Comparison of Source Zone Natural Attenuation Rates At Crude Oil and Ethanol–Blended Fuel Release Sites

Abstract

Understanding differences in source zone natural attenuation (SZNA) rates occurring among field sites impacted by the same contaminant and across field sites impacted by different contaminants is critical for developing management strategies. For example, unique site conditions can favor or inhibit biodegradation, whereas differences between contaminants can lead to variations in biodegradation potential. However, the implications of these effects for real-world release scenarios remain ambiguous. To better understand the implications of these effects, the authors investigated differences in SZNA rates at two crude oil and two denatured fuel-grade ethanol (DFE) spill sites using on-site measurements of surficial gas effluxes. At the crude oil sites, CH 4 effluxes were below detection, whereas CH 4 effluxes occurred at both DFE sites. Similarly, SZNA rates among sites impacted by the same contaminant were comparable, whereas order of magnitude differences existed between sites impacted by crude oil or DFE. At DFE sites, results also revealed source zone expansion in relation to the initial mass in place, suggesting that extended spatial monitoring may be required to characterize risk potential. Overall, key differences between crude oil and DFE release sites demonstrated the importance of site-specific interactions between hydrogeology and contaminant composition for mediating gas emissions and SZNA rates, and modulating gas transport regimes under field conditions.

Feasibility of greenhouse gas emissions offsets for natural source zone depletion of petroleum hydrocarbons

“…Multiple researchers have quantified average sitewide contaminant respiration rates using a simple background correction (i.e., subtracting an average unimpacted area flux from the flux measured above the contaminant footprint) and/or by radiocarbon correction (which accounts for hydrocarbon‐related flux because it is “radiocarbon dead”). For instance, Sihota and Mayer () presented a comparison of contaminant respiration emissions for two crude oil and two denatured fuel ethanol (DFE) spill sites in Minnesota, with average contaminant‐related CO 2 effluxes of 2.3 to 8.0 micromoles CO 2 per meters squared per second (μmol m −2 s −1 ) at the crude oil sites and 16 to 20 μmol m −2 s −1 at the DFE sites. The DFE sites also exhibited CH 4 emissions, with average values ranging from 1.4 to 24 μmol m −2 s −1 .…”

“…The DFE sites also exhibited CH 4 emissions, with average values ranging from 1.4 to 24 μmol m −2 s −1 . The higher CO 2 effluxes and added CH 4 effluxes at DFE sites are likely related to the highly bioavailable ethanol (Sihota & Mayer, ; Sihota et al., ), with the implication that these sites may act as a shorter term source of greenhouse gases than crude sites. Sitewide assessments at large former oil refineries indicated average contaminant respiration emissions of 3.4 μmol m −2 s −1 at one site in the Midwest United States (Eichert et al., ) and 0.9 to 4.4 μmol m −2 s −1 at one site in the Rocky Mountains United States (Sihota et al., ).…”

“…More recently, researchers have begun to quantify the hydrocarbon removal rates from the LNAPL bodies themselves by estimating fluxes of biodegradation reactants and products in nearby groundwater and soil gas (Interstate Technology & Research Council [ITRC], ; Johnson, Lundegard, & Liu, ; Lundegard, & Johnson, ). For instance, elevated CO 2 effluxes have been measured at ground surface above multiple LNAPL bodies (McCoy, Zimbron, Sale, & Lyverse, ; Sihota & Mayer, ; Sihota, Mayer, Toso, & Atwater, 2013; Sihota, Singurindy, & Mayer, ), and emitted CO 2 has been inferred by radiocarbon analysis (Eichert, McAlexander, Lyverse, Michalski, & Sihota, ; McCoy et al., ; Sihota & Mayer, ) and correlation to subsurface LNAPL distribution (Sihota, McAlexander, Lyverse, & Mayer, ) due to biodegradation processes. In cases where subsurface microbial demand is likely high and O 2 supplies are low (e.g., LNAPL associated with labile denatured ethanol fuel in a waterlogged setting), elevated CH 4 effluxes have also been observed (Sihota et al., ).…”

Methodology for the in situ characterisation of soil vapor contaminants and their impact on the indoor air quality of buildings

“…Different methods have been developed to characterise soil contaminant vapors: soil gas collection, by driving a tube or rod, often called a "probe", into the soil or by burying a small-diameter tube in the soil, passive soil gas collection (absorbent material designed to collect volatile chemicals). More particularly in recent years, flux chambers have been widely used for the assessment of soil gas pollutant emissions from the subsurface to the atmosphere [4,5,6,7,8,9,10,11,12,13].…”