Groundwater in the Great Lakes Basin

46
tive source of drinking water may be warranted. The
U.S. EPA has an outdated guidance level of 20 mg/L.
Although such a level might have a benefit at the population
level in reducing hypertension and cardiovascular
disease, it is unrealistic. Sodium has been put on
the Contaminant Candidate List for reevaluation (U.S.
EPA, 2007b).
Hardness
The hardness of water is expressed in terms of the
amount of calcium carbonate – the principal constituent
of limestone – or equivalent minerals that would be
formed if the water were evaporated. Water is considered
soft if it contains 0 to 60 mg/L of hardness, moderately
hard from 61 to 120 mg/L, hard between 121 and 180 mg/L
and very hard if more than 180 mg/L of dissolved solids.
Very hard water is not desirable for many domestic
uses; it will leave a scaly deposit on the inside of pipes,
boilers and tanks. Hard water can be softened at a fairly
reasonable cost, but it is not always desirable to remove
all the minerals that make water hard. Extremely soft
water is likely to corrode metals, although it is preferred
for laundering, dishwashing and bathing. Softened
water contains an increased concentration of sodium
that makes it unsuitable as drinking water. Hardness
levels for residential water are targeted at 80 to 100 mg/L
calcium carbonate. Levels above 500 mg/L are unacceptable
for domestic purposes (GCDWQ, 1979).
Calcium and magnesium are essential to human health.
A large body of scientific information demonstrates a
beneficial relationship between high levels of hardness
and the development of stronger bone structure. Some
studies have demonstrated a reduction in certain
types of cardiovascular disease in populations using
hard water as their drinking water supply (Monarca,
Donato, Zerbini, Calderon and Craun, 2006). This
effect may be related to the magnesium rather than
the calcium in the drinking water of populations with
overall low magnesium in their diet (Durlach, Bara and
Guiet-Bara, 1985). These effects are biologically reasonable
but have not been demonstrated conclusively.
Methyl Tert-Butyl Ether
Methyl tert-butyl ether (MTBE) is added to gasoline
to increase the octane level and to reduce carbon
monoxide and hydrocarbon emissions by vehicles.
MTBE has been the most commonly used fuel
oxygenate (GCDWQ, 07/2006). The release and
distribution of MTBE in the aquatic environment
has raised concern about the compound’s occurrence
in drinking water, due to its low taste-and-odour
threshold and the potential impact on human health
(Kolb and Puttmann, 2006). Potential and documented
contamination of water resources by MTBE has become
a cause for public concern and increasing controversy.
MTBE readily dissolves in water, can move rapidly
through soils and aquifers, is resistant to microbial
decomposition and is difficult to remove in water
treatment. Beginning in the early 1990s, when lead was
removed from gasoline, MTBE has been added in much
higher concentrations (up to 15%) to enhance gasoline
combustion and reduce tailpipe emissions in the United
States. Methylcyclopentadienyl manganese tricarbonyl
has been used for this purpose in Canada.
The U.S. EPA has not established drinking water
regulations for MTBE. In December 1997 the U.S.
EPA issued a Drinking Water Advisory for MTBE at
levels of 20 to 40 µg/L, primarily for taste and odour
considerations. Based on present knowledge, the U.S.
EPA believes this provides a wide margin of safety (U.S.
EPA, 1997). 30% of the United States population lives
in areas where MTBE is regularly used. Concentrations
in tap water in excess of 2 µg/L are unlikely (Stern and
Tardiff, 1997). A MAC for MTBE has not been set in
Canada (GCDWQ, 07/2006).
Manganese
Manganese is an essential element required for normal
body functions. Manganese deficiency is rare; the
body’s needs are fulfilled through dietary intake with
retention of 3% to 5% of ingested manganese. Most
knowledge on the effects of excess exposure comes
from studies of workers exposed to manganese dust
or patients with chronic liver dysfunction, which
results in higher manganese retention (for review see
Mergler and Baldwin, 1997). The nervous system is the
major target organ; effects are on a continuum: subtle
changes, particularly in motor functions and mood are
observed at lower levels of exposure and manganism, a
degenerative neurologic disorder with many similarities
to Parkinson’s Disease, at high levels of exposure. Until
recently, little attention has been paid to manganese in
drinking water, but reports suggesting increased infant
mortality (Hafeman, Factor-Litvak, Cheng, Van Geen
and Ahsan, 2007), intellectual deficits (Wasserman
et al., 2006), and increased hyperactive behaviour in
children (Bouchard, Laforest, Vandelac, Bellinger and
Mergler, 2007) associated with elevated manganese
in drinking water have given rise to new concern and
questioning of the current drinking water guidelines
(Ljung and Vahter, 2007).
Manganese at levels of 150 µg/L stains laundry and
plumbing fixtures and causes undesirable tastes in
beverages. This problem can occur at concentrations as
low as 20 µg/L, but it is difficult to reduce manganese
concentrations below 50 µg/L. The GCDWQ (11/1987)
set an aesthetic objective of 50 µg/L. The U.S. EPA
secondary standard is also 50 µg/L (2007a).