Bioventing Feasibility Assessment and System Design Using Subsurface Oxygen Sensors

Abstract

Vadose zone oxygen sensors can be effectively used to improve bioventing remediation design and monitoring. The capacity of the oxygen sensors for continuously monitoring oxygen concentrations unattended offers an improved approach for bioventing feasibility evaluation and respiration measurements. A new in-situ technique has been developed using these sensors for evaluating respiration rate at the startup of air injection or vapor extraction. This dynamic method imposes little restriction on the flowrate and soil conditions when the oxygen sensors are within the radius of influence, whereas a traditional in-situ oxygen uptake respiration test assumes a static condition. Oxygen uptake respiration tests measure a localized respiration rate. The dynamic method determines a spatially averaged respiration rate in the air flow path between injection point and a sensing point in air injection setup, or within the capture zone of a vapor extraction setup. Because respiration measurements can be made using the new technique at the startup of a remediation process, whether it is air injection or vapor extraction, it allows the process to run without interruption. Using the subsurface oxygen sensor, the dynamic method also allows the respiration rate to be continuously monitored. A case study at a site in Palm Springs, California is used to document the application of this new technique and its advantages. In addition to directly monitoring the radius of influence, the subsurface oxygen sensor can also provide necessary parameters to calculate a radius of influence for a given air injection rate. The Palm Springs case study demonstrates the application, as well as the limitation, of the radius of influence calculation.