Water is the basis of all life, all of nature and its habitats. When processed into food as drinking water, it's a substance that is strictly tested. However, water also has an important task in chemistry since it is a good solvent and absorbs many substances.
The most common way of water to enter the natural cycle is as rain into the ground and then into water bodies or deeper wells. It absorbs soil components and carries them in solution or as suspended particles. There are both visible and invisible portions.
Visible particles usually give the water an earthy color, an appearance that is normal for outdoor ponds or lakes. However, if a slight tint in a bottle of drinking water is found, it is perceived purely subjectively as deviating from norm and experience. Drinking water is expected to be colorless and clear.
The warm summer of 2018 brought a supply of still mineral water with a slight tint. This had to be investigated with fluorescence spectroscopy.
It is known from the literature that water can contain humic acid, a natural degradation product of organic sources such as leaves and grasses. This occurs naturally with open water, and the acid serves as food for living organisms in the water. Humic acid is not a stable molecule. It oxidizes through oxygen and decomposes chemically into tryptophan and L-tyrosine. It could be deduced from this reaction that if no more humic acid is present, the necessary amount of oxygen in the water is also consumed.