No abstract

Phosphogypsum (PG) is an acid by-product from the production phosphate fertilizers that is produced in large amounts all over the world. However, it is challenging to reuse this waste due to the quantities of various impurities in PG and technical restrictions, resulting in PG accumulation, which threatens adjacent environments. To reduce PG storage and safely utilize PG, the aim of our study was to define the leaching characteristics of the impurities present in PG under different conditions. We found that (1) different leaching experiments showed different leachate profiles (including pH and electrical conductivity values of leachate), which indicated that the liquid/solid (L/S) ratio, temperature, oscillation strength, particle size, and pH have remarkable effects on the concentration of leached impurities; (2) all leachate (i.e. soluble P, fluoride, Zn, Pb, As, and Hg) extraction concentrations were significantly more sensitive to the effect of the L/S ratio and pH than the effect of the other conditions; and (3) the leaching concentration was strongly influenced by pH, and a strong correlation among soluble P, F, As, and Hg concentrations in the leachate was demonstrated by a correlation analysis. In the present study, we revealed a systematic leaching mechanism of potential toxic elements in PG and have provided guidance and a reference for PG resource comprehensive utilization and surrounding environment treatment.Supplementary material: Chemical compositions of the phosphogypsum and pairwise Wilcoxon test results are available at https://doi.org/10.6084/m9.figshare.c.5088187

Knowledge of the spatial distribution characteristics of hydraulic parameters is essential for the management and protection of karst groundwater resources. In this study, we propose a workflow integrating the transition probability geostatistics (T‐PROGS) and the discrete cosine transform (DCT) with the ensemble smoother with multiple data assimilation (ES‐MDA) method to map the hydraulic conductivity of karst aquifers that usually follow a multimodal distribution. The priori parameter ensemble is constructed from the T‐PROGS and transformed into an approximately Gaussian distributed coefficient field containing critical spatial structural features about the aquifer using DCT. The ES‐MDA method is employed to update the DCT coefficients by assimilating the measurements. In addition, a postprocessing process based on cumulative distribution function (CDF) mapping is used to address the problem of parameters gradually tending to a Gaussian distribution after updating using the ES‐MDA and the inappropriate selection of initial parameter values. In practice, the limited amount of available data makes it difficult to fully capture the spatial distribution of the parameters in the initial ensemble of a single realization. Therefore, we suggest a new strategy for structuring the initial ensemble by mixing samples from multiple realizations. We then apply the proposed approach to four single realization models and a combined multiple realizations model in a field hydraulic tomography investigation of a real karst aquifer. The computed results show that this method can effectively identify the characteristics of the spatial distribution of hydraulic conductivity in karst aquifers. Compared with an individual realization model, the uncertainty of the hydraulic conductivity estimated by the combined multiple realizations model is significantly reduced. Therefore, including more uncertainty of the aquifer in the initial ensemble is beneficial to improve the accuracy of the parameter estimation.

In arid and semi-arid areas, groundwater flow is a potent geological agent. The typical profile of Aksu river basin was chosen as the research object. First, the survey region’s geological background and hydrological conditions were systematically analysed. Combined with geophysical and remote sensing characteristics, the thickness change of the Quaternary loose layer, boundary of the subaqueous system, and basement of the water circulation depth were revealed. Hydrogeochemistry and environmental isotopes were applied to explain the recharge-runoff-discharge process, water-rock interaction, movement law, and residence time of surface-groundwater. Owing to the dissolution of carbonate and gypsum minerals and evaporates, cation exchange between Ca (Mg ) and Na (K ), and the evaporation-concentration effect, specific ions (SO , Cl , Na ) and [total dissolved solids](javascript:;)(TDS) from the surface to groundwater gradually increased with the flow direction and gradually decreased with depth. This difference was more evident in the downstream discharge area, which indicated that they belonged to different groundwater flow systems(GFSs).Because of the elevation effect of the recharge source, the δ O values in different sections showed different degrees of depletion in the horizontal and vertical directions. In [association with](javascript:;) the structural ‘one convex and two concaves’ frameworks and the particle size variation of loose sediments, it reflected the distribution pattern of multiple local GFSs. The vertical zonation between the H and C isotope concentrations and the recorded groundwater [residence](javascript:;) time (modern-24000years) further illustrated the existence of intermediate and regional GFSs. Three surface-conversion boundary key zone (GFS ) were identified, and the GFS conceptual model was established. Finally, the corresponding relationship between the GFSs and the environmental effects, such as the distribution and aggregation of phreatic water with high F and As and soil salinization, were analysed, which had important theoretical significance for protecting the ecological balance of Aksu River basin.