Formatted Title
Nitrogen Biosparging: An Effective Catalyst for In Situ Bioremediation of Commingled VOCs
Background/Objectives
In situ bioremediation of commingled petroleum and chlorinated volatile organic compounds (VOCs) at soil saturation level concentrations is fraught with challenges. Even when microbial activity can survive and flourish in these punitive source areas, partial dechlorination can frequently occur, and degradation rates are mostly limited by mass transfer and desorption from the absorbed to the aqueous phases. Nitrogen biosparging at field scale has been implemented as a novel technique to foster conditions conducive to degradation of source level commingled benzene, toluene, ethylbenzene, and xylenes (BTEX) and chlorinated ethenes (particularly tetrachloroethene [PCE] and trichloroethene [TCE]) at a chemical manufacturing facility in Mason, Michigan. Nitrogen biosparging avoids the risks, costs, and intrusiveness of traditional direct push technology or groundwater push-pull remedial injection approaches. Grossly contaminated soil and groundwater at the site coincide with a local topographic low point serving as regional recharge adjacent to the Mason Esker, the largest esker in Michigan. Groundwater flow within the esker is bifurcated (creating two plumes) and recorded at bulk velocity as high as 13 feet per day. This complex hydrogeologic setting coupled with anthropogenic factors such as the aboveground intensity of an active chemical manufacturing facility in a densely populated, mixed-use neighborhood have made typical brute force methods of source area remediation infeasible. The presence of non-aqueous phase liquid (NAPL) containing both BTEX and PCE and TCE coupled with this dynamic hydrogeologic setting has caused a bifurcated groundwater plume of 40 acres that underflows at least 15 off-site properties and threatens public health via the vapor intrusion, drinking water, and groundwater/surface water interface exposure pathways.
Approach/Activities
Nitrogen biosparging activities were implemented to facilitate the desorption of contaminants from saturated soils (i.e., increased mass transfer), promote the destruction of the transmissive portions of NAPL bodies, and stimulate biodegradation of contaminants in the saturated and vadose zones by promoting a suitable redox and geochemical environment for bioremediation, with the overall remedial cleanup objective of reducing aqueous concentrations by two orders of magnitude (OOM). Nitrogen biosparging also proved to be an effective delivery mechanism for dispersion and distribution of remedial injectants (e.g., electron donor, nutrients) via sparge-induced pathways throughout the intended treatment zone(s). In one punitive source area, nitrogen sparging was used to distribute and support the degradative mechanisms of both reductive dechlorination and sulfidated micro zero valent iron. Robust field operational monitoring via a variety of real time techniques led to pulsed and sequential operation fine-tuned to each treatment area’s response to biosparging perturbations. A Remediation Vapor Collection System (RVCS) was installed to capture remediation-generated soil vapors (particularly methane), increase the efficiency of groundwater manipulation, and influence gradients/vectors within the treatment areas.
Results/Lessons Learned
Regular monitoring events incorporating submersible datalogging transducers and soil/water sensors, real-time gas chromatography/mass spectroscopy (GC/MS) via HAPSITE ER monitoring of RVCS exhaust stacks, sub-slab soil vapor, saturated soils and groundwater sampling of proximal monitoring wells identified biosparge-induced desorption and mobilization of contaminants in the vadose zone into the aqueous phase, mass transfer of VOCs into the vapor phase, destruction of NAPL bodies, and increased bioavailability of sorbed source contaminants for ongoing bioremediation. Post sparging groundwater samples displayed short-term, elevated dissolved phase concentrations, confirming accelerated mass transfer to allow for increased destructive mechanisms to occur in the aquifer. This short-term phenomena was followed by significant reductions (2-3 OOM) in both aqueous and saturated soil concentrations of BTEX, PCE and TCE impacted saturated soils in response to biosparging activities. A three-dimensional model will depict the time series changes of saturated soil mass throughout the nitrogen biosparging regime.