Oxygen profiling of the unsaturated zone using direct push drilling

A methodology for oxygen profile measurements in the unsaturated zone is developed based on direct push drilling using sampling liners equipped with homemade silicone septa. The oxygen measurement is carried out by puncturing the septum with a commercial retractable optode needle fitted with a fluorescent tip. Metrological characteristics and method validation were carried out under laboratory conditions using different levels of oxygen and various water contents. The relative standard deviations under dry and water saturated soil conditions were less than 0.3% and 5% for 0.5 mg L^-1 of oxygen and less than 2% and 3% for 9 mg L^-1. Field demonstrations in a calcareous-sandstone soil aquifer treatment system with a layered clayey, marl and sandstone lithology of widely different water contents provided down to 30 m deep profiles of the dissolved oxygen level with less than 1.5 m spatial resolution. A single sensor was used for over 50 field measurements, though recalibration was required after approximately 30 measurements due to the deterioration of the fluorescent tip.