Formatted Title
In Situ Bioremediation of Perchlorate in a Highly Heterogeneous Geologic Setting
Background/Objectives
A large-scale pilot study was designed and implemented to evaluate the effectiveness of in situ bioremediation (ISB) through injection of emulsified vegetable oil (EVO) to reduce the flux of perchlorate mass in groundwater within a highly heterogenous geologic setting. The site is located immediately upgradient of a surface water body at a primary discharge point for a major hydrographic basin. Prior to pilot study design, an investigation to characterize the local hydrogeology and extent of contamination revealed that elevated perchlorate concentrations were much deeper than originally anticipated and that the site geology was significantly more complex than expected. Perchlorate concentrations as high as 16,000 micrograms per liter were detected in groundwater across a large, saturated thickness from the water table at approximately 25 feet below ground surface (bgs) to depths of up to 235 feet bgs. The geology encountered within the study area across this depth interval was highly heterogeneous both vertically and horizontally and included alluvial deposits ranging from coarse-grained sand with gravel and cobbles to fine-grained sand interbedded with silt and clay overlying a significantly lower permeability, variably consolidated and faulted formation. Due to site complexity, injection well transects were designed and installed within three separate areas of the site, with each area targeting a specific geology and having a unique set of objectives. As a result, this pilot study was able to demonstrate and evaluate the implementability of ISB in areas with significant geological heterogeneity, large and deep alluvial paleochannels, secondary porosity associated with a complex fault zone, and large saturated thickness of contaminated alluvium and underlying low permeability formation.
Approach/Activities
As part of the pilot study, three EVO injection events were conducted at 64 injection wells within the three separate remediation zones. Both dual and triple-nested injection wells were used to evaluate delivery of EVO to large, saturated thickness where tight injection well spacing was required as a cost-effective option compared to multiple separate injection wells. Various aquifer tests were performed during the pilot study (slug testing, single-point borehole dilution testing, nuclear magnetic resonance logging) to delineate localized preferential flow pathways within different zones, determine pre-injection hydraulic conditions, and characterize groundwater flow rates downgradient of the injection well transects. Additionally, a tracer dye study was conducted as part of the injection design to estimate the effective porosity of the targeted formations, assess the radius of influence of the injection wells, estimate travel time of the injectate/dye in the various lithologic units targeted, and evaluate discharge from the deeper underlying zones to the overlying shallow alluvium zone and vice versa. An effectiveness monitoring network of more than 160 monitoring wells and six surface water sampling locations was used to evaluate ISB performance on a biweekly and/or monthly basis throughout the 18-month pilot study.
Results/Lessons Learned
Eighteen months of monitoring data indicated significant reductions in perchlorate concentrations to below the remedial goal of 15 µg/L. Although significant reductions in perchlorate and chlorate were observed in all targeted areas, better results were observed in intervals with lower groundwater velocities, which allowed for more residence time for complete perchlorate biodegradation. EVO distribution in the subsurface varied significantly between remediation zones due to the highly heterogeneous geology of the site. The presentation will discuss conceptualization and implementation of the project, field challenges, results of the tracer dye study (including animations), and effectiveness monitoring results.