This study was performed in the Linshui River Watershed to explore the contents of heavy metals in river water, their species, and health risks. The test results show that the contents of arsenic (As), chromium (Cr), copper (Cu), uranium (U), and thorium (Th) are relatively low and below the limit of class III of the environmental quality standards for surface water. According to the analysis through PHREEQC (version 2.18), the main species of As, Cr, Cu, U, and Th are H₂AsO₄⁻ and HAsO₄²⁻, HCrO₄⁻, Cr₂O₇²⁻ and CrO₄²⁻, Cu²⁺, CuSO₄ and CuCO₃, UO₂CO₃, (UO₂)₂CO₃(OH)₃⁻, and UO₂(CO₃)₂²⁻, Th(OH)₄, Th(OH)₂²⁺, and Th(OH)₃⁺, respectively. Health risk assessment demonstrates that the average total risk value is 5.68 × 10⁻⁵ year⁻¹, which is higher than the maximum acceptable level of 5.05 × 10⁻⁵ year⁻¹ recommended by International Commission on Radiological Protection (ICRP). Moreover, the average of total radionuclide risk (U and Th) is calculated to be 7.85 × 10⁻⁹ year⁻¹, which is five orders of magnitude lower than the standard value of 5.0 × 10⁻⁴ year⁻¹. The health risk of chemical non-carcinogenic (Cu) is lower than that of chemical carcinogenic (As and Cr). Human health risk values descend in the order of Cr, As, U, Th, and Cu. Therefore, As and Cr in the study area are labeled as the priority pollutants.

Anthropogenic activities have had diversiform impacts on the surrounding river and environments. A study on the surface water of the Lhasa River Basin was undertaken to assess its hydrogeochemical characteristics under the influence of anthropogenic activities. A total of 16 samples collected from the Lhasa River were analyzed for pH, electric conductivity (EC), oxidation-reduction potential (ORP), total dissolved solids (TDS), major cations (K⁺, Na⁺, Ca²⁺, and Mg²⁺), major anions (Cl⁻, SO₄²⁻, HCO₃⁻, and NO₃⁻), and trace metals. The pH of the analyzed water samples varied from 6.40 to 9.10, indicating alkaline in nature. The EC values varied from 77.3 to 213 μs/cm while the TDS values varied from 73.37 to 217.55 mg/L. HCO₃⁻ and SO₄²⁻ are the dominant anions while Ca²⁺ and Na⁺ are the dominant cations in the river water. The concentration of alkaline earth metals (Ca²⁺ + Mg²⁺) exceed the alkali metals (Na⁺ + K⁺) and HCO₃⁻ dominates over SO₄²⁻ + Cl⁻ concentrations in the majority of the surface water samples. Ca²⁺-Mg²⁺-HCO₃⁻ and Ca²⁺-Mg²⁺-Cl⁻ are the dominant hydrogeochemical facies in the surface water of the area. The water chemistry is mainly controlled by rock weathering with secondary contribution from anthropogenic sources. The content of trace elements in the Lhasa River is low, basically at the natural background value. The high content of individual elements is mainly due to the mineral resource exploitation and geothermal resources in the basin. Countermeasures and attention should be payed to these aspects, such as the construction of water conservancy facilities, urban economic development, the development of mining activities, and sewage and wastewater discharge, so as to protect the water quality and sustainable development of the Lhasa River Basin.

Numerical modeling has been adopted to assess the feasibility of centrifugal simulation of solute transport within the unsaturated zone. A numerical model was developed to study the centrifugal simulation of nonreactive, adsorption, radionuclide, and reactive solutes. The results showed that it is feasible to conduct centrifugal experiments for nonreactive solute transport. For the solute transport containing physical processes or chemical reactions, if the reaction is very rapid or slow, it is feasible to conduct centrifugal experiments. For the solute transport with a product B generated, if the reaction is relatively slow, the centrifugal prediction of solute is suitable. The centrifugal prediction of solute A matches the prototype quite well, but the prediction of B is in poor quality. If B is the focus, it is not feasible to conduct centrifugal experiments; but if B is not important, the centrifugal modeling is suitable. This has significant implications for the centrifugal modeling application to solute transport simulation within the unsaturated zone.

Centrifugal modelling, both physical and numerical, has been used for studying groundwater flow and transport processes in the past. However, there was disagreement in previous studies whether numerical models can be used in simulating centrifugal systems under unsaturated flow condition. In the present study, a numerical model based on Richards’ equation was developed to predict one-dimensional unsaturated flow in centrifugal systems. The validity of the model was tested using data from physical models in four published benchmark problems. The ability of the numerical model to close mass balance was also tested. It was shown that the newly developed numerical model was able to recreate the four benchmark problems quite successfully, indicating that using such a model under unsaturated flow condition is feasible. The mass conservation result shows that the model is more sensitive to spatial grid resolution than to specified temporal step. Therefore, fine spatial discretization is suggested to ensure the simulation quality. Additionally, adaptive temporal time stepping method can be used to improve the computational efficiency. It was found that the dimensionless factors used for scaling physical dimensions by 1/N, seepage velocity by N, and temporal dimension by 1/N² were useful parameters for scaling centrifugal systems.