Dense Nonaqueous-Phase Liquid Architecture in Fractured Bedrock: Implications for Treatment and Plume Longevity

Abstract

Partitioning tracer testing was performed in discrete intervals within a fractured bedrock tetrachloroethene (PCE) dense nonaqueous-phase liquid (DNAPL) source area to assess the fracture flow field and DNAPL architecture. Results confirmed that the partitioning tracer testing was able to identify and quantify low levels of residual DNAPL along flow paths in hydraulically conductive fractures. DNAPL fracture saturations (Sn) ranged from undetectable to 0.007 (DNAPL volume/fracture volume). A comparison of the fracture flow field to the DNAPL distribution indicated that the highest value of Sn was observed in the least transmissive fracture (or fracture zone). Application of a simple ambient dissolution model showed that the DNAPL present in this low transmissivity zone would persist longer than the DNAPL present in more transmissive fractures and would persist for 200 years (in the absence of any degradation reactions). Assessment of PCE mass distribution between the rock matrix and fractures showed that, due to the presence of DNAPL, the rock matrix accounted for less than 10% of the total PCE mass. The evaluation of PCE concentration profiles in the rock matrix and the estimated diffusional flux from the rock matrix suggest that the elevated PCE groundwater concentrations observed in the fractures likely are due to the presence of the residual DNAPL sources and that removal of the residual DNAPL sources within the fractures would result in a significant decrease in dissolved PCE concentrations in the source area.

Bioaugmentation in a Well‐Characterized Fractured Rock DNAPL Source Area

“…No remedial amendments or biodegradation impacts (e.g., decreases in PCE, chlorinated ethene daughter products, lactate fermentation products, increases in DHC) were observed at either of the extraction wells during the demonstration, and no treatment of PCE from these wells was observed (data not shown). These results are consistent with the previously performed tracer tests (Schaefer et al ). In addition, no remedial amendments or biodegradation impacts were observed at the shallow and intermediate intervals of 37‐B07, which was the second monitoring location downgradient from the injection well (although one sample at the shallow interval of 37‐B07s did show a 1 to 2‐log increase in DHC; data not shown).…”

“…DNAPL mass removal along the radial fracture flow path between the injection well, 37‐B06, and the circumference defined by the distance to 37‐B11 was determined using a mass balance approach based on the estimated DNAPL mass from the partitioning tracer test (Table ) and the chloride generated during active treatment. Using this approach, the DNAPL mass along the shallow and deep radial flowpaths was estimated at 1.1 and 1.3 kg, respectively (Schaefer et al ). The mass of PCE DNAPL removed was determined based on chloride generation (Schaefer et al ; Torlapati et al ) using the following equation: …”

“…The PCE extracted from the extraction wells and re‐injected into 37‐B06 is assumed to be completely converted to DCE upon re‐injection, and the chloride derived from this dissolved PCE mass is subtracted from the average molar increase in chloride so that C Cl is attributable to DNAPL dissolution only. The value for V f (for both the shallow and deep fracture zones) is 205 m 3 , which is determined based on the total recirculated flow during treatment multiplied by the fraction of flow going to both the shallow and deep fracture zones (Schaefer et al ). It is noted that the radial flow assumption used to determine the initial DNAPL mass by Schaefer et al would also be employed in the calculated value of M D * .…”

Effects of microarrangement of solid particles on PCE migration and its remediation in porous media

“…Unfortunately, with the rapid development of economic activities such as mining, agriculture, landfills and industrial activities (Bakshevskaia and Pozdniakov, 2016;Cui et al, 2016;H. Liu et al, 2016;An et al, 2016;Shen et al, 2017), more and more contaminants released from human activities are contaminating the precious groundwater resource and subsurface environment (Dawson and Roberts, 1997;Liu, 2005;Hadley and Newell, 2014;Carroll et al, 2015;Essaid et al, 2015;Huang et al, 2015;Schaefer et al, 2016;Weathers et al, 2016). Among the contaminants detected in groundwater, dense nonaqueous phase liquids (DNAPLs) such as perchloroethylene (PCE) and other polycyclic aromatic hydrocarbons (PAHs), are highly toxic and carcinogenic (Dawson and Roberts, 1997;Hadley and Newell, 2014).…”

Remediation of soil contaminated by PAHs and TPH using alkaline activated persulfate enhanced by surfactant addition at flow conditions

“…These contaminants generally have low solubility and persist as a separate liquid phase, termed non‐aqueous phase liquids (or NAPLs). When NAPL is less dense than water, as is the case in many petroleum products, they are known as light non‐aqueous phase liquids (LNAPLs), being found above the water table . Contamination derived from gasoline spills mainly consists of BTEX compounds, while heavier and less soluble compounds in the aqueous phase are found in fuel oil spills, compounds generally known as total petroleum hydrocarbon (TPH), which contain a mixture of carbon chain length C14–C40 …”