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Highlights: [•] Firstly highlight the importance of loess weathering in a semi-arid lake. [•] Loess weathering controls sediment compositions in loess-covered area. [•] Loess weathering results in similar water chemistry of rivers and groundwater. [•] Water draining loess areas has distinct Mg/Ca and 87Sr/86Sr from global rivers. [Copyright &y& Elsevier]

Water resources play a key role in the development of regional economy and stability of ecosystem in the middle Loess Plateau, China. Spatial variability of fluoride (F − ) and its geochemical control with other dissolved ions in the middle Loess Plateau were discussed. Samples were neutral to weak alkaline (pH = 7.4–9.6) with little variation of ion concentrations between river water and groundwater. The F − concentrations ranged from 0.2 to 3.1 mg/L, with an mean value of 0.9 mg/L. Approximately 31% of samples in northern and southern part of the study area were HCO 3 − –Na + water type with F − levels higher than the Chinese Drinking Water Standard of 1.0 mg/L. About 12% samples in the central part were HCO 3 − –Ca 2 + water type with F − content lesser than recommended value for caries control of 0.5 mg/L. The Quaternary deposits in the study area were the mainly F − sources of natural waters, and rainwater also contributed some F − . The alkaline characteristic of natural waters favored desorption of F − from hydrous metal oxides in loess, the climate of the study area, cation exchange, and CaCO 3 precipitation further increased F − concentrations in natural waters in the middle Loess Plateau. Moreover, the chemical data analyzed in the principal component analysis (PCA) also indicated that climate coupled with geochemical processes was the main controlling factor for high F − in natural waters in the area. For the supply of high quality drinking water with safe F − , short-term and long-term action plans as well as awareness/education programs for the public were advocated. [ABSTRACT FROM AUTHOR]

Landslide-dammed reservoirs are widespread on the Chinese Loess Plateau (CLP). Due to the contrasting seasonality of the monsoonal hydroclimate, sediment sequences in landslide-dammed reservoirs are characterized by deposition couplets of eroded loess with annual freeze-thaw layers that are ideal objects to reconstruct catchment erosion processes, hydrologic events, land use and human activity history. Identifying annual freeze-thaw layer from the deposition couplets in a given year is crucial for dating sediment sequences and for reconstructing erosion history using sediments because they are considered to be the boundary of the last hydrological event in an individual year. Using high-resolution XRF core scanning, grain size and mineralogy, fine-grained layers were identified in two representative core sections from the Jingbian and Heshui landslide-dammed reservoirs on the northern and southern CLP. The results show that both annual and intra-annual fine-grained layers have high Ca, Fe, and Mn contents but low Si contents and are dominated by clays, calcite and fine silt as a result of sorting and stratification during deposition. However, Ca exhibits two distinct trends with Fe (and Mn) in these fine-grained layers from both core sections. The steeper slopes of the relationship between them reflect further concentrations of both Ca and Fe (and Mn) that potentially result from freeze-thaw processes. The fine-grained layers with high concentrations of Ca and Fe and high slopes of their correlations are therefore identified as the annual freeze-thaw layers. The identification of annual freeze-thaw layers in landslide-dammed reservoir sediment sequences by high-resolution XRF core scanning provides new insights into their formation and mobile elemental enrichment processes as well as dating of the sediment sequences. [ABSTRACT FROM AUTHOR]