Numerical modeling of the impacts of reclaimed water recharge to the Chaobai River channel on the ambient shallow groundwater

Reclaimed water has been widely used to recharge river channels in Beijing, with 2.3×10⁸ m³ of reclaimed water recharge to the Shunyi section of the Chaobai River from the end of 2007 to 2017. It has a high background value of pollutants (Cl⁻ concentration ranging from 62 to 122 mg/L) and recharges the ambient aquifers, which has an impact on the ambient groundwater through riverbank filtration, particularly on the shallow groundwater. Based on 10-year series groundwater monitoring data (2007−2017), we conducte a 3D groundwater flow and solute transport model to simulate the variations of the shallow groundwater table and chloride concentrations, and analyze the variations in groundwater storage, Cl⁻ loads, and NO₃-N loads in the shallow aquifers near the Chaobai River receiving reclaimed water. The results demonstrate that the ambient groundwater table swiftly rose by about 3−4 m following the reclaimed water replenishment from 2007 to 2009, and stayed steady under the condition of continuous replenishment of reclaimed water. However, affected by the exploitation of deep groundwater, the groundwater storage in the unconfined aquifer still decreased overall from 2007 to 2014. After reducing the groundwater extraction since 2014, the shallow groundwater storage recovered from 3.76×10⁸ m³ at the end of 2014 to 3.85×10⁸ m³ at the end of 2017. Cl⁻ concentrations in the ambient shallow groundwater changed from a range of 5−75 mg/L before the recharge to 50−130 mg/L after the recharge (2007−2009), and then remained stable. The areas of shallow groundwater quality affected by reclaimed water infiltration expanded from 11.7 km² in 2008 to 26.7 km² in 2017. The Cl⁻ loads in the affected areas increased from 1.8×10³ t in 2008 to 3.8×10³ t in 2017. The NO₃-N loads decreased from 29.8 t in 2008 to 11.9 t in 2017. Although the results show that the shallow groundwater quality outside the affected area is not significantly affected by the reclaimed water, the potential salinization and pollution cannot be ignored and need to be further clarified in the subsequent studies.