Anomalous Sample Detection Tool and Its Application in Identifying Interbasin Groundwater Flow in the Yarlung Zangbo River Basin, Tibetan Plateau

Abstract Interbasin groundwater flow (IGF) is increasingly recognized as an important but difficult‐to‐diagnose component of regional groundwater flow systems. Recent studies highlight that major rifts within the Yarlung Zangbo River Basin (YZRB) may facilitate IGF, but the role of all six major rifts remains unclear. We developed the anomalous sample detection tool, a principal component analysis based diagnostic tool built on end‐member mixing analysis, to identify anomalous samples from 560 stream samples in the YZRB. The identified anomalous samples (∼5% of the data set) are statistically inconsistent with the basin's primary mixing model and are predominantly located near five of the six major rifts. These samples are influenced by a different set of end‐members, exhibiting distinct water types and elevated total dissolved solids compared with the majority, which are mixtures of groundwater, precipitation, and geothermal water. This suggests the major rifts may act as conduits for IGF into the YZRB. Crucially, the anomalous samples are primarily associated with the rifts other than particular lithology types or artifacts of different sampling campaigns, indicating that their presence reflects intrinsic hydrogeochemical characteristics of the YZRB. We conclude that the groundwater flow system of the YZRB is primarily local, with minimal impact from the IGF channeled from the Qiangtang Basin through the rifts. This conclusion is supported by geophysical rationale and isotopic evidence. The proposed method holds potential for application in other regions where IGF is suspected, enhancing the understanding of groundwater flow systems and informing water resource management strategies.