Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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. 6 1 of 2 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 7 11 8 12 13 2 of 2