Nitrate (NO3-) pollution of groundwater is a global concern in agricultural areas. To gain a comprehensive understanding of the sources and destiny of nitrate in soil and groundwater within intensive agricultural areas, this study employed a combination of chemical indicators, dual isotopes of nitrate (δ15N-NO3- and δ18O-NO3-), random forest model, and Bayesian stable isotope mixing model (MixSIAR). These approaches were utilized to examine the spatial distribution of NO3- in soil profiles and groundwater, identify key variables influencing groundwater nitrate concentration, and quantify the sources contribution at various depths of the vadose zone and groundwater with different nitrate concentrations. The results showed that the nitrate accumulation in the cropland and kiwifruit orchard at depths of 0-400 cm increased, leading to subsequent leaching of nitrate into deeper vadose zones and ultimately groundwater. The mean concentration of nitrate in groundwater was 91.89 mg/L, and 52.94% of the samples exceeded the recommended grade III value (88.57 mg/L) according to national standards. The results of the random forest model suggested that the main variables affecting the nitrate concentration in groundwater were well depth (16.6%), dissolved oxygen (11.6%), and soil nitrate (10.4%). The MixSIAR results revealed that nitrate sources vary at different soil depths, which was caused by the biogeochemical process of nitrate. In addition, the highest contribution of nitrate in groundwater, both with high and low concentrations, was found to be soil nitrogen (SN), accounting for 56.0% and 63.0%, respectively, followed by chemical fertilizer (CF) and manure and sewage (M&S). Through the identification of NO3- pollution sources, this study can take targeted measures to ensure the safety of groundwater in intensive agricultural areas.

Loess is a kind of soil that experiences a long period of deposition, and it is relatively stable under natural conditions. However, in the process of engineering construction in loess areas, the original soil structures of the loess are destroyed, inducing changes in the composition and water content in the loess. These changes may cause different environmental and engineering geologic problems. To reveal the engineering properties of disturbed losses in the Chinese Loess Plateau, the physical properties of 135 groups of disturbed loess samples in Yan’an City were analyzed statistically, and the compression properties of loess with different moisture contents and dry densities were studied by high-pressure consolidation experiments. We elucidate the compressive deformation law for perturbed solids at different moisture contents and dry densities. The experimental results show that the water content rate for the best compaction performance of the disturbed loess is 16%. The compressive deformation coefficient generally decreases with increasing dry density and water content. However, when the soil moisture is low, a small amount of water and salt is concentrated in the contact position of the powder, and the soluble salt is condensed into cement. The molecular forces between particles and the bonding forces of bound water and capillary water are larger. The soil forms a porous structure with coarse grains as the main skeleton, and the cement bonding strength is strong at the contact points of the coarse grains. As a result, the loess shows high intensity at low-water content. This results in a compression-deformation coefficient that increases with dryness density in the small load range.

The soil-water characteristic curve (SWCC) is an essential tool to determine hydraulic and mechanical properties of unsaturated soils. As an inherent influencing factor, soil texture controls the characteristics of SWCCs. Fractal theory can quantitatively describe the physical characteristics of soil. This study used particle size distribution data and water content data contained in the UNSODA2.0 database to explore the fractal characteristics of 12 soil types with different textures under different matrix suctions. The SWCC fractal model was adopted to characterize the hydraulic properties of soil with various soil textures. The findings revealed that the mass fractal dimensions of particles from these 12 different soil types significantly differed and were closely related to the clay content. Fractal dimension increased with increasing clay content. The fractal dimension established a good relationship between soil structure and hydraulic properties. Fractal analysis can be used to determine the connection between physical properties and soil hydraulic parameters. The estimated results of the SWCC fractal model indicated that it had a good performance regarding the description of SWCCs for the 12 soil textures. The soil structure could be described through fractal dimensions, which can effectively indicate soil hydraulic characteristics. The estimated fractal dimension of this model could be obtained by particle size distribution. Furthermore, using the SWCC fractal model, we found that the SWCC of coarse textured soil changed sharply in the low suction stage and its residual water content was small, and the SWCC of fine textured soil changed gently with a large residual water content. The water retention capacity followed the order clay > silty clay > sandy clay > clay loam > silty clay loam > sandy clay loam > loam > silt loam > sandy loam > silt > loamy sand > sand.

The application of hydraulic fracturing has transformed tight sandstone gas into a very important global un-conventional resource. The accurate description of geochemical processes during hydraulic fracturing can pro-vide essential information for improving tight gas production. In this study, field fracturing and laboratory water -rock interaction experiments were designed and performed in conjunction with geochemistry, isotope, XRF, XRD and SEM techniques to reveal the prevalent geochemical processes. Sixty-five flowback fluid samples were collected from a hydraulic fracturing well and a tight sandstone sample was drilled from gas reservoir to perform water-rock interaction experiments. The results indicate that hydraulic fracturing flowback fluid is a complex mixture, with high TDS values (0.93-33.04 g/L) and heavy isotopes (e.g., mean delta O-18 of -5.35 parts per thousand, mean Sr-87/Sr-86 of 0.714651). Geochemistry and isotope data reveal that the mixing process between fracturing fluid and formation water exerts an important influence on the flowback fluid chemistry, while water-rock and fluid-fluid interactions are also variably important during the hydraulic fracturing process. Pyrite oxidation process is confirmed during hydraulic fracturing in the tight sandstone reservoir, while the addition of gel breaker makes the process prominent. Due to high clay mineral contents (24%), ion exchange and adsorption/desorption are critical factors affecting the stability of the clay minerals during the entire fracturing and flowback processes. Approximately, 20%-43% of the strontium was removed from the solution by ion exchange between the 8 and 44 h intervals of the flowback process. The fractures derived from fracturing connect different types of high salinity formation water, leading to potential mineral precipitation (e.g., barite and calcite). The analysis of formation water needs to be taken into consideration during hydraulic fracturing operations.

Groundwater is critical for the sustainable development of the Loess Plateau, while groundwater quality is generally poor in this area due to natural factors and anthropogenic pollution. This study was carried out to investigate the suitability of groundwater for domestic and agricultural purposes in Yan’an City on the Chinese Loess Plateau and to assess its implications to sustainable groundwater management on the plateau. The index levels were compared with the threshold values established by the national and the WHO drinking water guidelines, and the suitability of groundwater for irrigation purposes was assessed using multiple agricultural water quality indicators. An entropy-weighted Technique for Order Preference by Similarity to an Ideal Solution (entropy-weighted TOPSIS) was adopted for overall groundwater quality assessment. The results indicate that the study area is characterized by saline, hard, and slightly alkaline groundwater, mainly of the HCO3–Ca·Mg type, accompanied by some minor SO4·Cl–Ca·Mg type. The dissolution of carbonates and gypsum and the leaching of soluble salts are important natural processes influencing the groundwater ion chemistry. The parameters TH, TDS, and SO42− are major indices, while Fe, Mn, F−, and NH4+ are minor contaminants affecting groundwater quality. The overall groundwater quality is generally acceptable for irrigation, and most of the water is suitable for drinking. Rainwater harvesting, water quality improvement programs, regular water quality monitoring, and multidisciplinary water research programs are suggested as measures for sustainable groundwater management on the Loess Plateau.