Quality of water is important for a healthy horse. Water quality panel tests include nitrate/nitrite, total dissolved solids (TDS), sulfates, and coliform bacteria. The best indicator of water quality is total dissolved solids (TDS). Coliform bacteria include total coliforms and fecal coliforms. These bacteria are considered indicators of the presence of animal wastes. Generally, the only time a horse gets exposed to nitrite is ingestion of high-nitrite hay that was baled wet and the microorganisms changed the nitrate to nitrite.
The table below shows published and accepted maximum safe levels for horses. Do your research. Published safe levels may nat be satisfactory for your situation.
Your local water agency or a private lab will conduct water tests for free or a small fee.
The Iowa State University Veterinary Diagnostic Laboratory will test the quality of water if you suspect a problem. The Water Quality Panel measures nitrate/nitrite, total dissolved solids, sulfates, and coliform bacteria. Link here.
Another resource, EWG’s Tap Water Database allows you to look up the safety of your local tap water. Link here.
Hydrogen Sulfide 0.1
Nitrate Nitrogen 100
Nitrite Nitrogen 10