Investigation of soluble indium chelates for groundwater and hydrothermal fluid tracing

Indium is an excellent tracer with a unique combination of good activation analysis sensitivity retaining most of the advantages of radioactive tracers without creating health or environemental risk, and low background concentration in surface and subsurface fluids. The organic chelating agents ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) were selected to enhance the solubility of In as a tracer within the pH range of groundwater and hydrothermal fluids. The thermal stability of indium chelates of EDTA and NTA was investigated at temperatures from 22 to 240°C. Degradation of In(III)-EDTA, observed at and above 200°C, is attributed to metal dissociation followed by thermal decomposition of the organic ligand. Degradation of In(III)-NTA, observed at temperatures greater than 150°C, is probably due to significant decrease of the thermodynamic formation constant. The results suggest that soluble indium chelates can be used effectively as tracers for surface and groundwater flow and transport studies. The applicability of In(III)-EDTA as a conservative tracer can be extended to low-temperature geothermal reservoirs where ionic chemical tracers are not fully conservative.