Review on the Research Progress of Natural Source Zone Depletion in LNAPL Contaminated Sites

Human health risks and potential environmental geological hazards caused by contaminated sites have been paid much attention. Monitored natural attenuation (MNA) is recognized as a preferred remediation technique. However, for sites with non-aqueous phase liquids (NAPLs), the problem of "tailing rebound" caused by the residue of NAPLs in the source area poses a challenge to the MNA technology. In recent years, the emergence of natural source zone depletion (NSZD) enriches the connotation of MNA remediation, and it is a potential way to solve the problem of "Tailing & Rebound".

To summarize the research process and latest achievements of NSZD for light non-aqueous phase liquid (LNAPL) contaminated sites.

A comprehensive review was conducted on the literature on NSZD for LNAPL contaminated sites from the end of the 1990's. The conceptual models of vertical zoning natural elimination in LNAPL source areas have been reviewed. The key control factors on NSZD and main scientific and technological challenges for future research have been fully discussed.

The research shows that: (1) since 2000, the research on MNA remediation has gradually shifted from groundwater pollution plume attenuation to natural depletion of vadose zone source areas; (2) The natural elimination process of aeration zones has proved to be the key biological process of NSZD, accounting for 90%-99% of the total mass loss of LNAPLs; (3) biodegradation in the volatilization process of LNAPL is the major research field of NSZD. The following research methods of NSZD are established: (1) the NSZD method can be divided into three parts: LNAPL source area plume identification, qualitative judgment and quantitative estimation; (2) The Concentration Gradient Method, CO₂ Fluxes Method (including Dynamic Closed Chambers and CO₂ Traps) and Thermal Gradient Method are three major methods for quantitative estimation.

Based on the existing research progress and challenges, the key scientific problems to be solved in the application and promotion of NSZD include identifying the composition change of LNAPLs in the source area, clarifying the speed limiting factor for natural elimination in the source area, and developing appropriate monitoring methods for degassing and bubble escape.