Understanding whether farmland or sewage effluent are the most important sources of phosphorus (P) in freshwaters is the first step in reducing levels. Over-enrichment or eutrophication is one of the major water quality problems which will have to be addressed before 2015 if the UK is to meet the water framework Directive's goal of good ecological status. Too much P leads to excessive growth of algae in freshwaters.
The researchers from CEH Dorset, led by Mike Bowes and Professor John Hilton, obtained Environment Agency data for the Warwickshire Avon catchment detailing sewage effluent inputs, river flows and phosphorus levels across 58 monitoring sites. In terms of land use and population, the catchment is typical of much of lowland England, with 44% arable, 30% pasture and 14% urban land.
Using a geographical information system, the researchers modelled the inputs from sewage works and agricultural land using export coefficients taken from the scientific literature. The agreement with actual total-phosphorus levels was good, and the researchers went on to look at the proportions of P expected from sewage and agricultural sources.
The researchers found that sewage sources dominated the picture, with a strong correlation between high P levels and the predicted proportion of P from sewage works. And 44 out of 45 occurrences of extremely high P levels (over 5 milligrams per litre) occurred at sites where sewage was predicted to contribute over 66% P.
"Diffuse inputs are completely dwarfed by the sewage inputs," Mr Bowes noted. "If you want to tackle nutrients in a big river like the Avon, you need to deal with sewage; it is pointless to tackle diffuse inputs. Sewage sources are also particularly important because they increase concentrations in the summer and that is when the algae are growing."
Increasingly, P removal from sewage effluents is required under a 1991 EU Directive on urban wastewater treatment. The Government must designate waters affected by eutrophication as "sensitive" and water companies are then obliged to fit P stripping at works discharging into them if the works serve populations of over 10,000.
The study went on to examine what impact P stripping would have on the Avon. Only seven works exceed the 10,000 population threshold. They serve settlements such as Coventry, Warwick, Stratford, Rugby, Redditch and Evesham.
Assuming that stripping would remove 80% of sewage P, the load in the river at Evesham would be reduced by 52% or 378 tonnes per year. But sewage sources would still be responsible for half of the P remaining in the river.
With stripping at the seven large works, the number of "highly contaminated" monitoring sites - with average P levels over 1 mg/l - was only reduced from 15 to seven, and 46 sites still had P levels in excess of an "interim target for heavily enriched rivers" of 0.2mg/l set by English Nature for river conservation sites in 1997 (ENDS Report 266, p 12 ).
The long-term target to prevent eutrophication in an alluvial lowland river like the Avon is a maximum of 0.1mg/l, English Nature considers. So action to remove more sewage P would be required to meet good ecological status in most of the river.
The study looked at the impact of removing P at eleven smaller sewage works. This reduced the P load in the river at Evesham by only another 4% or 29 tonnes per year. However, this had a "dramatic" effect on P levels in some of the Avon's highly enriched tributaries. The researchers noted that while larger sewage works tended to discharge to the main river or close to it, smaller works were close to the headwaters where they had a profound impact on nutrients along the tributaries.
Following the additional P stripping, modelling showed that there was no longer a strong correlation between P levels and the proportion of sewage-derived P, suggesting that diffuse sources like farmland were then having a significant impact.
"A lot of funding goes into tackling diffuse inputs but we don't know what their overall contribution is," Mr Bowes concluded. "It looks as if the easiest and obvious way to produce a step change in P is to tackle sewage."