Continuous and Remote Monitoring of Natural Source Zone Depletion of Light Non-Aqueous Phase Liquids With Temperature-Based Sensors

Natural source zone depletion (NSZD) of light non-aqueous phase liquid (LNAPL) has gained scientific and regulatory interest over the past decade due to the recognition that natural processes can remove LNAPL at rates that are comparable to or exceed active remediation technologies. While several technologies are available to quantify NSZD rates (passive CO₂ flux traps, the gradient method, the dynamic closed chamber method, and biogenic heat methods), biogenic heat methods, or Thermal NSZD, in particular provide the opportunity for continuous, remote monitoring of subsurface temperature data to automatically quantify NSZD rates, with data review and visualization available through a dedicated and secure web interface. After a one-time field installation, continuous, remote monitoring essentially eliminates routine site visits and operations and maintenance, which further improves the environmental impact of monitoring and health and safety to field personnel.

At each monitoring station, a vertical array of temperature sensors is installed belowground. Dataloggers continuously record in situ soil temperatures at high frequency (e.g., every 15 minutes) and calculate daily average soil temperatures at each depth. Data is conveyed remotely via cellular signal at regular intervals to a data warehouse. Thermodynamic algorithms have been developed to convert the temperature data to rates of degradation of LNAPL on a daily basis. Web-based dashboards provide a secure, intuitive, and information-rich interface for data review and visualization in real-time for interested parties (e.g., research partners, regulators overseeing site assessment and remediation activities, concerned citizen groups, etc.).

Preliminary temperature and NSZD data from a demonstration project at two Department of Defense installations will be presented, along with a nascent look at complementary ORP and water level data and a novel processing algorithm. These data demonstrate the viability of quantitative monitoring beneath paved surfaces. Ongoing research continues to improve sensor technology and available sensors, and further advances in temperature processing and NSZD algorithms can be implemented and incorporated into the existing data processing pipeline seamlessly.