Friday, 14 July 2006

The Use of Non-Invasive Time-Domain Induced Polarization for Diagnosis of Soil Metal Contamination.

Rebekah T. Brosky-Dorsey, Jim Cull, and Antonio Patti. Monash University, School of Geosciences and Centre for Green Chemistry, Clayton VIC, Australia

Conductivity and chargeability data, obtained in the field by Time-Domain Induced Polarization (TDIP), were employed as a non-invasive means of determining elevated soil zinc concentrations. A comparison of chargeability data with zinc concentration data, obtained by total soil digestion in the laboratory, showed soils with high chargeability were soils which also had elevated zinc concentrations (300 to 4,750 mg/kg soil). Three pairs of electrical transmission towers, situated in suburban Melbourne Australia, were chosen as field sites. The towers are excellent point sources of contamination, as the zinc protective layer leaches slowly over time and is deposited into the local surface soil (Jones 1984). Highest total soil zinc concentrations were found in soils within one metre of the tower legs, and decreased significantly with distance from the tower legs. Similarly there was a decrease in chargeability and conductivity with distance. Within the pH range of the soils investigated, 5.0-7.1, the major form of zinc present is Zn2+, as a free ion or ionic molecule (Lindsay 1972). Soil resistivity is primarily determined by soil porosity and permeability as well as electrolyte conductivity. Therefore, metals present in ionic form, as well as salts, will increase specific conductivity within the soil (Mirsal 2004). Chargeability is increased by disseminated metals (electrode polarisation) and clay content (membrane polarisation). Soil conductivity and chargeability, therefore, are expected to increase significantly with increasing soil metals. In the laboratory, experiments were performed to determine the threshold soil metal concentration necessary for IP signal change. Soil clay content and salt content were measured and found to be constant among the replicate samples. Soil moisture was also kept constant by weight (watered to field capacity). The single variable was soil metal concentration. IP laboratory results confirmed initial field results, in that trends in conductivity and chargeability increases mirrored absolute soil metal concentration increases. These results were further confirmed by forward modelling using RES2DINV. Jones, R. and M.S.E. Burgess, 1984, Environ. Sci. Technol., V. 18, No. 10, p. 731-734 Lindsay, W.L., 1972, Advan. Agron., V. 24, p. 147-186 Mirsal, I.A., Soil Pollution Origin, Monitoring and Remediation (Berlin: Springer, 2004).

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