The science

Water molecules are made up of two kinds of atoms, namely hydrogen and oxygen. In one water molecule, two hydrogen atoms and one oxygen atom are bound together, giving the famous chemical expression H2O for water.

Nature is, however, often not as simple as we humans would like to have it. There are actually different types of hydrogen and oxygen atoms around. Strangely as it sounds, these variants of the same atoms are just a tiny bit heavier, as their atomic core contains extra neutrons. We call such different atoms of the same brand "isotopes". Isotopes can be chemically stable over time, or they can fall apart spontaneously, which is the case for radioactive isotopes. Here, we work with the naturally ocurring stable isotopes of water.

The bottom line is that there are different types of water molecules everywhere. The regular (light) water, H2(16O), is by far the most frequent (>99 %). And then there is different heavy water, which is a bit more heavy, but also very rare: only less than 1 in 500 for H2(18)O, and 1 in 3000 for H(2H)O. But you are safe to drink it like any other water.

Now, how can we make use of the fact that there are different water molecules around? There is one important consequence from the little bit of extra weight in the heavy stable water isotopes. The extra mass, albeit small, makes ties between heavy and other water molecules stronger. This means they are harder to break apart once formed, but also more likely to form.

When water circulates freely in the oceans, air, clouds, snow, and rivers, the stronger ties have an important consequence. Water molecules sit about 1000 times closer together in liquid than in gas (such as air), allowing them to have more bonds - which helps heavy water to accumulate in the ocean, and liquid in general. Therefore, heavy water is also more likely to fall to the ground as rain and snow.

The longer it rains and snows from a cloud, the more rare the stable isotopes become in both the cloud and in the falling rain and snow. If we now look at snow and water on the ground that people collect across Scandinavia, then we expect to find most heavy isotopes along the coast. The further we go up into the mountains, the more heavy isotopes have been lost from before. The snow therefore contains increasingly less heavy water.

Since heavy water molecules are very rare in nature, we have to measure with very little error to detect these. The method for measuring how many heavy and light molecules are in a snow sample involves laser light. The liquid sample is first cleaned from debris, then stored in a small glass bottle with a septum at the top. A robot arm with a syringe then pierces through the cap of the bottle, draws up a tiny amount of the liquid, and injects this into a hot oven filled with completely dry air. Thus, we simply turn the sample into steam!

The steam is then sucked into our measurement device. In a small, very hot chamber with mirrors in both ends, a laser beam is first switched on, then off. Now the analyzer measures how quickly the light beam fades as its going back and forth between the mirrors. The more water molecules are in the chamber, the faster the light disappears. The light rays sent by the laser are so specific, that they get picked up differently by regular water and heavy water molecules. We typically repeat such a measurement 8-10 times to get a good enough result. Overall, we use about 2 h of measurement time for one sample.

All analyses in ISOSCAN are carried out at the FARLAB laboratory. FARLAB stands for Facility for Light Isotope Research in Earth Science. The water isotope analysis we use for citizen science is only one of many other chemical analyses that are done at FARLAB. Currently, FARLAB has two measurement stations going where liquid samples from ISOSCAN'ners can be analysed.

Here you can look around in a virtual tour in the laboratory where the ISOSCAN measurements are carried out. The two green/grey instruments contain the lasers. The robot arm sits on top. The syringe moves continuously between the crate with small sample bottles, and an opening in the ovens that sits on the top of the laser analysers.

So far, so good. But now comes a key question: how do we actually use the stable isotope information in the snow to better predict floods and droughts? To understand this, we again think about the fact that heavy water falls out first when clouds form. Since most of the weather along the west coast of Scandinavia comes from the West (from the Norwegian sea), the snow that falls at the coast is heaviest. Then, the further we get towards the east, the less heavy water is in the snow.

We use such differences in the snow between different places to distinguis between river water that comes from the high mountains, and river water that comes from further down. It is generally difficult for river forecast models to know how much water comes from which place. With the citizen science sampling, we gain this valuable information! Using additional isotope measurements in rivers, lakes, and ground water, water forecasts learn how much water comes from what part of the water cycle.