Horses are generally good at detecting when water sources are not safe, but not always, so it is important to know some basics about water quality. Water quality can be affected by the amount of compounds in the water (including minerals, such as sulfur or iron), any noxious agents such as pesticides, or pathogenic bacteria. Water can be tested easily for total dissolved solids, pH, individual minerals and other compounds.
Ponds can be at risk of accumulating any runoff from neighboring farms, and if they are the horse’s sole source of water, they should be tested regularly. Ponds can also be at risk for developing blue-green algae. This is a noticeable blue-green sheen or scum that tends to bloom in hot months. Fresh blooms often smell like newly mown grass, while older blooms may smell like rotting garbage. The algae is really a type of cyanobacteria that can contain toxins that are dangerous to the horse (and other pets or livestock), potentially causing sudden death. Ponds can also be at risk of accumulating nitrates, which is of greater concern for ruminants such as cattle, but also, to some degree, for horses. If high amounts of nitrates are ingested, it can be converted to nitrates in the body, which competes with oxygen on hemoglobin.
Streams are a little better than ponds because, typically, compounds are less likely to sit and accumulate. That said, there is no knowing where the water is coming from and what it could contain.