How exposed are the world’s rivers to pollutant chemicals discharged from domestic sources?

CEH scientists Andrew Johnson, Virginie Keller and Richard Williams blog about their new paper examining pollution in the world’s rivers.

All around the world we use more and more chemicals in toothpaste, drugs, shampoo, hormones for cancer treatment and as a means of contraception. All these chemicals go "down-the-drain" and a proportion escape from sewage treatment plants before entering rivers and potentially affecting wildlife. A good example is the issue of sex hormones discharged by the human population resulting in male fish displaying female characteristics (e.g. egg proteins in their blood, egg sites in their testes), which caused alarm around the world.

One way of finding out the scale of national exposure to such chemicals is to get chemists to test for every chemical in every river in their country. Such monitoring campaigns are extremely challenging and very costly to carry out. Even in countries such as the UK, which has a small geographical area and leads the world in environmental monitoring, the level of effort required is difficult to sustain. For developing nations the expense of such monitoring campaigns means such information and knowledge is beyond reach.

Our new study, recently published as an open access paper in the journal Environmental Toxicology and Chemistry, used a geographic-based model which compares population with river flow to assess the available natural dilution for the human wastes that contain chemicals. The less dilution available within a nation, the higher the exposure levels to these chemicals will be.

As we all know rainfall patterns can vary dramatically throughout the year, and from year to year. The model used long-term annual and monthly averages of river flow (based on up to 60 years of measured rainfall) to generate a range of dilution factors.

Many nations have poorly populated and densely populated regions. Thus, for each nation, a variety of dilution factors were examined (expressed as a range of percentiles). The spatial variability of the dilution factors can vary considerably depending on the percentiles – defining low to high flows – and types of flow considered (annual or monthly).

Predicted values of national annual median (50%ile) dilution factor

The results are interesting. In Europe, the UK median dilution factor of 37 puts it in the upper third of nations exposed to domestic chemicals. However, the 25%ile results – a measure of low flow dilution – give the UK a dilution factor of only 6, making it the most exposed country. This is a consequence of our relatively dry summers and high population densities in the South and Midlands. By way of contrast the French median dilution factor is 76 and 25%ile dilution factor is 47. This tells us that French fish have a lot less to worry about than British fish!

The paper is open access so readers can look up their exposure to chemicals by examining the tables we’ve created. We hope that environmentalists and regulators from countries across the world, as well as the general public, are able to use the results, to better address this important issue.

Andrew Johnson, Virginie Keller and Richard Williams

Additional information

The full paper reference is:

Keller V.D.J., Williams, R.J., Lofthouse, C., Johnson, A.C. (2014). Worldwide estimation of river concentrations of any chemical originating from sewage-treatment plants using dilution factors.
Environmental Toxicology and Chemistry, 33, 447-452. doi: 10.1002/etc.2441

Staff page of Professor Andrew Johnson

Staff page of Richard Williams