Nitrates in water can harm newborn babies and trigger the rapid growth of algae in rivers, killing fish and choking plants. As a result, nitrates are removed from water supplies...but the amount of nitrates in the River Thames has trebled since the 1930s.

Scientists wanted to know why - so a team from the universities of Durham, Cranfield and Bristol has been studying how nitrates move through the land to end up in rivers, springs and other sources of water. To do this, they used a paper trail of archives containing records of water quality dating back 140 years.

Planet Earth podcast presenter Sue Nelson met two of the team members on the bank of the Thames in Central London - Dr Nicholas Howden, a senior lecturer in water at the University of Bristol and Tim Burt, Professor of Geography and Dean for Environmental Sustainability at Durham University. Sue began by asking Nicholas where the nitrates in the Thames have come from...

Nicholas - A proportion of the nitrate in the Thames has come from sewage effluent discharges. That has largely been controlled in the last 20, 30 years, there have been lots of new treatment processes put in place. A lot of that comes from point source discharges, so we can see them. We know where they are, we know who's discharging them and they've been largely cleaned up as much as they can be and regulated. The component that we've been mostly interested in, is looking at the diffuse sources - so those are the sources that are widely distributed throughout the Thames catchment. That's one of the largest catchments in the UK, and it's some 10,000 square kilometres. So it's a very big area and that predominantly is coming from our agricultural activity to grow food, so ploughing, fertilizing, use of animals and the natural processes of biological fixation and also, some atmospheric deposition as well. So, it's a combination of factors that have come from our use and our more and more intense use of the river catchment from which all of this water drains to then end up in the Thames.

Sue - How does ploughing release more nitrates into the soil, Tim?

Tim - Well ploughing mineralises organic matter, it basically aerates the soil and you get organic matter being oxidised and that works through to first of all, ammonium salts and then though to nitrates. So it's really the aeration of the soil. It's what, in a sense, ploughing is actually about turning organic matter into nutrients which can then grow crops.

Sue - So does this mean then that during World War II, for example, when everybody was being encouraged to 'Dig for Victory', did you notice an accompanying spike of nitrate levels?

Tim - Yes, we did. In fact, in the first war as well, but the second world war in particular. There wasn't much use of fertiliser other than animal manures then, but the mere fact of ploughing and widespread ploughing, a huge increase in the area of land under the plough and that does show through. It showed through to an extent straight away, but it also showed through in a big delayed response which came several decades after the war. But yes, you can definitely see a link between 'Food for Victory' and all those sorts of campaigns and the water quality in the Thames decades later.

Sue - I know that, Nicholas, one of the important things here is about when you saw the link as well. It took time to get those nitrates from the surrounding land into the River Thames.

Nicholas - Yes, that's right. So what we've tried to do is to estimate what the inputs were in any particular year and then we've used this long record of what was going on in the river to try and match the inputs to the outputs using a series of stores. So in this case, two stores - one which is run off, so that's the water that hits the land and it makes its way to the river in that year. Then there's a second store which is a ground water pathway and so the water that falls in any particular year takes nitrate with it and then it ends up in the river some time later.

In producing our model, we allowed that delay time to vary up to a number of decades from zero to about 50 years or even more. What we found was, by using the estimated pattern of inputs that we'd come up by considering what fertilizers were used, how much ploughing there was, how many animals there were in any year, we were able to match that input to what we actually observed in the river, By doing that, we can estimate what these different delay times are and how long it takes for something we do in, say, 1940 to reach the river. It turns out the answer is, it reaches in about 1970 and the split is roughly 50:50. So a 50% of what goes onto the land comes out in that year and 50% is delayed by approximately 30 years.

Sue - That's incredible though isn't it?

Nicholas - Yes it is and we think this is very important because a lot of what we do, we think that if it doesn't have an effect shortly after we've done it then it doesn't have an effect at all. Whereas in actual fact to deal with this problem, we are going to have to think in terms of 50-year cycles, and that's because we're dealing with a natural system that has this natural time constant and it's very important that we start to understand this when we're setting policy objectives, when we're trying to understand how what we do now will affect the future.