Scientists have assembled a remarkable record of water quality in the Thames River that stretches back over 140 years.
The archive is thought to be the longest for any river in the world. The Bristol University-led team says the record provides a true baseline against which measures to keep the great river clean can now be judged.
But it also illustrates just how intractable the issue of nitrate pollution has become. Over the 140 years of the new time series, this nitrogen component of river chemistry is seen to climb relentlessly. The pollution will almost certainly have altered the ecology of the river, shifting the balance of plant life in the Thames. “Generally, Thames water quality has been improving – the fish are returning,” Nicholas Howden explained. “But the big challenge that remains is diffuse pollution, and the UK remains the global hotspot for nitrate flux. We have the highest export for nitrate for any country in the world per capita or per land area, depending on how you want to express it. And some of the highest concentrations are coming out through the Thames.”
The presence of nitrate in the water is a consequence of land management decisions and farming practices. Plowing fields opens up organic matter to oxidation and the release of nitrogen held in the soils. This will eventually wash through to the river, as will nitrogen applied to those fields in the form of fertilisers.
Clearly visible in the new time series are major land-use changes during the 20th century that gave enormous nitrate kicks to the hydrology of the Thames basin. The most obvious is the Dig for Victory policy of 1939 that saw 30 per cent of grasslands turned to arable production, as Britain sought to grow enough food to support the war effort. Also evident is the impact of the European Common Agricultural Policy in the 1960s and ‘70s that witnessed the intensification of farming powered by the widespread use of agrochemicals.
But the team’s analysis of the data throws up an astonishing feature in these kicks: They have two parts. One is an immediate effect, as surface runoff takes roughly 50 per cent of the free nitrate straight into Thames tributaries and the main river. The other is a delayed effect that sees the other roughly 50 per cent working its way through ground aquifers before washing out into the Thames.
“What we’ve found is that some of what we do to the land – we plow the fields and we add fertiliser – doesn’t go through to the river for decades,” Howden said. “There’s a time delay. The water has to percolate through the soil, it’s got to go through unsaturated rock, it’s then got to go to the aquifer, and then it’s got to be transported and discharged in the river.”
This pathway can be deferred for three decades or more. It means the 1939 kick seen in the nitrate “curve” also incorporates some of the delayed response from the land-use change brought about by the introduction of arable subsidies in 1917. The realisation that the effects of land-use changes in the Thames catchment take more than 30 years to show themselves fully has profound implications for policies to monitor and manage the river’s water quality. I
t means, for example, the benefits from the 1990s introduction of nitrate-vulnerable zones, which put controls on the application of manures to the land, cannot properly be assessed for at least another 15 years. The 140-year record (1868 to 2008) was assembled by drawing together and cross-calibrating at least six different data sets. The research, which also involved Cranfield University, was part funded by the National Environment Research Council.