Stable Isotope Probing (SIP) to Assess the Feasibility of Monitored Natural Attenuation (MNA)

Overview

The study site was a former retail gasoline station with dissolved benzene concentrations exceeding risk based closure levels. Historical groundwater monitoring generally indicated a stable or decreasing benzene concentrations. Monitored natural attenuation (MNA) can be an effective remediation strategy at sites impacted by petroleum hydrocarbons but is sometimes viewed as a “do nothing” solution in which decreases in contaminant concentrations result from physical processes (e.g. dilution) rather than biodegradation. Ultimately, acceptance of MNA as a remediation strategy rested upon demonstrating contaminant biodegradation under existing site conditions.

Questions

  • Is benzene biodegradation occurring under existing site conditions?
  • Is MNA feasible?

 

Stable Isotope Probing Study

A stable isotope probing (SIP) was conducted to conclusively determine whether benzene biodegradation was occurring at the site. Briefly, Bio-Traps® amended with 13C labeled benzene were deployed in select monitoring wells within the BTEX plume for 45 days. Following field deployment, the Bio-Traps® were recovered and two methods were used to conclusively demonstrate biodegradation of the contaminant of concern:

  • Quantification of 13C enriched phospholipid fatty acids (PLFA) which indicates contaminant incorporation into microbial biomass.
  • Quantification of 13C enriched dissolved inorganic carbon (DIC) which indicates contaminant mineralization.

SIP Results – 13C Incorporation into Biomass

  • For both monitoring wells, 13C enriched PLFA was detected indicating incorporation into biomass and conclusively demonstrating the benzene biodegradation occurred during the deployment period.

SIP Results – 13C incorporation into DIC

  • The background isotopic composition of dissolved inorganic carbon (DIC) is typically -25‰.
  • The detection of 13C enriched DIC in the Bio-Traps® deployed in monitoring well MW-12 (979‰) and MW-6 (670‰) demonstrated that mineralization of the 13C labeled benzene occurred during the field deployment.

Questions answered by SIP

  • Is benzene biodegradation occurring?

Yes. Detection of 13C enriched biomass and DIC demonstrated benzene biodegradation and mineralization.

  • Is MNA feasible?

Yes – For this site, MNA was a feasible remedy. Benzene biodegradation was occurring, trend analysis suggested a stable to decreasing plume, and the site posed no imminent threat to any receptors.