During the Leighton Beach development site's use as a railway yard from 1959, diesel leaked into the underground and contaminated soil and groundwater. It is suspected that ~ 1300 kg of hydrocarbons are currently present over an area of 600 m2 as residual phase within the groundwater fluctuation zone. Biosparging was suggested by GHD Pty Ltd as an enhanced natural attenuation remediation strategy to increase biodegradation rates of the residual hydrocarbon source at the groundwater table. The biosparging strategy is a hybrid method between air sparging and bioventing. Air sparging wells are used to deliver oxygen to the saturated zone. Nutrient wells distribute ammonium and phosphate in the capillary fringe. The soil gas composition (O2, CO2, and CH4) and the dissolved oxygen in the groundwater are measured prior to, during, and after the biosparging intervals. The CO2 concentrations decreased during the air injection and slowly increased after the biosparging was completed. The changes in CO2 concentrations are related to different reactions of indigenous bacteria to oxygen injection (e.g. enhanced biodegradation rates or repression of anaerobic bacteria).