Example analysis of a hazardous waste landfill storage area soil and groundwater remediation and treatment project
01.Background
Hazardous waste refers to solid waste listed in the National Catalog of Hazardous Wastes or identified as possessing hazardous characteristics according to national identification standards and methods. Landfilling is a major disposal method, particularly for non-recyclable, non-degradable, or otherwise untreatable waste. However, due to substandard construction and operational management, landfill cells often experience failures of the impermeable liner system and leachate leakage, directly threatening groundwater safety. This study examines a remediation project for a hazardous waste landfill cell, focusing on the implementation and effectiveness of the selected soil and groundwater remediation technologies, to provide a reference for similar projects.
02. Site Overview and Process Flow
The hazardous waste landfill cell covers an area of 2.6×10⁴ m² with a designed capacity of 2.57×10⁵ m³. It features a double-layer HDPE geomembrane liner system, complemented by leachate collection and removal systems, and groundwater monitoring wells. Operational from 2012 to 2019, the site received approximately 7.4×10⁴ tons of hazardous wastes, including spent catalysts, incineration residues, and arsenic-containing wastes, reaching a fill height of 8-12 meters. Improper management led to the acceptance of some wastes not meeting the GB 18598-2019 standard, resulting in high contaminant concentrations in leachate and increased pollution risk to surrounding soil and groundwater.
Topography and Hydrogeology: The site is a valley-type landfill situated in a basin surrounded by hills on three sides, opening to the southwest. Groundwater generally flows from northeast to southwest along fractures in the bedrock, discharging into a stream southwest of the site. Eight monitoring wells surround the cell.
Identified Problems: Monitoring data from 2019-2021 indicated exceedances of fluoride and volatile phenol in groundwater, suggesting soil and groundwater contamination. Maximum fluoride concentrations reached 1.73 mg/L and 2.27 mg/L (standard: ≤1.0 mg/L), and volatile phenol reached 0.058 mg/L and 0.151 mg/L (standard: ≤0.002 mg/L) in monitoring wells and the drainage outlet, respectively.
Remediation Process Flow: The project involved excavating and transporting the stockpiled waste for co-processing in licensed cement kilns. Key steps included site investigation/risk assessment, contaminated soil remediation, and contaminated groundwater remediation.
Preliminary Work
Waste Excavation: Mechanical excavation was conducted in sections and layers ("high to low, NW to SE") with controlled slope stability. Approximately 1.07×10⁵ tons of waste were excavated and transported via enclosed vehicles for cement kiln co-processing.
Soil Contamination Survey: Investigation covered 1.75 hectares. 536 soil samples from 154 sampling points identified 51 exceedance points. Primary soil contaminants were Arsenic (As), Chromium (Cr), Antimony (Sb), and volatile phenol, exceeding GB 36600-2018 risk intervention values for Class II land. Three main pollution zones were identified based on contaminants: a Western composite pollution zone (As, Cr, Sb, phenol, ~25 m³), a Northern phenol zone (~347.5 m³), and an Eastern phenol zone (~163.5 m³).
Groundwater Contamination Survey: Utilizing 8 existing and 8 new temporary monitoring wells, groundwater contaminants identified were fluoride, volatile phenol, arsenic, and thallium, exceeding GB/T 14848-2017 Class III standards. Exceedances were localized (e.g., fluoride/As mainly in well JC8) or widespread (phenol).
03. Remediation Targets and Strategies
Remediation Goals: Post-remediation, soil concentrations of As, Cr, Sb must meet GB 36600-2018 Class II screening values (≤60, ≤800, ≤180 mg/kg); volatile phenol must be below detection limit (ND). Groundwater concentrations of volatile phenol, fluoride, As, and thallium must meet GB/T 14848-2017 Class III standards (≤0.002, ≤1.0, ≤0.01, ≤0.0001 mg/L).
Soil Remediation Strategy: The selected approach was Excavation and Off-site Co-processing in Cement Kilns. This method destroys organic pollutants via high temperatures and immobilizes heavy metals in the kiln's alkaline environment, achieving reduction, stabilization, and resource utilization.
Groundwater Remediation Strategy: The Pump-Treat-Hydraulic Controlmethod was employed. This involves extracting contaminated groundwater via pumping wells, treating it above ground, and injecting clean water to control the groundwater flow field and prevent contaminant plume migration. It's suitable for large, deep plumes.
Pump-Treat-Hydraulic Control Parameters: The system used 5 pumping wells (including existing wells JC2, JC3, JC4, JC8 and new well B5) and 8 injection wells (remaining existing wells and new wells B1-B4). Injection wells were strategically placed upstream of the flow direction, and pumping wells downstream. Well depth extended 20-30m below the base. Submersible pumps (3-5 t/h capacity) operated continuously, pausing when water levels dropped critically. Treated water was discharged post-treatment. Injection used clean water, maintaining a slight extraction/injection imbalance to contain the plume.
04. Performance Analysis
Soil Remediation Effectiveness: Post-excavation verification sampling at 38 locations (0-0.2m depth) confirmed that concentrations of volatile phenol, As, Cr, and Sb were all below remediation targets, validating the effectiveness of the soil remediation strategy.
Groundwater Quality Trends:
Volatile Phenol: Concentrations peaked during excavation (e.g., JC1: 0.0058 mg/L) but showed a consistent declining trend post-remediation. By March 2023, all wells met the standard consistently.
Fluoride: Exceedances were primarily in JC8 (peak: 4.44 mg/L). Concentrations declined steadily post-remediation, meeting the standard by March 2023.
Thallium: Exceedances occurred in JC1, JC4, JC6 (JC1 peak: 3.2x standard). A general declining trend was observed, with JC1 and JC6 meeting standards by March 2023, though JC4 showed fluctuations.
Arsenic: Exceedances were confined to JC8 (peak: 0.0953 mg/L). Concentrations decreased continuously, meeting the standard by March 2023.
Contaminant Correlation: Contamination in JC8 (Fluoride/As) was likely linked to the adjacent western soil pollution zone. Discrepancies between soil and groundwater contaminants (e.g., Cr/Sb only in soil; Fluoride/Thallium primarily in groundwater) highlight the influence of pollutant speciation and soil properties on mobility.
05. Cost-Benefit Analysis
The project's construction and installation cost was 167 million CNY, including 85.6 million CNY for waste disposal. Post-remediation, the site regained its functionality, providing 2.23×10⁵ m³ of effective capacity for future hazardous waste disposal (~3.345×10⁵ tons). At a market rate of 1500 CNY/ton, potential revenue is estimated at approximately 502 million CNY, indicating significant economic benefit alongside environmental remediation.
06. Challenges and Outlook
The incomplete overlap between soil and groundwater contaminants (e.g., fluoride/thallium only in groundwater) presents a challenge, as unidentified sources can cause groundwater concentration rebounds. For such scenarios, soil leaching tests are recommended to better assess pollutant migration potential and define source areas more accurately for targeted remediation.
07. Conclusion
This case study demonstrates an effective integrated approach for remediating a contaminated hazardous waste landfill cell.
(1) The "Excavation and Cement Kiln Co-processing"strategy efficiently removed the primary contamination source.
(2) The "Pump-Treat-Hydraulic Control"technology successfully restored groundwater quality, with key contaminants meeting standards within a short period.
The project achieved a virtuous cycle of "environmental improvement - capacity restoration - economic return", delivering significant environmental, social, and economic benefits.