Processes governing the performance of oleophilic bio-barriers (OBBs) – laboratory and field studies
Date
2018
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Abstract
Petroleum sheens, a potential Clean Water Act violation, can occur at petroleum refining, distribution, and storage facilities located near surface water. In general, sheen remedies can be prone to failure due to the complex processes controlling the flow of light non-aqueous phase liquid (LNAPL) at groundwater/surface water interfaces (GSIs). Even a small gap in a barrier designed to resist the movement of LNAPL can result in a sheen of large areal extent. The cost of sheen remedies, exacerbated by failure, has led to research into processes governing sheens and development of the oleophilic bio-barrier (OBB). OBBs involve 1) an oleophilic (oil-loving) plastic geocomposite which intercepts and retains LNAPL and 2) cyclic delivery of oxygen and nutrients via tidally driven water level fluctuations. The OBB retains LNAPL that escapes the natural attenuation system through oleophilic retention and enhances the natural biodegradation capacity such that LNAPL is retained or degraded instead of discharging to form a sheen. Sand tank experiments were conducted to visualize the movement of LNAPL as a wetting and non-wetting fluid in a water-saturated tank. The goal was to demonstrate 1) the flow of LNAPL as a non-wetting fluid in sand, 2) the imbibition of LNAPL as a wetting fluid on the geocomposite, and 3) the breakthrough of LNAPL after saturating the geocomposite to the point of failure (sheens in the surface water). Dyed diesel was pumped through a tank with sand and geocomposite and photographed to document movement. Diesel was the non-wetting fluid in the sand and moved in a dendritic pattern. Diesel was the wetting fluid on the geocomposite and uniformly imbibed horizontally across the geocomposite before breakthrough to the overlying sand layer. A second set of laboratory experiments was designed to estimate the aerobic and anaerobic OBB degradation rates of LNAPL in field-inoculated sediment. Unfortunately, due to a flaw in the experimental design, the mass balance could not be completed, and degradation rates were not calculated. The setup was designed to emulate field conditions as best practically possible and to observe the effects of water table fluctuations, different loading rates, and iron. The effluent pumping system designed to remove water in the water fluctuation columns also inadvertently removed LNAPL, creating a mass balance discrepancy for the aerobic columns. Though degradation rates could not be calculated from this experiment, the experiment did visually document the changing redox conditions in the columns, such as formation of a black precipitant (likely iron sulfides) around LNAPL. Ideally, the limitations of this experimental design can be addressed for future research to eventually resolve degradation rates for OBBs. The success of a 3.8 m by 9.3 m demonstration OBB at a field site on a tidal freshwater river resulted in replacing the demonstration OBB with a 3.8 m by 58 m full-scale OBB. The construction event provided a unique opportunity to sample the demonstration OBB after a four-year deployment. The sampling results advanced the mechanistic understanding of how OBBs work to reduce LNAPL releases at GSIs. Sampling revealed the material was suitable for field LNAPL loading rates and was not compromised by field conditions such as ice scour or sediment intrusion. LNAPL analysis showed no LNAPL on the geocomposite or in the underlying upper sediment (0-10 cm). Diesel range organic (DRO) concentrations in the low 1,000s of mg/kg were observed in the sediment 10-20 cm below the geocomposite. LNAPL composition analysis suggests that the majority of the compounds are polar in the lower sediments (10-20 cm), providing a line of evidence that petroleum liquids have been oxygenated. Microbial data show the average number of bacterial 16s transcripts in the geocomposite is larger than in the sediment layers, confirming that the geocomposite is suitable substrate for microbe growth. The observation of ferric iron suggests that ferric/ferrous iron cycling may play a role in degradation processes, where the ferric iron acts as a "bank" of solid-phase electron acceptors. This sampling event suggests that LNAPL biodegradation rates in and below the OBB are comparable to the LNAPL loading rates.
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Subject
LNAPL
petroleum
sheens
OBB
hydrocarbons
remediation