Cumulative Risk of Heavy Metals in Long-Term Operational Rain Garden

With the advancement of sponge city construction, rain gardens, as key facilities for concentrating and infiltrating rainwater runoff, have been widely established. However, the accumulation of heavy metals (HMs) in the fillers and the associated pollution risks cannot be ignored, which have a significant impact on the operational lifespan of these facilities. This study took the observation point (P) within a rain garden that has been in operation since 2012 and the control point (CK), which is the soil sample collection point in the natural infiltration area, as samples. Based on the monitoring data of HM content from 2017 to 2022, the pollution characteristics of Cu, Zn, and Cd were analyzed using enrichment factors and the geo-accumulation index, and the potential ecological risks were evaluated to reveal the impact of concentrated infiltration of runoff. The results showed that Cu and Cd accumulated in the 0&ndash:10 cm depth, while Cu and Zn exhibited seasonal annual variations, and the variation of Cd was not obvious. The study found that Cu and Zn were in a non-enriched state, while Cd was slightly enriched. Among the single ecological risk factor indices, the pollution levels of Cu and Zn were low, while that of Cd was relatively high. Comparison of the data from the observation point and the control point reveals that 88.9% of the data points of single ecological risk factor indices at each soil depth at the observation point are higher than those at the control point, revealing the impact of concentrated infiltration of rainwater runoff on the soil. However, the comprehensive assessment indicated that the overall ecological risk of the soil in the rain garden and the natural filtration area was at a low level. Nevertheless, given that the long-term operation of rain gardens may still pose pollution risks to the soil and groundwater, it is imperative to take timely measures to control HM pollution to ensure the long-term stable operation of sponge city facilities and the safety of the ecological environment.