[Geochemical Characteristics and Ecological Significance of Carbon Isotopes in Groundwater Under the Influence of Different Land Use Types in Karst Areas].

Based on the seasonal characteristics of groundwater hydrochemistry and the carbon isotopes (δ13C) of dissolved inorganic carbon (DIC) in the Hongjiadu Basin, Guizhou Province, this paper discusses the natural processes and anthropogenic factors affecting the characteristics of δ13CDIC in karst groundwaters under different land use types. The results show that the main sources of DIC in groundwater are carbonate weathering and soil CO2. In winter, the δ13CDIC values for groundwater ranged from -14.8‰ to -4.1‰ with an average of -10.1‰ and, in summer, ranged from -14.5‰ to -6.3‰ with an average of -10.2‰. Sulfuric acid from sulfide oxidation in coal-bearing strata and acid rain is involved in carbonate weathering, resulting in the enrichment of groundwater with heavy carbon isotopes. Due to the soil CO2 effect, the δ13CDIC values of woodland groundwater experiencing less disturbance from human activities are lower in summer than in winter. The degradation of organic matter input from residential areas is a significant contributor of DIC to groundwater. The average values of δ13CDIC in winter and summer were -11.9‰ and -11.6‰, respectively, and the seasonal difference was relatively small in residential areas. During different seasons and for different types of land use, human activities could lead to differences in groundwater δ13CDIC values and hydrochemistry. Therefore, δ13CDIC can reflect the impact of human activities on karst aquifers, which has important ecological significance.

[Identification of Nitrate Pollution Sources Through Various Isotopic Methods: A Case Study of the Huixian Wetland].

In recent years, the ecological environment of wetlands has deteriorated under the predatory exploitation of human beings. Nitrate pollution is a major problem faced by wetland ecosystems. In this paper, Huixian karst wetland was selected as a study area to identify the main factors leading to the increase of nitrate in wetland water. The main sources of nitrate pollution in the area were determined by means of 15N(NO3-) and 18O(NO3-) isotopes. The contribution rate of all types of pollution sources was determined with the SIAR model. To explore the relationship between the spatial distribution of the nitrate concentration and the groundwater runoff conditions, the 13CDIC isotopes were used to qualitatively characterize the runoff conditions of the groundwater. The results show that there are three main factors affecting the nitrate concentration in the study area:animal manure and domestic sewage, NO3- in chemical fertilizer, and soil nitrogen. The average contribution rates to nitrate pollution are 39.1%, 32.2%, and 28.5% respectively. The 13CDIC isotope data clarify that a light 13CDIC reflects better groundwater runoff conditions. The concentration of NO3- is generally lower, while the NO3- concentration corresponding to heavy 13CDIC is generally higher. The condition of the groundwater runoff to a certain extent also affects the distribution of the NO3- concentration.