Total dissolved solids in drinking water serves as a very poor predictor of animal health. As noted above, TDS is a measure of all inorganic and organic substances dissolved in water. These individual solutes range in toxicity from relatively non-toxic substances, such as Ca2+, to extremely toxic (Hg2+, Se+4), but tests of TDS do not differentiate between them. Several early studies suggest no significant effects in sheep at TDS concentrations up to 13,000 mg/L or cattle and swine up to 5,000 mg/L, and the NRC661 accepted larger concentrations as tolerable "for older ruminants and horses," yet the authors have seen animals poisoned by waters in which the TDS was measured as slightly less than 500 mg/L,415,662 and there are reports of decreased productivity in dairy cattle at 2,000-2,500 mg/L. Early epidemiologic studies in people suggested high drinking water TDS decreased the incidence of cancer and heart disease in people. Later, however, studies narrowed the active component of TDS that was negatively correlated with heart disease, first to hardness, then finally to the Mg2+ ion concentration. In human health, the World Health Organization dropped health-based recommendations for TDS in 1993, instead retaining 1,000 mg/L as a secondary standard for "organoleptic purposes." The test is just too non-specific to be reliable. As noted by Chapman et al.663, in a study of aquatic toxicity, "Toxicity related to these ions is due to the specific combination and concentration of ions and is not predictable from TDS concentrations."
We do not recommend relying upon TDS to evaluate water quality for livestock and wildlife; however, if no other information is available, TDS concentrations less than 500 mg/L should ensure safety from almost all inorganic constituents. Above 500 mg/L, the individual constituents contributing to TDS should be identified, quantified, and evaluated.