Integrated, in situ gas/steam thermal desorption-chemical oxidation for volatile organic compounds and odorants in deep, contaminated soils.

Remediation of deeply contaminated legacy pesticide sites must often achieve risk-based contaminant control while also addressing persistent nuisance odors; however, field-scale performance is frequently limited by vertical heterogeneity and mismatch between odor drivers and standard risk indicators. At a deep (0-25 m) pesticide-manufacturing brownfield in Guangdong, China, a zoned and stratified in situ treatment train that coupled gas thermal desorption (GTD), steam thermal desorption (STD), and alkaline-activated persulfate in situ chemical oxidation (ISCO) was evaluated. The GTD was applied to heavily-contaminated shallow layers (0-12.5 m), STD to deeper layers (12.5-25 m), and ISCO to mildly formaldehyde-contaminated soils. Laboratory trials supported the operational settings for GTD/STD and showed marked reductions in volatile organic compounds and odor intensity, whereas ISCO displayed a non-monotonic response with an apparent optimum of approximately 2.5% persulfate at pH ≈ 11. During the pilot operation, the extracted off-gas odor intensity typically increased in the early stages and then declined, consistent with contaminant mobilization followed by source depletion, and post-treatment soils met the risk-based remediation and project-specific odor control targets established for the site. Post-treatment soil formaldehyde met the remediation target, whereas groundwater formate concentrations increased from ≤ 0.023 to 47.7-63.7 mg/L during staged ISCO and remained elevated at the final monitored stage, indicating aqueous-phase accumulation of a mobile transformation product. Secondary-impact monitoring identified fugitive NMHC, odor concentration, wastewater ammonia nitrogen concentrations, and early-stage nighttime noise as the main operational control endpoints. These results demonstrate pilot-scale feasibility under the site conditions examined and provide a field-based framework for similarly stratified pesticide brownfields requiring concurrent control of contaminant exposure and odor impacts.