BIOAUGMENTATION FOR MTBE REMEDIATION - Bioaugmentation for Groundwater Remediation - page 291

Facilities Engineering Service Center (NAVFAC ESC), Port Hueneme, California. Six pilot-test
plots were installed at the Naval Base Ventura County (NBVC) at Port Hueneme, California.
These plots examined the performance of 20-ft (6-m) wide biobarriers employing various
combinations of oxygen and air injection and the mixed- and single-cultures MC-100 and
SC-100. All were placed far downgradient of the source zone where groundwater contained
only MTBE and TBA.
Oxygen-rich biologically reactive treatment zones (the “biobarriers”) were established
in situ and downgradient of the source of dissolved MTBE contamination. The system was
designed so that groundwater containing dissolved MTBE flowed to, and through, the biobar-
rier treatment zone. The use of natural flow conditions is generally preferred, but one can
imagine groundwater pumping schemes directing impacted groundwater to a treatment zone.
As the groundwater passed through the biobarrier, microorganisms converted the MTBE
(presumably to CO 2 ). Ideally, groundwater leaving the downgradient edge of the treatment
zone contains MTBE at concentrations less than or equal to the target treatment levels.
A system designed to treat MTBE also will very likely reduce concentrations of other aerobi-
cally biodegradable chemicals dissolved in the groundwater (e.g., BTEX and TBA). It should be
noted that: (1) other contaminants will likely represent competitive oxygen sinks, (2) some
organisms like strain PM1 are known to be inhibited by certain BTEX compounds, and (3) the
presence of other organic compounds allows the possibility of cometabolic degradation.
Oxygenation of the aquifer was accomplished through periodic oxygen (or air) injection via
a line of gas injection wells spanning the width of the biobarrier. In this approach, gas injection
is at high pressure, but low volume (e.g., averaged gas flows of > 10 cubic feet per minute
[cfm]) for durations of about a minute and periodic (e.g., daily) to achieve sufficient gas
distribution while not altering the natural groundwater flow through the treatment zone. While
there are a number of ways to deliver oxygen to groundwater (e.g., in-well oxygenation systems
and oxygen releasing compounds), this section focuses on gas injection because that was the
approach successfully demonstrated at full-scale (Miller et al., 2003 ).
Other than groundwater monitoring wells, the only process components required for this
technology are associated with the oxygen delivery system. These might typically include an
oxygen generator (or air compressor), oxygen or air storage tanks, gas injection wells, and a
series of timers and solenoids to control and direct the oxygen to the gas injection wells. In
some cases, oxygen addition will stimulate the growth of indigenous MTBE-degrading organ-
isms, and the growth rate and activity of these organisms will be sufficient to effect the desired
reduction in concentration. At other sites, the microbial community may not contain the
necessary organisms, or the growth rate and activity may be too low to achieve the desired
concentration reduction within time frames acceptable to local regulating agencies. In those
cases, it may be necessary to bioaugment the aquifer with MTBE-degrading cultures.
Figure 10.3 shows a plot plan of the original test cells: a control cell, a biostimulation cell,
and a bioaugmentation cell. The control cell had 14 paired shallow and deep monitoring wells.
The biostimulation cell had 20 paired shallow and deep monitoring wells, and several lines of
oxygen injection wells installed transverse to the understood groundwater flow direction. This
oxygen delivery system was intended to provide a flow-through cell with at least a week of
elevated oxygen contact time. The bioaugmentation cell had 17 paired shallow and deep
monitoring wells. This test cell was set as a similarly robust oxygenated flow through cell,
with the addition of high concentrations of MC-100 injected on 1-ft centers over 21 ft (6.4 m).
There was only a single biomass injection event. Injections took place using a direct-push rig, a
grout pump, and several hundred gallons of MC-100 diluted to a total suspended solids (TSS)
of about 2.5 g/L. This concentration was found to be the best balance between distribution and
loading. A point was pushed to the bottom of the contaminated aquifer (20 ft [6 m] below
ground surface [bgs]) and 5 gallons (20 L) was injected at between 50 and 60 pounds per square