Skip directly to search Skip directly to A to Z list Skip directly to site content


Sulfide in Well Water



The U.S. Environmental Protection Agency (EPA), Region IV, asked the Agency for Toxic Substances and Disease Registry (ATSDR) to perform a public health evaluation of the sulfide levels in two private wells in the Copper Basin Mining District in Polk County, Tennessee. EPA suspects that both wells may have been contaminated by nearby mining waste leachate.

The Copper Basin Mining District Site (the site) is located in southeast Tennessee in Polk County, and northern Georgia in Fannin County, near the state border with North Carolina. The Copper Basin is the site of extensive former copper and sulfur mining operations that date back to the early 1800s. For more than 150 years, numerous companies and individuals were involved in various mining, refining, and manufacturing operations in the area. Mining operations ceased in 1987, and sulfuric acid production was discontinued in 2000 (EPA).

Mining and related activities have resulted in the environmental degradation of portions of the Copper Basin, including the North Potato Creek Watershed, the Davis Mill Creek Watershed, and parts of the Ocoee River. Acidic conditions and leaching metals have impaired water quality and deforestation has resulted in severe erosion. Abandoned and collapsing mine works and other deteriorating facilities and waste piles also pose significant physical hazards. In addition, the lack of a healthy soil structure and the poor quality of riparian and upland ecosystems contribute to poor surface water quality.


In the fall of 2001, the Tennessee Department of Environment and Conservation (TDEC), sampled several private wells within the site. Laboratory analyses indicated that two of the wells have elevated levels of sulfide. Well PWO1 had 1.08 milligrams per liter (mg/L) of sulfide and well PWO3 had 1.48 mg/L of sulfide. No other measured constituents were notably elevated. The concentrations of metals are low, varying between 41 micrograms per liter (µg/L) for iron to below the detection limit of 1 µg/L for arsenic, cadmium, copper, lead and other metals. None of the metals, nor any measured chemical, exceed any safe drinking water standard.

As noted by EPA and TDEC, no drinking water standard exists for sulfide. A secondary maximum contaminant level has been established for sulfate at 250 mg/L because of taste, odor, color, corrosivity, and staining properties. Sulfide in water shares some of the same offensive properties for taste, odor, and corrosivity but no particular concentration has been identified as a threshold limit for health or other issues such as corrosivity.

In regards to the toxicity of sulfide in drinking water, the World Health Organization (WHO, 1996) reports that "the taste and odor threshold for hydrogen sulfide in water has been estimated to be as low as 0.05 mg/liter. Although oral toxicity data are lacking, it is unlikely that anyone could consume a harmful dose of hydrogen sulfide in drinking-water. Consequently, no health-based guideline value is proposed. However, hydrogen sulfide should not be detectable in drinking-water by taste or odor. "

Literature from WHO (WHO, 1996) describes how soluble sulfides are hydrolyzed in water: " In water, hydrogen sulfide dissociates, forming monohydrogensulfide(1-) (HS-) and sulfide (S2-) ions. The relative concentrations of these species are a function of the pH of the water, hydrogen sulfide concentrations increasing with decreasing pH. At pH 7.4, about one-third exists as undissociated hydrogen sulfide and the remainder largely as the monohydrogensulfide(1-) anion. The sulfide is present in appreciable concentrations above pH 10. In well aerated water, hydrogen sulfide is readily oxidized to sulfates and biologically oxidized to elemental sulfur. In anaerobic water, microbial reduction of sulfate to sulfide can occur ."

The Cooperative Extension Service of the University of Nebraska reports that sulfide concentrations as low as 0.5 parts per million (pp) produce an odor detectable by most people. At concentrations of 1-2 ppm, the water is corrosive and produces a rotten egg odor. The rotten egg odor is produced by hydrogen sulfide gas generated by the sulfide contaminated water. The off-gassing of the hydrogen sulfide from the water is partially controlled by the level of acidity with more gas produced at neutral to acidic conditions (pH measurements below 7.5). According to Mike LeRoy with the Florida Department of Environment (personal communication), at pH 7.2, which is a common pH for Florida waters, less than 50% of the hydrogen sulfide is in gas form.

Mr. LeRoy, an expert on drinking water issues, provided detailed information to ATSDR on treatment options for reducing sulfide in well water. The information is contained in the Appendix of this document for use by EPA and TDEC in their decision process.

One of the well owners reported that the bad taste and odor problems only began a few years ago but that the problems have been increasing in intensity over time. Over the last few years, the sulfide concentrations or the acidity of the water may be increasing, or both. If sulfide concentrations and acidity are increasing, the water may be too odorous and corrosive for domestic purposes.

The source of the sulfide is suspected to be mining waste leachate because of the many sulfide minerals historically mined at the site. However, bacteria may also produce sulfide in well water by converting elemental sulfur and sulfate to sulfide.

If the mining waste leachate is the source of the sulfides, conditions in the two wells may change over time, with the possibility of increasing sulfide or sulfate levels and increasing acidity and corrosivity of the well water. Such changes may produce water unfit for consumption because of severe taste and odor problems.

A quantitative public health evaluation of the sulfide levels in the two wells cannot be made because health standards for sulfide contaminated drinking water have not been established. If the secondary drinking water standard for sulfate of 250 mg/l is used as a surrogate standard, the reported combined levels for both sulfide and sulfate are very far below the sulfate standard and not expected to cause either acute or chronic illness.


The reported sulfide levels in the two private wells are elevated above normal for the general area but are within the range of concentrations ingested by many people, such as many Florida residents, without any reports of acute or chronic ill health effects. No apparent health hazard is associated with ingesting the current reported levels of sulfide in the two wells. However, we cannot be certain that the concentrations of sulfide and the acidity of the well water will not increase in the near future to the point of rendering the water unfit for human consumption because of severe taste and odor problems.


  1. Recommend that EPA and TDEC conduct long term periodic monitoring of the two residential wells to detect and warn of rising levels of sulfides and/or other contaminants.

  2. Providing alternative water for the two residences with elevated sulfide in their well water should be considered by EPA and TDEC as a cost-effective, protective health measure if long term periodic monitoring appears too costly.


John Mann
Environmental Health Scientist
Division of Health Assessment and Consultation

Technical Reviewer of Report

Kenneth Orloff, Ph.D.
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry

Management Reviewer of Report

Donald Joe
Deputy Branch Chief
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultations
Agency for Toxic Substances and Disease Registry


  1. Environmental Protection Agency, Region IV Superfund site information Website .

  2. World Health Organization. Guidelines for drinking water quality, 2nd edition, Volume 2. Health criteria and other supporting information. 1996 .

  3. University of Nebraska, Cooperative Extension Service. Water quality guides, April 1996 .

  4. LeRoy, Mike, Florida Department of Environmental Protection, personal communication, February 2002 via email.


Treatment Options

The information on treatment options for sulfide contaminated water is provided to assist EPA and TDEQ in their remedial decision process.

Mike LeRoy, an expert in the Florida Department of Environmental Protection, provides the following information on treatment of sulfide in drinking water (LeRoy, 2002):

Tray aeration. This is a relatively common method of treating for H2S in Florida. It is only partially effective at the pHs of most of the state's groundwaters. The treated water is more corrosive and can cause additional undesirable problems. [Tray aerators are towers with trays in them. The water enters the top and cascades down to the bottom over a series of trays. The water is then pumped out of the bottom.]

Air stripping towers. This method can be very effective when used with pH adjustment and properly designed equipment and processes. The finished water usually requires additional treatment to stabilize it and reduce its corrosivity because the aeration process also removes carbon dioxide and increases its oxygen content. [Air stripping towers normally consist of a cylindrical shell containing a support plate for packing material. Packing material is usually individual chunks of plastic randomly dumped into the column. Water enters at the top of the tower and air is forced upthrough the bottom by blowers.]

Oxidation with chlorine. This method has been the historical method of choice in Florida for removing H2S from the water. It is relatively inexpensive so utilities prefer this method, but there are some major drawbacks. First, enough chlorine has to be used to oxidize all of the H2S. Disinfection does not begin to take place until all of the H2S has been consumed by the chlorine. Second, the chlorine oxidizes the H2S to elemental sulfur, which often creates significant turbidity consisting of very fine particles. The sulfur can then be converted back into H2S by sulfur reducing bacteria that are not completely destroyed through disinfection. Furthermore, the resulting water is corrosive and can contribute to the formation of copper sulfide (black water) in customers' home plumbing. [Oxidation of hydrogen sulfide with chlorine does not produce disinfection by-products. These by-products are formed as a reaction between chlorine and organic compounds that are present in the water.]

Oxidation with ozone. Ozonation is 100% effective in converting H2S to sulfates. However, the ozone is expensive. Also, there are other disinfection by-products that must be considered. If bromide is present in the raw water, ozone will convert it to bromate, which is a soon to be regulated contaminant. Other disinfection by-products are also possible.

Granular activated carbon (GAC). This method can be effective, but there are operational problems associated with its use. The gas saturates the GAC quickly, and the carbon material has to be replaced frequently. Orlando Utilities once used GAC as a treatment method for H2S, but they have since removed it. They determined that aeration is a cheaper treatment technique, is easier to control, and is more reliable.

Oxidation of H2S with chlorine (Cl) is probably the most common method of dealing with the contaminant. There are some small household units that use hydrogen peroxide.

The chlorine required to oxidize hydrogen sulfide to sulfur and water is 2.08 mg/L chlorine to 1 mg/L hydrogen sulfide. To drive the reaction to completion, 8.32 mg/L of chlorine are required to oxidize 1 mg/L of H2S to the sulfate form.

There are problems with this method even though it is so common, and that is, the process is reversible. Originally the H2S was generated under anaerobic conditions by sulfate reducing bacteria. These same bacteria are in the water when it is pumped to the surface. Some of the bacteria survive the disinfection process, make it to nice warm quarters like the hot water heater in the home, and regrowth takes place. The sulfur and sulfates in the water and the electrons provided by the hot water heater's sacrificial anode is all that is needed. The bacteria start producing H2S. The H2S in turn reacts with the copper pipes forming copper sulfide, which is a black material that causes real problems in the home.

Aeration is the next most common method of treating H2S. The normal pH of Florida groundwater is 7.2 to 7.4. In this range about 50-60% of the sulfides are HS-. This form is not volatile and cannot be removed by aeration. To be effective, the pH must first be lowered to about 6 or so. A drawback here is that the aeration process will also strip the alkalinity (CO2) and add oxygen. This results in an unstable corrosive water. The pH must be raised back to about 7.4 and CaCO4 or CO2 must be used to increase the alkalinity. The O2 will of course tend to increase the corrosiveness of the water.

More information on aeration and hydrogen sulfide is in "Water Quality and Treatment, A Handbook of Community Water Supplies", Fourth Edition, American Water Works Association, McGraw-Hill, Inc. ISBN 0-07-001540-6.

Information is available on the Internet at the following sites:

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

A-Z Index

  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #