Formatted Title

Natural Source Zone Depletion and the Challenges of Fractured Bedrock Assessments

Background/Objectives

A natural source zone depletion (NSZD) assessment was conducted in support of remedial alternatives planning for a large (~32 acres) light non-aqueous phase liquid (LNAPL) plume in fractured bedrock at a former fuel storage and dispensing facility in California. The carbon dioxide (CO₂) flux method was employed with the goal of approximating the average rate of LNAPL biodegradation on an annualized basis. The depth to the LNAPL, between 30 to 90 feet, and the complexities of variably fractured bedrock presented some unique challenges in approximating the biodegradation rate. 

The geology is characterized by extensional (normal) faulting and steeply dipping and often deeply weathered fractures. There is no appreciable porosity in the quartz monzonite bedrock and LNAPL is found principally in the fractures.  Plume geometry has been shaped by the variable transmissivity related to these structures.  LNAPL skimming has been employed since 1996, and current recovery rates are about 500 gallons annually.

Approach/Activities

Several factors were considered to inform the placement of the CO₂ traps within the footprint of the LNAPL plume. This included LNAPL thickness, and the distribution of dissolved methane and well-head methane vapor concentrations. The working hypothesis assumed that thicker LNAPL and higher concentrations of dissolved and well-head methane concentrations would correlate directly to higher CO₂ flux rate. These data were used to map areas of potentially high, medium, and low CO₂ flux. Six traps were placed in this regard following the vendor's standard operating procedures, two each in the high and medium areas, one in the low potential area and one to evaluate background conditions. Two sampling events were conducted, one in November 2022 and another in April 2023, to evaluate seasonality.  The traps were left in place for between 14 and 17 days per the vendor specifications.

Results/Lessons Learned

Using the site-specific data on the LNAPL composition and density, an average order of magnitude annual LNAPL biodegradation rate between the two events of about 3,600-gal year^-1 was estimated. The factors employed to indicate areas of high to low CO₂ flux did not correlate well to the results. Fundamentally the cause of the poor correlation is likely overlying variability in the fracture density, vertical permeability, and connectedness.  Poor correlation may have resulted from differences in the depth to the LNAPL where higher rates in areas of shallower LNAPL. In these areas the CO₂ flux travel path is shorter and there is less dispersion through the fractured bedrock.  In the deeper parts of the plume longer travel paths through the variably fractured bedrock may have allowed for more dispersion and “diluted” CO₂ gas concentrations. Lastly, some higher-than-normal precipitation that occurred during the April 2023 event may have created a wetting front where invading porewater may have locally influenced CO₂ flux. Going forward, future NSZD assessments may consider more traps to better evaluate spatial distribution in the CO₂ flux rate. Additionally, sampling of nested wells may be performed to profile vertical changes in the CO₂ concentrations and evaluate dispersion in the deeper parts of the plume.