Effects
Sulphate is a substance that occurs naturally in drinking water. The lethal dose for humans as potassium or zinc sulphate is 45 g. The reported minimum lethal dose in mammals is 200 mg/kg (Arthur D. Little Inc., 1971). Sulphate doses of 1000 to 2000 mg (14-29 mg/kg bw) can have a laxative effect on humans (McKee and Wolf, 1963) causing diarrhea, especially when switching abruptly from drinking water with low sulphate to drinking water with high sulphate concentrations ( US EPA, 1999). Water containing 1000 mg/L MgSO4 acts as a purgative in normal humans, but concentrations below this are apparently physiologically harmless to the general population (McKee and Wolf, 1963; Arthur D. Little Inc., 1971). Humans are able to adapt to higher concentrations with time ( US EPA, 1985).
Dehydration has also been reported as a common side-effect in humans following the ingestion of large amounts of MgSO4 or Na2SO4 (Fingl E., 1980).
The taste threshold concentrations for the most prevalent salts are 250-500 mg/L (median 350 mg/L) for Na2SO4, 250-900 mg/L (median 525 mg/L) for CaSO4, and 400-600 mg/L (median 525 mg/L) for MgSO4 (National Academy of Sciences, 1977). Concentrations of sulphate salts at which 50% of panel members considered the water to have an offensive taste were approximately 1000 and 850 mg/L for CaSO4 and MgSO4, respectively (Zoeteman, 1980).
Guidelines from the Literature
The Office of Water in the United States specifies a secondary maximum contaminant level (MCL) for sulphate of 250 mg/L in drinking water, based on aesthetic effects (i.e., taste and odour). This regulation is not a Federally enforceable standard, but is provided as a guideline for States and public water systems.
Recommended Guideline
The aesthetic objective for sulphate of 500 mg/L in water to be used for drinking is adopted from the Guidelines for Canadian Drinking Water Quality (Health Canada, 1996), which has been accepted by the BC Ministry of Health1 for application in British Columbia.
Rationale
This drinking water objective for sulphate is based on taste considerations. Because of the possibility of adverse physiological effects at higher concentrations, it is also recommended that health authorities be notified of drinking water that contain sulphate concentrations in excess of 500 mg/L (Health Canada, 1996). Health Canada advises by footnote that there may be a laxative effect in some individuals when sulphate levels exceed 500 mg/L.
1 The BC Ministry of Health regulates the quality of purveyed water.