The idea of a pond to provide a constant water supply to your horses is appealing, but it might not be what we consider clean or safe. Obviously feral horses drink from ponds, lakes and streams, but that doesn't mean they never have illness related to poor-quality water.
In addition, their water sources are more likely to be rapidly flowing rivers and streams. Even lakes differ from ponds, especially manmade ones. That doesn't mean that ponds can't be used as a water source for horses. You simply need to know what the risk can be and how ponds should be managed.
Disease Transmission: A pond will be the watering site for more than just your horses. Wildlife will be attracted to them, too, and will leave behind droppings and urine to contaminate the water and vegetation around the pond. Many wild animals, both land-dwelling and aquatic such as muskrats or beaver, may be carrying Leptospira, a common contaminant in ponds. Leptospira infection can cause an acute febrile illness, abortion in pregnant mares or periodic ophthalmia (moonblindness). Other widespread organisms that may contaminate the water or surrounding grass are Listeria, Salmonella and E.coli. Opossums carrying Sarcocystis, the organism that causes EPM, may also be drawn to your pond, as will deer whose ticks may be harboring Lyme disease. The organism responsible for Potomac horse fever requires aquatic insects in its life cycle.
Agricultural Runoff: If there are livestock operations nearby, fecal matter and fertilizers in the surface runoff or shallow underground water sources can contribute to high-nitrate levels in the pond water and heavy algae growths. Industrial wastes recycled as soil treatments and even heavy manure contamination can also cause build up of toxic minerals in the pond.
Toxic Algae Blooms: Algae are an important part of the ecosystem within your pond. While they're not necessarily pleasant to look at, fortunately most are harmless to the horse. However, rapid proliferation of some forms of algae can indeed be highly toxic, and livestock are lost to these poisonings every year.
Some blue-green algae, such as Microcystis and Anabaena, can cause extreme illness or death when an animal ingests contaminated water during a bloom, a period of rapid growth when the algae can produce a toxin. Not all blue-green algae growths are toxic, of course, but can turn so, causing experts to advise you to avoid letting animals have access to the water during blooms.
The two characteristics most often associated with blooms is the water turning a deep green so that it looks more like pea soup than normal greenish tinted pond water, and an "earthy" smell to the water. Symptoms range from skin rash and GI upset/diarrhea to rapid death.
Safety Issues: Unfortunately, farm ponds are a common site for drownings. Slippery footing and holes from muskrats or groundhogs make the area around the pond treacherous for horses. Horses, people and other animals may also venture out on frozen ponds and fall through.