The Science: Natural Source Zone Depletion of LNAPL


What is NSZD?

Petroleum hydrocarbons, which are common light non-aqueous phase liquids (LNAPL), can seep into soils as a result of oil spills, storage leaks and other handling incidents. Risks such as groundwater contamination and toxicity can result from petroleum hydrocarbon releases. These LNAPL bodies pose a unique set of remediation challenges, from both environmental and regulatory perspectives.

Natural source zone depletion (NSZD) is term that encompasses the processes that decrease subsurface LNAPL concentrations. These processes include sorption, volatilization, dissolution and biodegradation. Biodegradation, which is considered the most significant process, is carried out by naturally occurring soil microbes that readily consume and degrade LNAPL, resulting in total contaminant mass reduction. Conditions such as temperature, water and LNAPL saturation determine the rate of LNAPL attenuation.

Recent and ongoing research suggests that the NSZD rates of LNAPL can be significant. Therefore, NSZD should be considered a viable remedial option and/or a benchmark for other remediation efforts. LNAPL degradation rates at oil spill sites can range from hundreds to thousands of gallons per acre per year. These rates are similar to those achieved by active remediation efforts, such as hydraulic recovery. Quantifying NSZD can be a valuable tool in determining the appropriate timing, scale and aggressiveness of LNAPL remediation efforts. The Interstate Technology and Regulatory Council (ITRC) recommends including NSZD as part of a comprehensive remedial site plan. For more information, download the ITRC guidance document, pictured at right.

E-Flux offers an easy, cost-effective solution for monitoring NSZD.

Soil-Gas Transport

Agronomy experts have long recognized that soils emit carbon dioxide, known as CO2 efflux. In many soils, this CO2 flux is the result of natural soil respiration processes due to microbial and plant activity that occurs in the "root zone." Microbial degradation of LNAPL in soils also results in a net contribution of CO2 to the efflux. Petroleum hydrocarbons can be degraded aerobically (in the presence of oxygen) or anaerobically (without oxygen). Methanogenesis is a common anaerobic degradation pathway that produces methane, which moves upward through the soil until it encounters oxygen diffusing downward and is oxidized. Other common anaerobic degradation processes include sulfate and iron reduction.

The chemical equations below describe degradation processes using octane (C8H18) as a model compound.

Anaerobic Degradation (Methanogenesis):

C8H18 + 3.5 H2O -> 6.25 CH4 + 1.75 CO2

Aerobic degradation:

C8H18 + 12.5 O2 -> 9 H2O + 8 CO2

CH4 + 2 O2 -> 2 H2O + CO2

CO2, which is produced as the final LNAPL degradation product, moves upward through the soil until it is eventually released at ground surface. LNAPL degradation rates can therefore be quantified by measuring the flux of CO2 at grade. E-Flux Fossil Fuel Traps are deployed at the soil surface and contain sorbent that captures CO2 without disturbing concentration or pressure gradients.

Soil CO2 flux changes throughout the day, according to fluctuations in temperature and moisture. Therefore, a one-time CO2 flux measurement during the day does not provide a representative idea of NSZD. E-Flux Fossil Fuel Traps account for this by measuring CO2flux over an extended period of time. Traps are deployed for 2 - 4 weeks, producing time-averaged results and an idea of actual NSZD on contaminated sites.

How the Fossil Fuel Trap Works

The E-Flux Fossil Fuel Trap is a passive sampler that captures soil CO2 coming out of the ground with CO2 sorbent. The top of the trap is open to the atmosphere so that gas flow is not disturbed. The trap contains two layers of sorbent; the top layer traps ambient CO2 while the bottom layer catches CO2 originating from the soil. The trap does not alter the diffusion gradient because the CO2 that enters the trap is captured by the sorbent and does not build up within the head space. Prior to field use, the technology was tested in the lab to demonstrate quantitative recovery of the sorbent and confirm that the trap hardware does not interfere with gas transport in soils.

For more information on this topic, please see the Case Study page.

After deployment in the field, the Fossil Fuel Traps are sent back to E-Flux for analysis. The sorbent is recovered from the trap housing, dried, and homogenized. Next, it is analyzed for carbonate and fossil fuel carbon content.

E-Flux uses industry-accepted practices and methodologies, including quality assurance and quality control protocols. In combination with our proprietary technology, the tests are based on ASTM methods:

  • D4373-02, Standard Test Method for Rapid Determination of Carbonate Content of Soils
  • D6866-12, Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis

Using travel-blank corrected CO2 fluxes and assumptions about LNAPL carbon content and density, E-Flux estimates LNAPL losses as gallons per acre per year equivalents. The analysis process takes approximately four weeks, after which E-Flux provides a confidential final report, including soil CO2 fluxes and estimated LNAPL degradation rates. Although no measurement tool is perfect, the Fossil Fuel Traps have consistently provided data that confirm the site conceptual model and do so in a uniquely quantitative basis.

Radiocarbon Analysis

The bottom sorbent sample in each E-Flux Fossil Fuel Trap undergoes radiocarbon (14C) dating in order to determine the amount of fossil fuel derived carbon in the sample. Total CO2 flux captured by the trap represents a two-source model, with CO2 generated from natural soil organic matter (modern carbon) and LNAPL (fossil fuel carbon).

Modern sources of CO2 can be significant and should be subtracted off of a net CO2 flux measurement for accurate biodegradation estimation.

Fossil fuel carbon can be differentiated from modern carbon through radiocarbon dating with the carbon isotope 14C, which has a half-life of about 5800 years. Fossil fuel contains an undetectable amount of 14C since it is millions of years old. E-Flux therefore analyzes sorbent samples for 14C to determine the amount of modern carbon in a sample and the percentage of the total CO2 flux that is fossil fuel derived.

For more information on 14C dating see "Basis for 14C Analysis".

References

  • ITRC 2009 Evaluating Natural Source Zone Depletion at Sites with LNAPL, LNAPL-1, Technology Overview, Interstate Technology & Regulatory Council, Washington D.C., 76 pp. [Additional references in: http://www.itrcweb.org/Documents/LNAPL-1.pdf]
  • Sihota, N.J., O. Singurindy, and K.U. Mayer. 2011. CO2 Efflux Measurements for Evaluating Source Zone Natural Attenuation Rates in a Petroleum Hydrocarbon Contaminated Aquifer. Environmental Science and Technology. 45:482-488.