Prediction of the impact of benzo[a]pyrene on shallow groundwater during natural infiltration of reclaimed water-receiving rivers: A case study of Liangshui, China.

The quality of groundwater along rivers is greatly affected by long-term infiltration from surface water, especially reclaimed water-receiving rivers. To predict the degree of influence of contaminated river water on groundwater quality, the spatiotemporal distribution and migration evolution prediction of benzo[a]pyrene (B(a)P) was monitored and simulated by Hydrus-coupled Groundwater Modeling Systems (GMS) model in terms of reclaimed water-receiving Liangshui River. The prediction results indicated the goodness-of-fit of this coupled model, according to the model efficiency (E: 0.78-0.93), the mean absolute error (MAE: 0.01-0.32 m) and the root-mean-square error (RMSE: 0.06-0.35 m). 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. 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. However, once B(a)P intersects with groundwater, the migration of B(a)P is dominated by horizontal migration due to downward movement along the groundwater flow direction. The migration rate of B(a)P in groundwater was 6.65 m/y in the horizontal direction, which was 2.42 and 16.22 times higher than the dispersion rate in the longitudinal and vertical directions, respectively. The spatiotemporal distribution indicated that the B(a)P concentration decreased with the crow-fly distance from river with attenuation rate constants of 1.19 × 10-4, 3.05 × 10-4, and 3.67 × 10-3 m-1 over horizontal, longitudinal, and vertical direction, respectively, which were negatively correlated with migration rate. However, the 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 health risks to groundwater-based drinking water safety, which resulted in the groundwater safety utilization range decreasing from 450 m, 283 m, and 20.1 m-583 m, 338 m, and 28.2 m far from the river over the horizontal, longitudinal, and vertical directions, respectively, 20 years later. This study provides a numerical modeling solution for the viable spatiotemporal evolution of B(a)P in groundwater and an effective decision-making tool for the safe utilization of groundwater as drinking water.