Application of 129I/127I Ratios in Groundwater Studies Conducted at Los Alamos National Laboratory, New Mexico

Abstract:

Los Alamos National Laboratory (LANL) is an operating nuclear site that has released treated effluents from three plutonium-processing facilities since the mid 1940s. The radioisotope ¹²⁹I (T_1/2 = 15.7 Myrs) derived from²³⁵U and ²³⁹Pu processing at LANL is locally detected in groundwater above background concentrations. This isotope provides a unique tracer for groundwater investigations conducted at LANL that helps to identify source releases linked to groundwater-flow paths in aquifers subject to binary and ternary mixing of natural- and industrial-derived waters containing chromate and other chemicals. Bromide, chlorate, chloride, nitrate, perchlorate, sulfate, and tritium were associated with multiple outfalls at LANL and, therefore, do not provide unique chemical signatures identifying a specific point of release or source. Natural and anthropogenic ratios of ¹²⁹I/¹²⁷I measured in groundwater samples collected at LANL were quantified using accelerator mass spectrometry at Purdue Rare Isotope Measurement Laboratory, Purdue University. Anthropogenic ratios of ¹²⁹I/¹²⁷I range from 1,531 X 10^-15 to 10,323 X 10^-15 within perched-intermediate groundwater present in volcanoclastic and basalt aquifers (210 – 216 m depth). Anthropogenic ratios of ¹²⁹I/¹²⁷I range from 359 X 10^-15 to 4,350 X 10^-15 within the regional aquifer (280 m depth) consisting of volcanoclastic sediments of variable hydraulic properties. Local background ratios of ¹²⁹I/¹²⁷I have a narrow range of 171 X 10^-15 to 378 X 10^-15 in the regional aquifer. Dissolved iodide measured in groundwater at LANL is stable dominantly as iodate. Background concentrations of dissolved iodate (0.1 to 33.2 nM) are less variable compared to anthropogenic iodate (8.0 to 246 nM) in groundwater at the site. Variability in concentrations of anthropogenic iodate is controlled by heterogeneous source releases of iodate over time and non-uniform mixing of groundwater in the different aquifers.