Spatial-temporal Variations and the Regulators of Nitrate Status in Shallow Groundwater of the Typical Mountainous Agricultural Watershed in the Upper Reaches of the Yangtze River

Abstract

Shallow groundwater is the main drinking water supply for the mountainous area in the upper reaches of the Yangtze River, while its quality is often degraded by nitrate (NO3--N) pollution due to intensive agricultural production activities. In the present study, we selected a mountainous agricultural watershed in the upper reaches of the Yangtze River to investigate the land use, management, and hydrogeological conditions, aiming to clarify the spatial-temporal variations in NO3--N concentration of shallow groundwater, thereby exploring the key regulators. The results showed that the NO3--N concentrations of the groundwater ranged from 0.40 to 12.51 mg ·L-1in the study area, and the exceeding ratio was nearly 30%. On an average, NO3--N concentrations for the wet season were higher than that for the dry season, indicating great variations in NO3--N concentration across different seasons. In addition, the spatial variations of NO3--N concentration in groundwater were also significant among different sub-catchment, which was mainly due to variations in the groundwater table depth and land uses. Furthermore, the significant correlations between the NO3--N concentrations and concentrations of Cl-, NH4+-N, DOC, and SO42- in the shallow groundwater were also explored in this study, suggesting that the NO3--N concentrations were likely to depend on the chemical factors of the shallow groundwater in the study area. Overall, our current study highlights that the characterization of spatial-temporal variations of NO3--N status in shallow groundwater and illustration of key regulators are essential to mitigate NO3--N pollution and prevent quality degradation of shallow groundwater in mountainous rural areas of the upper Yangtze River watershed.