Accelerated remediation of chlorinated solvents using a horizontal ZVI PRB in combination with ISCR for enhanced Anaerobic remediation
Description: Industrial Site in Apeldoorn, Netherlands
Consultant and Contractor: NTP Enschede
Approach: Severe soil and shallow groundwater contamination by metals and chlorinated solvents (cVOCs) was detected at the site. A comprehensive site remediation plan was developed by NTP and Adventus Group to provide accelerated site clean-up, that included:
• Installation of two extraction wells at the downgradient site boundary and a groundwater re-circulation system to control the plume migration;
• Excavation of highly contaminated unsaturated soil and shallow aquifer material in three delineated source areas;
• Backfill of the deep excavated areas using a mixture of zero valent iron (5% vol.) and sand;
• Installation of three rows of infiltration wells perpendicular to the groundwater flow direction for groundwater re-circulation and EHC-A substrate injections; and
• Installation of shallow infiltration drains within the iron-sand backfilled areas.
In total, about 35,000 m3 of soil material was removed to a depth of 4.5 m bgl. After completing the soil excavation and backfill activities, a monitoring well was installed at a depth of 7 m bgl in each of the three source areas (wells A, B and C). Even though dewatering was used during those activities, concentrations of up to 26 mg/L cDCE were measured in the monitoring wells after completion of this remedial stage.
The groundwater flow within the property was controlled by two extraction wells located at the downgradient site boundary. The wells each extract water at about 3 m3/hr. The extracted water was re-circulated through the network of three rows of injection wells and infiltration drains installed in the iron-sand backfilled areas. Groundwater levels were monitored by a telemetric control to adjust the extraction rates to prevent any overflow.
Based on previous site investigation, background conditions in the site aquifer were favourable for natural attenuation of cVOCs. The horizontal infiltration beds containing zero valent iron and sand mixture were used to create anaerobic conditions in the re-circulated water and to provide partial mass removal of dissolved cVOCs. To enhance anaerobic bioremediation of cVOCs, EHC-A; a soluble combination of a carbon source and ferrous iron, was periodically injected through the vertical infiltration wells. EHC-A was dissolved in a 1m3 vessel using a paddle mixer. The dissolved EHC-A was then pumped into the injection wells at a rate of about 5m3/hr over 2 to 6 hour injection events. A total of about 1,500 kg of EHC-A was injected in four events. As a result, DOC concentration increased from <5mg/L to 38 mg/L, EHC was lowered to about -250 mV, ferrous iron concentration increased to up to 13 mg/L and methane levels to 2-4 mg/L directly following EHC-A injections. Levels of these reductive indicator parameters decreased with time between the EHC-A injection events, as expected with a souble substrate application. However, the decreasing trends in cVOC concentrations has continued in all wells since the last EHC-A application at the beginning of 2008.
Significant reductions in cVOC concentrations were observed in all monitoring wells in the first sampling monitoring event, within two months of the first EHC-A application. PCE and TCE concentrations were reduced completely following the first EHC-A application. Concentrations of cDCE have decreased consistently to below 100 µg/L within 6 months in all wells, including well A were 26 mg/L cDCE was detected after source zone excavation. After some VC build-up in wells C and B, concentrations of VC in June 2008 were below 30 µg/L in all wells. A test in Summer 2008 for Dehalococcoides population density in the monitoring wells detected cell counts 2 to 3 orders or magnitude higher than those measured before remediation. Based on the observed success of this remedial effort, the site is now being prepared for redevelopment.
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