Tests Of Ground Penetrating Radar And Induced Polarization For Mapping Flwial Mine Tailings On The Floor Of Coeur D’Alene River, Idaho

Abstract

SUMMARY In order to investigate sequences of mine tailings that have settled in the bed of the Coeur d’Alene River, we improvised ways to make geophysical measurements on the river floor. To make ground penetrating radar (GPR) profiles, we mounted borehole antennas on a skid that was towed along the river bottom. To make induced polarization (IP) profiles, we devised a bottom streamer from a garden hose, Pb strips, and insulated wire. Both expedients worked well. GPR showed shallow stratigraphy, but did not directly detect the presence of contaminating metals. IP showed a zone of high chargeability that is probably due to pockets of relatively higher metal content. Neither method was able to define the base of the fluvial tailings section. BACKGROUND Starting in 1884, Pb-Zn-Ag was produced from the historic Coeur d’Alene mining district, ID (Hobbes and Fryklund, 1968). Tailings from the old mining operations have often flushed down the Coeur d’Alene River, especially during times of heavy winter floods, depositing in the river channel, flood plain, and on the bottom of Lake Coeur d’Alene as far as 50 miles away. These fluvial deposits are reworked by each big flood, and some contain enough’heavy metals, Pb in particular, to be dangerous to organisms that ingest the sediment. Box and others (1994) discuss some of the resulting environmental problems and prospects for their remediation. Drilling and assaying are reliable for investigating the fluvial tailings from spot to spot, but are slow, difficult, and expensive. A good, fast, and cheap method is needed to map their location, thickness, and compositions. This paper reports on tests we made to do that using ground penetrating radar (GPR) and Induced Polarization (IP). LABORATORY MEASUREMENTS Before going to the field, we measured samples from cores of river-bottom sediments at Colorado School of Mines Petrophysical Laboratory (Roth, 1996). The samples included pre-mining river bottom materials, as well as two kinds of sediments containing fluvial mine tailings: deeper tailings from the early mining days that often contain unrecovered metal ore; and shallower tailings from flotation mills introduced in the 192Os, which typically contain relatively lower metal concentrations. Seven of the 11 samples (Fig. 1) were from a sequence including all three of the above-described types, taken from a single corehole, 95PCK-1 (location shown on Fig. 4). The remaining 4 samples were from other coreholes in the Coeur d’Alene river bed. Samples marked (8~) on Fig. 1 are pre-mining sediments that contain negligible metals. The two samples marked (!) contain especially high metals; the one from 297-326 cm depth contained over 5% Pb and Zn, and that from 262-297 cm depth had about 3% Pb and Zn. The remaining, still-shallower, fluvial tailings from 95PCK-1 contained about 1% total Pb and Zn. Fig. 1 shows measured values of relative dielectric permittivity (RDP) and of resistivity for the 11 samples. The figure shows that RDP tends to increase with depth over the measured interval of 95PCK- 1, and that resistivity is generally low there. At other places, where the pre-mining sediments are sandier and less silty or clayey, resistivities are much higher. The generally-low resistivities at the 95PCK-1 site probably arise from high amounts of dissolved metals in the pore water, and were bad news to us, for they meant that GPR penetration depths there would be limited. There is a vague tendency in this sample set for RDP to go up as particle size goes down. This indicated to us that GPR might be