In this lecture we'll talk about water quality. Even the clearest water still contains more than just water. As water flows through the soil to the ground water or the stream, it dissolves substances it encounters along the way, or it may deposit some of its constituents. Soils contain high concentrations of carbon dioxide. This creates a weak acid that dissolves many silicate minerals. The eventual chemistry of the water depends on the rock and soil formations through which the water flows, the temperature, and how long the water stays in the ground. Usually, faster flowing water dissolves less material, as a result most natural water contains minerals. The most common minerals are bicarbonate, sulfate, sodium, chloride, calcium, magnesium, and potassium. You can see which minerals are in the water when you pick up a bottle of water. If you look at the label, it will tell you immediately which minerals are in the water, as well as whether there's any nitrate that usually comes from fertilizers. Which minerals are in the water depends on the type of aquifer that the water comes from. Water flown through carbonate aquifers is mainly rich in calcium and bicarbonate. Water from crystalline aquifers can contain more sulphate, but water from the alluvial aquifers, and the mollase aquifers may contain more magnesium. Hard water is water that has a high mineral content. Hard water is formed when water flows through limestone and chalk, they are calcium and magnesium carbonates. Hard drinking water may have a moderate health benefits, but it can cause serious problems in industrial settings because it leads to costly breakdowns of boilers and cooling towers. Municipalities may soften the water by using an ion exchange resin. This replaces some of the calcium ions by sodium or potassium. So even though this map tells you roughly where you can find hard water, your local tap water may actually be soft water. If your water doesn't really create suds or foam when you add soap to it, and if you have a lot of limescale in your kettle, then your drinking water is probably hard water. Some minerals are in the water in only very low concentrations, for example fluoride, which may have a health benefit, because it's good for your teeth. Other minerals are less healthy, for example uranium and arsenic. Arsenic itself is not toxic but when it becomes oxidized it create compounds that are toxic. You may have heard about arsenic contamination of ground water in Bangladesh. In Bangladesh one in five wells is contaminated with arsenic above the government's drinking water standards. But relatively high arsenic concentrations can also be found in some areas in Switzerland. For example, in the thermal waters in northern Switzerland, iron rich chalk layers in Asura, and certain areas of the Alps that and fibrillate bedrock, and also sometimes in granite bedrock areas. The color of the water can sometimes tell us what's in the water. For example, a reddish color may indicate that there is some iron in the water. Similarly, a tea-like brown color near wetland areas can tell us that there is a lot of carbon in the water. But usually the color of the water tells us more about the solid particles that are in the water. As water flows over the soil surface, it can pick up soil particles and transport them to the river. Here we see the confluence of the Limmat with clear water from Lake Zurich and a Sihl that has brown water and high sediment concentrations after several days of rain. Here in Geneva, the River Arve comes from the northern side of the Mont Blanc massif and has much higher concentrations of silt than the Rhone that comes from Lake Geneva. This results in the striking contrast between the two rivers. Because the Arve receives the water from many glaciers in the Chamonix Valley, its color is sometimes milky white. Rivers from glaciers contain rock flour and glacier flour, which consist of fine grain silt-sized particles. They are generated by the mechanical grinding of the bedrock by the glaciers. Because these rock particles are very small they are suspended in the water which makes the water look milky or cloudy. The rock particles in these glacier rivers contain almost no clay minerals or organic matter. This makes the river look white and not muddy-brown like the other rivers that contain more clay minerals and organic matter. Alpine lakes are often turquoise or emerald green. This is not because they do not contain any particles, but in fact they are so blue because the particles are so small that they float in the water. The glacial flour doesn't sink, and it reflects the wavelengths of the light that fall in the bluish and greenish end of the spectrum. That makes these lakes appear blue or green. Water clarity is usually measured with a Secchi disk. A Secchi disk is a disk that has two black and two white areas. You lower the disk in the water until you can no longer distinguish the black and the white areas from each other. You can easily make such a disk, and start taking water clarity measurements in your favorite lake. Basically, you measure how deep you can see in the lake to make water clarity measurements, you could also have walked into the lake and check at what depth you can no longer see your feet. In this lecture we've talked about natural water equality. In the next lectures, we'll talk about pollution. We look forward to seeing you there.
