Characterize groundwater vulnerability to intensive groundwater exploitation using tritium time-series and hydrochemical data in Shijiazhuang, North China Plain

• Mean residence times of shallow groundwater were aged by tritium time-series data. • A negative correlation between groundwater age and ions was demonstrated. • Groundwater became more vulnerable to anthropogenic contaminants under intensive exploitation. The deterioration of water quality in shallow groundwater systems over the past decades has attracted increased amounts of attention from hydrogeologists. Groundwater vulnerability and its changes are the factors that cause groundwater deterioration. Combining of the mean residence times (MRTs) with chemical characteristics of groundwater can be applied to understand groundwater vulnerability and its changes. The groundwater MRTs ranged from 6 to 180 years, with MRTs of 6 to 77 years in the shallow aquifer. Spatial distribution of groundwater MRTs suggested that young groundwater trended to extend. Relatively uniform MRTs and concentrations of ions were found above the main pumped well depth. High concentrations of ions (K + , Na + , Ca 2+ , Mg 2+ , HCO 3 − , Cl - and SO 4 2- ) were mainly distributed in the western piedmont plain areas with short turnover time but with low productivity, meaning that the aquifer is susceptible to human activities and hardly migrate out. Elevated concentrations of nitrate and others ions (such as K + , Ca 2+ , Cl - and SO 4 2- ) also were observed in moderately old groundwater (20 < MRTs < 80 years), indicating that some mixing with recent water within MRTs < 20 years likely occurs through vertical drainage from the upper to deeper in the single aquifer and that lateral flow occurs along flow paths towards the piezometric depressions caused by the pumping in urban areas. Slightly elevated HCO 3 − and K + were found at east of the areas with long residence times suggested that water-salt equilibrium had also been disrupted. Therefore, the anthropogenic influence exceeded the natural attenuation in the current groundwater environment on the piedmont plain. And the Shijiazhuang groundwater system was identified to be tending more vulnerable under anthropogenic influence. Some measures must be taken to significantly reduce groundwater withdrawal and pollutant discharge.