downstream indicated advanced PCE dispersion, reaching the downstream limit, with migration to the external area through conductive layers. In the most impacted region, the results showed that the contamination has already extended to the deepest aquifer evaluated. A total retained mass of 35.24 t was estimated, being 34.42 t of PCE and the mobile mass, associated with the dissolved phase, accounts for about 64.2 kg, distributed in 26.58 kg associated with the upper aquifer level, 20.45 kg at the intermediate level and 18.17 kg at the deepest level. The mass flow in the region of land downstream limits shows outputs of 116.802 mg/year of PCE by the shallow level to the external area, 55.782 mg/year by the intermediate level and 1.203 mg/year by the deep aquifer level, with main outputs associated to the central portion of the terrain. The human health risk assessment to was performed and currently the area is at the stage of consolidation of remediation techniques that will be applied from 2024.

Rodrigo Cunha

Affiliation

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(Group 2, Poster Board #121) High-Resolution Investigation to Improve CSM, Determinate Hidroestratigraphic Model and Mass Flux to External Areas