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Abstract

Surface geophysical measurements were successful in monitoring a groundwater tracer<br>experiment within the municipality of Gray, Maine. Low levels of volatile organic compounds<br>have been detected in the town’s water-supply well field, which is located 900 meters away from<br>the municipal landfii. In order to discover the relationship between the well field and la&ii, a<br>tracer experiment was conducted at an accessible location between them. The tracer, consisting of<br>minor amounts of salt mixed with water recovered from each injection point, was injected at two<br>locations 10 meters apart within a sand pit; one injection was in a monitoring well screened from<br>3.4 to 6.4 m below ground surface, and the other was a surficial injection, within a pit dug into the<br>saturated zone of the aquifer approximately 1 m below ground surface.<br>Geophysical methods tested included spontaneous polarization (SP), resistivity, induced<br>polarization (IF’), frequency- and time-domain electromagnetics (EM), and ground-penetrating<br>radar (GPR). Resistivity methods included the dipole-dipole array, radial array, and mise-a-lamasse.<br>The radial array was a modified pole-dipole array where one transmitter electrode was<br>down the injection well at the screen and the other was some distance away. Potential measuring<br>points were located at n-spacings of 1,2,3, and 4, using an a-spacing of 5 m. IP data were also<br>acquired along with resistivity data. GPR data were collected using both 200 MHz and 100 MHz<br>antennas.<br>Initially, the most effective and efficient geophysical method for location of the tracer was<br>the SP method. Radial array resistivity, GPR, and n-&e-a-la-masse methods also proved effective<br>in detecting the tracer. Frequency-domain EM (terrain conductivity) measurements proved<br>excessively noisy. This paper describes SP, radial array resistivity, and GPR methods, since they<br>appear at this stage of our analysis to be the most informative.

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/content/papers/10.3997/2214-4609-pdb.204.1997_051
1997-03-23
2024-03-29
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