The Accumulation and Transformation of Heavy Metals in Sediments of Liujiang River Basin in Southern China and Their Threatening on Water Security

Abstract

Heavy metal (HM) pollution in sediments is tightly related to the security of water quality in rivers, but the accumulation and conversion of HMs are poorly researched, so that a field study was conducted as an example in the Liujiang River Basin. Seven HMs were analyzed to determine between the overlying water and sediments. Moreover, the regulation of HMs speciation and environmental factors in their accumulation and conversion were identified. The obtained results suggested the HM concentrations in water are far below the primary standard of water quality, but in sediments, the contents of Cd and Zn are significantly higher than their corresponding baseline of soil. Only Cd and Pb are dominantly in non-residual form (carbonate-bound fraction and reducible fraction, respectively). The non-significant correlations suggested pH and Eh may be hard to influence HMs in water, while the significant correlations highlighted the regulations of Eh, organic matter and mean grain size on the accumulation of metals in sediments. The opposite correlations between EC, TDS, pH and Cd confirmed the emission of acid wastewater contributed to the accumulation of Cd in sediment. The conversion of metals between water and sediments were found to be significant only in specific forms of Cd, As, Cu, Zn and Pb, suggesting the conversion of HMs in sediments should be largely regulated by their specific forms. The very high risk disclosed by the higher values of Eri and RI are only found upstream, while the higher risk of Cd should be treated as a critical environmental threat.

The Influence of the Reduction in Clay Sediments in the Level of Metals Bioavailability—An Investigation in Liujiang River Basin after Wet Season

“…Metals in reducible and oxidizable forms were generally stable and relatively non-bioavailable, so it could be very hard for them to exert their impacts on threatening environmental security directly. However, once the destabilization relevant to the reversed transformation of metals take place with the influence of environmental fluctuations [ 26 ], the aggregation of metals in exchangeable form and carbonate-bound form, which resulted from the destabilization of reducible and oxidizable forms, would have an indirect impact on metals’ bioconversion. In view of the great differences of metals’ transformation, the reversed transformation of metals in reducible and oxidizable forms should be detailed to clarify their interactions with metals’ bioavailability ( Table 3 ).…”

“…For Cr, the non-significant correlation suggested its bioavailability had not received the support from its oxidizable form, so that the elevation of its bioavailability should be blamed on the aggregation of exogenous Cr. In fact, the emission of exogenous Cr was reported to be widespread in Liujiang River Basin [ 25 , 28 ], but it preferred to stay in the water column, instead of sediments [ 26 ], which resulted in the relatively low ratio of Cr in non-residual forms. However, with the enhanced recharge from the alkaline groundwater in the karst catchment after the wet season, the elevation of carbonate in the water column aggravated the combination of exogenous Cr with carbonate ions, so that the carbonate-bound Cr in the water column would gradually aggregate to sediments in light of their low solubility, which just corresponded to the significant increase in Cr only in carbonate-bound form.…”

“…Despite the fact that the regulation of water conservancy may work in avoiding the excessive aggregation of contaminated sediments, the coarsening sediments after the regulation of water conservancy may also aggravate the destabilization of metals in sediments, and then increase metals’ bioavailability in sediments, which should be treated as a potential threat in the environmental security of Liujiang River Basin. In addition, metals’ bioavailability in sediments was susceptible to the elevation of Eh and DO in river [ 26 ], so that the fluctuation of water chemistry should be not overlooked during the regulation of metals’ contamination in the watershed of Liujiang River. The monitoring of water chemistry and metals’ bioavailability should be simultaneously incorporated into the regulation of metals’ contamination in the watershed of Liujiang River, which is of great value to improve the regulation of metals’ contamination in river and safeguard the environmental security.…”

“…More than 350 million tons of wastewaters from the extensive industrial operations were discharged into the watershed of Liujiang River annually, which eventually elevated metals’ contamination in the Liujiang River and its tributary, the Luoqingjiang River [ 25 ]. In recent years, the strict restriction on sewage discharge has substantially improved the water quality in these waterways, but exogenous metals still largely remained in the sediments, most of which were primarily preserved as non-residual forms [ 26 ]. It is well-known that metals in non-residual forms were treated as the critical parts to support their bioconversion [ 12 ], so that the contamination of metals in sediments gradually elevated metals’ bioaccumulation in the fish of the Liujiang River Basin [ 1 ].…”

Environmental Assessment Impact of Acid Mine Drainage from Kizel Coal Basin on the Kosva Bay of the Kama Reservoir (Perm Krai, Russia)

“…The content of each individual TE in sediments is calculated using the individual and ecotoxicological indices. The values of geoaccumulation index (I geo ), enrichment factor (EF), single-pollution index (PI), contamination factor (CF), and potential ecological risk factor (E i r ) can be used to classify sediments into several classes based on pollution level [28][29][30][31][32][33][34]. Complex pollution and ecotoxicological indices (sum of contamination (PI sum ), Nemerow pollution index (PI Nemerow ), pollution load index (PLI), average single-pollution index (PI avg ), background enrichment factor (PIN), multi-element contamination (MEC), contamination security index (CSI), degree of contamination (Cdeg), and potential ecological risk (RI)) are used to determine sediment contamination based on the content of more than one TE or a sum of individual indices [34][35][36][37][38][39].…”