An Environmental Chemistry Experiment The Determination of Radon Levels in Water Lawrence E. welch' and Daniel M. Mossman Knox College, Galesburg IL 61401 Radon was diswvered by Rutherford as an "emanation" from the known radioactive element thorium in 1899 and confrmed the followingyear by Dorn as being produced in the same manner from uranium (I).Until the last decade, it has been an element of little note to those outside the nuclear chemistry field. Inhalation of radioactive radon was linked to lung cancer in miners in the 1940's (21, but this caused little stir outside the mining community. A minor scare was set off in 1957 when a well in Raymond, Maine, was found to have high radon levels, setting off a flurry of subsequent well tests in the area (I).Our perceptions of radon as a health threat largely are a result of a 1984 incident in Limerick, Pennsylvania, where a nuclear power plant worker named Stanley Watras tripped a radiation alarm on his way into the plant. The source of the radiation was traced to his home, which was found to contain 2700 picocuriefi (pCi/L)of radon, 675 times greater than the EPA recommended limit. Although most of the neighboring homes were not highly contaminated, a number of other residences atop a local granite belt known as the Reading Prong, extending throughout eastern Pennsylvania and stretching into New Jersey and New York were found to be contaminated (I).The flames from this incident were fanned considerably when in 1986 the EPA announced, "Radon is the biggest public health problem that radiation experts have acknowledged for years," speculating that lung cancer from radon exposure was killing up to 20,000 Americans per year (3).Since then public awareness of radon and its health risks has been high, and the percentage of homes tested for radon levels has increased dramatically. Radon Radon is a colorless, odorless noble gas that is almost completely chemically unreactive. Although 31 isotopes have been identified (41, it exists primarily in isotopic forms with masses 219, 220, and 222, with half-lives of 2.96 s, 55.6 s, and 3.8235 days, respectively Of these, the most prevalent is "'Rn, as it is part of the uranium series decay pathway, four u and two P decays from the abundant parent isotope 238U.None of the isotopes has a long enough half-life to allow a radon pocket unsupported by parent radioisotope to endure for long. However, as a gas, radon is unique from the other radioisotopes produced by decay of naturally occurring species. The diffusional transport of radon is much more efficient, allowing it to be moved through rock, soil, and building materials and concentrated in regions that are remote from its source. Radon is soluble in wld water, but the solubility decreases as the temperature increases. This allows transport of radon through groundwater aquifers and into homes through indoor plumbing. The high density of radon (9.73 g L , 4) leads it to be most commonly concentrated in low regions such as basements. 'Author to whom correspondence should be addressed.
The gaseous nature of radon also makes it a greater risk in other forms. Much lareer to human health than isoto~es quantities can be transported into the lungs cornparegto the particulate form of other emitters. The skin is normally efficient at blocking penetration from a particles, but the inner surfaces of the lungs are not as effective. In addition, the progeny of radon are no longer chemically inert, and some of them, especially the polonium isotopes, can stick to the bronchial mucosa within the lungs and greatly enhance the localized radiation damage and cancer risk (3). Some relatively simple and inexpensive methods have been develooed to monitor airborne radon. Activated charcoal canistrk can be used to absorb radon from the air(51. After allowine eauilibration with the air samole. the canister is closedkd the time of sealing recorded. The canister is sent to a facility where scintillation counting is used to measure the gamma ray activity of the radon progeny, typically '14Pb and '14Bi for the "'Rn parent isotope (6). These levels can be extrapolated backward to predict the '"Rn level at the time the canister was sealed. Another simple method is the alpha track or track etch method (7). A container is built that only allows diffusion of gases through a semi-permeablemembrane. This effectively limits radioisotope entry to radon, which upon decay emits an alpha particle that etches a special internal membrane (typically made of cellulose acetate). At the end of the collection period the membrane is treated with a caustic solution to etch the alpha tracks into larger craters that can then be counted with a light microsco~e.More so~histicated versions have cmplo~edelectrochemical etching 181 and solid state dielectric detectors ~ Y I . During the development of a new environmental chemistry laboratory curriculum, we sought to develop some sort of radiation ex~erimentto com~lementthe lanze role this subject played in the lecture p k i o n of the cou;e. We had howd to identifv radioactive s~eciesbased on identification of gamma ray energies wit6 a NaI scintillation crystal and a multichannel analyzer. However, due to balky electronics within this system, it was deemed too unreliable to use. From an instrumental perspective, one instrument that was available and reliable was a liquid scintillation counter. We decided to use the scintillation counter to measure radon in water, adapting a laboratory procedure from a literature article by Prichard and Gesell(10). The procedure relies on the fact that toluene will preferentially extract radon from water, much like charcoal does from air. Radon in water does provide health risks upon ingestion, but this is much less of a concern than airborne radon, primarily on the basis of daily ingestion volumes. Despite being less of a health risk than its airborne counterpart, radon carried in water is often a major transportation mechanism leading to high airborne concentrations. Radon is carried into homes via laundry and bathroom facilities, often in the basement, where exposure to air allows outeassine into the home. The solubilitv resDonse with tem&rat&e exacerbates this effect, because a; the water is brought into the house it in general is warmed, reducing Volume 71 Number 6 June 1994
521
radon solubility. The presence of abnormally high aqueous radon levels has been found to correlate strongly with high airborne levels (11. Procedure Enough sample material is available within the laboratory to perform the experiment, but it is always of interest to obtain a sample of well water to test, because typically these will produce the highest radon levels. Samples ofcity water within the same water system tend to give homogeneous results, and as a result show little difference from the laboratory tap water. To avoid significant decay, the well water samples are collected the morning of the experiment. The sample containers are filled to the brim and sealed tightly. Glass vials were used for counting, done in racks containing 12 samples each. Typically, we would use four different water samples and a blank, each run in triplicate for a total of 15 sample vials, requiring two sample racks. A scintillation cocktail was made by adding 15.00 g of PPO (2,5-diphenyloxazole)and 0.90 g dimethyl POPOP (1,4-bis[4methyl-5-phenyl-2-oxazolyllbenzene), diluting to a total volume of 3 L with reagent grade toluene. During sample preparation particular care was made to keep the sample solutions sealed as much as possible, so as to minimize any outgassing of radon. TWOhundred milliliters of each sample was measured out using a volumetric flask. This solution was then transferred to a 250-mL separator, funnel and capped. Then 20 mL of toluene was pipe&d into the funnel: The mixture was shaken weU to allow extraction ol'thc radon into the toluene. Once thoroughly mixed, 10 mL of the toluene layer was removed from the funnel. In the past, we have found that the simplest way to do this was to use a large calibrated syringe with a long flat-tipped needle, withdrawing toluene from the layer on top of the water. The toluene portion was transferred to one of the scintillation vials. Next 5 mL of the scintillation cocktail was added, the vial capped, and the solution mixed well. Each vial was then labelled and daced in the countina rack for the scintillation counter. hlank samples were made by mixing 10 mI. oftoluene rewent with 5 mI.of the srintillation cocktail. Once all ofthe vFals were prepared, the samples were taken to the counter and acquisition was initiated immediately. It can be noted that an effort was made to intersperse the replicate trials throughout the counting rack. Our experiments and others (10)had shown us that the scintillation counts per minute (CPM) values did not decline as the time lag between extraction and counting was increasedup to 5-6 h. However, we still felt that it was best to distribute the samples evenly in case any sort of time dependent sensitivity change was to occur. The scintillation counter used was a Beckman LS 6000IC. It has an external data station/controUer that allowtd a customized counting prokvum to be produced. A countinr window of 18.6 to 1710 KrV wasused for thesamples that were counted for 15 min each. The wide energy window was necessary due to the short half-life of 222Rn, which insured that significant fractions of daughter radioisotopes also would be present. The first five decay steps from "'Rn involve three a and two P decaysto give the relatively long-lived "Pb (half-life22.6 yr, 4). Although all but one of these five decays were of greater energy than the counting window (41,oxygen quenching of the scintillation fluid caused a shift to lower energy, resulting in most of these decays being recorded. Prichard and Besell (10) found that the energy window in use was 87% efficient for "'Rn and its first four daughter isotopes. They also counted for 40-min periods rather than 15. We found that this improved precision, but not by a considerable amount. 522
Journal of Chemical Education
Table 1. Raw Data in Scintillation Counts per Minute Sample #
Sample ID
CPM
DDI Water, trial 1
72.93
Tap Water, trial 1 Blank, trial 1
109.87 70.07
Aerated Tap Water, trial 1
89.47
DDI Water, trial 2
73.60
Tap Water, trial 2 Blank, trial 2
111.20
Aerated Tap Water,trial 2 DDI Water, trial 3 Tap Water, trial 3 Blank, trial 3 Aerated Tap Water, trial 3
76.07 81.07 77.07 104.07 77.87 74.93
Summary: DDI Water
Avg = 74.53
a = 2.22
Tap Water
Avg = 108.38
o = 3.79
Blank
Avg = 74.67
o = 4.08
Aerated Tap Water
Avg = 81.82
Our major motivation for shortening the window was that the scintillation counter was a heavily used facility, and counting 15 samples for 40 min each blocked out too much time. Results and Dlscusslon The raw data from a typical experiment is given in Table 1. The tap water samples were collected directly from a laboratory faucet. The aerated tap water had been collected in the same manner, transfemed to an open beaker, and stirred vigorously for 15 +n. The DDI samples were doubly deionized H20. They had been purified by the building HzO system, a Culligan S Series Reverse Osmosis system followed by a Culligan Mark 812 Automatic Water Conditioner. Polishing treatment was done with a final stage through a Barnstead NANOpure I1 system. No well water samples were done with this trial, but from previous laboratory trials it has been observed that these always contained more radon than our local tap water samples, often twice as much. Although the CPM values are enlightening on their own, some further calculations add better perspective to the data. It should be noted, for those favoring computerized data treatment, these calculations work well on a spreadsheet, as we have done for the tap water and aerated tap water samples in Table 2. We have not included the calculations for DDI water in this table because its activity is virtually the same as the blank. This is typical behavior, because the DDI treatment is effective at removing the radon from the water. The aerated tap water also was scrubbed of a portion of its radon content, but the crude aeration procedure was much less effective than the DDI process. The most important conversion is to change the CPM values to pCiiL, the standard unit for expressing radon activity. First of all, the CPM values from all three trials were averaged. Corrected CPM values are produced by
Table 2. Sample Spreadsheet Calculations
Aerated Tap Water
Tap Water
Avg CPM
81.82
108.38
Corrected CPM
7.15
33.71
CPM for Entire Sample CPMIL
14.3
67.42
71.5
337.1
CPSA
1.19E+00
5.618E+W
CurieWL
3.22E-11
1.518E-10
pCi/L
3.22E+Ol
1.518E+02
p C i corrected for counting eniciency
3.7E+01
1.7E+02
pCR corrected for 5.7Et01 earaction efficiency
2.7Et02
riubtracting the blank CPM value from each of the samples. The corrected CPM was multiplied by two to give a CPM value for the entire sample, compensating for the fan that only halfof the toluene extract wascounted. Next, the corrected CPM values are multiolied bv five to eet CPML because we used only 200 mL df wate; This vilue is con: verted to Curie&, knowing that a Curie is 3.7*1010disintegrations per second. Because this value is typically small, a unit conversion is made, multiplying by 10" to express radon in pCi/L. While the activity value is now expressed in proper units, further correction is required. Because the counting window used on the scintillation counter is known to be 87% efficient, a better value of activity is found by dividing through by 0.87. Further correction can be made for the inefficiencyin the extraction process. Based on solubilities at 20 'C, the proportions of radon distributed among equal volumes of water, air, and toluene would be 0.255 : 1.00 : 12.7 (10). Further correction for unequal volumes during extraction was necessm. The "250-mL" seoaratorv funnel was found to have a voi-e of 305.0 mL, ;esultini in volumes during extraction of 200.0 mL water, 85.0 mL air, and 20.0 mL toluene. Putting this together with the solubilitv data (10). we would exoect followine an extraction that"l3.1% of the radon wouli remain in tge water, 21.8% in the air. and 65.1% in the toluene. Therefore. the DCVL value is divided by 0.651 to correct for extradive Lefficienw. This should result in the best oossible measure of radon activity. For the sake of com~arison,it has been noted that the average radon activity in U. S. puhlic drinking water supplies is in the range of 200-600 pCi/L (11). Other calculations can be made that may be of interest. An estimate of a household's airborne radon level can be
made based on its water activity. The rule of thumb is "1 pCi/L in air for every 10,000 pCVL in water (12)." It also has been stated that based on the wnsumption of 300 mL H20 per day containing 1000 PC&, a radiation dose of 100 milliremly will result to the subject's stomach (13).Assuming a linear function, a dosage can be calculated for one of the laboratory samples by setting up a proportion using its actual activity. While it takes some time actually to complete the laboratom. much of the time is soent countine scintillations. w h h does not require opeitor presenceT~reparationof the samoles is comoleted easilv in less than 90 min. The major tike concern usually has been t o coordinate with other users of the scintillation counter such that a lag time between sample preparation and counting is avoided. Likewise, for large lab sections arrangements may be needed to stagger starting times for the different lab groups. As can be seen from the sample data, the expected precision is not tremendous. Time permitting, it would be beneficial to increase the counting time per sample and to run a greater number of trials from each sample solution. It also should be noted that this procedure was designed for 222Rn.Different decav enereies bv the other isotooes will alter the collection efficiency of t i e wunter. ~ow&er,the extremelv wide enerm window beine used should allow it to be highly efficient-iounting for these other isotopes too, and a large change in the collection percentage would not be expected. Due to the extremely low level of radioactivity extracted from typical water samples, no special handling of the radon extracts is necessary, and they may be treated as toluene waste for disposal purposes. Acknowledgment
Partial support for this work was provided by the National Science Foundation's Instrumentation and Laboratory Improvement Program through grant # USE9051124. Facultv release time for LEW to work on this project was funded by the Lilly Research Enhancement Program from the Lilly Endowment, Inc. ~
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Literature Cited 1. Cothem, C. R.; Smith, J. E., Ed.Enuirmenfo1 Rodon; Plenum:New York,1987. 2. Loren=,E. J. Nuti Cancer Ins1 lBM, 5.1-15. 3. Wagner, H. E.; Ketchurn, L E. Living Wlth Ridlotion: Johns Hopkina: Baltimore, 1989. 4. Lids, D. R., Ed. Hondbmh of Chernrshy and Physics, 72nd ed.; CRC Pms: Baea Raton, FL, 1991. 5. Ceolge, A. C. Hoolth Physim 1534,46,867-872. 6. Gray, D. J.; Windham, S. T EPAPuMimlion 5201587-W5 1987.140. 7. Alter, H. W;Flaischer, R. L Health Physies lsB1,40,693-702. 8. Urban.M;Piesch, E. Rodlolion Platection D a s i m t q 1981, 1.97-109. 9. Love% D. B.Hdfh Phyeics 1969,16,623-828. lo. Richard, H. M.:Gesell, T E Health Physics 1977,33,577-331. 11. Cothem. C. 8.: Lsn~mbusch.W L:Miehel. J. H d t h Phvsics 1953 50.3347
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13. Cram, F T.;Har1ay.N. H.; Hofmann, WHpolth Physles l W , 48,649-670.
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