Water resources used for irrigation are subject to many processes that cause their chemical composition to change over time.
The composition of water is affected by natural climatic processes such as rainfall, freezing, evaporation, as well as biological processes such as algal blooms, which can be harmful to human and animal health. The biggest influence on the quality of freshwater in watercourses and in the ground is human management and, in the case of agriculture, this will be the way organic and mineral fertilisers are stored, the choice of doses, timing and method of fertilisation, crop protection and irrigation, and the way wastewater and other waters are collected, neutralised and discharged, including washings from the washing of tanks and equipment used for the application of fertilisers and RWM. It is very difficult to realistically calculate the risk to the quality of water used for irrigation, so it is advisable to carry out water composition tests, especially of surface water, which, due to its proximity to fields, the open nature of the water table and the direct impact of high temperatures, is subject to very rapid changes in its chemical composition and biological state. Leaky tanks and on-farm sewage systems can promote the movement of pathogenic bacteria, such as Escherichia coli, into the water. The use of biologically contaminated water, especially during the pre-harvest period for green crops or fruit, can result in infectious diseases for both harvesters and consumers, especially in plantations of crops that are not washed before sale. There is also a risk of toxin poisoning as a result of contamination of crops with water from reservoirs where cyanobacteria have bloomed. Toxins secreted by some strains of cyanobacteria can cause skin irritation, kidney and liver damage to muscle paralysis in humans. The risk can be too high a content of natural or man-made minerals in the water, which, by accumulating in plants or soil, can lead to a deterioration of the quality of the crop or even long-term deterioration of soil fertility.