A 150-year phosphorus budget for the Thames catchment, UK

Phosphorus (P) is critical for food production but rising P inputs to agricultural land have contributed to eutrophication of fresh and marine waters. Concurrently, wastewater effluent from increasing populations has also become a major P input to natural waters, particularly in urbanised catchments. This study considers the long-term phosphorus budget of the River Thames catchment from 1867 to the present. We combine databases of agricultural land use, human population and river monitoring to develop a phosphorus budget model for the gauged catchment area (9,948 km²) and identify key inputs, outputs and transfers over the period. We quantify P imports and exports of fertilizer, food, feedstuffs, and industrial products (1867-2017), along with direct discharge of fluvial P at the tidal limit (1936-2017).

Net P input to land from animal production was essentially stable at ~1,700 tonnes P until 1940, after which there was a steady rise, peaking at approximately 3,800 tonnes P in the early 1970s. Since then, P inputs to land have fallen to a current stable level of ~2,200 tonnes P. This represents a cumulative net input to land of 350 kT P since 1867. Whilst this input is somewhat counterbalanced by losses to the fluvial system and crop harvest, there is nevertheless a large P legacy in catchment soils.

Net inputs from wastewater (urine and faeces) rose steadily from 0.8 kT in 1936 to 2 kT in 2010, whilst the marked change occurred in relation to P in detergents rising from zero in 1950 to a peak of ~2kT in 1987, since when there has been a gradual decline to <1 kT at present. The total wastewater effluent contribution rose from 0.8 kT in 1936 to a peak of 3.4 kT at the end of the 1980s. The Urban Waste Water Treatment Directive (91/271/EEC) enforced enhanced removal of P in wastewater from the early 1990s, which led to an immediate, sharp decrease in wastewater contribution of 1 kT P since when there has been a steady decline to 0.4 kT at present. This has shifted the environmental pathway of wastewater P from discharge to rivers to accumulation in sludge which is now largely disposed of by application to agricultural land thus adding to the P legacy in catchment soils.

 

Our analysis of the Thames P budget will end with a discussion of uncertainties in the P model, and the sensitivity of our overall conclusions to assumptions about model structure and parameters applied to our historical records.