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

The uppermost two feet of the soil column at the site is primarily fill materials, including some
broken asphalt and concrete. Below this is a thick layer of coarse-to-medium sand that is fairly
homogeneous. The water table is approximately 8 feet below ground in this sand layer. Annual
fluctuations in the water table are on the order of six inches above and below the average depth.
Through this fluctuation and other factors, a smear zone of free product has developed above and
below the water table a distance of two feet. Groundwater flow direction is consistently to the south.
Impacted Receptors
None identified at present. However, the property is slated for redevelopment, raising the possibility
of exposures in the future.
Remedial Objectives
The objective of this pilot test was to clean up the remaining plume of groundwater contaminated
with light nonaqueous phase liquid (LNAPL). Contaminants encountered include benzene, toluene,
ethylbenzene, and xylene (BTEX) and naphthalene. The plume is about 100 by 150 feet in size.
Selection of Technology and Design Parameters
T his sit e was selected for implementation of in situ chemical oxidation in part because of its
relatively small, well-defined plume, and the suitability of the site hydrogeology to injection of
oxidants (permeable sands with relatively high hydraulic conductivity). In addition, the presence of
monitoring wells on-site for evaluation of the process, and the extensive data on historical
cont aminant concentrations, allowed detailed evaluation of the effectiveness of the oxidation
process. Finally, the site is vacant, so gaining access to the necessary injection points was not an
issue.
The technology implemented at this site was BioManagement Services’ BiOx process, which uses
a modified Fenton’s reagent chemistry. A solution of 5% hydrogen peroxide, an iron catalyst, and
other proprietary ingredients is mixed on-site and injected into the ground through a direct-push
probe at the desired depth or depth range. The mixture of chemicals can be tailored for specific site
conditions.
Implementation of Technology
Two days prior to implementation of the in situ treatment, NMED performed a complete sampling
of all monitoring wells on the site for volatile organic compounds (VOCs), total cations, nitrate,
phosphate, and total dissolved solids. Dissolved oxygen was not measured, as the necessary field
equipment was not available.
As this was a technology demonstration for state regulators, the contractor selected all parameters
for the test to achieve complete remediation of the plume, based on his experience at previous sites.
Wit hin a 25-foot circle around MW-8, the monitoring well with the highest concentrations of
contaminants (assumed to be the center of the plume), 39 injections of the oxidant solution were
p laced in a square grid array. At each injection point, the injector probe was pushed to a depth of four
feet while injecting water through the probe to aid in penetration of the soil. At a depth of four feet,
the oxidant solution began to be injected, and the probe was advanced while injecting to a depth of
twelve feet. This provided oxidant to the soil zone four feet above and below the water table.
The spacing of the grid of injection points, and the amount of oxidant injected per hole, was adjusted
in the field based on observation of other injection points. Bubbling or “geysering” of fluid from
p revious, adjacent holes was used as an indication that overlap of the radii of influence of the
injections had been achieved. The final grid of injections was spaced 3 feet apart. A total of
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