Spatial variability of shallow groundwater level, electrical conductivity and nitrate concentration, and risk assessment of nitrate contamination in North China Plain

摘要

In recent years, nitrate (NO_3) contamination of groundwater has become a growing concern for people in rural areas in North China Plain (NCP) where groundwater is used as drinking water. The objective of this study was to evaluate groundwater resource level, to determine groundwater quality and to assess the risk of NO_3 pollution in groundwater in Quzhou County in the NCP. Ordinary Kriging (OK) method was used to analyze the spatial variability of shallow groundwater level, groundwater electrical conductivity (EC) and NO_3-N concentrations, and Indictor Kriging (IK) method was used to analyze the data with NO_3-N concentrations equal or greater than the groundwater NO_3 pollution threshold (20 mg L~(-1)). The results indicated that groundwater level averaged 9.81 m, a level 6 m lower than in 1990. The spatial correlation distances for groundwater level, EC and NO_3-N concentration were 21.93, 2.19 and 3.55 km, respectively. The contour map showed that shallow groundwater level areas extended from north to south across the County. Groundwater EC was above 3 dS m~(-1) in the most part of the northern county. Groundwater NO_3 pollution (NO_3-N ≥ 20 mg L~(-1)) mainly occurred in the County Seat areas due to wastewater irrigation and excessive fertilizer leaching from agricultural fields. At Henantuang town, besides suburban of the County Seat, groundwater was also contaminated by NO_3 shown by the map generated using the IK method, which was not reflected in the map generated using the OK method. The map generated using the OK method could not reflect correctly the groundwater NO_3 pollution status. The IK method is useful to assess the risk of NO_3 pollution by giving the conditional probability of NO_3 concentration exceeding the threshold value. It is suggested that risk assessment of NO_3 pollution is useful for better managing groundwater resource, preventing soil salinization and minimizing NO_3 pollution in groundwater.