Formatted Title
Steam-Enhanced Extraction for Jet Fuel Remediation in Heterogeneous Environments: Optimization Strategies and Residual Risks
Background/Objectives
Steam-enhanced extraction (SEE) is a technology that has been widely and successfully implemented for the in situ remediation of kerosene-based jet fuels in permeable geologies. This is predominantly due to the ability of the SEE approach to aggressively desorb and mobilize non-aqueous phase liquids (NAPLs) from the soil matrix, boil lighter aliphatic hydrocarbons and some volatile organic compounds (VOCs) such as benzene, and to enhance the volatilization of midrange aliphatic hydrocarbons for removal using soil vapor extraction. However, a high degree of heterogeneity and the presence of low permeability soils at these sites can limit the effectiveness of SEE in achieving stringent soil cleanup goals for middle-range hydrocarbons such as those found in many jet fuels. This is because the complete vaporization of midrange distillates does not occur at steam temperatures, and mass removal via an increase in vapor pressure and enhanced soil vapor extraction relies more heavily on achieving a high number of pore volume exchanges at the elevated temperatures. The presence of low-permeability lenses and layers in a heterogeneous formation can therefore result in preferential treatment of the more permeable zones, and can leave middle-range aliphatic hydrocarbons sorbed to the lower permeability soils. When attempting to meet stringent soil cleanup standards, this can prolong the active remediation timeframe, increase costs, and can make traditional measures of remedial progress for in situ thermal remediation sites such as asymptotic mass removal conditions less reliable.
Approach/Activities
At a U.S. government facility in southern California, SEE was implemented at full scale at three separate bulk, field-constructed underground storage tank (BFCUST) sites where kerosene-based jet fuels were historically stored. At each BFCUST site, pre-design investigations were performed to update the conceptual site model to delineate the extent of hydrocarbon impacts relative to hydrogeologic conditions and the BFCUST location. Each site was predominantly characterized by poorly graded sand with some interbedding of silt and silty sand, with a siltstone bedrock zone encountered at deeper intervals. The conceptual site models were developed using three-dimensional modelling software, and the layout and design of each of the three SEE systems was optimized to effectively heat the entire zone of impacts above soil cleanup goals. At one of the three BFCUSTs, permeability enhancement was also implemented in the very low permeability siltstone layer where interbedded limestone fractures with flowing NAPLs were observed. During active heating operations at the three sites, multiple optimization strategies such as pressure cycling and zone operation were implemented to enhance recovery of the hydrocarbons in order to meet soil cleanup goals.
Results/Lessons Learned
At all three BFCUST sites, average subsurface temperatures of 90 °C were achieved across the vertical and lateral extent of the treatment zones and were sustained at the elevated temperatures for multiple months while the soil vapor extraction system continued to remove mass. At all three sites, asymptotic mass removal conditions were achieved as measured by both well-head vapor concentrations as well as diminishing total mass removal. The combined mass removal across the three sites exceeded 370,000 pounds of petroleum hydrocarbons. Also at all three sites, post-remediation soil sampling indicated residual midrange hydrocarbons above soil cleanup goals predominantly in lower permeability soils, indicating that the optimization strategies implemented had only a limited effectiveness in removing midrange hydrocarbons, and that the other lines of evidence of remediation completion such as asymptotic mass removal conditions did not accurately reflect progress towards achieving soil cleanup goals.