SVE at a Clay Till Dry-Cleaner Site
A former dry-cleaning site has soil contamination from 0 to 4 m below grade. The soil is clay till with thin sand lenses; groundwater is at 3 m. Concentrations are PCE 850 mg/kg, TCE 120 mg/kg, and vinyl chloride 15 mg/kg. The soil cleanup standard is 10 mg/kg for PCE. The affected area is about 2,000 m3, with bulk density 1.6 t/m3. The consultant proposes soil vapor extraction (SVE) because PCE is volatile. A pilot test shows low airflow and rapidly declining vapor concentrations. Groundwater PCE is 8 mg/L near the source and the MCL is 5 µg/L.
1. Estimate the PCE mass in soil and assess whether SVE is likely to achieve the 10 mg/kg standard. Identify the key limiting factors.
Read the worked answer
Soil mass = 2,000 m3 x 1.6 t/m3 = 3,200 t = 3.2 x 10^6 kg.
PCE mass = 850 mg/kg x 3.2 x 10^6 kg = 2.72 x 10^9 mg = 2,720 kg. TCE is about 384 kg and vinyl chloride about 48 kg.
A concentration of 850 mg/kg is high enough that separate-phase or residual dense non-aqueous-phase liquid is likely, especially near the dry-cleaning equipment area. SVE removes contaminants from the vapor phase; it is much less effective when the source is NAPL or when mass transfer is limited by low-permeability clay. The pilot data support that concern: low airflow means poor sweep efficiency, and a rapid vapor decline indicates that the readily accessible vapor-phase mass was removed while the bulk mass remains in the clay.
SVE alone is unlikely to reach 10 mg/kg across the whole volume. It may work in the sand lenses but not in the clay matrix. The design should not rely on SVE as the sole remedy for the source. Additional characterization is needed to delineate NAPL and the sand lenses.
2. Compare excavation, in-situ chemical oxidation (ISCO), and monitored natural attenuation for this site, and recommend a remedy.
Read the worked answer
Excavation is the most predictable way to remove the 2,000-m3 source in clay till. It can achieve the 10 mg/kg standard with confirmatory sidewall and bottom sampling, and it removes NAPL. Costs and disruption are high, and excavation below 3 m will require dewatering and shoring, but the volume is manageable. Contaminated soil would need proper characterization and disposal or treatment.
ISCO can destroy PCE and TCE in situ, but oxidant delivery in clay till is difficult. Contaminants in the low-permeability matrix may be bypassed, and rebound is likely. ISCO may be useful in the sand lenses or as a polishing step, not as a stand-alone guarantee.
Monitored natural attenuation is not appropriate as a source remedy. The groundwater concentration is 8 mg/L PCE, far above the 5 µg/L MCL, and the source mass is large. Natural attenuation may eventually manage the dissolved plume, but it will not meet the soil standard on a reasonable schedule.
Recommend excavation of the source area, with groundwater treatment or monitoring for the dissolved plume. Use ISCO only as a targeted supplement if excavation cannot reach a hot spot.