In situ biostimulation of petroleum hydrocarbon degradation by nitrate and phosphate injection using a dipole well configuration

• Crude-oil biodegradation in an anoxic aquifer was successfully stimulated.
• A dipole well configuration was operated for 114 days.
• Nitrate as electron acceptor disappeared with 9.2 day half-life.
• Dissolved benzene and other hydrocarbons decreased totally in 49 days.
• A mass balance suggests that 115 kg of hydrocarbons were degraded.

The main aim of this study was to explore the feasibility of source zone bioremediation by nitrate and nutrient injection in a crude-oil contaminated aquifer using a recirculating well dipole. Groundwater pumped from a downgradient well at a rate of 2.5 m3 h− 1 was enriched with bromide (tracer), nitrate and ammonium phosphate and injected in a well 40 m upgradient. The test was run for 49 days with solute injection, followed by 65 days of dipole operation without solute addition. The resulting bromide breakthrough curve allowed quantifying a first-order leakage coefficient of 0.017 day− 1 from the dipole, whereas from the nitrate data a first-order nitrate consumption rate of 0.075 day− 1 was determined. Dissolved hydrocarbon concentrations including benzene decreased to non-detect in 84 days but experienced important rebounds after ending circulation. Nitrite accumulated temporarily but was consumed entirely when solute injection stopped. The mass balance calculations revealed that about 83% of the nitrate was used for hydrocarbon degradation, with the remaining being used for oxidation of reduced sulfur. A reactive transport model was used for the delineation of the treated zone. This model suggested that denitrification influenced flow and transport in the dipole. It is concluded that successful promotion of denitrifying hydrocarbon degradation is easily obtained in this aquifer and enables to abate dissolved concentrations, and that dipole configuration is a good option.