Drinking Water Treatment Wastes
Certain rock types naturally contain radioactive elements referred to as NORM (Naturally Occurring Radioactive Materials). When a source of drinking water comes in contact with NORM-bearing rocks, radionuclides may accumulate in the water to levels of concern. The predominant radionuclides found in water include:
- radium (and its decay products)
- thorium (and its decay products)
- uranium (and its decay products)
As water is treated to remove impurities, radionuclides may collect and eventually build up in filters, tanks, and pipes at treatment plants. The small amounts of NORM present in the source water may concentrate in sediment or sludges. Because the NORM is concentrated due to human activity, it is classified as TENORM (Technologically Enhanced Radioactive Material). Most of this waste is disposed in landfills and lagoons, or is applied to agricultural fields.
Most drinking water treatment sludges are thought to contain radium (Ra-226) levels comparable to typical concentrations in soils. However, some water supply systems, primarily those relying on groundwater sources, may generate sludge with much higher Ra-226 levels. Furthermore, some water treatment systems are more effective than others in removing naturally-occurring radionuclides from the water.
The table below lists the current radionuclide standards for drinking water.
|Radionuclide||Maximum Contaminant Level (MCL) Allowed by EPA|
|combined radium 226/228||5 pCi/L|
|beta emitters||4 mrems|
|gross alpha standard||15 pCi/L|
|radon||300 pCi/L (proposed standard)|
On this page:
- Radiation Levels
- Waste Generation
- Disposal and Reuse
- What is being done about these wastes?
Of the 160,000 public water systems serving the U.S., approximately 700 of these systems treat water with elevated TENORM levels. The National Inorganics and Radionuclide Survey(NIRS), performed by the EPA, found large numbers of water treatment plants in the midwestern and southeastern states processing water with radium concentrations in excess of EPA's Maximum Contaminant Levels (MCLs). The concentration and distribution of radionuclides in drinking water varies from one area to another depending on location variables:
- geological location
- groundwater pH
- redox potential
- radionuclide solubility and half-life
- water withdrawal rates
- amount of sludge produced per unit volume of water processed.
Radium levels are typically found in higher concentrations in groundwater than in surface water. Concentrations in groundwater typically range from 0.5-25 pCi/L compared to 0.01-1 pCi/L for surface water. Radium concentrations as high as 200 pCi/L have been reported, although this is rare; levels are seldom seen above 50pci/L.
Ra-226 average concentration in community drinking water supplies is estimated to range from 0.3 to 0.8 pCi/L . The results of the National Inorganics and Radionuclide Survey (NIRS), performed by the U.S. EPA's Office of Drinking Water, indicate a higher weighted average of 0.905 pCi/L. Five states—Georgia, Illinois, Minnesota,Missouri, and Wisconsin— have been noted to have still higher average concentrations, ranging from 1.27 to 5.29 pCi/L.
Uranium activity as high as 652 pCi/L has been observed in both surface and groundwater samples. The average uranium concentrations in surface and groundwater are believed to be about 1 and 3 pCi/L, respectively. The population-weighted average uranium concentration in community drinking water supplies is estimated to range from 0.3 to 2.0 pCi/L.
The presence of radon in groundwater is known to vary significantly, with the concentration being over six orders of magnitude different from that of Ra-226. The geometric mean of groundwater radon concentrations is nearly 1,000 pCi/L. For radon, the population-weighted average is believed to range from 194 to 780 pCi/L.
Approximately 260,000 metric tons (MT) of TENORM wastes are generated each year by U.S. water treatment facilities. This is equivalent to 600 MT of TENORM per utility. Eighty-three percent of the contaminated waste is filter sludge and the remaining 17 percent is ion exchange resins and charcoal.
The table below lists typical radiation levels in drinking water treatment residuals:
|Wastes||Radiation Level [pCi/g]|
|Treatment Sludge [pCi/l]||1.3||11||11,686|
|Treatment Plant Filters||40,000|
The Radiation in TENORM Summary Table provides a range of reported concentrations, and average concentration measurements of NORM associated with various waste types and materials.
Not all treatment facilities produce TENORM wastes. The degree of contamination varies according to the natural abundance of radionuclides in the water and on the treatment methods employed. Only 28 percent of water treatment processes remove radionuclides from water. Those of most concern are lime softening and ion exchange and activated charcoal.
Lime softening is used on large water supply systems to soften water by the addition of calcium hydroxide. Calcium hydroxide raises the pH of water causing calcium and magnesium to settle out and form a solid sludge. 80 to 90 percent of the radium in the water becomes trapped in the sludge, which is removed by sedimentation and filtration. This process typically produces about 4 cubic yards (3.1 m3) of dewatered sludge per million gallons of processed water.
Ion-Exchange and Activated Charcoal
Ion-exchange resins are used on smaller water supply systems to soften water by replacing Ca2+ and Mg2+ ions with Na+ ions. In the process, about 95 percent of the radium is also removed . However, the resins are usually back washed for reuse rather than being disposed. The backwash water, which contains radium, is typically discharged to storm sewers, underground injection wells or septic tanks, or is back washed to another ion-exchange column for the selective removal of radium. Radionuclide content eventually builds up in the resin after prolonged usage.
Disposal and Reuse
A variety of methods are employed for disposing of drinking-water treatment sludge, including lagoons, landfills, sewer systems, deep well injection, and land spreading/soil conditioning.
Approximately 42 percent of sludge is disposed of in lagoons. Any radium present in the sludge will settle in bottom sediments which may have to be periodically dredged and properly disposed of.
Approximately 30 percent of generated sludge is disposed of in landfills. Contaminated materials are typically covered and compacted on a daily basis. Features such as clay layers are emplaced above and below the buried waste to prevent radon emissions and radionuclides from leaching into the groundwater.
Approximately five percent of sludge is discharged into sewer systems. Sludge and other wastes released into sanitary sewers are ultimately treated at sewage treatment facilities. Some of the discharged sludge may wind up in storm sewers where it is typically routed to natural bodies of water where sludge may accumulate in aquatic sediments.
Deep-well injection involves the pumping of sludge into a stable geologic formation. Deep-well injection is not commonly used and is specifically prohibited in the states of Wisconsin and Illinois. Because of its potential adverse impact on groundwater aquifers, EPA uses its authority under the Safe Drinking Water Act to control and also discourage this practice.
Land Spreading/Soil Conditioning
About 20 percent of sludge is disposed of by land application to improve soil conditions or to fertilize the soil. The sludge is plowed directly into the soil to limit water runoff and for sanitary reasons. Recently proposed rules may prohibit this practice on agriculture land.
What is Being Done About These wastes?
EPA has developed guidance on managing drinking water treatment waste and is conducting a study to learn more about the levels of radioactivity in these wastes.
A Regulators' Guide to the Management of Radioactive Residuals from Drinking Water Technologies (PDF) (81 pp, 561K About PDF)[EPA 816-R-04-005]
This report includes guidance on managing water treatment wastes and other topics:
- technologies for treating and removing radionuclides from drinking water
- general principles of radioactivity
- recommendations for occupational safety of workers at the Community Water Systems (CWS) including guidance for radiation protection and protection from radon
- regulatory requirements governing the radioactive wastes generated by the CWSs.
Study of the Levels of Radioactivity in Drinking Water Wastes
In a limited radiation study at several Community Water Systems around the country, EPA is evaluating radiation levels and concentrations of radionuclides in facilities which treat water for radionuclides. The study will measure radiation levels the facilities:
- tanks, filters, and other equipment
- piping and possibly pipe scales
- source water
- finished drinking water
- sludges and sediments