Formatted Title

Combined In Situ Conductive Heating, Steam Injection and Air Sparging for Remediation of Fractured Chalk at a Former Chemical Facility in Kent (UK)

Background/Objectives

The historical activities at a former chemical facility in Kent (United Kingdom) contaminated underlying unsaturated soils and the chalk aquifer with a mixture of organic and inorganic contaminants. Benzene and trimethyl-benzene were identified as the contaminants of concern (CoCs). Fissures within the chalk promoted localized vertical migration impacting the water table resting at approximately 25 m bgl creating a large contamination plume. Significant seasonal fluctuations of the water table (up to a range of 8 m) resulted in a smear zone within the capillary fringe. To add complexity, a band of horizontal fissures within the aquifer resulted in occasional breakthrough of benzene contamination in monitoring wells located off site. Pump and treat remediation, SVE and excavation works carried out over several years significantly reduced the extent of the contamination plume in the aquifer and shallow soils. However, the Environment Agency (EA) deemed the risk associated with the residual contamination near the point of loss, fissure flow and groundwater seasonal fluctuation to be too high and requested further remediation. An intensive groundwater monitoring regime was carried out over a period of 12 months to assess the hydraulic properties, site-specific physico-chemical parameters and microbial population dynamics. This was complemented by additional site investigation works and Membrane Interface and Hydraulic Profiling Tool probe (MIHPT) assessment to fully characterize the conceptual site model as part of the remediation options appraisal.

Approach/Activities

Upon review of these new findings and historical data trends, a remediation design combining steam injection to heat the aquifer and in situ thermal remediation (ISTR) using Haemers Smart Burnersä to treat the capillary fringe and the unsaturated soils at the point of loss was identified as the best approach to meet the programme timescales set by the client. Hydraulic control of the aquifer and VOC capture within the treatment area was achieved by the deployment of a dual phase extraction (DPE) system with a combined GAC filtration and catalytic oxidizer (CATOX) system to abate VOCs in the off-gases. Air sparge wells were also installed in the deep aquifer to create aerobic conditions following completion of the thermal treatment to allow for biodegradation of residual benzene contamination. The remediation design objective was to heat soil and groundwater within the treatment area to a temperature >80°C and provide lines of evidence for site betterment under optimum conditions in agreement with EA requirements.

Six steam injection and air sparging wells were installed at a depth of 35 m and 36 m bgl, respectively. Sixteen ISTR wells were installed to heat the capillary fringe (ranging from 20-27 m bgl) and the unsaturated soil at the point of loss (between 8-15 m bgl). Seven dual purpose wells (dewatering and SVE) were installed to be operated by the DPE system at a depth of approximately 29 m bgl. Eight SVE wells were installed to a maximum depth of 27 m bgl. Soil samples were collected throughout the installation of the remediation wells and tested for CoCs contamination in order to create a 3-D contamination map and adjust the installation of the remediation system in accordance with the site findings. A Leapfrog Geoä software was used for 3-D modelling and to calculate contamination mass within the treatment area to assess remediation performance. Over a period of 9 months, in situ steam injection and ISTR were operated sequentially maintaining soil and groundwater average temperature of >80°C. Soil vapor extraction and air sparging were operated continuously for an additional 4 months and then at intervals over a further period of 6 months. The site is currently undergoing a 12-month rebound monitoring period.

Results/Lessons Learned

The remediation works revealed the following:

• Hydraulic control was maintained throughout remediation works (drawdown of approximately 2-5 m in the source area compared to expected seasonal fluctuations).
• Average temperature in the central treatment area in excess of >80°C was achieved in soil and groundwater following completion of the thermal treatment.
• Emulsified free product was recovered in the oil water separator (OWS) during the remediation works.
• Benzene concentrations in groundwater increased from an average of 2 mg/L prior to remediation to a peak of 68.6 mg/L during heating, followed by a decrease to <0.2 mg/L during rebound monitoring post remediation.
• VOC recovery was monitored continuously using a TVOC machine and validated using a PID and GC-MS measurement using Summa canisters sampling, which showed a typical recovery response curve associated with ISTR. Approximately 1,300 kg of benzene and trimethyl benzene were recovered from VOC and groundwater which is approximately 2x times more than the initial mass balance calculation using the software model (619.09 kg).
• Headspace testing in individual wells during remediation suggests gross contamination in the shallow soils at the point of loss have been successfully removed – peak reading of >5,000 ppm during ISTR/steam injection, followed by a steady decrease to ~100 ppm (following ISTR).
• Presence of inorganic mercury contamination was recorded in the off-gases preventing the use of the CATOX.
• Air sparging maintained prevailing aerobic conditions (dissolved oxygen > 1 mg/L) within the treatment area following completion of the thermal treatment; however, since sparging operations have ceased in August 2023, the aquifer within the target area is steadily dropping back to anaerobic conditions.
• Temperature drop within the unsaturated soils and groundwater following the thermal treatment was much slower than anticipated. This delayed the implementation of a number of microbial and degradation tests we anticipated to perform as line of evidence.

These findings and the results of the post remediation monitoring, including bacteria population assessment and half-life calculations for benzene degradation, will be presented at the conference.