Lead Isotope Analysis Using Thermal Ionization Mass Spectrometry (TIMS) at Isotopic Geology Laboratory (UFRGS) and Its Application in an Environmental Study in Candiota-RS

The Isotopic Geology Laboratory at UFRGS – LGI, traditionally conducts Pb isotope analyses in rocks to investigate petrological evolution. Recently, it has expanded its analyses to include environmental matrices, aiming to identify anthropogenic and/or geogenic sources of Pb in areas affected or potentially affected by lead contamination and the results of one study case in Candiota (RS) area will be briefly reported. Pb has four stable isotopes - ²⁰⁴Pb (1%), ²⁰⁶Pb (24%), ²⁰⁷Pb (23%), and ²⁰⁸Pb (52%). The isotopes ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb are considered primordial and radiogenic, formed from the radioactive decay series of ²³⁸U, ²³⁵U, and ²³²Th, respectively. Isotope ²⁰⁴Pb is exclusively primordial, maintaining a constant concentration in nature. Due to minimal mass differences between isotopes, natural processes have insignificant effects on isotopic composition across different matrices. Consequently, the isotopic signature, for example, of groundwater and surface waters reflects their source or a mixture of sources, enabling differentiation in many cases. The isotopic composition of Pb is expressed in isotopic ratios, with the ²⁰⁶Pb/²⁰⁷Pb ratio being widely used in environmental studies as an indicator of anthropogenic Pb. However, the distinction between different anthropogenic sources may not be achieved solely with the ²⁰⁶Pb/²⁰⁷Pb ratio. Thus, other isotopic ratios normalized to ²⁰⁴Pb, such as ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, are frequently used due to their greater variability among reservoirs. At LGI, Pb isotopic analyses are performed using a Thermal Ionization Mass Spectrometer TRITON - Thermo Scientific. Samples undergo acid digestion procedures using HNO₃, HF, and HCl acids, which are pre-purified by sub-boiling distillation. Pb separation then occurs on a chromatographic column packed with pre-cleaned Anion Exchange Resin 200 - 400 mesh (Eichrom - Analytical Grade - Chloride Form). Subsequently, the samples are mixed with a combination of 3 µL of 0.25 M H₃PO₄ and 3 µL of 0.4% silicagel water before being deposited on a single Re filament. Each batch includes at least one analysis of the NBS981 reference material. The samples are ionized at a temperature between 1250 and 1320°C. The resulting ions are focused, accelerated to 10 kV towards the magnetic sector, where mass separation occurs based on the mass-to-charge ratio, and then directed to the collectors for detection. Eighty measurements of the isotopic ratios are made for each sample. The results are corrected based on the average deviation of the isotopic ratios of the NBS981 reference material historically obtained for this equipment in the lab. The percentage deviations, per atomic mass unit, are currently as follows: ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁶Pb, ²⁰⁶Pb/²⁰⁷Pb, and ²⁰⁷Pb/²⁰⁶Pb are 0.082%, 0.096%, 0.059%, 0.008%, and 0.008%, respectively. In a recent environmental study in the Candiota-RS region (Martins et al., 2024), geological samples such as sandstones, coal, marbles, and anthropogenic samples such as coal ash, pesticides, were analyzed at LGI to assess the sources of Pb contamination in groundwater and surface water samples around a thermoelectric plant. The data revealed that surface waters are influenced by coal compared to groundwaters, which exhibit signatures matching sandstones as well as those from surface waters in some locations, regarding geogenic sources of Pb. The study identified anthropogenic sources of Pb related to marble mining, coal ash, and pesticides, along with their potential impact on the isotopic composition of analyzed waters and sediments in the region. Finally, the isotopic signature ²⁰⁶Pb/²⁰⁷Pb of groundwaters from two monitoring wells also indicates the influence of fuels and domestic sewage on their composition. This demonstrates, for the first time, the complex distribution of Pb in the area originating from both geogenic and anthropogenic sources.