Does sand mining affect the remobilization of copper and zinc in sediments? - A case study of the Jialing River (China).

It is generally accepted that the sand mining industry causes severe destruction in river basin environments. In this study, six sediment cores were collected, and sequential extraction was applied in conjunction with the diffusive gradients in the thin films (DGT) technique to explore the effect of sand mining on the remobilization of Cu and Zn in the sediments. The results showed that Cu and Zn were mainly bound in the residual fraction in the sediments. CDGT-Cu/Zn in the sediments presented obvious increasing trends at the bottom (-9 to -12 cm) at the four sites that experienced sand mining and a decreasing trend at the sites with no sand mining disturbance. Cu and Zn also tended to be transported from the sediments to the overlying water at the four sand mining sites. A correlation analysis found that F1 and F3 correlated well with CDGT-Cu/Zn, indicating that the water/exchangeable fraction and oxidized fraction were the main fractions that led to increases in DGT-labile Cu and Zn in the sediments. Further analysis found that the introduction of oxygen (O2) was the main reason for the simultaneous release of sulfur (S), Cu and Zn in the sediments, as indicated by the "dark area" of AgI gel appearing at the same position as the "hot spot area" of Chelex gel. Two main sand mining effects on the release of Cu and Zn were hypothesized: (1) intense sand disturbance leads to the transfer of the water/exchangeable fraction (F1) to the DGT-labile fraction and (2) O2 introduction promotes the reaction of stable sulfide (F3), thus transferring it to the DGT-labile fraction. The above results indicated that the sand mining industry should be paid much attention in the Jialing River, as it can obviously cause labile Cu and Zn release into the water.