Heavy metals in the upper reaches of the Thames River flow downstream towards the sea. But, what happens when these pollutants reach the estuary? Do they stay or do they go? The answer is less intuitive than you may think.
Imagine that you have to commute using two busy subway lines during the rush hour. To make it worse, you are carrying big and heavy luggage that slows you down. It happened once during my PhD. in London, I had to change from Central line to the Northern Line at the busy Bank station. In this situation, you basically can decide which channels inside the station will take you to the next platform and then you simply keep the pace with the streams of people around. Finally, you get to the desired platform and see the next train arriving completely full. You know that there will be a flush out of people and then a flush in, which followed by a short period to accommodate before the door closes. You are still far from the train. Thus, if you’d manage to get closer to the door, the time could be exactly just enough to allow you to board the train. Uncertain whether you have gotten close enough, every time the train arrives you ask the question: should I stay, or should I go? Surprisingly, a somehow similar situation happens with metal pollution in estuaries. There are tides flushing in and out and with various forms of metals slower than others depending on whether they are attached to sediment or any other particulate matter. But what is the fate of metals, do they stay or do they go?
Only on the late 1970s people realized the seriousness of the pollution of London water bodies, which were used as open sewers. As a result, many contaminants accumulated on the Thames water and sediments.
London is perhaps one of the most iconic and creative of European cities, with many of its landmarks like the Big Ben, Palace of Westminster, London Eye, and London Bridge directly connected to the Thames estuary. So, one might expect the river to be the focus of environmental concern. However, only around the late 1970s people really started to realize the seriousness of the pollution of London main water bodies, which were basically used as open sewers for domestic and industrial effluents. As a result, many contaminants accumulated on the Thames water and sediments. The organic matter that originated in the toilets of the millions of Londoners and polluted the Thames waters was slowly degraded by microbes and the river started to revive. However, the majority of metals are stable chemical elements. They might participate in several chemical reactions and still not degrade. That means that metal pollutants could stay for many years within the estuary, changing their chemical form, and eventually impacting the estuarine plants, fishes, birds, and the Londoners themselves. On the other hand, the problem would not happen if the riverine water and tides in the Thames would flush metals out. Thus, the whole issue of potential impacts on metals on the Thames depends on a single question: what forces drive the fate of metals in that estuary?
Someone might think: Well the Thames is a river, water flows from the river to the sea, then metal pollutants from the river should flow to the sea, right? Not exactly. There are numerous ways metal fate is affected in estuaries, as discussed in our review at STOTEN. Let’s take as an example the patterns in water flow, i.e. hydrodynamics. While the river does flow towards downstream, water dynamics in an estuary is much more complicated. In the estuary, water parcels face the tides that change completely the direction of the flow every few hours. The flow in (flood tide) and flow out (ebb tide) often occur at different speeds, and in between the two phases, there is a moment of accommodation, in which the heavier particles settle down. To complicate things, the salinity of the sea mixing with freshwater from the river also can exert a force that moves water (we call baroclinic pressure) that affects the flow. Add to this complexity, that metals are changing between dissolved (faster and mobile) and particulate (slower and immobilized) fraction because of the estuarine geochemistry. Thus, estimating whether a heavy metal is flowing to the sea or staying within the estuary might be not that straight.
Thames is a river, water flows from the river to the sea, then metal pollutants from the Thames river should flow to the sea, right? Not exactly.
That was exactly the question studied by Valentina Premier, an Italian engineer that collaborated with me during her master studies under the supervision of Prof. Marco Tofolon. Valentina developed a numerical model using the computer program Delft3D to resolve what was happening with the hydrodynamics and the water quality in the Thames Estuary. The grid for her model divided the Thames into thousands of units, and within each unit, she calculated the major physical and chemical forces affecting the fate of the metals. Using the powerful computers of the University of Trento she could compute the effect of hydrodynamics and water quality every fraction of second for the whole estuary during an entire year. Her findings are now published in the scientific journal Estuaries and Coasts under the title “A model-based analysis of the metal fate in the Thames Estuary”.
The results of this research were surprising! It seems that, within this period, most of metal pollution tend to accumulate in the central part of the estuary. That means that the natural behavior of the estuary concentrates heavy metal pollution within the city of London, close to major landmarks as the London Bridge. In her model, Valentina did not consider the city of London as a source of contaminants to the Thames, so the high contamination in the most urban reaches of the Thames was determined by the hydrodynamics. In fact, you can see in the models when tides come in and out as the areas highly contaminated (dark red) shift position. Thus, considering the time-scale that Valentina investigated, not only the metal mostly stays but also increases in concentration within certain areas of the estuary.
Valentina developed a numerical model to resolve what was happening with the hydrodynamics and the water quality in the Thames Estuary. The results of her research are surprising and have direct implications for managing heavy metal pollution in those waters.
Valentina’s findings have direct implications for managing the pollution in the Thames estuary. The findings mean that even without pollution inputs from London city, the water of the Thames within central areas will remain polluted for years because of the interaction between water hydrodynamics and polluted sediments. The sediments, that buffered much of the toxic metals from water during centuries of contamination, now can release them back to the water. This, in turn, may delay the environmental recovery of the whole ecosystem. The reality might be even more critical as the Thames receives many more historical and current pollution sources that we did not include in the analyses.
The concentrations of metals and many other pollutants are decreasing in the Thames since the 80s when authorities started to act on the issue. But it seems that we may need to do more if we want to see this marvelous ecosystem recovered to its full health soon. If you follow the debate on plastics and microplastics pollution you know that a big part of the problem is that plastics take a very long time to degrade, so even a low toxicity matters for environmental impacts. But now think of metals, they have high toxicity and take just as long (or longer) to degrade. Some metal pollution that was caused by early humans from Iron Age and even Neanderthals leave their footprints of until today. Let’s hope that the Thames Estuary would never become one of those places in which metal pollution simply does not go away.