Seasonal recharge mechanism of the upper shallow groundwater in a long-term wastewater leakage and irrigation region of a river alluvium aquifer
Abstract. Understanding the mechanisms that control seasonal groundwater recharge at local and intermediate scales is critical for understanding contaminant transport. Preferential flow accompanied with intensive and seasonal recharge allows contaminants to migrate rapidly through the unsaturated zones to underlying aquifers. In this study, we investigated the recharge mechanism from multiple water sources to the upper shallow groundwater of alluvial aquifers at the Tanghe Wastewater Reservoir (TWR) with a length of 17.5 km in the Xiong'an New Area, North China Plain. To do so, we sampled 30 m deep soil porewater profiles and groundwater boreholes perpendicular to the TWR. We traced the recharge processes and related pollutants transport pathways based on stable water isotopes (2H and 18O). The stable isotopes in porewater of the soil profiles revealed vertical recharge rates ranging from 63 to 109 cm/year for the layered unsaturated zone with silt and silty clay loams. However, fast flow (i.e., preferential flow or lateral flow) occurred in sand aquifers with the mixing of slow translatory flow with multiple recharge sources (i.e., precipitation, irrigation water, and wastewater remaining in porewater). The distribution of δ2H and δ18O relationship of the upper shallow groundwater showed two hysteresis loops which reflected the impact of fast flow with seasonal variation and translatory flow with legacy water in soil: 1) groundwater in regions affected by the TWR wastewater leakage shows a narrow loop and a nearly straight line with end-members of precipitation, which recharged to groundwater by fast flow, and evaporated porewater crossing with the TWR evaporation line; and 2) groundwater in irrigated farmlands with low and high irrigation amounts and strong evaporation shows a concentrated loop overlapping with shallow porewater, suggesting the impact of porewater persists in shallow soil on groundwater. The recharge type of fast flow determines seasonal variation of groundwater SO42- and NO3-. The SO42- concentrations in groundwater were diluted after recharge of fast flow and then increased due to the contribution of slow flow in porewater. However, NO3- increased as the fast flow carried pollutants from shallow soils to the aquifer. Measures are needed to prevent contamination caused by preferential flow in river alluvium aquifers at local scales which can extend to regional scales.