Manganese and Arsenic in New Hampshire’s Drinking Water Supplies – What You Need To Know

Author: Jim Vernon, PhD, PG, CG – Director of Water Supply Services

Manganese and arsenic are metals that are commonly found in New Hampshire’s drinking water wells. Commonly referred to as “contaminants”, these metals typically derive from natural sources. (Manganese and arsenic that are elevated due to human-caused contamination or treatment for other contaminants is a subject for a future article.)

Concentrations of both manganese and arsenic in New Hampshire groundwater vary greatly, depending on many factors including the type of underlying bedrock and other water chemistry factors, such as the pH (measure of water’s acidity) and oxidation/reduction potential. Elevated levels of one metal does not necessarily indicate elevated levels of the other. Both metals can be found in bedrock wells, while manganese is also found in some dug wells and screened wells tapping overburden groundwater. Shallow wells near swamps have a greater likelihood of elevated manganese than wells not located near swamps or wetlands, but there are many exceptions.

Long considered an aesthetic parameter (possibly staining bathroom fixtures or laundry), manganese is now known to present health risks at higher concentrations, including neurotoxic effects on infants and children. Manganese concentrations in drinking water are subject to a series of standards that can be measured in micrograms per liter (ug/L), which is equivalent to parts per billion (ppb). Here are the current New Hampshire standards for manganese:

  • Secondary (aesthetic) drinking water standard = 50 ug/L; not a health standard; established by EPA in 1974
  • EPA Health Advisory for Infants = 300 ug/L
  • NH ambient groundwater quality standard (AGQS) = 300 ug/L as of December 2020 (formerly 840 ug/L). This is a health-based standard for both drinking water and water that is discharged to the ground.

Depending on the groundwater chemistry in a well, manganese can either be dissolved in the water or suspended as particles or colloids. The New Hampshire Department of Environmental Services (NHDES) advises testing for both dissolved manganese and total manganese before selecting a water treatment option. For more information, consult NHDES Fact Sheets on manganese and on water treatment, available on the NHDES website.

Arsenic is potentially more dangerous at lower concentrations in groundwater than manganese. Arsenic concentrations are higher in some parts of NH than others, depending on bedrock type and other factors. Arsenic can occur in two forms in well water: As-III (Arsenite) or As-V (Arsenate). Long-term consumption of water with elevated arsenic increases the risk of certain cancers and other disorders; additional risks are found in infants who consume water with elevated arsenic. For additional information, consult NHDES Fact Sheet DWGB-3-2, available on the NHDES website.

Research has determined that lower concentrations of arsenic can produce health risks, so standards have progressively lowered (become more stringent) over the years:

  • 1975 – 2001: 50 ug/L; EPA standard for Public Water Systems
  • 2001 – 2021: 10 ug/L; still the EPA standard in many states for Public Water Systems
  • July 2021 – present: 5 ug/L; new standard passed by NH Legislature for Public Water Systems

Water tests are recommended for private wells, particularly bedrock wells. If the result is greater than 5 ug/L NHDES recommends testing for the specific types of arsenic (As-III vs As-V) because treatment methods vary depending on which type of arsenic is present. For more information, see NHDES Fact Sheet DGWB-3-2.

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