Understanding the implications of vadose zone processes across spatial and temporal scales is challenging. At petroleum release sites, biodegradation of hydrocarbon compounds contributes to biogeochemical cycling through natural source zone depletion (NSZD). Considerable gaps remain in characterization at large sites. An evaluation of NSZD rates at a >80ha decommissioned oil refinery was conducted using a dynamic closed chamber (DCC). Across the site, we characterized spatial variability and compared radiocarbon ( 14 C) and background-corrected NSZD rate estimates. At a high-resolution (0.5-ha) area, temporal variability in NSZD rate estimates was characterized and a method comparison was conducted for DCC, the concentration gradient method (CGM), and CO 2 traps. The spatial variability of sitewide NSZD rates was significantly different from smear zone thickness, suggesting that additonal factors affect NSZD variability at a sitewide scale. The estimated sitewide average NSZD rate using the locationspecific 14 C correction was significantly different from the average by the background correction method (1.2 vs. 3.0 mmol CO 2 m −2 s −1 , respectively). In the high-resolution area, NSZD rates estimated by DCC varied temporally across seasonal and daily time scales. Higher temperatures were correlated with increased NSZD, and precipitation was important in dampening effluxes following rain events. The method comparison identified high sensitivity of the CGM vs. the other methods to input parameters, while CO 2 traps showed relatively high intersample variability. Possible limitations to DCC, including measurement timeframe and 14 C sampling method, were considered. The findings assist in setting NSZD expectations at other large sites.
Natural source zone depletion (NSZD) is increasingly being integrated into management strategies for petroleum release sites. Measurements of NSZD rates can be used to evaluate naturally occurring hydrocarbon (HC) mass losses, and provide a baseline for evaluating engineered recovery systems. Here, nominal saturated and unsaturated zone HC losses were quantified by groundwater sampling and ground surface CO2 effluxes approximately monthly over a 1‐year period. In addition, subsurface gas profiles and temperature, precipitation, and groundwater elevation were evaluated to elucidate dominant environmental factors controlling NSZD rates. Results showed that NSZD rates estimated by surface CO2 effluxes were, on average, more than a factor of 3 greater than those estimated by uptake of electron acceptors (primarily sulfate) in extracted groundwater. This may indicate that vadose zone mass loss mechanisms (e.g., volatilization and subsequent biodegradation) were dominant in this system, but possible transfer of gases from the saturated zone to the vadose zone confounds this interpretation. Results for this semiarid site revealed that increasing NSZD rates tended to occur with increasing ambient monthly precipitation and temperature when accounting for time lags associated with subsurface transport. However, groundwater elevation was not found to be significantly related to NSZD rates. This result is counter to an expectation that increased smear zone exposure increases HC mass losses, and suggests that the pump‐and‐treat system did not greatly influence total NSZD rates directly through smear zone flushing or indirectly by lowering the regional water table.
Soils previously treated with landfarming to reduce petroleum hydrocarbon concentrations are often left with a less biodegradable residual fraction that can present challenges for additional treatment. Four possible polishing technologies were tested on the bench scale for weathered hydrocarbons present in fine grain soils obtained from a previously landfarmed area at an active oil refinery. The technologies included additional bioremediation (both biostimulation and bioaugmentation tested), soil washing, chemical oxidation, and low temperature thermal desorption. Multiple parameters were tested separately for each technology to identify possible factors that were relevant across technologies. Extractable hydrocarbons comprised only approximately 35% of the organic carbon in the soils, and this component was considerably less affected by biological, surfactant, and oxidant treatment than organic materials that are not Downloaded by [Texas A & M International University] at 15:45 13 August 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 quantified by the TEH analysis. Treatment testing of thermal desorption indicated removal of large quantities of extractable hydrocarbons despite the presence of high organic matter. The additional demand to the system would likely result in considerably large timeframes (biological treatment), reagent quantities (soil washing and oxidation), or energy input (thermal desorption) for treatment of target hydrocarbons on a full scale.
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