Formatted Title
Natural Source Zone Depletion Study at a Former Central California Refinery Site: Evaluating the Influence of Vapor Extraction System and Air Sparging on NSZD Rate Determination and Study Duration
Background/Objectives
This study aims to determine the natural source zone depletion (NSZD) rate at a former refinery site in central California. Prior to this investigation, the site underwent comprehensive remediation involving a large-scale soil vapor extraction system (VES) and air sparging (AS) activities to address petroleum hydrocarbon (PHC) fractions in both soil and groundwater. The objective of the NSZD study was to ascertain the NSZD rate and employ this data to guide future site management strategies for the remaining PHC impacts at the site.
Approach/Activities
The NSZD study, conducted from September 23, 2022, to June 30, 2023, utilized the "Single Stick" computational method (Askarani and Sale, 2020). This method enabled continuous data collection and daily NSZD rate calculation throughout the study. The extensive dataset revealed distinctive thermal behavior at the site, predominantly influenced by the recent operation of VES and AS activities. In the initial 3 months of the NSZD study, the data indicated a significant decrease in NSZD rates. In response, the study duration was extended to observe if the site's thermal characteristics would stabilize, allowing for a stable and consistent NSZD rate to be determination
Results/Lessons Learned
The study unveiled the substantial impact of prior remediation efforts on the site's thermal conditions. Over a period of approximately 3 to 6 months, the site transitioned from aerobic to methanogenic conditions, eventually stabilizing and reaching a new equilibrium in thermal behavior. This stabilization enabled the determination of a steady NSZD rate, no longer influenced by past remedial efforts. The study underscores the necessity of considering prior remediation impacts and their potential to affect NSZD rate determination. Additionally, it highlights the importance of collecting data over an extended duration at sites where active remedial approaches have been recently employed, ensuring that NSZD rates are stable and not skewed by past activities. These insights provide vital guidance for effective site management, particularly in understanding how certain forms of active remediation can significantly impact the results of practices used to determine NSZD rates at sites.