Global silicate weathering flux overestimated because of sediment–water cation exchange

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removesCO2from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the netCO2consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid EarthCO2degassing fluxes need to be further investigated.

Linkages between catchment characteristics and river water chemistry at the locality scale in the Ikpoba River, Southern Nigeria

This study explored the linkages between catchment characteristics (topography, land cover, soil and geology), average water chemistry (pH, calcium, magnesium, sulphate, nitrate, ammonium, orthophosphate, iron, zinc and lead) and rainfall in the Ikpoba River catchment, southern Nigeria, using statistical and locality-based GIS analyses. The results show that sites with high rainfall and percentage cover of arable and sand-gravel-clay lithologies were characterized by high acidity. There were strong links between average nutrient (𝑃𝑂43−-P, 𝑁𝐻4+-N and 𝑁𝑂3−-N) concentrations and diffuse agricultural sources in the catchment. Rainfall was strongly related to 𝑆𝑂42−, 𝑃𝑂43−-P and 𝑁𝐻4+-N suggesting that atmospheric deposition may influence their riverine concentrations. Results also suggest that decomposition of organic matter from forest stands was a significant driver of nutrient concentrations. Although metals (Fe2+ and Zn2+) were positively related to bedrock geology of sand-gravel-clay, there was no clear link between Pb2+ and the catchment characteristics investigated. Wetlands was found to be attenuating river water chemistry especially 𝑆𝑂42−, 𝑃𝑂43−-P and 𝑁𝐻4+-N concentrations. To underpin current environmental protection strategies, there is need to integrate a GIS-based analysis approach with monitoring data to fully identify the variability patterns in river water chemistry dynamics at local and multiple scales of water resource management in Nigeria. Key words: river catchment, water quality, metals, nutrients, GIS.

Spatiotemporal hydrochemical variations in river water in the Qilian Mountains and their sources: a case study of the Binggou River Basin

The headwater region of inland China is immensely important for sustaining livelihoods and maintaining ecological balance, highlighting the need to characterize and assess water quality in this region. The aim of this study is to acquire a comprehensive understanding of the spatiotemporal variation characteristics of river water chemistry and to identify the sources of major ions in the Binggou River Basin, Northwest China. The results show that the water of the Binggou River is neutral freshwater. SO42− is the major anion, Ca2+ is the major cation and the river type is Ca–Na–SO4–HCO3. The concentrations of Na+, Mg2+, SO42−, NO3−, and total dissolved solid are slightly higher than the global average. The seasonal variations of major ion concentrations in the river are highest in winter but lowest in autumn, whereas the spatial variations are greater in the east branch than in the west branch and upstream compared with downstream. Source analysis of the ions indicates that rock weathering is the main source of ions, followed by human activities such as farmland fertilization and coal burning. Water quality analysis shows that the river water is suitable for drinking and irrigation, but the water quality is relatively poor in areas with more human activity, which indicates that human activity greatly influences water quality.