The fluvial flux of total reactive and total phosphorus from the UK in the context of a national phosphorus budget:comparing UK river fluxes with phosphorus trade imports and exports

A national river water quality database of total reactive phosphorus (TRP) and total phosphorus (TP) and flow was used, together with catchment characteristic datasets (soils, land use and hydroclimatic properties), to derive national fluvial phosphorus (P) flux estimates for Great Britain (England, Wales and Scotland) from 1974 to 2012. These fluvial P fluxes were compared with P imports and exports, in fertilizer, food, feedstuffs, and industrial products, along with coastal direct discharge of wastes, at the British national boundary from 1990 to 2012. The results showed that: (i) Average annual river TP concentrations in Great Britain have declined from a peak of 0.27–0.1 mg P/l and annual river TP flux has declined from 120 to 16 ktonnes P/year (0.49–0.06 tonnes P/km2/year); (ii) Average river TRP concentration has declined from a peak of 0.19–0.05 mg P/l and annual river TRP flux has declined from 71 to 10 ktonnes P/year (0.29–0.05 tonnes P/km2); (iii) Over the period 2003–2012, even after the introduction of the Urban Waste Water Directive, 60 % of UK’s TP flux was still from urban areas; and (iv) In 1990, the fluvial flux of TP from the UK was equivalent to 41 % of imports; by 2012 this had decreased to 15 %. The UK (relative to its boundary) continues to accumulate P and, over the last 15 years, this accumulation has increased at an average rate of 0.6 ktonnes P/year2. Enhanced removal of P in waste water treatment has shifted the environmental pathway of sewage P from discharge to rivers to accumulation in sewage sludge, which is largely disposed of on agricultural land, and which could eventually provide a sustained legacy source of P to rivers for decades. However, a substantial proportion of P accumulation is via food waste into landfills.