Despite being the world's most widely used synthetic nitrogen fertiliser, urea remains virtually unmonitored as a distinct aquatic pollutant. Although it accounts for approximately 60% of global nitrogen fertiliser consumption, monitoring programmes predominantly quantify total nitrogen, ammonium, and nitrate, overlooking dissolved urea despite its distinct transport pathways and biogeochemical behaviour. This gap is especially significant in India, the world's second-largest fertiliser consumer and third-largest producer, which consumed 20.46 million metric tonnes (MMT) and produced 31.41 MMT of urea in 2023–24, approximately 62% of total fertiliser output. China and India together account for nearly half of global urea consumption, concentrating nitrogen-loading risk in South Asian river systems. Yet no published baseline measurements of dissolved urea exist for Indian rivers, preventing assessment of its contribution to riverine nitrogen export and coastal nutrient enrichment.
High urea concentrations adversely affect aquatic ecosystems through multiple pathways. Urea is directly assimilated by bloom-forming phytoplankton at concentrations as low as 1 µmol N L⁻¹, stimulating harmful algal blooms (HABs), reducing water transparency, and promoting hypoxia following microbial degradation. Evidence from comparably intensive systems illustrates the scale of risk: tributaries feeding Chesapeake Bay recorded urea concentrations up to 50-fold above baseline during spring fertilisation, while rivers in Mexico's Yaqui Valley lost 20–40% of applied urea-nitrogen to runoff within days of application. Urea hydrolysis also produces ammonia, impairing fish growth, feeding, and metabolism, while urea-derived nitrate poses recognised health risks, including methemoglobinemia, via contaminated drinking water. These impacts threaten aquatic biodiversity, fisheries productivity, and water security.
This review compiles evidence on urea production and consumption, focusing on its transport from agricultural areas to aquatic environments, and the ecological consequences along the land-river-ocean nitrogen continuum. It highlights the impact of monsoon-driven runoff and extreme rainfall on urea export to coastal waters, where it contributes to eutrophication and hypoxia, similar to inland effects. However, its role in India’s coastal nutrient loading is not quantified due to a lack of specific monitoring. Therefore, recognizing urea as a priority pollutant in national water quality frameworks is crucial. Establishing baseline urea concentrations in Indian rivers can enhance nitrogen source attribution and aid in managing coastal eutrophication.