Wednesday, April 2, 2008 : 8:20 a.m.

Forensic Modeling to Optimize Bioaugmentation to Treat Recalcitrant Low Level Contamination

Chad D. Drummond, P.E., Jason Christopherson, E.I., Robert Bretnall, P.G., Ravi Nalamothu, E.I. and Kenneth Watson, Ph.D., HSW Engineering

Forensic groundwater flow and solute transport modeling was performed to facilitate full-scale remediation at a site near Gainesville, Florida.  The project goal was to decrease low level groundwater concentrations of 1,1‑dichlorothene (DCE) and other VOCs in the lower surficial aquifer, thereby reducing their flux to the H1 intermediate aquifer (potential drinking water aquifer).  Three dimensional numerical models, GMS 6 (MODFLOW/RT3D), were used to optimize the injection scheme, calculate the contaminant dilution rate, and identify an inferred area of connectivity between the lower portion of the surficial aquifer and the H1 intermediate aquifer below.  The full-scale bioaugmentation design consisted of 30 injection locations located 40 to 55 feet below ground surface (bgs).  Treatment lines were placed perpendicular to groundwater flow, with an approximate 2-year travel time between the lines.  Injections were spaced on 20-foot centers, assuming a 10-foor radius of influence.  Injectates included an electron donor, emulsified edible oil (EEO), and a dechlorinating inoculum containing Dehalococcoides (Dhc).

The targeted lower portion of the surficial aquifer is comprised of damp to moist, soft medium to fine grained sand containing up to 10% clay, with clay content increasing with depth to 20% and the sediment becoming dry and very stiff to hard.  Groundwater within the lower portion of the surficial aquifer predominately contains 1,1-DCE at concentrations up to over 1,000 micrograms per liter (mg/L). 

In November 2006, 750 gallons of EEO and 1 liter of inoculum were injected in each interval.  Following injection, groundwater VOCs, geochemical parameters and Dhc were monitored to evaluate EEO/bioaugmentation performance.  Six-months post-injection, 1,1-DCE concentrations were reduced by 89% with transient production and degradation of vinyl chloride.  One year post-injection, 1,1-DCE concentrations were reduced by over 98%.  Modeling predictions indicate that bioaugmentation reduced the plume attenuation time in the H1 aquifer from 18 years to less than 2 years.

Chad D. Drummond, P.E., HSW Engineering Mr. Drummond is HSW’s vice president in charge of corporate engineering operations and is the Ground Water Manager of HSW’s corporate Integrated Hydraulics & Hydrology (H&H) Group. His technical responsibilities include managing and executing various environmental restoration, water resources, and civil engineering projects. These projects typically consist of technically challenging numerical modeling and environmental restoration. He is the former NGWA Chairperson of the Groundwater Modeling Interest Group Committee (2004 and 2005) and is currently serving a three-year appointment to the NGWA’s Technical Group Oversight Committee.


2008 Ground Water Summit