Which activity contributes to water pollution more than any other throughout…

Concept — how water-pollution sources are classified and compared. Sources of water pollution are classified by the pathway a pollutant takes to reach water. A point source discharges at one identifiable outlet — an effluent pipe, a sewer outfall — so its load can be metered and intercepted at the place of release. A non-point, or diffuse, source is washed off broad land surfaces across a whole catchment, so there is no outlet at which it can be metered or intercepted. Which source dominates globally is settled by the loads recorded in international water assessments, not by land area alone: a diffuse source becomes the dominant one only when a very large area is also treated intensively with the pollutant in question.

Application — what the global water assessments record. Farming applies nitrogen and phosphorus fertiliser, manure and pesticides across the world's cropland and pasture, and part of that input leaves the land as diffuse runoff and leaching into rivers, lakes and groundwater. The FAO/IWMI global review "More People, More Food, Worse Water? A Global Review of Water Pollution from Agriculture" (2018) records that in most high-income countries and many emerging economies agricultural pollution has overtaken contamination from settlements and industry as the major factor in the degradation of inland and coastal waters, and that nutrient runoff from farming is the leading cause of eutrophication of surface waters. UN World Water Development Report assessments likewise identify agriculture as the principal source of nitrate contamination of groundwater. The activity asked for is therefore Agriculture, which is also the value recorded in the official answer key for this paper.

Cross-check — what each of the other activities contributes.

• Hydroelectric power generation: a dam alters the flow regime, traps sediment upstream and changes the temperature and dissolved-oxygen profile of the water it stores and releases. These are physical and habitat changes concentrated at the impoundment, and almost no nutrient or chemical load is added, so it is not a candidate for the largest global contribution.

• Industry: effluent carries heavy metals, solvents, dyes and waste heat, and it can be severe close to a discharge point. It is generated at a finite number of identifiable sites, however, and the assessments cited above place industrial contamination below agricultural pollution as a factor in the degradation of inland and coastal waters.

• Urbanisation: sewage and stormwater carry organic matter, nutrients, detergents and oils, and a large share of the world's wastewater is still released without treatment, which makes settlements a major source in their own right — but the same assessments place agriculture ahead of settlements as the dominant contributor.

Result. Agriculture contributes to water pollution more than any other activity worldwide.