Water is the number one equine nutrient; horses can drink 10 to 15 litres a day in winter and between 50 and 70 litres during very hot weather and sustained work.
Horses which do not drink enough will pass hard, dry stools, and be at greater risk of gastrointestinal disorders.
Regularly ensure that drinking troughs and buckets are clean.

If horses are given well-water, microbiological analyses should be carried out in order to determine the nitrate and nitrite content etc. so as to ensure it is suitable for horses.
Water treatments are also available in order to improve water quality. You should preferably contact a water-treatment professional.