Shallow Groundwater Manganese Merits Deeper Consideration

Manganese, the twelfth-most abundant element in the earth’s crust and an essential micronutrient, is not frequently viewed as a major drinking water contaminant. Yet, a quarter of all U.S. groundwater wells surveyed by the U.S. Geological Survey (USGS) between 1991 and 2010 contained manganese at concentrations above the secondary maximum contaminant level (SMCL) of 0.05 mg/L set by the United States Environmental Protection Agency (USEPA).¹ Over the same period, 7% of the wells sampled by the USGS had manganese concentrations above the health-based screening level (HBSL) of 0.3 mg/L. In the past decade, there have been recurring reports of manganese contamination in local groundwater supplies around the U.S., frequently at levels approaching or exceeding the HBSL.¹ However, because manganese is classified as a secondary contaminant, the SMCL is only an aesthetic guideline, and neither the SMCL nor the HBSL are enforceable standards.²

A growing body of research points to the neurotoxic effects of ingesting excessive manganese in drinking water: chronic overexposure is shown to damage intellectual function and motor skills in young children.^3,4 Because these setbacks are not easily reversed, overexposure to manganese poses a significant health threat to children. Although water is the principal route of manganese exposure for pregnant women and young children,⁵ the EPA does not currently oblige public water systems or private well users, to monitor and report manganese levels. Private wells provide drinking water for approximately 43 million people, which is 15% of the U.S. population. Given the toxicity concerns, the frequent exceedances of advisory limits coupled with potential exposure for a large proportion of the U.S. population, there is reason to believe that manganese may warrant a stricter regulatory standard in the U.S. Manganese is an essential nutrient with a recommended maximum daily intake of 10 mg/day for an adult, with most manganese consumed via diet.⁶ Concentrations in drinking water need to be monitored in order to prevent excessive manganese intake.

A specific challenge to keeping manganese concentrations at safe levels, particularly in groundwater, is to understand the spatial heterogeneity of geogenic sources of manganese and the processes influencing subsurface geochemistry and manganese oxidation states.^7,8 Manganese can be found in a wide range of primary minerals and is generally more abundant in finer fractions of soils and sediments where it is associated with layered silicates and carbonates.⁹ Weathering of primary minerals releases Mn²⁺ which undergoes oxidation to form secondary Mn(III/IV) oxides. These secondary Mn(III/IV) oxides can subsequently be reductively dissolved under suboxic or anoxic conditions. Dissolved Mn(II) can then be mobilized through sediment pore-networks and eventually transported into aquifers. Though Mn²⁺ is the predominant species under low oxygen conditions, it can be readily converted to insoluble Mn(IV) oxides during water treatment via chlorination and removed by filtering. If ingested, Mn(II) can become oxidized to Mn(III) in the body. While both Mn(II) and Mn(III) are biologically relevant species, Mn (III) is more readily transported across membranes and the blood brain barrier.

Mobilization of manganese as Mn(II) into groundwater occurs most readily in shallow groundwaters where subsurface strata possess Mn content, with enough dissolved oxygen to trigger manganese reduction (e.g., Mn(IV) to Mn(II)), while still maintaining a high enough redox status to allow manganese to remain dissolved in well water without precipitating manganese carbonate or other reduced manganese solids. Therefore, under certain conditions, shallow depths can engender well water with significant concentrations of manganese.¹⁰ As a consequence, private wells, which are often drilled to shallower depths than public supply wells, may put their users at greater risk of manganese overexposure than users served by public water systems. There are very few studies on the quality of drinking water infrastructure in the U.S. and on the socioeconomic characteristics of the users served by each infrastructure type; however, even assuming that on an average, users of private wells are no poorer than users served by community water systems, private wells users still carry the onus of maintaining their own water quality. Where these users have the resources to test their wells, they might prioritize primary contaminants over manganese. In light of these gaps, regulatory policy ought to consider how best to protect private well users from overexposure to manganese in shallow groundwater.

Drinking water quality monitoring programs instituted in accordance with EPA rules test regularly for arsenic, among other primary contaminants, but not for manganese. Whereas arsenic concentrations are suppressed under oxic and suboxic conditions, manganese concentrations can be poorly correlated with redox potential, with high concentrations found in wells exhibiting oxic conditions.¹ As a result, monitoring programs might deem (shallow) wells that meet the arsenic standard as safe for drinking water needs without ever testing and ensuring that the wells also meet the manganese standard. Thus, an unenforced manganese standard could also leave community water system users open to elevated concentrations of manganese.

Although the USGS has analyzed several thousands of domestic well samples between 1991 and 2010, it has usually tested a given domestic or monitoring well only once over this time-span. Ideally, a population’s overexposure to manganese in drinking water is gauged from longitudinal data on manganese exceedances in both private and public water systems. Toward this end, targeted studies of populations who obtain drinking water by tapping groundwater from shallow glacial and sandstone aquifers could prove useful.⁷

Should further evidence of manganese contamination come to light, and the manganese drinking water standard is made mandatory and enforceable, the good news is that the cost of treating manganese contamination is relatively inexpensive. For private well owners, the application of a number of point-of-use treatment methods that combine oxidation with filtration (e.g., greensands filters, chlorination followed by filtration) can remove dissolved manganese from raw groundwater effectively. For public supplies, manganese can be readily removed through traditional water treatment techniques, for example, chlorination. The availability of cost-effective treatment solutions are helpful insofar as they can be implemented widely and thus, ensure that a stricter manganese standard is attainable universally.