Prediction of the impact of benzo(a)pyrene on shallow groundwater during natural infiltration of reclaimed water-receiving rivers

To predict the influence of reclaimed water-receiving rivers on groundwater quality, the spatiotemporal distribution and migration evolution prediction of benzo(a)pyrene B(a)P was conducted by investigating and monitoring its levels in the Liangshui River, which received reclaimed water, with Hydrus-1D coupled GMS model. The research results were as follows: The vertical infiltration rate of B(a)P in the vadose zone was 0.102 m⁻¹, which was only 0.73% that of water migration. B(a)P penetrated the 16 m depth vadose zone for 63 years owing to the attenuation function of adsorption and biodegradation, with contribution ratios of 78.4% and 19.3%, respectively. When B(a)P intersected with groundwater, driven by groundwater flow, the migration of B(a)P was mainly in the direction of groundwater flow. The migration rate of B(a)P in groundwater along the direction of groundwater flow was 6.65 m/a, which was 2.42 times and 16.22 times of the diffusion rate in the vertical groundwater flow direction and vertical downward direction, respectively. The spatiotemporal distribution indicated that B(a)P concentration decreased with the crow-fly distance from the river with attenuation rate constants of 1.19×10⁻⁴, 3.05×10⁻⁴, and 3.67×10⁻³ m⁻¹ in parallel groundwater flow direction, vertical groundwater flow direction and vertical downward direction, respectively, which were negatively correlated with migration rate. However, B(a)P content increased over the extension of infiltration time with an accumulation rate of 7.3×10⁻² d⁻¹. The migration and accumulation of B(a)P induced potential harm to coastal residents taking groundwater as drinking water, which would result in the groundwater safety utilization range decreasing from 438, 276, and 19.8 m to 568, 324, and 27.7 m far from the river in parallel groundwater flow direction, vertical groundwater flow direction and vertical downward direction, respectively, 20 years later.