Agricultural water pollution: key knowledge gaps and research needs Alexandra EV Evans 1 , Javier Mateo-Sagasta 2 , Manzoor Qadir 3 , Eline Boelee 4 and Alessio Ippolito 5 While water pollution is starting to receive the attention it deserves, the contribution of agriculture requires greater consideration as current agricultural practices have an unprecedented impact on water quality. This paper reviews knowledge in selected areas of agricultural water pollution (AWP) and identifies future research needs. These include source attribution, emerging contaminants, costs and incentives for adoption of pollution reduction measures. Future research priorities include identification and testing of locally appropriate markers; modelling the effects of contaminants on biota and pathways of microbial contaminants; harmonization of data collection and calculation of economic costs of AWP across countries and projects; and how to better share relevant knowledge to incentivize improved agricultural practices. Addresses 1 Independent Researcher, United Kingdom 2 International Water Management Institute, Colombo, Sri Lanka 3 United National University Institute for Water, Environment and Health, Canada 4 Deltares, PO Box 177, Delft, The Netherlands 5 European Food Safety Authority (EFSA), Via Carlo Magno 1A, 43126 Parma, Italy Corresponding author: Mateo-Sagasta, Javier (J.Mateo-Sagasta@cgiar.org) Current Opinion in Environmental Sustainability 2018, 36:20–27 This review comes from a themed issue on Global water quality Edited by Nynke Hofstra, Carolien Kroeze, Martina Flo ¨ rke and Michelle van Vliet Received: 23 June 2018; Accepted: 08 October 2018 https://doi.org/10.1016/j.cosust.2018.10.003 1877-3435/ã 2018 Elsevier Ltd. All rights reserved. Introduction Water pollution has worsened in many regions of the world with high pollution threats identified across Eur- ope, India, China, South America and parts of Africa [1]. This deterioration is expected to escalate increasing the threat to human health, the environment and sustainable development [2–4,5  ,6]. Cities and industries are well known sources of pollution, but recent assessments suggest that agriculture is among the leading causes of water pollution globally [4,5  ] being responsible for the discharge of large quantities of agrochemicals, nutrients [7], organic matter [8], drug residues [9], sediments [10], saline drainage inducing salinization and alkalinization [11,12], microplastics [13–15] and pathogens [16]. Water pollution is exacer- bated by the reduction of flow in water bodies [17,18], which is largely driven by agricultural needs for crop irrigation [19,20]. In some high-income countries agriculture has overtaken contamination from settlements and industries as the major factor in eutrophication of inland and coastal waters, and groundwater pollution with nitrates and salts [21,22]. For example, in the European Union (EU) 38% of water bodies are under significant pressure from AWP [23], in the United States of America (USA) agriculture is the main source of pollution in rivers and streams [24] and in China agriculture is almost exclusively responsible for groundwater pollution by nitrogen [25]. It is also an increasing concern to health and environment in many low-income countries, with pollution from pesticides a case in point [5  ,6]. The increasing contribution of agriculture to pollution can be attributed to several root factors including popu- lation growth, changes in demand for food and fuel, and climate change [22,26] (Figure 1). These have led to several changes in agricultural production including intensification, expansion of agricultural land and irri- gated areas, and greater use of agricultural inputs. As examples: the area equipped for irrigation has more than doubled since the 1960s to over 320 million hectares (Mha) [27]; the number of livestock has more than tripled since the 1970s to over 24.2 billion [28]; aquaculture has grown more than twenty-fold since the 1980s [29]; and land, water and other agricultural inputs (notably fertili- zers and pesticides), are used more intensively than ever before [21]. Globally, irrigation is the single largest pro- ducer of wastewater by volume, and livestock produce more excreta than humans [21]. Managing pollution and minimizing risk from agriculture will require great understanding of:  Sources and fate of pollutants  The changing landscape of pollutants – contaminants of emerging concern (CEC) and microplastics Available online at www.sciencedirect.com ScienceDirect Current Opinion in Environmental Sustainability 2019, 36:20–27 www.sciencedirect.com