Characteristics of groundwater level and nitrate variability at typical sites in the Piedmont Region of the North China Plain

The North China Plain is an important agricultural production area. Agricultural production in this region has depended on the overexploitation of groundwater and overfertilization, resulting in a continuous decline in the groundwater level and deterioration of groundwater quality, which seriously threatens the safety of the regional ecosystem, food security, and public health. In this study, using a set of long-term observations at a typical site (Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences) in the Piedmont Plain of the North China Plain, we analyzed the characteristics of interannual changes in shallow groundwater depth and water quality. The results indicated that: 1) The groundwater level exhibited a decreasing trend, dropping 32.6 m from 1974 to 2019, with an average annual decline rate of 0.7 m·a⁻¹. By the recent comprehensive management of groundwater overdraft, the groundwater level has recovered as a result of an increased rate of 0.2 m·a⁻¹ in the last three years (2020−2022). 2) The electrical conductivity of shallow groundwater fluctuated considerably within the year, ranging from 425 μS∙cm⁻¹ to 776 μS∙cm⁻¹, indicating that groundwater exploitation for agricultural irrigation resulted in intense mixing between deep groundwater (with lower electrical conductivity and relatively better water quality) and shallow groundwater (with higher electrical conductivity and relatively poorer water quality) within the year. 3) The water quality observation data for the past 20 years indicated a trend of deterioration in the shallow groundwater quality, with elevated concentrations of chloride and nitrate ions. This study is of great scientific value for the sustainable utilization of groundwater resources and promotion of groundwater quality protection in the region.