Download Manganese in Groundwater: Research and potential risks

FACT SHEET 6
International Manganese Institute
Manganese in Groundwater:
Research and potential risks
This fact sheet covers manganese in groundwater: how it
gets there, the likely concentrations and the factors that
influence those concentrations. Further, we describe some
of the current research findings on the potential risks posed
by manganese in groundwater, and contextualise those
risks in terms of current regulatory positions.
1.Introduction
Manganese is a naturally occurring and abundant element
that is essential in biological systems. The chemical behaviour
of manganese is dominated by pH, reduction and oxidation
reactions (see Fact sheet 4). As a naturally occurring element,
manganese is also ubiquitous in the environment, and so is
found in soils, sediments, surface water and groundwater.
Groundwater may be defined in
a number of ways, depending on
the technical field of interest. One
of the simplest definitions is that
from the European Groundwater
Daughter Directive (2006/118/
EC) that states: all water which is
below the surface of the ground
in the saturated zone and in
direct contact with the ground or
subsoil1. Environment Canada
gives a similar definition: the
entire region below the water
table is called the saturated zone,
and water in this saturated zone
is called groundwater2.
2.How does manganese get into
groundwater and how much is there?
Manganese occurs naturally in surface water and groundwater,
especially in oxygen depleted or anaerobic systems. The
concentrations of manganese in groundwaters are dependent
upon a number of factors such as rainfall chemistry, aquifer
lithology, geochemical environment, groundwater flow paths
and residence time. Some of these factors can be highly
variable over relatively small spatial and temporal scales.
Manganese can be leached from overlying soils and minerals
in underlying rocks as well as from the minerals of the aquifer
itself. Figure 1 shows the hydrological cycle and the position of
groundwater within that cycle.
A monitoring survey of groundwater assessing baseline
geochemistry in England and Wales assessed the variability in
manganese concentrations in many aquifers2. Figure 2 shows
the range of manganese concentrations in similar source
rock from different locations across England and Wales. The
findings here are typical of many areas around the world in that
the greatest concentrations are found in confined groundwater
where conditions are reducing (anaerobic). For example
manganese concentrations in groundwater in Minnesota4 have
been noted to be from below the detection limit (0.1 µg l-1) to
5050 µg l-1, although the median for all aquifers was 93 µg l-1.
Also a United States Geological Survey study of groundwater
chemistry in New England coastal basins noted concentrations
(medians) of manganese ranging from 0.03 µg l-1 to 5.88 µg l-15.
Baseline ranges of manganese concentrations vary both within
aquifers and between different aquifers over several orders
of magnitude, controlled largely by prevailing Eh (reducing
conditions) and pH, which respond to seasonal water table
fluctuations in the aquifer.
For some countries and regions of the world, groundwater is
used extensively as a source of water for public supply, for
example in Europe nearly 70% of water supplied for public
consumption is groundwater sourced3.
Figure 1.
Stylised diagram
of the hydrological
cycle, showing
the percolation
of water through
geological strata
to contribute to
groundwater.
Figure 2.The cumulative frequency distribution of manganese
concentrations in groundwater in Permo-Traissic
sandstone in England (from Shand et al. 2009)6
http://www.groundwateruk.org/Summary-of-Framework-Directive.aspx
http://www.ec.gc.ca/eau-water/default.asp?lang=En&n=300688DC-1#sub1
3
Shand, P., Edmunds W.M., Lawrence, AR, Smedley, P.L. Burke, S. 2007. The
natural (baseline) quality of groundwater in England and Wales. British Geological
Survey Research Report No. RR/07/06.
1
4
2
5
http://www.health.state.mn.us/divs/eh/water/swp/manganese/iamemo.pdf
http://pubs.usgs.gov/wri/wri994162/pdf/6relation.pdf
Shand, P., Edmunds W.M., Lawrence, AR, Smedley, P.L. Burke, S. 2007.
The natural (baseline) quality of groundwater in England and Wales. British
Geological Survey Research Report No. RR/07/06.
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FACT SHEET 6
International Manganese Institute
3.Potential risks from concentrations of
manganese in groundwater?
The ecological risks associated with manganese in groundwater
are relatively few, and may occur most significantly only when
manganese rich groundwater substantially feeds surface
waters.
For humans, manganese occurs naturally in many food
sources, and the greatest exposure to manganese (aside from
occupational exposure) is usually from food, however, exposure
via drinking water can be important. This route of exposure has
recently become of more interest
to researchers and regulatory
organisations, especially for
those using private water wells
(e.g. The State of Wisconsin7,
Connecticut Department of
Public Health, Drinking Water
Section8,
British
Columbia
Ground Water Association9).
Not all drinking water is from
groundwater, but it tends to be
the well water, i.e. groundwater,
that attracts the closest scrutiny,
especially where concentrations
approach 50 µg l-1 (the point at
which taste may be affected).
The neurological effects of inhaled manganese have been
well documented in humans chronically exposed to elevated
levels in the workplace. By the oral route, manganese is often
regarded as one of the least toxic elements, although there is
scientific debate as to whether the neurological effects observed
with inhalation exposure also occur with oral exposure, i.e.
potentially via drinking water. Epidemiological studies have
shown equivocal results10, with some showing associations
between neurological effects and exposure to manganese
through drinking water11 at concentrations below the current
WHO guideline value12 (0.4 mg Mn L-1).
What is clear is that the form (or species) of manganese and its
consequent bioavailability are important factors in the potential
effects (WHO 2011)13. Manganese in water is readily oxidized
by water treatment to insoluble manganese VI oxide which is
considered to be less bioavailable. Epidemiological studies
that indicate neurological effects have looked at manganese
exposures on untreated anaerobic groundwater where the
majority of the manganese was in solution (i.e. manganese
II). Differences in valency play an important role in the level of
bioavailability hence in the degree of potential risk.
The fourth edition of the WHO Manganese Guidelines, published
in 2011, concluded that as the calculated health-based value
is well above concentrations of manganese normally found in
drinking-water (including groundwater), it was not necessary
to derive a formal guideline value. The studies highlighted
previously in this sheet support this view but, nevertheless, it
looks likely that this decision will be revisited in the near future
http://www.dhs.wisconsin.gov/eh/water/fs/manganese.pdf
http://www.ct.gov/dph/lib/dph/drinking_water/pdf/manganese.pdf
9
http://www.env.gov.bc.ca/wsd/plan_protect_sustain/groundwater/library/ground_
fact_sheets/pdfs/fe_mg(020715)_fin2.pdf
10
E.g Kondakis et al. 1989, Archives of Environmental Health 44, 175-178; Vieregge
et al. 1995, Canadian J. Neurological Sciences 22, 286-289.
as research in this area has increased over the past decade.
Researchers hope to increase the understanding of the
potential risk to manganese exposure via drinking water and
also provide a better understanding and a robust estimate of a
safe concentration of manganese in drinking water.
4. Summary
Manganese in groundwater
comes from rainfall, dissolution
of manganese in minerals
from surrounding rocks and
leaching of manganese in
percolating through soils.
Greater concentrations of
manganese are found in
groundwater that are acidic
(low pH) and are in a reduced
(anaerobic) condition.
Groundwater is an important source of drinking water in many
parts of the world.
Natural variability in groundwater manganese concentrations
is large, often spanning orders of magnitude, sometimes in the
same aquifer.
Concentrations of manganese in groundwater generally have a
greater range than those in surface water, and would normally fall
between 1 and 20 µg Mn l-1. However, there are circumstances
when manganese concentrations in groundwater may exceed
300 µg Mn l-1.
Water treatment tends to readily oxidise manganese II to
manganese IV, which reduces concentrations in water.
However, if the water is from wells or private supplies with no
treatment, the manganese may remain in manganese II form.
Recent regulatory concern has been raised by research
indicating the possibility that oral exposure to manganese,
via drinking water from groundwater sources, may cause
neurological impairment. The form of the manganese in water
is likely to be critical in such exposures. With the new research
focus, the WHO is likely to revisit its decision to discontinue
their drinking-water guideline for manganese (previously set at
0.4 mg l-1) in the very near future.
Further information:
There are more fact sheets in this series: Fact Sheet 1. The
derivation of limit values for manganese and its compounds
in freshwater: data availability. Fact Sheet 2. Construction
of the biotic ligand models for manganese, Fact Sheet 3.
Accounting for bioavailability in assessing potential risks
of manganese in freshwater, Fact Sheet 4. Assessing the
potential terrestrial risks from manganese, and Fact Sheet 5.
Life Cycle Assessment (LCA) aimed at measuring the overall
environmental performance of the global manganese alloy
industry. You can find them at: http://www.manganese.org. For
more information please contact: [email protected].
November 2013
http://ehp.niehs.nih.gov/wp-content/uploads/119/1/ehp.1002321.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2072823/pdf/ehp0115-001533.pdf
http://www.who.int/water_sanitation_health/dwq/chemicals/manganese.pdf
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