Importance of Radon In Water

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There are 3 possible sources of airborne radon in households:

Soil gas
Water supply
Building/construction materials

Although soil gas is generally recognized as the largest natural source of radon in homes, the water supply can contribute a significant fraction. In fact, in certain homes the water supply has been shown to be responsible for essentially 100 percent of the elevated levels of airborne radon. Additionally, the localized concentration of radon released from water during showering or similar activities can be extremely high.

Water vs. Air Radon Concentration

Waterborne radon concentrations are generally much higher than those found in air. The unit picocuries per liter (pCi/L) is used to express an amount of radon per liter of air (or liter of water in this instance). Until recent years, the levels at which these concentrations became a cause for concern were thought to be very different for the two media. Ingestion of radon through drinking water was not considered to pose a significant health threat until the levels reached several hundred thousand pCi/L. Recent experimental work, however, suggests the number of fatal cancers from radon ingestion in drinking water may be significant. Although the uncertainty of this work is large, it has raised the possibility that radon ingestion may represent a major pathway for risk when stomach cancer is included in the risk analysis.

It takes relatively high levels of waterborne radon to result in significantly elevated levels of airborne radon. This is due to the partial transfer of radon from water to air and, more importantly, the relatively small volume of water used as compared to the large volume of diluting air inside the home. Some researchers have determined that. On Average, 1 pCi/L of airborne radon will result from the normal use of a water supply containing 10,000 pCi/L. Whereas this number is only an average and is subject to extreme variations, it does illustrate the large difference in the significance of water and air concentrations. It is important to explain this consideration to the homeowner, as it is sometimes difficult to convince a person to be concerned about 10 pCi/L in the air when the water supply contains 10,000 pCi/L.

Water to Air Radon Transfer

Levels of radon observed in household well water supplies range from near 0 to over 1 million pCi/L. Using the assumption that 10,000 pCi/L in the water will add 1.0 pCi/L to the overall accumulation of airborne radon in the home, it takes a waterborne radon level of 40,000 pCi/L to result in the EPA guideline level of 4 pCi/L in the air.

Water to Air Radon Transfer

However:

The 10,000:1 ratio is not valid in all cases
Water is rarely the only contributor to the airborne radon level
The conditions at a specific household may dictate concern about levels significantly lower than 40,000 pCi/L of radon in its water supply
Levels far greater than the whole house average can be found in areas of hot water use, such as showers, laundry rooms etc.

The amount of radon transferred from water to air is a function of:

The waterborne radon level
The amount of water used
The type of water activity, e.g., shower (high transfer) vs. running water in a sink (low transfer)
The water and air temperatures

Radon in Drinking Water Analytical Methods

Liquid scintillation has an obvious advantage over de-emanation: its low labor requirements are consistent with the fact that typical commercial liquid scintillation counters are designed for automated sample processing. This is an important consideration for the implementation of the future radon rule, since it increases the ability of a laboratory to handle larger sample loads and it decreases per sample analytical costs.

The liquid scintillation method has been published in the 19th edition of Standard Methods(3) as method "7500-Rn". This method was collaboratively tested at radon concentrations of 200 and 400 pCi/L. It requires: a minimum of fifty minutes counting (detection) time, a maximum of four days sample holding between collection and analysis, and on-site sampling using standard glass liquid scintillation vials with teflon (TFE) or foil-lined caps.

In 1991, the U.S. Environmental Protection Agency (EPA) proposed both liquid scintillation (1) and de-emanation (2) as analytical methods for radon in drinking water. This choice was based on the following factors:

Reliability of the methods to detect and measure radon in water over a range of concentration including the proposed Maximum Contaminant Level (MCL) of 300 pCi/L
Specificity of the methods to detect radon in water in the presence of potential interferences
Availability of equipment and trained personnel
Potential of the methods to quickly measure radon in water consistent with anticipated data quality objectives
Potential of the methods for routine use, consistent with the anticipated sample load
cost of analysis.

Radon Removal Methods

Radon removal water treatment methods for individual households should be point-of-entry devices that treat the entire water supply, rather than point-of-use devices that treat only the drinking water. This is because the water used for non-drinking purposes contributes the most radon to the air.

There are 3, basic, water treatment methods that could be used to remove radon from water supplies:

Decay storage
Aeration
Granular activated carbon (GAC) adsorption/decay

Decay Storage

Decay storage is typically a good method, but is impractical for both space and cost reasons. GAC generally is very effective at removing radon from water, but it suffers from gamma emission problems and eventual disposal problems related to the accumulation of the radioactive radon decay products. Aeration is also an effective approach, but generally more mechanically complex and requires more maintenance than GAC. Decay Storage One approach to reducing radon levels in water would be to hold water in a storage reservoir, and allow radon to decay prior to use. This actually occurs to a varying extent in public water supply storage reservoirs. Since radon has a half life of 3.8 days, after 27 days, 99% of the radon originally present would have decayed. On the average, a person uses approximately 60-100 gallons of water per day in the home. For a typical family of four, that amounts to 240-400 gallons per day, or, 6400-10,800 gallons for 27 days. A tank this large is normally considered impractical and extremely expensive for 99% removal. If low levels of removal are needed, 50-70%, storage may be practical.

Aeration

In the same way as radon can easily de-gas from water sprayed during use of a shower, spraying water into an enclosed tank with air blowing through it can affect a controlled separation of radon from water. This process occurs inside of a tank at atmospheric pressure. Therefore, a pump is required to increase the water pressure; such that the water can be delivered throughout the house. Basically, variations of aeration systems involve different methods of contacting air and water.

The four approaches are:

Spray: where water is sprayed into a tank with air blown upwards and out to a vent and discharged into the atmosphere.
Diffused Bubble: where a reservoir of water is held and air is injected below the water and bubbles up through the water. He air leaving the surface is then discharged to the outside.
Packed Column: where water trickles down through a large pipe or "column" that contains loosely filled material (packing). The water forms a falling film on the packing which increases the surface area for de-gassing. Simultaneously, air is blown into the bottom of the column so it can rise through the column and sweep the de-gassed radon away.
Shallow Aeration: where water is allowed to pass over a plate or tray inside a tank. The tray has perforations in it that allows for air to be blown up through the water on the tray, thus affecting the air/water contact.

Granulated Activated Carbon (GAC)

Activated carbon canisters are widely used devices for determining the radon in air by virtue of the fact that radon can be adsorbed onto and held in the pores of the activated charcoal. Similarly, when water containing high concentrations of radon is passed through a bed of activated carbon the radon is adsorbed onto the carbon, thereby affecting a separation of radon from water.

Once the radon has been collected onto the carbon, it is held there long enough for the radon to continue through its normal decay chain. In essence, the radon is trapped in the carbon bed and its decay products are also firmly held in place. Carbon style filters, when used for removal of other organic contaminants, exhibits what is referred to as "breakthrough" (when the bed becomes fully loaded with the contaminant, and it can be no longer removed). Because the radon on the bed is constantly breaking down, this loading or breakthrough does not occur. This allows carbon units to operate until they become plugged with contaminants, rather than becoming saturated with radon.

A significant advantage of this technique is that the carbon beds are within pressure tanks that allow for easy insertion into the water supply to the house, without the need for a pump or pressure tank to deliver the water to the point of use. The major disadvantage is the amount of gamma radiation that emanates from the tank due to the radon decay products that have been trapped in the bed from the decaying radon.

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