Abstracts Chemistry

Water tracing with soluble metal chelates and neutron activation analysis : a laboratory and field study

by Peter James Hanson

Research on the application of soluble, metal chelates for water tracing using thermal neutron activation analysis for tracer determination has led to the development of several tracers with promising solution stabilities (conservative behavior) and determination sensitivities. The solution stabilities of the ethylenediamine tetraacetic acid (EDTA) and the diethylenetriamine pentaacetic acid (DTPA) chelates of the activable elements Cr, Co, Eu, In, Sb, Sc and Tb were evaluated in a series of laboratory, radiotracer experiments conducted under simulated field conditions. They showed that 1) chelation of metals produces highly conservative solution behavior, 2) DTPA is generally better than EDTA for improving tracer behavior, 3) non-chelated metals are rapidly sorbed onto sediments and 4) chelation reduces sorption of metals onto sediments. Field tests of EuDTPA, TbDTPA, InDTPA and Rhodamine B fluorescent dye in a small eastern Washington stream demonstrated that the metal chelates have significantly improved solution stabilities in natural water compared to widely used Rhodamine B. Minimum detection levels in natural waters for the activable tracers were determined to be 0.075 ppb for Eu, 0.10 ppb for Tb and 0.55 ppb for In in comparison to 0.49 ppb for Rhodamine B. Associated determination uncertainties (one standard deviation) amount to ±12% for Eu at 12 ppb, ±12% for Tb at 13 ppb, and ±13% for In at 10 ppb compared to ±7.3% for Rhodamine B at 1-10 ppb. Activation procedures are feasible which decrease the minimum detection levels to 0.0015 ppb for Eu, 0.0019 ppb for Tb and 0.0057 ppb for In in comparison to 0.075 ppb for Rhodamine B. Anion-exchange proved uniquely suited for recovery and concentration of metal-chelate tracers from natural water and as a low background irradiation and counting matrix. A method evaluation and economic analysis clarify the potential application of activable metal chelates for water tracing.