Topic Area: 3a. Bioremediation in Complex Geological Settings

Formatted Title:

Bioremediation of Deep PCE Plumes at the North Railroad Avenue Plume Superfund Site

Background/Objectives:

At the North Railroad Avenue Plume Superfund Site in the semiarid Rio Grande valley, in situ bioremediation of tetrachloroethene (PCE) in groundwater using emulsified vegetable oil (EVO) from 2007 to 2013 reduced concentrations by about three orders of magnitude to achieve remediation goals (RGs) in most of the shallowest impacted aquifer, where risks were highest. In four deeper aquifer units from 35 to 285 feet below ground, where PCE concentrations were up to nearly 200 times the RG, EVO injections to selected wells through 2012 created small reducing zones. Five years later, groundwater at deep injection wells met most RGs and had robust Dehalococcoides (DHC) populations, but monitoring wells 30 to 40 feet away still had PCE up to 86 times the RG and rapidly declining DHC. Evaluations found that monitored natural attenuation alone may not be adequate to limit PCE migration, motivating expanded characterization and remediation of the deep zone. Objectives included limiting contaminant migration, degrading contaminant mass, improving plume delineation, and better characterizing deep zone hydrogeology.

Approach/Activities:

Remediation in better-characterized zones with high PCE concentrations or zones where aquifer tests and fate and transport evaluations suggested higher migration risk proceeded in tandem with drilling new monitoring wells in poorly understood zones. Amendments including 3-D Microemulsion (3DME^®) and sulfidated micro-scale zero valent iron (S-MicroZVI^®) were blended with water and injected with and without addition of bacteria inoculum. Pilot testing determined each aquifer unit could sustain adequate injection flowrates under moderate pressures below the formation fracture pressure. Chase water injections using separate equipment were used to reduce well fouling and increase the radius of influence while minimizing time required for amendment application resources at each well. Rotosonic techniques were used to drill new wells with continuous core logging to allow interpretation of aquifer units and aquitards within the complex depositional sequence. Modeling core data with environmental sequence stratigraphy methods facilitated conceptual site model updates. For new installation, clusters of wells were preferred over nested or multiport wells, with wells designed and permitted to allow transition from use for monitoring to amendment injections if necessary. Deeper wells were typically drilled first, and push-ahead groundwater sampling devices were used to screen plume concentrations and boundaries in shallower aquifer units. Iterative implementation of remediation and further characterization as necessitated by annual budgets, though not ideally cost efficient, was used to address multiple objectives simultaneously and promptly leverage new information to design each new remediation event.

Arun Wahi

Affiliation

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(Group 2, Poster Board #64) Bioremediation of Deep PCE Plumes at the North Railroad Avenue Plume Superfund Site