The hydrogeochemical characteristics of groundwater are an important element in the study of the spatial and temporal variation of groundwater resources, which is crucial to water resources utilization, ecological environmental protection, and human development. Water samples were collected at eight observation of Nandong Karst Water System (NKWS) sites in each month of 2019, and the main ions and isotopes of the water samples were examined. The hydrogeochemistry characteristics of groundwater and its differences with surface water were explored by using the methods of multivariate statistics, Gibbs model. Results showed that the water chemistry types of groundwater were mainly HCO₃–Ca and HCO₃–Ca·Mg. The analysis of hydrogen and oxygen isotope showed that the initial recharge source of surface water and groundwater were atmospheric precipitation, and the measured hydrogen and oxygen isotopes of surface water were heavier due to the strong evaporation effect. The natural and anthropogenic processes contributed to the chemical composition of surface water and groundwater in the study area. However, the main factor affecting the quality of surface water and groundwater was the input of anthropogenic contaminants. In terms of natural factors, the main chemical ions of surface water and groundwater were mainly controlled by water-rock action originating from weathering and hydrolysis of rocks and soils. Ca²⁺, Mg²⁺, and ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ mainly originated from natural dissolution of carbonate rocks. K⁺, Na⁺, ${\mathrm{S}\mathrm{O}}_4^{2-}$, and Cl⁻ were partly from atmospheric precipitation. For human activities, Na⁺ and Cl⁻ were partly from domestic water for local residents. ${\mathrm{S}\mathrm{O}}_4^{2-}$ in surface water mainly came from mining. ${\mathrm{N}\mathrm{O}}_3^{-}$ in groundwater mainly came from chemical fertilizers, and ${\mathrm{N}\mathrm{O}}_3^{-}$ in surface water were mainly from human waste and domestic sewage.

In fragile karst environments that have seen past and current human exploitation of agricultural and forest resources, the quantification of underground riverine sediment has been widely used to evaluate subterranean stream basin erosion. These measures are highly influenced by both precipitation and anthropogenic factors; therefore, soil erosion control measures must be urgently designed and applied. In this study, 17 years of sediment discharge across the Nandong underground river system in southwest China was monitored. To achieve this goal, the Mann–Kendal mutation test and proxy indicators were used to estimate the general influence of human activities and precipitation on sediment discharge. The results showed that: (1) Both anthropologic disturbance and rainfall have impacted the sediment discharge, although the influence of the anthropologic factor on sediment discharge was greater (61.53%), and (2) rainfall showed a hysteresis effect on sediment discharge. We obtained three different stages based on the mutation points and variation characteristics of the studied sediment discharge resulting from different driving forces, from 1998 to 2014. Prior to 2004, in the whole basin, the decrease of sediment yield was the result of the Natural Forest Protection Project. During the period from 2004 to 2008, due to continuous droughts, flood disasters, and intensive cultivation practices on the steeper hillslopes, the total sediment discharge of the whole basin increased. After 2009, the sediment discharge decreased due to the development of soil conservation projects and mushrooming reservoirs. These findings are expected to provide insights into watershed management and ecological restoration in fragile karst ecosystems, specifically, in southwestern Chinese river systems. More research must be conducted to monitor, with in situ measurements and observations, possible extreme events that can determine the exact erosion control measures that need to be designed and applied.

Since 2008, soil and water treatment and ecological restoration have been applied in the karst areas of Southwest China, but the effect of the treatments in karst fault basins is not clear. As a typical watershed of a faulted basin, studying the influence of landscape pattern changes on runoff and sediment yields in the Nandong underground river system (NURS) helps to establish the relationship between watershed and runoff and sediment changes. It provides a theoretical basis and effective method for water and soil management assessment, and soil and water treatment in karst fault basins. The vegetation topographic factor (VTF) was constructed using the normalized vegetation index (NDVI), digital elevation model (DEM) and water-system map from 2000 to 2018. On the basis of VTF classification, the vegetation topographic landscape index (VTLI) was calculated using the FRAGSTATS software, and the effects of VTLI changes on NURS runoff and sediment yield were analyzed. The study found the following: (1) PD, IJI, LSI and SHDI were positively correlated with runoff and sediment yield (p < 0.01), and the correlation coefficients were 0.693, 0.668, 0.551 and 0.582 and 0.62, 0.635, 0.627 and 0.63, respectively. AI and CONNECT were significantly negatively correlated with runoff and sediment yield (p < 0.01), and the correlation coefficients were −0.551 and −0.596, −0.627 and −0.446, respectively. The LPI and DIVISION index were not significantly correlated with discharge, but were significantly correlated with sediment yield (p < 0.05), and the correlation coefficients were −0.179 and 0.271, respectively. (2) The interpretation of VTLI for runoff increased from 0.639 to 0.778, and the interpretation of sediment transport decreased from 0.809 to 0.613. (3) In urban areas, VTF decreased and was persistent. VTF increased in the basin mountain edge area. In mountainous areas, VTF was unchanged, but has an antipersistence trend. The NURS ecological restoration project had achieved obvious results, and the change in the watershed increased runoff production and reduced sediment production. The capacity of soil and water conservation in the high slope area of the mountain edge of the basin recovered and showed a trend of sustainable development. Due to the urban expansion brought about by economic development, the capacity of soil and water conservation around the city has declined, and it showed a sustainable development trend. Policymakers should strengthen the ecological environment of urban areas and coordinate development within mountainous areas.