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 (Figure 1)
- 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
- 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 (Figure 1). 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.
Results
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 (Figure
2). PCE ant 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 (Figure
2). After some VC build-up in wells C and B, concentrations of VC in
June 2008 were below 30 µg/L in all wells (Figure 2). 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.
Consultant and Contractor: NTP Enschede
Companies On This Project
ADVENTUS GROUP
