Controls on the Spatial Distribution of Trace Metal Concentrations along the Bedrock-Dominated South Fork New River, North Carolina

In marked contrast to alluvial rivers, few studies have examined the physical and geochemical controls on the spatial distribution of toxic trace metals along bedrock channels. This study examined the factors controlling the geographical pattern of selected trace metal (Cu, Cr, and Zn) concentrations along the bedrock-dominated channel of the South Fork New River (SFNR). The SFNR is located in the Blue Ridge Physiographic Province of North Carolina, and is representative of many rivers in mountainous terrains that are often subjected to the influx of toxic trace metals from historic and contemporary mining operations. The topography of the SFNR’s channel bed is highly variable and can be subdivided into pool and shallow bedrock reaches. The latter contained localized cascades characterized by topographically higher bedrock ribs that are separated by topographic lows, both of which are oriented oblique to flow. Accumulations of bed sediments are predominantly associated with the traverse bedrock ribs that generate high hydraulic roughness. Except for a few localized zones of enrichment, sediment-associated trace metal concentrations tended to vary within a narrow range of background values over the 36 km study reach. Elevated trace metal concentrations were closely linked to zones of high Fe and Mn concentrations, and were associated with pools located within or immediately downstream of bedrock cascades. The elevated concentrations of the metals appear to be derived from the erosion of lithologic units within the cascades that contain sulfidic layers or zones of mafic mineral enrichment, and which are known to occur in the underlying bedrock. Once eroded, these minerals and/or rock fragments were deposited within low-velocity zones created by the transverse ribs or within downstream pools. The enrichment of trace metals downstream of the cascades may also be due to the formation of Fe and Mn oxyhydroxides as turbulent flows aerate river waters as they traverse the cascades. Chemically reactive fine-grained (<63 µm) sediments had a relatively limited influence on the downstream variations in metal concentrations, presumably because the channel bed sediments are composed primarily of sand-sized and larger particles. Although a principal component analysis (PCA) suggested that reach-scale variations in channel and valley morphology may have partly influenced downstream variations in trace metal concentrations, the geographical patterns were primarily controlled by local geological and geomorphic factors associated with the bedrock cascades. The design of future sampling programs along such coarse-grained, bedrock rivers should consider the significance of these local controls on trace metal storage to effectively characterize and interpret downstream patterns in metal concentrations.

Univariate and Multivariate QTL Analyses Reveal Covariance Among Mineral Elements in the Rice Ionome

Rice provides more than one fifth of daily calories for half of the world’s human population, and is a major dietary source of both essential mineral nutrients and toxic elements. Rice grains are generally poor in some essential nutrients but may contain unsafe levels of some toxic elements under certain conditions. Identification of quantitative trait loci (QTLs) controlling the concentrations of mineral nutrients and toxic trace metals (the ionome) in rice will facilitate development of nutritionally improved rice varieties. However, QTL analyses have traditionally considered each element separately without considering their interrelatedness. In this study, we performed principal component analysis (PCA) and multivariate QTL analyses to identify the genetic loci controlling the covariance among mineral elements in the rice ionome. We resequenced the whole genomes of a rice recombinant inbred line (RIL) population, and performed univariate and multivariate QTL analyses for the concentrations of 16 elements in grains, shoots and roots of the RIL population grown in different conditions. We identified a total of 167 unique elemental QTLs based on analyses of individual elemental concentrations as separate traits, 53 QTLs controlling covariance among elemental concentrations within a single environment/tissue (PC-QTLs), and 152 QTLs which determined covariation among elements across environments/tissues (aPC-QTLs). The candidate genes underlying the QTL clusters with elemental QTLs, PC-QTLs and aPC-QTLs co-localized were identified, including OsHMA4 and OsNRAMP5. The identification of both elemental QTLs and PC QTLs will facilitate the cloning of underlying causal genes and the dissection of the complex regulation of the ionome in rice.

Assessment the Seasonal Variability and Enrichment of Toxic Trace Metals Pollution in Sediments of Damietta Branch, Nile River, Egypt

This work appraises the extent of toxic trace metals and seasonal pollution degree in Damietta branch sediments of the River Nile of Egypt. The toxic trace metals Fe, Mn, Cd, Co, Cu, Ni, Pb, and Zn were analysed in sediments from six sites during the summer and winter seasons. The metal concentrations and organic matter were determined using inductively-coupled-plasma mass spectrometry and loss-on-ignition, respectively. Multivariate statistical methods were used in order to allocate the possible metals sources and their relationships in sediments. The seasonal mean sequence of toxic trace metals was: Fe > Mn > Zn > Pb > Cu > Ni > Co > Cd. The mean Cd, Pb, and Zn values exceeded the sediment quality guidelines and average shale and they represent severe potential toxicity for aquatic organisms. Cu and Co were enriched during winter. The geo-accumulation index stipulated that metal pollution degree in the sequence of: Pb > Zn > Cd > Co > Cu > Mn > Ni > Fe. The highest metal pollution index reported in winter in sites S4/S5 and during summer in sites S4–S6. Different agricultural, wastewater discharge, fisheries, and industrial activities, as well as the effect of dilution/concentration during summer/winter seasons, are the main factors that contributed to metal accumulations in Damietta branch sediments. Continuous monitoring and evaluation of toxic trace metal concentrations of the Damietta sediments and similar localities worldwide can help to protect the ecosystem from harmful metal contaminations.