This study evaluated the microbiological quality of wash water used in dairy farms from Galicia (NW Spain) based on the total coliform, faecal coliform and faecal streptococci counts. The paper aimed to identify geographical areas that are at a high risk for bacterial contamination of wash water and to study the temporal distribution of positive samples. Water samples from 120 farms, whose water supply came from a private well, were tested for the presence of the three mentioned bacterial groups by the membrane filtration method. To investigate whether the presence of positive herds for a particular bacterial group in water samples was higher in some areas or during a specific time period, data were tested using the Bernoulli model. In farms without a purification system, the prevalence of positive samples for total coliform, faecal coliform and faecal streptococci was 36.7%, 13.3% and 24.5%, respectively. In farms using hydrogen peroxide, these figures were 15.4%, 7.7% and 7.7%, respectively. In farms using chlorine or ultraviolet radiation all the samples were negative for the three bacterial groups. The spatial analysis identified one statistically significant spatial cluster of herds with positive samples of faecal streptococci in an area characterized by a high cattle density. Moreover, one most likely temporal cluster was identified for each of the three groups between July and August 2017, which were the driest months in the studied area.