Featured Study: Could lead leach into tap water?

Featured Study: Could lead leach into tap water?

About one-third of more than 8,300 wells tested across the U.S. had groundwater with chemical characteristics that could cause lead, if present in plumbing, to leach into tap water at levels above the EPA Action Level.

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Corrosivity describes how aggressive water is at corroding pipes and fixtures. Corrosive water can cause lead and copper in pipes to leach into drinking water and can eventually cause leaks in plumbing.  Surface water and groundwater, both sources of drinking water, can potentially be corrosive.

Corrosion on water tap

Corrosion on water tap

► All About Corrosivity - Potential Corrosivity of Untreated Groundwater report and FAQ


Corrosion in water-distribution systems is a costly problem, and controlling corrosion is one of the most important ways that concentrations of lead (Pb) and copper (Cu) can be reduced in tap water. Naturally corrosive water is not dangerous to consume in itself, but it can cause health-related problems by reacting with pipes and plumbing fixtures in homes. If plumbing materials contain lead or copper, corrosive water can cause these metals to leach into the water supply. Signs that corrosive water is causing leaching of metals may include bluish-green stains in sinks, metallic taste of water, and small leaks in plumbing fixtures.

The USGS National Water-Quality Assessment (NAWQA) Project has been studying where and why surface water and groundwater that are sources of drinking water are corrosive. Many factors contribute to corrosivity, including elevated concentrations of chloride, pH out of neutral range, elevated concentrations of dissolved and suspended solids, and lower alkalinity. The potential for water to be corrosive is measured by three different indexes: the Langelier Saturation Index (LSI), the Potential to Promote Galvanic Corrosion (PPGC), and the Larson Ratio (LR).  

  • The LSI is a measure of the balance between pH and calcium carbonate (CaCO3)—as the LSI value becomes more negative, the water is increasingly under-saturated with CaCO3 and therefore has a greater corrosion potential. 
  • The PPGC is based on the ratio of chloride to sulfate; the higher the PPGC, the greater the potential for galvanic corrosion of lead in the plumbing system.
  • The LR is defined as defined as the sum of equivalents of chloride and sulfate divided by equivalents of bicarbonate. The LR indicates the corrosivity of water to iron and steel.
Urban streams have an elevated potential to cause corrosion

See Increasing chloride in rivers of the conterminous U.S.  for more information. (Graphic by Edward Stets, USGS)

Surface Water

Chloride, PPGC, and LR have been increasing in urban streams across much of the United States, and especially in the most snowy areas in the study, which are most likely to use road salt.  In a study of trends in chloride and indexes of corrosivity, seasonal changes in chloride, CSMR, and LR varied greatly among sites, and site-specific conditions make it difficult to generalize seasonal patterns in corrosivity. However, water-quality monitoring protocols do not emphasize characterization of the seasonal or long-term changes in surface water corrosivity or metal release, and corrosion driven by changes in chloride concentration, CSMR, or LR would not be detected under regulatory requirements or typical voluntary monitoring.


Image shows a map of all 50 states color-coded for their potential for groundwater corrosivity

Potential for groundwater corrosivity in groundwater wells in all 50 states. (From Potential corrosivity of untreated groundwater in the United States, USGS Scientific Investigations Report 2016-5092.)


An assessment by the NAWQA Project of more than 20,000 wells nationwide showed that 25 states have groundwater that either has high or very high potential to be corrosive. The states with the largest percentage of wells with potentially corrosive groundwater are located primarily in the Northeast, the Southeast, and the Northwest. About 24 million people in these areas rely upon groundwater from private water systems for their source of drinking water. Read more about the assessment here.