Wetland is a transition zone between terrestrial and aquatic ecosystems, and is the source and sink of various biogenic elements in the earth’s epipelagic zone. In order to investigate the driving force and coupling mechanism of carbon (C), nitrogen (N) and phosphorus (P) migration in the critical zone of lake wetland, this paper studies the natural wetland of Dongting Lake area, through measuring and analysing the C, N and P contents in the wetland soil and groundwater. Methods of Pearson correlation, non-linear regression and machine learning were employed to analyse the influencing factors, and to explore the coupling patterns of the C, N and P in both soils and groundwater, with data derived from soil and water samples collected from the wetland critical zone. The results show that the mean values of organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in groundwater are 1.59 mg/L, 4.19 mg/L and 0.5 mg/L, respectively, while the mean values of C, N and P in the soils are 18.05 g/kg, 0.86 g/kg and 0.52 g/kg. The results also show that the TOC, TN and TP in the groundwater are driven by a variety of environmental factors. However, the concentrations of C, N and P in the soils are mainly related to vegetation abundance and species which influence each other. In addition, the fitted curves of wetland soil C-N and C-P appear to follow the power function and S-shaped curve, respectively. In order to establish a multivariate regression model, the soil N and P contents were used as the input parameters and the soil C content used as the output one. By comparing the prediction effects of machine learning and nonlinear regression modelling, the results show that coupled relationship equation for the C, N and P contents is highly reliable. Future modelling of the coupled soil and groundwater elemental cycles needs to consider the complexity of hydrogeological conditions and to explore the quantitative relationships among the influencing factors and chemical constituents.

Diclofenac (DCF) is one of the most frequently detected pharmaceuticals in groundwater, posing a great threat to the environment and human health due to its toxicity. To mitigate the DCF contamination, experiments on DCF degradation by the combined process of zero-valent iron nanoparticles (nZVI) and nano calcium peroxide (nCaO₂) were performed. A batch experiment was conducted to examine the influence of the adding dosages of both nZVI and nCaO₂ nanoparticles and pH value on the DCF removal. In the meantime, the continuous-flow experiment was done to explore the sustainability of the DCF degradation by jointly adding nZVI/nCaO₂ nanoparticles in the reaction system. The results show that the nZVI/nCaO₂ can effectively remove the DCF in the batch test with only 0.05 g/L nZVI and 0.2 g/L nCaO₂ added, resulting in a removal rate of greater than 90% in a 2-hour reaction with an initial pH of 5. The degradation rate of DCF was positively correlated with the dosage of nCaO₂, and negatively correlated with both nZVI dosage and the initial pH value. The order of significance of the three factors is identified as pH value > nZVI dosage > nCaO₂ dosage. In the continuous-flow reaction system, the DCF removal rates remained above 75% within 150 minutes at the pH of 5, with the applied dosages of 0.5 g/L for nZVI and 1.0 g/L for nCaO₂. These results provide a theoretical basis for the nZVI/nCaO₂ application to remove DCF in groundwater.

Based on the observation of a complete hydrological year from June 2014 to May 2015, the temporal and spatial variations of the main inorganic nitrogen (MIN, referring to NO₃^--N, NO₂^--N, NH₄⁺-N) in surface water and groundwater of the Li River and the Yuan River wetland succession zones are analyzed. The Li River and the Yuan River are located in agricultural and non-agricultural areas, and this study focus on the influence of surface water level and groundwater depth and precipitation on nitrogen pollution. The results show that NO₃^--N in surface water accounts for 70%-90% of MIN, but it does not exceed the limit of national drinking water surface water standard. Groundwater is seriously polluted by NH₄⁺-N. Based on the groundwater quality standard of NH₄⁺-N, the groundwater quality in the Li River exceeds Class Ⅲ water standard throughout the year, and the exceeding months’ proportion of Yuan River reaches 58.3%. Compared with the Yuan River, MIN in groundwater of the Li River shows significant temporal and spatial variations owing to the influence of agricultural fertilization. The correlation between the concentrations of MIN and surface water level is poor, while the fitting effect of quadratic correlation between NH₄⁺-N concentration and groundwater depth is the best (R²=0.9384), NO₃^--N is the next (R²=0.5128), NO₂^--N is the worst (R²=0.2798). The equation of meteoric water line is δD =7.83δ¹⁸O+12.21, indicating that both surface water and groundwater come from atmospheric precipitation. Surface infiltration is the main cause of groundwater NH₄⁺-N pollution. Rainfall infiltration in non-fertilization seasons reduces groundwater nitrogen pollution, while rainfall leaching farming and fertilization aggravate groundwater nitrogen pollution.