STAR (In Situ) Pilot Test to Treat Coal Tar LNAPL and DNAPL in a Fine Sand Aquifer
Former Manufactured Gas Plant in Michigan
The subject site is a former Manufactured Gas Plant (MGP) in Michigan. Investigations have shown that the subsurface geology consists of medium-grained sands to a depth of approximately 35 feet below ground surface (ft bgs) with a reddish brown silty clay beneath. The sand unit is impacted with both Light Non Aqueous Phase Liquids (LNAPL) and Dense Non Aqueous Phase Liquids (DNAPL). LNAPL is encountered in a smear zone located near the groundwater table (15 to 24 ft bgs) and typically extends one to three feet above and below. DNAPL is generally encountered on top of the clay unit up to a maximum thickness of eight feet.
Pre-Design Evaluation (PDE)
A STAR Pre-Design Evaluation (PDE) involving a four-well ignition/air injection system targeting both the LNAPL and DNAPL zones (two wells per zone) was conducted to evaluate key full-scale design parameters such Radius of Influence (ROI), combustion front propagation rate, and volatile mass loading.
Self-sustaining smoldering combustion was achieved in both the LNAPL and DNAPL zones. Combustion was maintained in a self-sustaining manner until the ROI was reached and the objectives of the test were met.
The coal tar mass destruction rate averaged 46 kilograms per day over the 20 day test period (including down time between ignition events) for a total mass of coal tar destroyed of 933 kilograms. The ROI was determined to be approximately 8 feet and the combustion reaction propagated at a rate of approximately one foot per day.
'Before' and 'After' soil cores show the degree of treatment with Total Petroleum Hydrocarbon (TPH) concentrations decreasing from pre-PDE levels on the order of tens of thousands of milligrams per kilogram to post-PDE levels of only a few hundred milligrams per kilogram.
The fraction of contaminant mass volatilized (as opposed to destroyed via combustion) that was subsequently captured and treated was approximately 1%.
Before and After Photographs of Soil from the Combustion Zone
The STAR technology is a rapid, safe, and low cost remedial alternative for source areas.
The STAR PDE at the Michigan former MGP Site:
- Demonstrated self-sustaining smoldering combustion (i.e., no energy input into the system following ignition);
- Targeted and treated both the DNAPL and LNAPL zones;
- Showed an ROI of approximately 8 feet with a combustion front propagation rate of approximately one foot per day; and
- Resulted in TPH concentration reductions of two to three orders of magnitude in the treatment zone.