Groundwaters from chalk aquifers which are used as a supply for drinking water are often contaminated with pesticides ‐ in particular, atrazine. This paper discusses the use of an industrial‐scale ultraviolet chamber to reduce the concentration of atrazine in a chalk‐derived water which is used for drinking water supply. The concentration of atrazine varied between 0.1 μg/l and 0.5 μg/l, and the raw water was spiked when necessary. Results for other pesticides contaminants are also presented.The efficiency of atrazine removal is dependent only on the energy input and is constant, regardless of the initial concentration. Hydrogen peroxide improves the efficiency of ultraviolet irradiation but requires high doses.