Arsenic variability in water-supply wells

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The USGS, in cooperation with the Town of Seabrook, New Hampshire and a private well owner, is assessing the variability of arsenic over multiple time scales. A network of three wells is being used to monitor changes in arsenic, arsenic species, a host of other chemical constituents, and the distribution of ages of groundwater entering the wells. The wells—one domestic bedrock aquifer well, one public bedrock aquifer well, and one public glacial aquifer well—were monitored from 2014-2018.

Assessing the variability of arsenic over multiple time scales study area map

Location of wells in this study in southeastern, New Hampshire. (Public domain.)


The variability of concentrations of arsenic in water supply wells over time is a concern for water supply systems and for private well owners. Public suppliers are required to ensure that Maximum Contaminant Levels (MCLs), such as those for arsenic, are not exceeded.  Private well owners are not required to reduce arsenic levels but many seek to ensure that exposure to arsenic through their drinking water is minimized. The three wells are located within an area previously identified as having an a higher-than-average number of high-arsenic wells (Ayotte and others, 2003). Monitoring to identify the time scales of change of arsenic in the three wells was done as part of the U.S. Geological Survey’s National Water Quality Assessment Project Trends Assessment.


The Town of Seabrook has several public-supply wells that collectively serve its citizenry. Some of those wells have arsenic concentrations that approach or exceed the MCL of 10 micrograms per liter of arsenic.  As a result, the water department has a large treatment system that removes the arsenic before the water is delivered to its customers.  Knowing the range of concentrations of arsenic entering the treatment plant, how those concentrations vary over time, and the factors that control that variability are important to the water department for effective management of their system. Factors such as season, water table elevation, pumping rates, and mixing of water among aquifers can strongly affect the concentration at any given time. In a domestic bedrock aquifer well in our network, season and water level are major controls on arsenic concentrations and on the particular arsenic form (i.e. As(III) and As(IV)) that is present in the water. The dominant form of arsenic has implications for treatment system efficacy for removing arsenic and can vary markedly by season.



Collection samples hourly at a private domestic bedrock well, Seabrook, New Hampshire

Collection samples hourly at a private domestic bedrock well, Seabrook, New Hampshire. (Public domain.)

The results from the detailed analysis of arsenic variability and controlling factors can help other well owners and operators around the region better manage their systems to ultimately reduce exposure to arsenic through drinking water. Working in cooperation with well operators, states, and health professionals, USGS science can help inform policy and decision makers to help ensure that safe drinking water is available to all. Two recent publications that present the major results of this work are available: Degnan and others, 2020 and Levitt and others, 2019

Ongoing work includes the evaluation of rapid measurement technologies to determine arsenic concentrations in well water, through a technical assistance agreement with a private company.