Risk Assessment of Radon in Drinking Water (1999) / Chapter Skim
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8 Mitigation
8 Mitigation
Pages 141-179
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From page 141... ...
More complete discussions of radon transport in soils and entry into buildings can be found in the literature (Sextro 1994; Nazaroff 1992; Nazaroff and others 1988~. Advection Bulk flow of soil gas that contains radon is the main mechanism of radon entry into buildings.
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From page 142... ...
In response to the slightly lower air pressure in the building, "makeup" air flows in through openings in the building shell, some of which might provide direct contact with soil air. The effects of the thermal stack and the wind can independently result in indoor-outdoor pressure differences of about the same size at the lower portions of the building shell.
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From page 143... ...
Overall, these effects are estimated to be small and to yield overall radon entry rates roughly the same as that due to the second source, infiltrating outdoor air. The latter, considered in more detail in chapter 2, provides an irreducible "baseline" indoor radon concentration.
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From page 144... ...
Assuming a typical value of about 40,000 Bq m-3 for the concentration of radon in soil gas, the soil-gas entry rate is about 1 m3 in-i, which is about 0.4% of the overall air flow rate into the house. Mitigation Methods for Existing Houses Conceptually, there are two approaches for mitigating indoor radon concentrations (or most other indoor pollutants, for that matter)
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From page 145... ...
Source Control When high indoor radon concentrations in houses were found in various locations in North America in the middle 1970s, initial research on reduction methods was based on two key assumptions: that the source term was high concentrations of radium in soil materials derived from uranium mining and that the principal means of radon transport and entry was diffusion. Thus, initial attempts at source control focused on removal of the materials, typically uranium mill tailings used as back fill under floor slabs or adjacent to basement walls.
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From page 146... ...
Almost all the retrofitted ASD systems are successful in reducing indoor radon concentrations to less than 150 Bq m-3 and often concentrations are reduced to about 75 Bq m-3. In some cases when the basement walls are constructed of blocks, Repressurization pipes are inserted into the hollow cores of the blocks themselves.
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From page 147... ...
In this case, the system is providing ventilation of soil gas, thus reducing the radon concentration in the soil region adjacent to the building. Rather than reducing or reversing the pressure gradient across the building shell, this method actually increases the interior-to-exterior pressure difference and so increases the flow of gas from the soil into the building.
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From page 148... ...
For this system to be effective, the pressure field developed by the stack below the floor slab needs to be sufficient to reverse or at least substantially reduce, the pressure gradient between the soil and the building interior, which drives advective flow of gas from the soil into the building. The soil-to-buildinginterior pressure difference will be greatest when the inside-the-stack-to-outdoor temperature difference is the largest, for example, during the winter in cold or moderate climates.
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From page 149... ...
Because most radon entry occurs through the basement floor and walls, basement radon concentrations are often higher than elsewhere in a house. Use of an HRV to reduce radon concentration in this space, as opposed to the whole house, means that the effective ventilation rate of the space is higher (which affords more control)
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From page 150... ...
As a result, the unattached fraction of airborne activity increases, especially of Typo. Because the unattached fractions of the radon progeny have been considered to be far more effective in depositing their radiation dose to lung tissue, concerns have been raised regarding the efficacy of air-cleaning as a means of mitigating the hazards arising from indoor radon.
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From page 151... ...
Thus, there is no reasonable likelihood that the use of an air cleaner will increase the hazards posed by indoor radon. In studies of different types of air-cleaning devices, reductions in exposure have always exceeded reductions in dose.
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From page 152... ...
Radon-Resistant Buildings Most of the elements required for making a building radon-resistant have already been described. In principle, if all entry routes through which soil gas can flow are eliminated or the pressure gradient that drives air flow through such openings is reversed, advective transport will not contribute to indoor radon concentrations.
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From page 153... ...
In 1991, Washington state adopted a radon provision as part of the Washington State Ventilation and Indoor Air Quality Code (WSBCC 1991~. It provides specific details for houses built with crawlspaces for the entire state and specifies radon-resistance features for eight counties thought to have potential for high indoor radon concentrations.
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From page 154... ...
Interestingly, in this analysis, the presence of a passive stack to Repressurize the subslab region was rated less effective than the other features of the radonresistance system (Nielson and others 1994; Rogers and Nielson 1994~. That is due in part to the assumed effectiveness of the features thought to block radon entry by reducing or eliminating air pathways between the subslab region and the house interior.
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From page 155... ...
In part, that is due to the low soil-gas radon concentrations in one study. Thus, with one exception, all the initial stack-closed indoor radon concentrations are low.
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From page 156... ...
It is not clear whether the new houses were occupied at the time the indoor radon was measured. Overall, in the 143 Colorado houses, the average basement radon concentration was 190 Bq m-3, compared with 230 Bq m-3 in 94 control houses in the same ZIP codes.
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From page 157... ...
In another study, the author concluded that radon-resistant techniques could be used in Florida for soil-gas concentrations up to 310,000 Bq m-3, as long as indoor air-exchange rates were kept above about 0.3 h-i (Hintenlang and others 1994~. The modeling done in support of the development of a radon potential map identified areas where the soil radon-potential was high enough that the reduction factors used for radon resistance were not sufficient to ensure that indoor radon concentrations would remain below 150 Bq m-3.
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From page 158... ...
To the extent that radon-control systems also rely on passive radon-resistance techniques to ensure control of radon entry, failures of these features will be much harder to detect without, for example, directly measuring the indoor radon concentration. Even then, the establishment of baseline radon concentrations in a local region is necessary if one is to be able to estimate the overall effectiveness of radon-resistance construction techniques.
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From page 159... ...
Because these alternatives reduction of radon concentrations in the drinking water and reduction of radon in indoor air can be compared only on the basis of health risks (not just indoor radon concentrations) , long-term airborne-radon measurements are essential, in that they are the only basis for assessing the health risks associated with airborne radon.
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From page 160... ...
reported the removal efficiency, flow range, and construction cost of 34 mitigation systems now being used in small and large communities to remove radon from drinking water (table 8.1~. The purpose of this section is to present an overview of existing and emerging technologies for removing radon from drinking water.
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From page 161... ...
Evaluations of aeration methods removing radon from drinking water are presented in Lowry and Brandow (1985) , Cummins (1987)
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From page 162... ...
used worstcase scenarios from four treatment facilities whose raw water radon concentrations were 49,000 to 4,074,000 Bq m-3. In only one facility was there a significant potential increase in cancer risk associated with radon emissions when a single point source was assumed.
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From page 163... ...
has noted that if the radon NESHAP were applied to a 93 m3 d-i water-treatment plant (serving about 250 people) with an influent radon concentration of 18,500 Bq m-3 in its water, radon would be emitted in the off-gas at 222 Bq ~2 S-~.
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From page 164... ...
estimated that less than 2% of radon would be removed by typical vapor-phase GAC units currently used at watertreatment plants to remove VOCs (where EBCT are in seconds)
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From page 165... ...
As a result, EPA has established potency values for these compounds. Disinfecting the effluent of aeration systems that remove radon from water increases the risk of exposure to disinfection byproducts.
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From page 166... ...
At one very small water-supply system in Colorado, aeration of the water to remove radon actually eliminated the need for addition of lime to prevent corrosion (Tamburini and Habenicht 1992~. At a small system in New Hampshire, aeration resulted in a decrease in corrosivity and a reduction in the lead and copper measured in the drinking water (personal communication, D
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From page 167... ...
Suggested Guidelinesfor Disposal of Drinking Water Treatment Wastes Containing Radioactivity. Special treatment of the residuals can increase the cost of operation and the risk to workers who must oversee the disposal process.
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From page 168... ...
The radon-removal efficiency of some GAC units also decreased with time (Kinner and others 1993; Lowry and others 1991; Kinner and others 1990; 1989~. It was the data from the studies in the late 1980s that led EPA to question the use of GAC as a best available technology in its 1991 proposed rule.
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From page 169... ...
The gamma exposure rates measured at about 1 m were considerable, in all but one case, because the radon concentration removal was very large (Cne~= 611,000 to 27,926,000 Bq m-3~. Except for the 27,926,000-Bq m-3 case, the measurements are in agreement with the range of the calculations from the extended source model.
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From page 170... ...
. In many cases, it is unlikely that water-treatment plant personnel would need to spend hundreds of hours per year near the GAC units.
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From page 171... ...
Equivalent gamma doses for larger GAC units should be predicted with an extended-source model that can address morecomplex geometries. Disposal Issues.
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From page 172... ...
. There have been three detailed reviews of federal and state guidelines and regulations regarding NORM and how they might apply to disposal of GAC used to remove radon from water (Cornwell and others 1999; Drago 1998; McTigue and Cornwell 1994~.
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From page 173... ...
The CARBDOSE model (Rydell and Keene 1993) makes a similar prediction for POE GAC units.
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From page 174... ...
These higher values suggest that GAC could be a much more cost-effective option at some sites than originally thought. For example, with a raw-water radon concentration of 111,000 Bq m-3, a flow of 39 m3 d-i, and a KSS of 4.5 in-i, the EBCT and amount of GAC needed to achieve an MCL of 11,000 Bq m-3 (90% removal)
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From page 175... ...
Although some iron precipitation has been observed in field evaluations of GAC units that treat radon with low iron (Cornwell and others 1999; Kinner and others 1990; 1989) , the problem is usually less acute than in aeration treatment; the water is not usually oxygenated and exposed to atmospheric conditions, so, much less oxidation of the iron occurs.
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From page 176... ...
The tanks usually are vented to the atmosphere, which would increase the risk of air emission as with aeration methods. However, this risk would probably be low because use of storage as a treatment method would be limited to very small water supplies that have relatively low radon concentrations.
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From page 177... ...
. Loss in the Water Distribution System There have been several studies of this method of radon removal from drinking water.
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From page 178... ...
In both technologies, the radon removed is trapped in a sidestream of water rather than being released to the air. Therefore, these systems have the potential to fill a niche where radon concentrations in the raw water are high (precluding use of GAC)
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From page 179... ...
Equivalent gamma doses from GAC units that remove radon from public water supplies should be predicted with an extended-source model that can address more-complex geometries. Accumulation of radionuclides can also lead to potential disposal problems for the spent GAC.
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