Technical and Regulatory Guidance for In Situ Chemical ... - ITRC

ap p roximately 1000 gallons of oxidant solution was injected for an average of 25.5 gallons per
injection point. The complete injection program took place in one day.
Operational Performance
Forty-eight days after injection, all of the monitoring wells at the site were purged and resampled,
and the same suite of analytes as performed prior to the injection was run by the same analytical
laboratory. The results for MW-8 indicated a 35% decrease in benzene concentrations, from 2300
to 1500 ppb. However, concentrations of toluene, ethylbenzene, xylenes, and naphthalene increased
by amounts ranging from 69% to 560%. A similar pattern was found at the next most contaminated
well, with benzene and naphthalene concentrations decreasing somewhat and the remaining
contaminants’ concentrations increasing. At more distant, less contaminated wells, effects were
mixed, but the overall changes in concentration were relatively small.
Problems
As indicated by the comparison of analytical results from before and after treatment, injection of the
oxidant solution did not significantly reduce the overall contaminant concentrations in the
groundwater. NMED believes that, due to the injection of oxidant above the water table, some degree
of soil washing or flushing occurred which removed adsorbed contaminants or free product from the
vadose zone and delivered it to the groundwater. The additional mass of contaminants delivered to
t he groundwater through this process may have counteracted any reductions in groundwater
concentrations due to oxidation. In addition, the rapid introduction of the oxidant solution into the
sandy aquifer may have formed preferential pathways that allowed the solution to bypass some
volumes of the soil. Finally, the rapid reaction of the oxidant solution with its catalyst, the
contaminants, and naturally occurring organic compounds in the soil may have prevented “active”
oxidants from reaching all contaminated areas of the aquifer.
Although NMED purged three to five well volumes from each monitoring well prior to sampling,
the contractor is concerned that the samples taken from the wells were not truly representative of the
overall groundwater concentrations.
Costs
As this pilot project was performed by the contractor in part to gain state acceptance for the process,
the true costs were not calculated. However, the reagents are fairly inexpensive, and labor costs are
low (one person completed the injection program in one day). Monitoring well installation and
analytical costs for follow-up sampling are expected to account for the majority of project costs.
Follow-up Actions
NMED is currently in negotiations with the contractor for additional treatment of the area using the
BiOx p rocess. If the effects of soil washing or flushing by the oxidant solution are largely
responsible for the observed contaminant concentrations in the groundwater, as is suspected, then
subsequent treatments with this technology should produce more dramatic reductions in groundwater
concentrations as the mass of contaminants in the vadose zone is reduced. Future groundwater
sampling methodology may be modified to deal with concerns about the representativeness of the
samples taken.
Point of Contact
Patrick DeGruyter
New Mexico Environmental Department
(505) 841-9188
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