Although there is hardly any acid rain any more, many inland waters around the world are still polluted with sulphate. Researchers have now compiled where this sulfur compound comes from today and what consequences the pollution has. According to this, the sulfate comes mainly from agriculture, but also from industry and open-cast lignite mining. And some consequences of climate change can also increase sulphate pollution.
“Acid rain” was a phenomenon of the 1980s: the burning of fossil fuels and industrial exhaust fumes released large amounts of sulfur into the atmosphere and reacted with water to form droplets containing sulfuric acid, which acidified the soil and water. After factories and power plants were retrofitted with flue gas desulphurisation systems, atmospheric sulfur emissions fell – in Germany by as much as 90 percent. Nevertheless, the sulfur pollution from sulphate in inland waterways remains almost unchanged – in some regions it has even increased.
Where does the sulfate come from?
Researchers led by Dominik Zak from the Danish University of Aarhus have now compiled an overview study where the sulphate measured today comes from. To do this, they evaluated global data on current sulfur sources from over 300 studies and created a general overview. Afterwards, the research team also formulated the negative global effects of sulfate on nutrient cycles, ecosystems and drinking water production. It is known that the weathering of minerals, volcanism or the decomposition of organic material naturally creates dissolved sulphate in inland waters. However, the researchers’ results suggest that to this day, human influences in particular play a role in the rise in sulfate concentrations in water.
As the scientists explain, fertilizer leaching from fields and arable land as well as wastewater from agriculture contribute to this: Worldwide, the agricultural use of sulfur-containing fertilizers accounts for around 50 percent of the sulfur released into the environment each year. In some regions of the world, more sulfur finds its way into the soil and water today than at the height of acid rain, as Zak and his team report. But industry also continues to emit considerable amounts of sulfur worldwide. For example, for the production of edible oil, potato starch and paper as well as through tanneries, textile factories and their waste water.
According to the research team, the consequences of climate change also increase sulphate pollution. The increasing heavy rain washes more and more sulphurous soils and fertilizers into bodies of water, and more and more wetlands are falling dry. “And the rising sea level causes sulphate-rich salt water to get into the groundwater and rivers, where it can significantly increase sulphate concentrations,” adds Zak. In addition, the drainage of bogs releases sulfur and sulfur-containing iron compounds.
Open pit mining is also a source of sulphate
Another source of sulfur is open-cast lignite mining. There the sulfide minerals pyrite and marcasite are increasingly exposed in the rock and oxidize in the air. The resulting sulphate is then washed into the water. This is exemplified by the Spree, a river whose sulphate concentrations have increased as a result of these mining activities. In some sections it already exceeds the drinking water limit of 250 milligrams per liter.
“This is problematic, as these bodies of water are used as a source of drinking water – typically via groundwater extraction and bank filtration,” explains Zak’s colleague Tobias Goldhammer. “Lignite mining continues to play an important role in many regions of the world, and sulphate pollution in bodies of water and drinking water is an issue everywhere there. Even if we have decided to phase out lignite mining in Germany, sulphate inputs into our waters will remain an environmental problem for us in the long term. “
However, the sulfate not only influences the quality of drinking water, but also the cycle of carbon, nitrogen and phosphorus, as the researchers report. Among other things, this increases the nutrient content in water and thus the growth of plants and algae as well as the food supply for aquatic organisms. The result is a lack of oxygen in the water, so that even more phosphate is released from the sediment – a vicious circle. Sulphate and its breakdown products – especially sulphide – can also have a toxic effect on aquatic organisms.
“Great need for action”
“There is a great need for action to reduce the sulphate concentrations in water,” emphasizes Zak. For example, the researchers suggest biological remediation using living organisms such as prokaryotes, fungi or plants as a way of removing pollutants from ecosystems. These store or decompose the sulphate and thus naturally filter the soil. Possible systems for such bioremediation include so-called plant sewage treatment plants, bioreactors and permeable reactive barriers. The scientists see the renaturation of moors as another possibility: rewetting these areas can stop the release of sulfur, so that sulfur is even stored again there. In addition, these wetlands filter sulphate-rich water that reaches them via ground or surface water.
“Since the problems in connection with high sulphate inputs have mainly been perceived regionally so far, the effects on inland waters have not yet been recognized as a global environmental problem,” said Zak. “Many countries have not defined any environmental standards for this. With our study, we would like to draw attention to the problem, show the current status of sulfate pollution and at the same time point out the numerous gaps in knowledge, ”the researcher concludes.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB), Article: Earth Science Reviews, doi: 10.1016 / j.earscirev.2020.103446