The soil and groundwater of an operative chemical plant is impacted with BTEX (toluene and ehtylbenzene), Methyl IsoButyl Ketone (MIBK), Methyl Ethyl Ketone (MEK, known as butanone) and TPH (mineral oil) due to past site activities. The source area is located underneath and on the southside of a tank farm, over an area of about 40 x 40 m. A contaminant plume of 400m x 60 m has developed and is moving towards the surface water.
The soil is characterized by a top layer of a mixture of gravel, sand/silt followed by silty, clayey and chalky layers with low permeability (in the order of 10-7 m/s). The low permeability makes this a challenging aquifer and results in wells running dry quickly when pumped at actively.
The groundwater is characterized by strong reducing conditions, extreme electrical conductivity (>100,000 uS/cm) and very low Dissolved Oxygen (DO). High pH values (up to pH 12) have been recorded in monitoring wells. High arsenic concentrations, caused by reduction of iron oxides, are found in wells spread over the site. The arsenic concentrations mainly in the form of As (V), are not related to site activities but originate from natural soil layers.
To assess the possible effects of the challenging soil conditions (high Ec, reducing conditions, high arsenic concentrations) on full scale remediation techniques, Greensoil R&D was consulted to conduct preliminary laboratory and field tests to test the feasibility of stimulated aerobic biodegradation and to optimize the design the remediation installation.
1. Biodegradation test: to investigate the degradation rate, nutrient demand, what final concentration levels may be achieved for MIBK/MEK.
2. Biosparging test: To test the efficiency of biosparging at the site.
3. Arsenic Precipitation test: to test the potential of chemical precipitation of arsenic under aerobic conditions so to assess if off site migration of the arsenic can be stopped. simultaneously with the biodegradation of the solvents.
The remedial approach for this site consisted of a combination of two treatment methods:
- Removal of the contamination by a groundwater circulation system and aboveground groundwater treatment unit.
- Stimulation of the in-situ aerobic biodegradation using a biosparging system.
Greensoil supervised the installation of 26wells inside the tank farm (manual drilling) and 26 wells outside the tank(mechanical drilling) for extraction and infiltration of groundwater.
The GWTU consist of a control unit(incl. remote access) – extraction/infiltration pumps – bioreactor –settlement/buffer tank – nutrient dosage system and granular activated carbon (GAC)
Installation of 44 biosparging wells both inside and outside the tank farm (installed by pneumatic hammering). The biosparging wells inject compressed air at relative low flow and pressure, to preventstripping of the contaminants from the groundwater to the air phase, thereby creating aerobic conditions in the soil.
Oxygen consumption by biological activity is a measure for the degradation potential of the contaminants. The biodegradation test (conducted at different concentrations) showed high oxygen consumption in standard conditions but especially high when nutrients were added (nitrogen andphosporus), proving that biological degradation is ongoing under aerobic conditions and stimulated. Due to the strong reducing conditions and high contaminant mass, the oxygen demand is high. Measured COD was up to 1,407 mg/l.
Concentration levels of MIBK/MEK were measured at the end of the test. The MIBK/MEK concentration in the stimulated sample with nutrients was < 1 mg/l. Given the starting concentrations, the test demonstrated that MIBK/MEK can be reduced > 99% by enhanced biodegradation. Concentration of BTEX were below detectable values and showed 100% mass removal.
In these specific site conditions increasing the redox potential does not remove arsenic from groundwater to the soil. Further, If reduced conditions are caused by organic impact in the tank farm area, the remediation of this impact will not lead to reduction of arsenic impact in groundwater. However, due to extensive research no active remediation methods are deemed necessary for the arsenic groundwater contamination. The evolution of the arsenic concentration levels will be monitored.
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