First strontium isotope map of groundwater in South Korea: applications for identifying the geographical origin

Abstract

In recent, strontium isotope ratios (87Sr/86Sr) have been commonly used as a geochemical tracer to investigate the geographic origin in various research fields, just comparing the data between samples. However, in forensic contexts it is much more important to predict and identify the origin for unknown samples rather than to compare the samples. Here, we collected about 500 groundwater samples from the National Groundwater Monitoring Wells in South Korea and measured their 87Sr/86Sr ratios representative of bioavailable 87Sr/86Sr ratios. The 87Sr/86Sr ratios of the samples ranged from 0.70540 to 0.86108, with a mean of 0.71890 ± 0.01509 (n = 504), reflecting the bedrock lithologies. Furthermore, the best interpolation methods were investigated using the measured 87Sr/86Sr ratios. The result indicates that the radical based function including spline with tension (RBF-ST) is the best interpolation method in terms of the prediction errors. Resulting strontium isotope map interpolated with RBF-ST mimics lithological distribution in South Korea. In order to evaluate the accuracy of the interpolation map, two animal bone samples were collected from two different provinces and measured their 87Sr/86Sr ratios. Although the estimated and measured 87Sr/86Sr ratios are not completely constant, the estimated 87Sr/86Sr ratios are within the range of those of groundwater samples near the site where the bone samples were collected, indicating that the interpolation map generated in this study can be used as a back-tracking technique to identify the geographical origin of unknown samples.