The Maoniuping Rare Earth Elements (REE) deposit, the second largest light REE deposit in the world, has been mined for decades, with serious impacts on the surrounding environment. However, the impact of mining on heavy metals in the downstream area (Nanhe River Basin) has not been systematically documented. To address this issue, this study explored the extent, transport, and accumulation of heavy metal contamination in the Nanhe River Basin through field surveys (2946 topsoil samples and four vertical soil sections) and regional geographic attributes (e.g., mining area, river, and elevation) combined with a variety of methods such as statistics, geostatistics, spatial analysis, geo-accumulation index, and potential ecological risk index. The results showed that soils in the Nanhe River Basin presented different degrees of heavy metal pollution, with Pb and Cd being the most abundant, and the soils as a whole showed moderate-heavy ecological risks. The spatial distribution and correlation of heavy metals exhibited similar distribution patterns and sources. Further analyses revealed that mining of REE in Maoniuping was the main source of heavy metal pollution in the Nanhe River Basin, with heavy metals entering the soil through runoffs. At the same time, mining activities led to the migration of heavy metals in different directions in the Nanhe watershed, i.e., about 1.3 km horizontally, 16 km longitudinally, and more than 1 m vertically. In addition, about 38.1 km² of the watershed is contaminated by mine wastes, which is 6.6 times the size of the mining area. In order to mitigate the threat of heavy metals, the local government has implemented water diversion projects and crop conversion in the Nanhe River Basin. This study provides a reference for research on the environmental problems caused by the exploitation of REE mines and other mineral resources.

The geochemistry of karstic paddy soils significantly influences Cd bioavailability and its accumulation by rice. However, there is limited research on Cd translocation and accumulation in rice, grown in paddy soil with elevated CaO and Zn levels. Addressing this, a field-based study in Hanyuan County, Sichuan Province, China, examined the Cd distribution and translocation patterns across the soil–root–straw–grain continuum, aiming to elucidate how elevated CaO and Zn levels regulate Cd accumulation in grains. The geochemical characterization of the soil revealed elevated concentrations of cadmium (Cd), calcium oxide (CaO), and zinc (Zn), with mean values of 2.12 ± 2.93 mg/kg, 5.77 ± 5.80 wt.%, and 359 ± 472 mg/kg, respectively. The sequential extraction results (by Tessier) demonstrated that the distribution of Cd followed the general order: residual > Fe-Mn oxide-bound ≈ exchangeable ≈ carbonate-bound > organic matter-bound. The exchangeable Cd content ranged from 0.063 to 0.93 mg/kg, accounting for up to 21.4% of the total Cd on average. The correlation analysis, Bioconcentration Factor (BCF), and Transfer Factor (TF) of Cd in different rice tissues, and structural equation modeling (SEM) revealed that elevated CaO and Zn levels in soil exhibit synergistic effects in inhibiting Cd accumulation in grains. Specifically, the CaO in soil reduces the Cd bioavailability by the formation of carbonate-bound specie (CdF2), thereby inhibiting Cd uptake by roots. In rice tissues, Zn could limit Cd translocation, particularly from roots to straws. Consequently, the regional rice Cd exceedance rate (25.8%) was significantly lower than that reported for other similar karst soils in China (over 50%).