Geophysical Monitoring Of A Groundwater Tracer In The Gray Delta, Maine

Abstract

Surface geophysical measurements were successful in monitoring a groundwater tracer experiment within the municipality of Gray, Maine. Low levels of volatile organic compounds have been detected in the town`s water-supply well field, which is located 900 meters away from the municipal landfill. In order to discover the relationship between the well field and landfill, a tracer experiment was conducted at an accessible location between them. The tracer, consisting of minor amounts of salt mixed with water recovered from each injection point, was injected at two locations 10 meters apart within a sand pit; one injection was in a monitoring well screened from 3.4 to 6.4 m below ground surface, and the other was a surficial injection, within a pit dug into the saturated zone of the aquifer approximately I m below ground surface. Geophysical methods tested included spontaneous polarization (SP), resistivity, induced polarization (IP), frequency- and time-domain electromagnetics (EM), and ground-penetrating radar (GPR). Resistivity methods included the dipole-dipole array, radial array, and mise-a-la-masse. The radial array was a modified pole-dipole array where one transmitter electrode was down the injection well at the screen, and the other was some distance away. Potential measuring points were located at n-spacings of 1, 2,more » 3, and 4, using an a-spacing of 5 m. IP data were also acquired along with resistivity data. GPR data were collected using both 200 MHz and I 00 MHz antennas. Initially, the most effective and efficient geophysical method for location of the tracer was the SP method. Radial array resistivity, GPR, and mise-a-la-masse methods also proved effective in detecting the tracer. Frequency-domain EM (terrain conductivity) measurements proved excessively noisy. This paper describes SP, radial array resistivity, and GPR methods, since they appear at this stage of our analysis to be the most informative.« less