Radionuclides, Radioactivity and Radiation Units
Radionuclides are radioactive atoms that break down to release energy (radioactivity). The energy is released in one of three forms: (1) Alpha radiation, (ά), which consists of large, positively charged, helium nuclei; (2) beta radiation, (β), consisting of electrons or positrons; or (3) gamma radiation, (γ), consisting of electromagnetic, wave-type energy similar to X-rays. Each of the radiation forms reacts differently within the human body. Alpha particles travel at speeds as high as 10 million meters per second, and when ingested, these relatively massive particles can be very damaging. Beta particles travel at about the speed of light. Their smaller masses allow greater [tissue] penetration, but create less damage. Gamma radiation has tremendous penetrating power, thus can be very dangerous. Fortunately, it has limited effect at low levels.
Radiation is generally measured and reported in units of Curies (Ci), or in rems. One Curie equals 3.7 x 1010 nuclear transformations (disintegrations) per second. A common unit used in drinking water regulations is the picoCurie (pCi), equivalent to 10-12 (0.000000000001) Curies. By definition, 1 gram of radium has 1 Ci of activity. By comparison, 1 gram of uranium-238 has 0.36 x 10-6 Ci. A rad quantifies the absorbed dose given to tissue or matter, such that a rad of alpha particles creates more damage than a rad of beta particles. Conversely, a rem quantifies radiation in terms of its dose effect such that equal doses expressed in rem produce the same biological effect, regardless of type of radiation involved. An effective equivalent dose of 0.1 mrem (millirem) per year from any type of radiation corresponds to about a 10-6 (1 in 1 million) excess lifetime cancer risk.
Radioactivity can be naturally occurring, from elements in the earth’s crust or from cosmic ray bombardment in the atmosphere. Man-made radiation has three general sources: nuclear fission from weapons testing, radiopharmaceuticals, and nuclear fuel processing and use. About 200 man-made radionuclides occur, or could occur, in water. However, only strontium and tritium have been detected on a consistent basis.
Humans receive an annual dose of radiation of about 200 mrem (millirem) from all sources. The United States Environmental Protection Agency (USEPA) estimates that drinking water contributes about 0.1 to 3 % of a person’s annual dose. Local conditions, however, can alter this considerably. It is estimated that between 500 and 4,000 public water systems have greater than 10,000 pCi / L of radon, a level that corresponds to an annual effective dose equivalent of 100 mrem per year.
Based on occurrence in drinking water, and on health effects, the radionuclides of most concern are radium-226, radium-228, uranium-238, and radon-222, all of which are naturally occurring. Radium-228 is a beta emitter whose decay gives rise to a series of alpha- emitting daughters (an atomic species that is a product of radioactive decay), while all the other radionuclides are alpha emitters. “Natural uranium” encompasses uranium-234, uranium-235 plus uranium-238. Of these, uranium-238 accounts for 99.27% of the occurrence.
Radioactivity can cause developmental, teratogenic (biol., abnormal formation or growth), genetic (affecting the genes), and somatic (cells pertaining to the formation of a body) health problems, including carcinogenesis (formation of cancerous growth). The carcinogenic effects of nuclear radiation (alpha, beta and gamma) on the cell are thought to be the ionization of cellular constituents, leading to changes in the cellular DNA and thence to DNA-instigated cellular abnormalities. All radionuclides are considered to be carcinogens, however, their target organs differ. Radium-228 is a close relative of calcium and is stored in the bones as a “bone seeker”, causing bone sarcomas. Radium-226 induces head carcinomas. Radon-222 is a gas and can be inhaled during showers, washing dishes, and so forth, or it can be ingested. A direct association between radon-222 and lung cancers has been shown. Because of its high concentration in various water systems, radon carcinogenicity is of great concern. Uranium is not a demonstrated carcinogen but accumulates in the bones, similar to radium. Uranium has, however, a demonstrated effect on the human kidneys, leading to inflammation and changes in urine composition.
Radium (Ra) Occurrences and Uses
Radium, atomic number 88, atomic weight 226 (most abundant isotope), a radioactive element, is a decomposition product of uranium. Radium decays by emission of alpha, beta and gamma radiation. The element was discovered in 1898 by Pierre and Marie Curie, and has been prepared as a white metal which immediately becomes black upon exposure to air. Radium, due to its decay, is constantly emitting heat. The temperature of radium and its salts is about a degree and a half (C) higher than that of its surroundings. During the 2,300 years of its life, a gram of radium gives off 250,000 times as much energy as the burning of one gram of coal. Radium forms water-soluble compounds. Radium and its salts are extremely poisonous to all life.
Table No. 1
Man-made Radium Isotopes
|Isotope||Half Life||Primary Emission||Decay Series|
|Radium-226 (226Ra)||1,600 years||Alpha||Uranium|
|Radium-228 (228Ra)||5.7 years||Beta||Thorium|
|Radium-224 (224Ra)||3.64 days||Alpha||Thorium|
Uranium (U) Occurrences and Uses
Uranium, atomic number 92, atomic weight 238 (most abundant isotope), is a dense silver-colored solid. Uranium occurs in pitchblende ore (essentially as UO2), a variety of uranite, coffinite and carnotite. Uranium is a highly toxic radioactive material, a source of ionizing radiation. Since uranium ignites spontaneously in air, it is a dangerous fire risk. Uses of uranium include pyrophoric armor-piercing ammunition. The isotope 235U is fissionable and is used in atomic weaponry and for nuclear power generation. Neutron bombardment of the (non-fissionable) 238U in a nuclear (breeder) reactor creates the fissionable synthetic radioactive metallic element, plutonium.
Table No. 2
Naturally Occurring Uranium Isotopes
|Isotope||Natural Abundance, %||Half Life, Years||Emission|
|Uranium-238 (238U)||99.276||4.51 x 109||Alpha|
|Uranium-235 (235U)||0.7196||7.1 x 108||Alpha / Gamma|
|Uranium-234 (234U)||0.0057||2.47 x 105||Alpha|
Regulatory Background (Radium and Uranium)
Public Law 93-523, passed by the 93rd congress in 1974, established the “Safe Drinking Water Act” (SDWA), which limits certain constituents (“contaminants”) in drinking water. The law is administered and enforced by the EPA, unless individual states qualify for and take “primacy.” The Act establishes Maximum Contaminant Levels (MCL’s) for the constituents, along with their required analytical detection limits. In keeping with EPA policy, any known or suspected carcinogen has an MCL “Goal” (MCLG) of “zero”. States may not relax any of the provisions or limits of the Act, but may impose stricter requirements. The SDWA has been amended a number of times. In 1986 congress passed a major amendment to include a greater number of contaminants, based on more current knowledge of chemicals and their health effects. The amendment, for the first time, also required monitoring for “Unregulated Chemicals”. Under the SDWA, each public water system must monitor each water source, or every entry point to the distribution system that is representative of all sources being used under normal operating conditions; must conduct all monitoring at the same site(s) unless a change is approved by the EPA or responsible state primacy agency, based on a review of the system and its historical water quality data. The SDWA states the initial monitoring for radionuclides shall be accomplished by taking four consecutive quarterly samples, which may be combined into one annual composite sample. The initial monitoring must have been completed by December 2007.
Regulated Beta and Photon Emitters: Strontium-90 and Tritium
The EPA, in November 2008, added strontium-90 (90Sr) and tritium (3H) to the list of regulated radionuclides. Both radionuclides are presently covered under the gross beta requirement (Federal and CA MCLs are identical at 8 pCi / L for strontium-90, and 20,000 pCi / L for tritium).
Strontium-90 is a man-made product of atomic fission from spent reactor fuel and nuclear tests. Strontium-90 has a half-life of 28.8 years and decays exothermically with beta emission to the short-lived ytterbium-90 then to the stable zirconium-90. Because of the exothermic decay, 90Sr has been, and continues to be, used in small nuclear reactors as a power source for space vehicles. Like its relative, radium, strontium is a “bone seeker”, and is implicated in cancers and leukemia.
Tritium is a “heavy” isotope of hydrogen. Tritium occurs in nature only rarely and is mostly produced in nuclear reactors by neutron activation of lithium-6 (6Li), or as a fission product of uranium-235, uranium-233 or plutonium-239. Tritium, a gas, combines with oxygen to form “tritiated water” (T2O).
Strontium-90 can be removed from water by the same unit operations that remove radium and uranium.
Except for Reverse Osmosis (RO), Tritium does not lend itself to removal by any of the EPA listed removal processes. A February 2009 document by the US Department of Energy (DOE) on tritium removal lists Distillation, Liquid Phase Catalytic Exchange (LPCE), Thermal Diffusion (TD), Electrolysis Catalytic Exchange (ECE), Evaporation, and the Girdler Sulfide Process (GS) as possible removal methods. All of these processes are extremely sophisticated, and expensive, and are not suitable for general waterworks applications.
Table No. 3
Radionuclide Maximum Contaminant Levels (MCL’s) and Detection Levels for Purposes of Reporting (DLR’s)
|Radium–226||5 pCi / L (Combined Radium-226 & -228)||1 pCi / L|
|Radium–228||1 pCi / L|
|Gross Alpha Particle Activity (Excluding Radon and Uranium)||15 pCi / L||3 pCi / L|
|Uranium||20 pCi / L**||1 pCi / L|
*DLR = Detection Limit for Purposes of Reporting (Nomenclature differs between
federal and states’ detection limits. Examples in above table are California DLR’s).
** Some states regulate uranium at 30 µg / L (micrograms per liter) which, after applying the conversion factor (µg x 0.67 → pCi), coincides with the federal MCL.
Radium and Uranium Removal Processes
Although possessing differing rates and modes of radioactive decay, the chemical and physical behavior of all isotopes of an element are the same. Therefore, any process that is capable of removing a given percentage of 226Ra, will remove the same identical percentage of the 228Ra isotope. The same applies to the removal of isotopes of uranium.
Before discussing possible removal methods, it is important to understand in what form radium and uranium are present in groundwater. The speciation tends to be a function of water pH, as is shown in the table below.
Table No. 4
Radium / Uranium Forms
|pH Range||Predominant Species||Predominant Species Charge|
5 to 6.5
6.5 to 7.6
|> 7.6||UO2(CO)34-||Tetravalent Anion|
Radium Removal Methods
Since radium is a close ‘relative’ of the (positively charged) divalent cations calcium and barium, it can be removed, like calcium, by ion exchange or by chemical (lime) softening, as well as by a number of other treatment methods. The following processes are deemed “Best Available Technology” (BAT) by the EPA. (List rankings are not indicative of preference.)
- Lime Softening
- Reverse Osmosis (RO)
- Cation Exchange
- Sorption onto Preformed Hydrous Manganese Oxide HMO
- Precipitation with Barium Sulfate
- Sorption onto Barium Sulfate-Impregnated Media
- Coagulation / Filtration with HMO Pretreatment
Table No. 5
Radium Removal Efficiencies and Operational Commentary
|Treatment Method||Removal Efficiency||Comments|
|Ion Exchange Softening, Na+||>95%||
Operate to Hardness Breakthrough
Barium (Radium) Sulfate Precipitation
Add BaCl2 To Feedwater Before Filtration
|Manganese Dioxide Floc Adsorption
(“Hydrous Manganese Oxides”)
Use Preformed MnO2 Or MnO2-Coated Filter Media
Reverse Osmosis (RO)
High Capital Expense
Waste Sludge Intensive
Coagulation / Filtration with HMO Pretreatment
|50 – 95%||Use with Permanent Media, e.g. Manganese Dioxide or MnO2 – Coated Media|
Uranium Removal Methods
As shown in Table No. 4, uranium, depending on the water’s pH, may be present either as a (positively charged) divalent cation, a neutral molecule, or as a (negatively charged) divalent, or tetravalent, anion. Uranium removal methods for water treatment plants include not only anion or cation exchange and chemical softening, but also enhanced coagulation / filtration, as shown in the Table No. 6:
- Anion Exchange
- Lime Softening
- Coagulation / Filtration with Iron / Aluminum Salts
- Reverse Osmosis (RO)
- Activated Alumina Adsorption
Table No. 6
Uranium Removal Efficiencies and Operational Commentary
|Treatment Method||Removal Efficiency||Comments|
|Coagulation / Filtration With Iron Or Aluminum Salts||50-90%||Effective At pH Near 6 And 10|
|Lime Softening||80-99%||Higher pH = Greater Removal Mg+2 Helps At pH >10.6|
|Anion Exchange||>99%||Regenerate With 2-4 M NaCl After 10-50,000 BV|
|Reverse Osmosis||>99%||Effective, High Capital Expense|
“M” = Molar Solution, “NaCl” = Sodium Chloride, “BV” = Bed Volumes
Radium and Uranium Removal Processes – Overview
Lime Softening (Radium / Uranium Removal)
Radium, a close relative of calcium and barium, responds to lime, or excess lime, softening, as does some uranium, by precipitating as a solid carbonate. The process is capital, and also operations and maintenance intensive. Because the process is not specific for radium, it also precipitates calcium and other divalent ions, producing considerable sludge volume. This sludge, if it contains a substantial amount of radium, may require disposal as a hazardous material.
Sorption onto Manganese Dioxide Based Floc
(Preformed Hydrous Manganese Oxides, HMOs) (Coagulation / Filtration)
In this process, hydrous manganese dioxide* (aka “amorphous manganese dioxide”), is used as the substrate onto which ions of radium will be adsorbed, and the HMO’s, along with the adsorbed radionuclides are removed by media filtration. In the presence of iron, the process is capable of also removing uranium, although as stated above, the iron must be fully oxidized, or it will adversely affect the removal efficiency for radium. While the HMO process is not specific for radionuclide removal, it is the closest to being specific of any BAT under discussion. The process also lends itself to coagulation / filtration with HMO pretreatment. Radium removal is not pH dependent.
*The hydrous manganese dioxide is created either from the reduction of permanganate, as follows:
3Mn+2 + 2MnO4 + 2H2O → 5MnO2(s) + 4H+
or by the oxidation of manganous sulfate, or by the permanganate oxidation of ferrous sulfate
Radium / uranium cannot be removed by oxidative processes. Oxidants such as sodium hypochlorite, chlorine, ozone, etc. are fed to oxidize raw water iron and / or other contaminants such as manganese, hydrogen sulfide, ammonia, etc. but will not affect the radium or uranium removal efficiency.
Coagulation / Filtration Removal of Uranium with Iron or Aluminum Salts
Uranium removals of 50 to 90% can be achieved by coagulation / flocculation / filtration with iron and / or aluminum salts. The mechanism appears to be the adsorption of uranium on the iron and / or aluminum hydroxide floc with subsequent removal by filtration. The process is pH dependent in that a low pH (approximately 6.5) is necessary for effective alum (Al2[SO4]3) floc formation, while ferric hydroxide (Fe[OH]3) (iron) flocs form at higher pH levels. If this process is to be used for the simultaneous removal of radium and uranium, the iron must be completely oxidized before the water contacts the HMO.
Reverse Osmosis (Radium / Uranium / Strontium-90 Removal)
A process originally designed to purify seawater, and brackish water, by the application of hydrostatic pressure to overcome osmotic pressure and drive water molecules through a semi-permeable membrane designed to exclude other molecules. The process requires fragile and expensive stacks of cellulose-acetate or thin film composite membranes. Since the recovery of product (treated) water, as a percentage of feed water, is a function of applied pressure (up to 400 psi or more, depending on membrane type), this process tends to be energy-intensive. Reverse osmosis is capable of the removal of radium and / or uranium, but is, of course, non-selective. EPA deems RO as BAT for the removal of beta and photon emitters including strontium 90 and tritium.
Cation Exchange Softening (Radium / Uranium / Strontium-90)
Originally designed for the removal of hardness (which is due to the presence of divalent metallic ions in the water), cation (zeolite) softening will also remove the divalent cation of radium, as well as some of the uranium ions, and exchange them for (generally) sodium ions.
2RNa+ + Ra2+ → R2Ra2+ + 2Na+ [R = Ion Exchange resin]
Since the process tends to be relatively expensive, it would seem to be warranted for radium removal only if there was also a need for hardness reduction as well.
Precipitation with Barium Sulfate / Sorption onto Barium Sulfate-Impregnated Media
The addition of barium chloride to precipitate barium sulfate results in the co-precipitation of radium, a close relative of barium. The process, simplified, is:
(Excess) Ba2+ + (Trace) Ra2+ + SO42- → Ba(Ra)SO4
This process, because it requires an excess of barium, tends to be not only expensive, but may also result in exceeding the barium MCL in the treated water. A “safer” approach is to “sorb” Ra2+ directly onto barium sulfate-impregnated filter media. This process will not remove uranium.
Electrodialysis (ED) and Electrodialysis Reversal (EDR)
(Radium / Uranium Removal)
ED is an electrochemical membrane process initially developed for the purification of brackish and saline waters. Instead of hydrostatic pressure, as in the case of reverse osmosis, the process uses an applied direct current (DC) voltage to move dissolved anions and cations from alternate cells through semi-permeable membranes. This results in the purification of a portion of the feed water, and the concentration of another. While capable of removing most contaminants, including radionuclides, the process is equipment, energy and operator intensive. The process creates a concentrate stream that requires disposal and is quite wasteful of water.
EDR is an ED process that reverses the polarity of the electrodes on a controlled, predetermined time cycle, thus reversing the direction of ion movement in the membrane stack. This provides automatic flushing of scale forming minerals from the surface of the membrane. ED and EDR systems are generally not considered economically viable for any but very small installations.
Activated Alumina for Uranium Removal
Activated alumina is capable of removing a great number of inorganics, and some organic molecules. The adsorptive capacity of many media, particularly activated alumina, is pH dependent. Generally, as pH decreases, removal increases. Activated alumina can be regenerated, or it can be replaced once the selected breakthrough point is reached. The process is not selective for uranium, and the regenerant may require disposal as a hazardous waste, as may the activated alumina itself, if it is replaced, rather than regenerated. Activated alumina will not remove radium unless it is barium sulfate – loaded.
Anion Exchange for Uranium Removal
As previously noted, uranium can also occur in water as either a divalent or a tetravalent anion. An anion exchange system, operating on a chloride cycle, is capable of removal of these anions, as shown:
4RCl– + UO2(CO3)4 ↔ R4UO2(CO3)4- + 4Cl– [R = Ion Exchange resin]
The process has very good removal efficiency but is, of course, not specific to uranium, and will try to exchange other anions for chloride ions as well. Radium is a cation and is not removed by anion exchange.
Iron Coagulation / Filtration for Uranium Removal
Sometimes practiced in conjunction with “enhanced coagulation” (for disinfection byproduct control), ferric hydroxide [Fe(OH)3] floc is capable of removing uranium though, again, it is not specific. Adding an HMO will also remove radium.
Radium / Uranium Concentrations in Treatment Process Wastes
The higher the concentration of radium / uranium in the raw water, and the more effective the removal process, the higher will be the level of radioactivity in the process wastes (treatment residuals). In the absence of universally applicable national (federal) standards; individual states, and various agencies within those states, set the levels of radioactivity in solid wastes below which a treatment residual may require disposal in a landfill, and above which they will require disposal in a hazardous waste site. In many states, (including California) the Radiological Health Branch (RHB) (or its equivalent agency) will set the upper limit of radioactivity for on-site accumulation and storage of radioactive waste. If the waste activity is below the specified limit, the generator may continue to accumulate the waste on site. If a waste exceeds the limit, it must be hauled away to an approved disposal site, by a hauler specifically licensed to transport low-level radiologic wastes. Many states, and their agencies, base their criteria on the results of laboratory tests, such as:
TTLC, “Total Threshold Limit Concentration”, a measure of the total concentration of a contaminant in a solid waste.
TCLP, “Toxic Characteristics Leaching Procedure”, a measure of the portion of a waste contaminant that may be leached if waste disposal is to a landfill.
STLC, “Soluble Threshold Limit Concentration”, a procedure similar to, but more stringent then, TCLP. Generally, state agencies will require STLC tests if the contaminant concentration found in a TTLC test exceeds the STLC limit by a factor of ten (10).
In the case of liquid treatment residuals, if a sewer connection is available, it is the local sewerage authority that will determine the maximum radioactivity permissible to be discharged into the sewer system. If no sewer connection is available, or if the activity exceeds the sewer discharge limit, disposal of the liquid waste into a lined evaporation pond, or other means of disposal, will be necessary. Under no circumstances should radioactive wastes be discharged to surface waters without an National Pollutant Discharge Elimination System (NPDES) permit from the local Water Pollution Control Agency (WPCA). Thus, it is very important that initial pilot testing results in, among other data, an estimate of the potential radioactivities expected in the wastes. And, it is equally important that municipalities and private water companies work very closely with, and adhere to, the requirements of local regulatory authorities.
The following, very general, guidelines are offered to estimate radioactivity in treatment process wastes (treatment residuals):
Ion Exchange Softening (Process Wastes)
In any ion exchange treatment, there will eventually be solid wastes in the form of spent ion exchange resins, and there will be liquid wastes of regeneration brine, backwash water and rinse water. Disposal of backwash and rinse waters containing radium / uranium wastes into sanitary sewers may be accepted, however a permit will be required. The brine from a typical system, treating a water containing approximately 10 pCi Ra / L, at steady state, (i.e., the resin has been exhausted and regenerated at least five times), will contain approximately 600 pCi radium per liter, while the resin itself will retain about 20 pCi of radium per gram.
When using cation exchange to treat a water with 40 micrograms (~ 27 pCi / L) uranium per liter, and regenerating after approximately 30,000 bed volumes, the waste brine may contain 80,000 (or more) pCi U per liter.
Barium Sulfate-Loaded Activated Alumina Radium Removal (Process Wastes)
A 10 pCi / L of radium feed will result in a radium concentration of approximately 220 pCi / gram of radium in the (dry) alumina, after processing 20,000 Bed Volumes (BV).
|NOTES:||1.|| Unlike the unit operations above, (ion exchange softening and barium-sulfate coated activated alumina) in which radioisotopes accumulate on the resin, or media, with increased water throughput, in the coagulation / flocculation / sedimentation processes below, the relative concentration of radioisotopes (in pCi / gram) in the sludge does not increase with increased water volume treated. The only increase will be in the volume of radioisotope-carrying sludge generated.
|2.||The maximum concentration of radioisotopes (radium / uranium) in pCi / liter in the decanted backwash water is the maximum radium / uranium concentration in the raw water.|
Coagulation / Filtration with HMO Feed / Radium Removal (Process Wastes)
Systems treating a water with approximately 25 pCi radium / L, with a dosage of approximately 1 mg / L manganese dioxide HMO, will produce a (dry) manganese dioxide sludge that can be expected to contain approximately 21,000 pCi radium per gram.
Lime Softening Uranium Removal (Process Wastes)
Lime softening of a water with approximately 40 pCi / L uranium, at 90% removal, will produce a dry calcium carbonate sludge (regardless of what else is removed) that will contain approximately 135 pCi uranium per gram.
Iron Coagulation / Filtration Uranium Removal (Process Wastes)
When operated as a typical coagulation / flocculation / filtration plant (i.e. typical iron dosages) the process can be expected to produce a dry sludge containing approximately 800 pCi uranium per gram.
Radon, an inert gas, cannot be removed by either chemical means, or filtration. The Best Available Technology (BAT), for removal is aeration to “strip” radon from the water. The MCL for radon is 300 pCi / L.
Field Pilot Testing
Since raw water quality can be quite variable, both geographically and seasonally, and since such variability will affect contaminant removal efficiency, field pilot testing of the chosen removal system(s) is strongly recommended. Raw water quality analyses should be performed, and the data evaluated, prior to pilot testing to determine all of the constituents which may affect radium / uranium removal. (For example, in cation exchange processes, common divalent ions contained in most natural waters, [i.e., calcium, magnesium, divalent iron or manganese, etc.] will compete with radium / uranium for active exchange sites.)
The piloting process should include all necessary pretreatment steps (such as pH adjustment for RO and activated alumina). The pilot process must verify radionuclide removal throughout the process run cycle, and removal must be verified by radiochemical analyses performed by a laboratory certified for such analyses. The piloting should be performed in a manner that will allow a reasonable estimation of pretreatment chemical requirements and costs for operation, labor, media replacement and / or disposal, membrane replacement, regenerant brine and / or treatment of sludge or waste water disposals. And, as previously noted, pilot testing must include the expected radioactive concentration of the process wastes such as decanted sludge, brine, resin, media, membranes, etc.
Pilot test protocol should be reviewed by the municipal / consulting engineers and all parties should agree to the protocol before the test begins (this should include details such as which party will pay the laboratory test costs). Duplicate samples of raw and treated water should be sent to two (2) independent laboratories for verification of the laboratory test results.
The pilot test report should include field test data, charts and graphs of important data and also a summary of the field and laboratory test results. The report should also include final treatment plant recommendation.
Radium, uranium and radon, because of their known, or suspected, implication in carcinogenesis, have been regulated by the EPA to extremely low levels. The low MCL’s dictate and drive the installation of removal processes. With the possible exception of radium removal with preformed hydrous manganese oxides (HMO’s), none of the available unit operations are selective or specific for the radionuclides only. It is very important for water purveyors to carefully choose, and pilot test, processes to maximize radionuclide removals without incurring extra treatment expenses for the co-removal of innocuous ionic species.