Groundwater, Engineering, Environmental & Mining
Groundwater Plume (Time Domain Electromagnetics)
A groundwater plume containing anomalously high total dissolved solids is present at a depth of about 20 feet and generally extends in a southwestern trend from an uranium mill tailings cell in the southwestern U.S. The time domain electromagnetic (TDEM) geophysical method provides a measurement of the subsurface conductivity (or its inverse, resistivity). The measured resistivity is converted to a true resistivity and is a function of both the minerals and pore water present in subsurface formations. The formation resistivity of a saturated layer with relatively large amounts of dissolved solids will be fairly low (and, thus, conductivity will be high) and identification of such anomalous zones is possible with the TDEM method.
Raw field data acquired with a Zonge GDP-16 "Nanotem" TDEM system were entered as sounding curves (TDEM magnitude versus time) and forward and inverse models of the curves were generated. The conductance (conductivity-thickness product) of the first conductive layer was calculated and is shown in Figure 1.
Figure 1: TDEM conductance of first conductive layer, uranium mill tailings site, southwestern U.S.
Working in concert with the project geochemist and referencing the dissolved solids values within selected boreholes, an arbitrary value of 1.0 Siemens was considered appropriate as a lower boundary for dissolved solids contamination. As seen in Figure 1, this level is indicated by the darker shaded (purple to red) areas and extends predominantly from the southwestern corner of the mill tailing cell but is also present along the entire length of the southern boundary of the cell. After inspection of core and geophysical logs from boreholes in the vicinity of the cell, it was assumed that the anomalies mapped by the TDEM method were due to high concentrations of total dissolved solids versus geologic sources given the resistive and ubiquitousness nature of the Navajo Sandstone in the survey area.
The need for areal coverage by surface geophysical methods was confirmed by the identification of a contaminated zone in areas without boreholes. The size of the contaminated zone indicated by the TDEM data will influence the type, cost, and schedule for remediation. Before proceeding with a remediation plan, it was proposed that additional geophysical data be acquired to better define the contaminated zone and one or more strategically placed boreholes be drilled using the results of those surveys. In addition to acquiring additional TDEM data, refraction seismic data were also proposed to be acquired in specific areas to confirm or deny the presence of a limestone deposit. Limestone outcrops within the general area and will have a seismic velocity three to four times faster than indigenous sediments.
Hasbrouck, J.C., 1996. Reinterpretation of Time Domain Electromagnetic Survey at Tuba City, Arizona UMTRA site, Rust Geotech Inc., Grand Junction, Colorado.
Hasbrouck Geophysics, Inc.
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Prescott, Arizona 86305
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Last Modified: 10 November 2004 @ 15:50