Abstract Defining moisture sources and evolution of precipitation is of significance for further exploration of complex hydro‐climatic interactions, especially under global warming with alternations of evapotranspiration capacity and precipitation moisture source structure. As a sensitive indicator, d‐excess has been widely used to quantify the proportion of recycled vapor to precipitation ( f re ). However, existing d‐excess‐based models ignore to take transpiration vapor into account and the calculated f re tends to be lower than the true value, thus underestimating the importance of recycled vapor in precipitation. Herein, the existing model was modified with transpiration vapor considered and applied in Guyuan, China located in a monsoon marginal zone with complex precipitation moisture sources. After modification, the estimated annual average f re was increased from 7.5% to 14.8%, well calibrating the existing model. This study highlights the contribution of transpiration vapor to precipitation and provides more information on the formation and evolution of precipitation to better serve future hydro‐climatic research.
The lowest reaches of a large-scale basin could be the discharge areas of local, intermediate and regional groundwater flow systems with significantly distinct travel distances and travel times. This study aims to delineate the groundwater chemical characteristics and the mechanism controlling the chemical evolution in the lowest reaches of the Wushenzhao Cretaceous basin, NW China. A total of 38 groundwater samples were collected and were chemically classified into five distinct water types by means of a Piper Plot. According to the hydrogeological setting and groundwater age, the spatial distribution of these water types is found to be associated with hierarchically nested groundwater flow systems (local and regional system): Types 1, 2, 3 and 4 belong to the local groundwater flow system, while type 5 belongs to the regional flow system. Graphical plots, stable isotopes and geochemical modeling techniques were used to interpret the observed compositions. The results show the dominance of carbonate and gypsum dissolution in type 1 waters; ion exchange in types 2, 3 and 4; and evaporite dissolution in type 5. In addition, human activities in the form of extensive irrigation also affect the chemical compositions of type 1 water. These findings are important for the sustainable management of groundwater resources in the study area.
Abstract The use of the sulphate mass balance (SMB) between precipitation and soil water as a supplementary method to estimate the diffuse recharge rate assumes that the sulphate in soil water originated entirely from atmospheric deposition; however, the origin of sulphate in soil and groundwater is often unclear, especially in loess aquifers. This study analysed the sulphur (δ 34 S‐SO 4 ) and oxygen (δ 18 O‐SO 4 ) isotopes of sulphate in precipitation, water‐extractable soil water, and shallow groundwater samples and used these data along with hydrochemical data to determine the sources of sulphate in the thick unsaturated zone and groundwater of a loess aquifer. The results suggest that sulphate in groundwater mainly originated from old precipitation. When precipitation percolates through the unsaturated zone to recharge groundwater, sulphates were rarely dissolved due to the formation of CaCO 3 film on the surface of sulphate minerals. The water‐extractable sulphate in the deep unsaturated zone (>10 m) was mainly derived from the dissolution of evaporite minerals and there was no oxidation of sulphide minerals during the extraction of soil water by elutriating soil samples with deionized water. The water‐extractable concentration of SO 4 was not representative of the actual SO 4 concentration in mobile soil water. Therefore, the recharge rate cannot be estimated by the SMB method using the water‐extractable concentration of SO 4 in the loess areas. This study is important for identifying sulphate sources and clarifying the proper method for estimating the recharge rate in loess aquifers.
Abstract Mehlich‐1 and DTPA extractants are frequently used to predict metal availability in soils. Metal extractability by the acid or chelate extractant reflects the metal characteristics and metal‐soil interactions. In this study, samples of eight topsoils from the southeastern United States were incubated with added lead (Pb) at the rate of 40 mg#lbkg‐1. After five months in the greenhouse, Mehlich‐1 and DTPA extractants were employed to extract Pb in both metal‐amended and natural soils. For the natural soils, Pb concentration in the DTPA extractant was always higher than that in the Mehlich‐1 extractant. This indicates that the DTPA chelate extractant is able to dissolve some Pb in soils which is not solubilized by protons. The negative correlation found between Mehlich‐1‐extractable Pb and soil clay content might result from two mechanisms: i) strong association between Pb and soil surfaces, or ii) readsorption of Pb during extraction. None of the correlations between DTPA‐extractable Pb and soil properties was significant, suggesting that the DTPA‐extractable Pb is not heavily dependent on soil properties. The DTPA extractant showed a high ability to solubilize Pb in the natural soils possibly due to a high affinity of Pb for soil organic matter.
Acid rain with a relatively high concentration of ammonium and nitrate can accelerate rock weathering. However, its impact on groundwater nitrate is uncertain. This study evaluated the dual isotopic composition of nitrate ( δ 15 N-NO 3 - and δ 18 O-NO 3 - ) from precipitation to groundwater in a rural mountainous area affected by acid rain. The average concentration for NH 4 + is 1.25 mg/L and NO 3 - is 2.59 mg/L of acid rain. Groundwater NO 3 - concentrations ranged from <0.05 to 11.8 mg/L (baseline), and NH 4 + concentrations ranged from 0.06 to 0.28 mg/L. The results show that groundwater δ 18 O-NO 3 - values (-4.7‰ to +4.2‰) were lower than the values of rainfall δ 18 O-NO 3 - (+24.9‰ to +67.3‰), suggesting that rainfall NO 3 - contributes little to groundwater NO 3 - . Groundwater δ 15 N-NO 3 - values (+0.1‰ to +7.5‰) were higher than the values of δ 15 N-NO 3 - derived from the nitrification of rainfall NH 4 + (less than -4.7‰ in the study area), suggesting that nitrification of rainfall NH 4 + also contributes little to groundwater NO 3 - . This implies that rainfall NO 3 - and NH 4 + have been utilized. The dual isotopic composition of nitrate shows that baseline groundwater NO 3 - is derived mainly from nitrification of soil nitrogen. The denitrification process is limited in the groundwater system. This study shows that the rainfall NO 3 - and NH 4 + contribute little to groundwater NO 3 - , improving the understanding of the nitrogen cycle in areas with a high concentration of NH 4 + and NO 3 - in rainfall.
Abstract A detailed study on geochemical processes following hydraulic fracturing can provide important information on the origin of solutes and potential improvement of fracturing technology. However, this remains difficult due to the low resolution of flowback water and high salinity of formation water. To fill this knowledge gap, a shale‐gas well was drilled and freshwater was used to fracture the shale. In parallel, laboratory water‐rock interaction experiments were conducted. The intensive sampling for flowback water and the use of multiple isotopes provided novel and detailed insights into the water‐rock interactions after hydraulic fracturing. The results showed that beyond mixing processes, cation exchange, adsorption/desorption, and barite precipitation were observed both in the laboratory and field studies. Although oxidation of pyrite was observed in most of the laboratory experiments, our findings demonstrate that this process was not evident in field flowback samples that were dominated by mixing of fracturing fluids and formation water.
Nitrate pollution is a global environmental problem, and mean nitrate levels have risen by an estimated 36% in global waterways since 1990. Tracing nitrate sources is important for water quality management, and nitrate isotopes (δ15N-NO3− and δ18O-NO3−) are commonly used for this purpose because of the different isotopic compositions of different sources. However, the impact of nitrate sorption on matrix and desorption from matrix on N and O isotopic composition of nitrate in liquid phase has not been well clarified. To explore the mechanism for the changes in nitrate concentration and isotopes in liquid phase during sorption and desorption, this study took a shale sample (enriched in clay minerals and commonly exposed in the Earth), conducted a series of laboratory experiments for nitrate sorption and desorption, and studied the impact of sorption and desorption on nitrate N and O isotopic composition in liquid phase. The results showed that the shale sample exhibited a rapid sorption and desorption rate for nitrate in the surface water samples, with the nitrate concentration in the solution decreasing from 14.3 mg/L to 4.1 mg/L within 5 min. The sorption data fit the Langmuir model better than that of the Freundlich model. The maximum possible sorption (Qmax) for the shale sample was estimated to be 46 μg/g. Preliminary laboratory experiments showed that changes in δ15N-NO3− values were not obvious, and changes in δ18O-NO3− values in liquid phase were minor during sorption and desorption of the shale sample, suggesting that nitrogen isotopic fractionation can be neglected, and the sorption of nitrate by the shale sample has a very limited impact on the distribution of nitrate isotopes in liquid phase. However, the impact of nitrate desorption on the nitrate isotopes in liquid phase depends on the isotopic composition of exchangeable nitrate in the solid phase, which may be related to antecedent water–rock interactions. This study provides important information for elucidating the evolution mechanism of nitrate and its isotopic compositions following sorption-desorption, and is conducive to revealing the nitrogen cycle law in the environment.
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