Old data series provide new answers about brown water

In a prevailing hypothesis, the so-called “brownification” (the gradual change in the colour of water in certain lakes and streams) is explained by biogeochemical processes in the soil, which were affected by the high sulphur deposits that occurred in the 1970s and 1980s. The hypothesis is that the recovery of the soil after acidification has caused increasing levels of dissolved organic matter (DOC) to “brown” the water in certain lakes and streams. 

“The acidification made us aware of the large-scale airborne environmental degradation and contributed to the expansion of environmental monitoring during the 1970s and 1980s. In the decades since then, we have seen the colour intensify, and the conclusion that has been drawn is that the water was unnaturally clear back then, when the pH was low,” explains Emma Kritzberg.

The increasingly brown colour was assumed to be due to the fact that the acidification-affected soil was slowly recovering. Mechanistic studies have indicated that there is such an effect – the more acidic the soil water, the less soluble the DOC. The reduction in acidification is one factor behind brownification – another being climate change, which contributes to faster degradation of organic matter and increased transport from soil to surface water.

A treasure chest at SLU

Since a great deal of data collection was initiated during the era of acidification, few analyses include water quality data from the years prior to that period. Emma Kritzberg was looking for a way to go further back in time. In connection with another research project, she had come into contact with SLU’s environmental analysis data for lakes and began to realise what a treasure chest they constituted.

“I was completely bewitched. I sat up at night and couldn’t stop plotting different variables from environmental analysis data,” says a visibly enchanted Emma Kritzberg. “All the research on the brownification had to do with DOC leaking from the soil. But here I saw that iron had also increased. What was the explanation for that?”

The increasing levels of iron aroused Emma Kritzberg’s curiosity, and she asked herself if it might be possible to find out what watercourses and their surrounding natural environments looked like before acidification.

Chance contributed

In a successful research study, the pieces of the puzzle can find their way to the researcher in unexpected ways. With evident fascination, Emma Kritzberg describes how a haphazard meeting with an engineer at a waterworks led her to a data series on the water quality in the Lyckeby drinking water supply that stretched all the way back to the 1940s. Furthermore, after an interview in Sydsvenskan newspaper she was contacted by a researcher at SLU (Matts Lindbladh) who tipped her off about compilations of the Swedish National Forest Inventory’s data from the same area and time period.

Suddenly it was possible to put a new piece of the puzzle in place – one that indicated that the increased distribution of spruce forests in southern Sweden probably had a greater impact on water quality than previously thought. In this part of Sweden, the coverage of spruce has risen from 15% before the turn of the last century to 75% today.