| Groundwater is an indispensable resource for human survival and social economic sustainable development in arid and semiarid regions. Under the pressure of social and economic development in many areas, the excessive exploitation and utilization of groundwater resources, and the rapid deterioration of groundwater quality, seriously threaten human health and restrict the development of society and economy. To protect and improve the groundwater quality and achieve the goal of sustainable development, understanding the processes and the mechanisms of the evolution of the chemical environment of groundwater is imperative.The evolution of groundwater geochemical environment associated with the development of society and economy is the typical problem in metropolitan regions. Numerous studies have been conducted in this area to address groundwater chemical evolution and identify the sources and behaviors of nitrate in the groundwater. The evolution mechanisms of various components in groundwater might be relatively similar, but also could be very different. Therefore, identifying multiple possible sources and geochemical processes of sulfate (SO42-), chloride (Cl-) and dissolved organic carbon (DOC) in natural waters not only can provide a better understanding of regional hydrologic and elemental cycles, but also can help evaluate the impacts of natural processes and anthropogenic activities on the evolution of groundwater quality. The traditional chemical method is very difficult to accurately identify various sources of solutes and complex water-rock interaction processes in groundwater. Multiple isotopes not only contain large amount of "fingerprint" of different solutes, but also can record the hydrogeochemical and biogeochemical processes in groundwater. This study aims to construct the conceptual model of isotopes evolution of groundwater sulfate, chloride and DOC in the Quaternary aquifer system, make up the deficiency in knowledge of the evolution of regional groundwater geochemical environment and expand the the application of sulfur and oxygen isotopes, chlorine isotope and carbon isotope in similar researches.The groundwater in Shijiazhuang area mainly occurs in the eastern piedmont regions and is rich in the modern riverbed and the axis of the alluvial fan of Hutuo river. The Quternary groundwater system can be divided into shallow and deep units. The two upper aquifers (I and II) with a close hydraulic connection are considered as the shallow unit, while the aquifer III and aquifer IV are taken as the deep unit. At present, aquifers II and III are the primary source of water supply for domestic, industial and agricultural demands. The recharge in the piedmont region consists of precipitation, irrigation return flow, infiltration from rivers and canals, and lateral inflow from the Taihang Mountains.Based on this regional hydrogeological model, aquifers II and III were considered as the objects of the research. The groundwater samples were collected from the drinking water and irrigation wells along the Hutuo river and around Shijiazhuang City. The samples of surface water and groundwater in the mountain area also were collected as the background references. The chemical composition of the samples and the isotopic composition of water, sulfate, chloride and DOC were determined. By the analysis and discussion of the chemical and isotopic results, the study were expected to identify the relations and differences between recharge and water-rock interaction processes in the study area, revealed the hydrogeochemical and biogeochemical processes and evaluated the influences of natural and anthropogenic processes on the evolution of the groundwater geochemical environment. The major results and conclusions are as follows:The primary chemical types of the groundwater were HCO3.SO4-Ca.Mg, SO4.HCO3-Ca.Mg and HCO3.Cl-Ca.Mg. The surface water and the shallow groundwater in or near the mountain area were mainly HCO3.SO4-Ca.Mg and SO4.HCO3-Ca.Mg types. The surface water and shallow groundwater in the eastern part of the study area were HCO3.Cl-Ca.Mg type. The variation of the chemical type of the samples collected in the different parts of the study area suggested that the groundwater near the mountain area was significantly affected by the lateral inflow from the Taihang mountain. The shallow groundwater collected from the easter part of the study area was influenced by urban sewage and wasterwater irrigation.The chemical composition of the groundwater was mainly controlled by atmospheric precipitation and surface water leaching processes. Dissolution of carbonates, silicate rocks and gypsum minerals were the primary geochemical processes in the groundwater. In addition, cation exchange and evaporation were also occurred affecting the chemical composition of the groundwater. The atmospheric precipitation, lixiviation water from the mountain area and anthropogenic inputs were the main sources of the chemical components in groundwater. The mixing effects between these end-members was the main processes for the evolution of the groundwater chemical composition.In the plain area, the SO42-concentrations of the surface water (178.2 to 266.1 mg/L, mean 232.6 mg/L) and shallow groundwater (80.5 to 350.2 mg/L, mean 197.7 mg/L) were generally higher than those of the deep groundwater (54.6 to 125.1 mg/L, mean 66.7 mg/L). In addition, the SO42-concentrations of water samples collected in the mountain area (332.7 to 469.4 mg/L, mean 418.5 mg/L) were mostly higher than those in the plain area (54.6 to 350.8 mg/L, mean 165.7 mg/L).The isotopic composition of SO42-in surface water showed no clear variation, with δ34S and δ18O values ranging from+7.2%o to+9.7%o (mean+8.6%o) and +7.2%o to+9.0%o (mean+8.0%o), respectively. Sulfate in the surface water is mainly derived from oxidation of inorganic sulfide minerals and dissolution of gypsum. In addition, atmospheric precipitation is one of the important sulfate contributors in the surface water. The sulfur and oxygen isotopic composition of sulfate in the shallow groundwater ranged from+8.7%o to+16.7%o and+5.9%o to+11.7%o, respectively. The sulfate in the shallow groundwater is a mixture of various sulfate sources. The lateral inflow from the mountain area is an important source of sulfate in the shallow groundwater. Dissolution of the carbonate and gypsum minerals dispersed in the vadose zone is another major source of shallow groundwater sulfate. The isotopic composition of sulfate in several shallow groundwater is significantly influenced by the import of washing wastewater. The deep groundwater in the plain area had the most positive δ34SSO4 values ranging from+18.1%o to+21.7‰ (mean+20.0‰), while their δ18O values (+8.7%o to+11.4‰, mean+10.2‰) had no apparent differences with other samples. Bacterial SO42-reduction is an important reaction affecting the SO42-concentrations in the deep groundwater of the plain area. The sulfate in the deep groundwater may be sourced from atmospheric precipitation and leakage of shallow groundwater.The sulfate concentration in the groundwater is mainly influenced by three factors. On the one hand, human activities accelerate the release of gypsum as well as inorganic sulfide minerals. The high SO42- concentration of the surface water in the mountain area could be attributed to a mass of mining activities such as coal and limestone mining causing the release of sulfate and sulfide minerals. Second, anthropogenic pollution is another important reason for the elevated sulfate concentration in the groundwater. Domestic and industrial wastewaster is the main pollution source of sulfate in the groundwater. In the third, the sulfate concentration in the deep groundwater is affected by the bacterial sulfate reduction which can protect the deep groundwater from the sulfate contamination.The isotope composition of chloride in the shallow groundwater was in the range of-0.01‰-+0.64‰. The potential main chloride sources were Cl-containing minerals, wastewater, and atmospheric precipitation. The elevated chloride concentration in the shallow groundwater mainly attributed to the import of wastewater in the study area. The leakage of polluted shallow groundwater was the main reason for the elevated chloride concentratin in the deep groundwater. The surface water samples in the plain area had negative 837C1 values (-0.18‰ and -0.02‰). The input of atmospheric HCl derived from acidification of the marine aerosol or industry emission via rainfall was probably an important source of Cl-with low CI-concentrations and negative 837C1 values for natural waters. In addition, the low Cl-concentration and negative 837C1 value of the deep groundwater indicated that pore water released from compacted clays containing Cl-, during diffusive-flux or advective-flux.The δ13CDOC values of DOC in the surface water and groundwater ranged from -32.2‰ to-24.7‰ and -32.4‰ to -25.1‰, respectively. DOC in the groundwater originated from soil organic matters which mainly derived from C3 plants.The carbon isotopic compositions of DOC were well correlated with the renewal rates of the groundwaters. The samples collected in the northwest of Shijiazhuang area (near huangbizhuang reservior) with high TOC concentration, low DOC content and negative 837C1 values revealed that The concentration and composition of DOC in the groundwater were mainly influenced by hydrodynamic processes. The groundwater near Shijiazhuang City with relatively high 813CDOC values indicated that the concentration and composition of DOC in these groundwates were subjected to the biogeochemical processes. Furthermore, the relationships among 813CDOC values, δ15NNO3 values and 834SSO4 values suggested denitrification effect occurred in the shallow groundwater; The deep groundwater was under reducing condition. The denitrification and sulfate reduction processes significantly affected the chemical composion of the deep groundwater.Overall, the integrated use of sulfur and oxygen isotopes, chlorine isotope and carbon isotope reveals the cycle and evolution processes of sulfate, chloride and dissolved organic carbon in groundwater, makes the understanding of groundwater geochemical environment evolution more deep, confirms that the multi-isotope method can provide more effective information for the understanding of recharge, runoff and discharge processes and hydrochemical and biogeochemiscal reactions for the groundwater. This study further expands the application of various isotopes in the research on groundwater environment evolution, especially for the application of chlorine isotope and carbon isotope in related studies. |
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