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 Verification
procedures of the in situ constructed geometry of the permeable
reactive barrier (PRB) are important for a high confidence in the
quality control and quality assurance of the constructed PRB. Difficulties
in direct sampling of the iron PRBs have been experienced at both
shallow and moderate depth due to the inability to contain the iron
filings with the sampler tool. At significant depth and/or in flowing
ground conditions, direct inclined samplings of undisturbed iron
PRBs have been found to be extremely difficult. Under these conditions
the ground in front of the sampler flows into the outer protective
casing when the sampler is withdrawn and thus disturbs the ground
for the next sampling interval. The next sampling attempt then samples
disturbed ground consisting of a mixture of native soils and iron
filings.
Such direct PRB sampling difficulties have led to the utilization
of an inclined direct push soil electrical conductivity/resistivity
probe to determine indirectly the iron PRB geometry. The electrical
resistivity contrast between native soils and the iron PRB are generally
sufficient to clearly identify the iron PRB geometry. The electrical
conductivity probe is approximately 6" in length and 1.5"
in diameter. The probe contains four electrodes spaced approximately
1" apart. The probe electrodes are excited by a low voltage
current and soil conductivity can be quantified by the instrument.
When the probe penetrates an iron PRB wall, a significant change
in conductivity is recorded, thus providing a measurement of its
thickness.
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