Context
An air sparging and soil vapor extraction installation had been operating for several years for a volatile TPH and BTEX soil and groundwater pollution in the marl geology of the Paris Bassin. Last year, the monitoring showed a stagnation in groundwater concentrations around 20.000 µg/L for TPH and 8.000 µg/L for BTEX (mainly benzene). Injectis was contacted by the contractor to investigate possible optimization methods for this problem. Several technical optimizations of the available air sparging/soil vapor extraction system had been worked out. Besides, instead of a physical removal technique to remove the remaining volatile pollutants, an in situ degradation alternative was elaborated by Injectis, using the SPIN® injection technology to apply the reagents in the low permeability marl. After thorough analysis, the client opted for this proposed in situ treatment for the capillary fringe zone and the saturated soil between 7 and 10 m-bgl.
Reaction
As volatile TPH and BTEX can be degraded by oxidation both chemically and biologically, it was opted for a combination of chemical and biological oxidation for these pollutants at this site. Persulfate was selected as the main oxidant and afterwards as electron acceptor. In a first stage, the persulfate serves as a chemical oxidant that chemically degrades the pollution. After the chemical oxidation reaction, a high concentrations of sulphate remains available in the groundwater, which serves as alternative electron acceptor for the anaerobic biological oxidation of the TPH and BTEX compounds. The advantage of this combination is that both the oxidant and the electron acceptor can be dosed in a concentration range of grams per litre, whereas oxygen (gaseous electron accepter) has only a maximum solubility in water of approximately 10 mg/L. Since marl and partly weathered marl has a low permeability, the SPIN® injection technology was chosen to distribute the reagent homogenously in the impacted layers and to guarantee contact for an optimal result.
Reagent
Base activated sodium persulfate has a high oxidation potential and serves as a vigorous chemical oxidant. The base used as activator compensates for the acidifying chemical oxidation reaction and keeps the pH close to neutral. After chemical oxidation, the high concentration of remaining sulphate can be used by the anaerobic bacteria as electron acceptor for the metabolic degradation of the TPH and BTEX compounds.
Location: Paris, France.
Geology: Calcareous marl
Pollutant(s): Total petroleum hydrocarbons (TPH), mainly C5-C10 : 20 mg/L ; C10-C40 : 500 µg/L Benzene: 6.000 µg/L ; toluene 700 µg/L ; ethylbenzene 1.300 µg/L ; xylene : 400 µg/L
Reaction: Chemical oxidation (ISCO) + anaerobic bio-oxidation
Reagent(s): Base-activated sodium persulfate
Application type: Grid application in the source zone
Surface/length: 200 m²
Number of points: 10
Depth interval: 7,0 – 10,0 m-bgl
Dosage: 200 l/m