Source zone natural attenuation microbial processes and overview of SZNA flux-based methods

Full text: Introduction - The biodegradation of petroleum is an oxidative process which requires the use of terminal electron acceptors (TEAs) and ultimately produces carbon dioxide. If petroleum soil contamination is shallow, oxygen is the preferential TEA used by microbes. At deeper locations, oxygen is scarce, and microbes resort to TEAs: sulfate, nitrate, and oxidized forms of iron and manganese. As these electron acceptors are depleted in the soil, microbes revert to a process called methanogenesis, which produces methane (CH₄) and carbon dioxide (CO₂), and does not require external electron acceptors. As methane gas moves upwards in the unsaturated zone, it encounters ambient oxygen and under such conditions is used as a carbon source by methanotrophic (methane oxidizing) microorganisms, producing CO₂. Methods - The measurement of SZNA rates is conducted by performing mass balances of biodegradation by-products or reactants in the dissolved phase on in the vadose zone. LNAPL losses associated with electron-donors in groundwater are estimated based on the groundwater flux of such reactive species. For SZNA losses associated with the vadose zone, three methods exist: The Gradient Method estimates diffusive fluxes based on soil gas concentrations and in situ effective diffusion coefficients; The Dynamic Closed Chamber Method (DCC) is a short-term soil gas flux measurement (typically of c) done at the ground surface, based on measured changes in concentration on a closed chamber emplaced directly over the soil; and the passive CO₂ flux traps (Traps) estimate the longterm CO₂ flux at ground surface over multiple days using passive sorbents, followed by laboratory analysis of total and carbon-isotopic corrected CO₂ fluxes to determine the fossilfuel related fraction. This presentation will describe these different methods, including the assumptions made on each method, their best use, advantages and disadvantages. Case studies exemplifying their use and typical results will be described, including validation studies comparing results from multiple methodologies. Results and discussion SZNA - processes are consistently present at most LNAPL contaminated sites. Quantification of these processes reveals that LNAPL loss rates are in the range from 1000s to 10,000s of kg/Ha/yr. Most of these LNAPL losses are expressed in the vadose zone. This is the result of mass transfer limitations in the groundwater electron acceptor supply at sites, and also due to the volatility of the methanogenesis by-products CH₄ and CO₂. Different vadose-zone methodologies have been validated in laboratory experiments, and have measured SZNA rates of similar magnitude at field sites. Conclusions - SZNA-related processes are present at most LNAPL contaminated sites. Given the availability of methods to conduct SZNA measurements, and the significant rates measured at sites, SZNA is becoming a cornerstone tool of the LNAPL practice, from site characterization to final remedy. This includes screening-level mapping of source zones, informing the conceptual site model, quantifying NSZD rates at field sites, and evaluating and comparing remedies. NSZD itself has been recognized as a potential remedy for some low risk sites, provided other conditions are met (such as lack of LNAPL mobility). (author)