Background River sand is better suited for use in concrete, cement and mortar rather than sand from deserts or the seabed. Sand grains from deserts are often too rounded, due to the effect of wind erosion, to bind concrete well and oceanic sand is expensive to extract and process as it contains salt, which must be removed to prevent metal corrosion. While sand and gravel can be thought of as renewable resources that are formed by erosive processes over thousands of years, they are being mined at a greater rate than they are being formed. The amount of sand mined each year is not accurately recorded, but the United Nations Environment Programme conservatively estimates that more than 40 billion tonnes of sand and gravel are consumed globally each year, which makes sand mining the largest extractive industry in the world. Increased urbanisation, particularly in India and China, is driving demand for construction materials, especially cement and mortar. The extraction of sand from rivers, deltas and coastal regions also has an effect on the water security of regions in which large amounts of sand are extracted. In those regions, erosion has increased, water flows have changed, the water table has lowered and, in some cases, become polluted with surface water contaminants. Removing sand from rivers also deprives deltas of the sediment that holds back seawater and reduces the amount of nutrients, such as carbon, nitrogen and phosphorus, which are carried with the sediment. That decreases the fertility of delta regions and increases the risk of saltwater intrusion. Comment The Mekong, Yangtze and Ganges-Brahmaputra-Meghna river systems have experienced the sharpest decline in sand and sediment flow due to the construction of dams, which prevent sand and sediment from flowing downriver, and sand mining. The effects of that decline are most severe in the river deltas, where accelerated subsidence, erosion and seawater intrusion are likely to undermine food and water security. China is a voracious consumer of cement, which gives an indication of the amount of sand that it uses. From 2011 to 2013, it used more cement than the United States used in the entire twentieth century. Poyang Lake in Jiangxi Province is the site of the world’s largest sand mine, with an estimated 236 million cubic metres of sand extracted from it annually. The lake is part of the Yangtze River system and, partly because of the vast amount of sand that has been extracted from it, the amount of water flowing out of the lake into the river has almost doubled. That, in turn, raises the risk of riverbank erosion and waterlogging further downstream, which undermines agricultural land. The lake also supplies water to nearby rice fields and is a source of freshwater fish. Water levels fluctuate seasonally and parts of it often dry out entirely, however, it has dried up more frequently and earlier in the year since the 1950s. Sand mining is worth an estimated 150 billion rupees ($2.86 billion) per year in India and the industry is dominated by organised crime groups, known as “sand mafias”. While there is no data available on the amount of sand extracted each year, domestic demand for cement has increased from 1.4 billion tonnes in 1994 to 4.8 billion tonnes in 2016. Increased sand mining in the Ganges-Brahmaputra-Meghna river system contributes to the subsidence of the delta region in Bangladesh, one of the most densely populated regions of the world. As a result, ground and surface water supplies are at risk of saltwater intrusion. Erosion has increased in the Mekong Delta over the past decade as a result of the construction of dams and an increase in sand mining. Sand mining has increased so significantly that the Vietnamese Institute of Transport Science and Technology warns that the supply of sand is nearing an end and could be depleted by 2022. Vietnamese food security is closely linked to the health of the Mekong Delta, as half of its food supply is sourced from the region. With 90 per cent of its rice crop produced in the delta region, any decline in the health of the delta is also likely to affect that important component of the Vietnamese food supply. As Vietnam is the second-largest rice producer, it is also likely to have implications for the global rice supply. Global demand for sand is likely to continue to increase, mainly as a result of rising demand for construction materials. That is likely to reduce the amount of sediment in delta regions, which will reduce agricultural productivity and threaten water supplies due to an increased risk of saltwater intrusion.