Environ. Sei. Technol. 1994, 28, 534-537

COMMUNICATIONS

Radon Release from Water Distribution System Deposits

Richard L. Valentine" and Scott W. Stearnst

Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242

Introduction

The 1986 Safe Drinking Water Act (SDWA) amend- ments include a requirement for the U.S. EPA to issue regulations for a number of radionuclides. The new radionuclide rules will apply to all community water systems and nontransient, non community water systems. Regulated species include 222Rn, 226Ra:28Ra, natural uranium, LY emitters, and /3 and photon emitters. While not yet final, the US. EPA has released important information on the proposed rules and has discussed the basis for their recommendations, which include among others (1) a 300 pCi/L maximum contaminant level (MCL) for radon enforceable a t the point of entry into the distribution system, (2) specification of MCLs for 226Ra and zz8Ra each at 20 pCi/L, and (3) application of best available technology or equivalent if required (1, 2). While the Becquerel (Bq) is now the standard unit for radio- acitivity, this paper adopts the older convention of using Curies (Ci) to be consistent with specifications in the proposed EPA regulations. For comparison, however, 1 pCi is equivalent to 3.7 X Bq, or 300 pCi/L is equivalent to 11.1 Bq/L.

Although the exact impact of the radon rules is disputed, it is generally agreed that meeting a 300 pCi/L MCL will cost a great deal. EPA estimates a cost of approximately 180 million dollars per year while an independent study (3) suggests that the cost is closer to 2.5 billion dollars per year. This latter estimate is based upon an analysis indicating that over 32 750 public water supplies or approximately 30 % of all groundwater supplies would be required to implement treatment to comply. Unlike the radon MCL, the proposed radium MCLs result in cost savings to the water industry compared to the cost of complying with the existing MCL of 5.0 pCi/L for 226Ra i- zz8Ra. Under the proposed radium regulations, relatively few utilities would be required to implement a control strategy to meet the new MCLs.

A number of technical, economic, and health related factors were considered in the development of the proposed radionuclide regulations (1, 2, 4). In particular, it is assumed that all radon enters the water from geological sources in the ground before it enters the treatment and distribution systems. Enforcement of the radon MCL at the point of entry is thought to ensure that radon concentrations do not exceed 300 pCi/L at the tap since radon, with a half-life of 3.82 days, is expected to decay

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* Corresponding author. + Present address: Strand Associates, Madison, WI 53715.

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in the distribution system. Likewise, radium is also a concern only if it is in the raw water a t concentrations exceeding the MCL. While radon comes from the decay of 226Ra, no linkages exist in the regulations between radon and radium.

A number of studies have shown that radium adsorbs and/or coprecipitates to many substances that commonly exist in water treatment plants and distribution systems (5-13). Valentine (10, 12) found that linear sorption isotherms described the adsorption of radium to hydrous iron and manganese oxides. Distribution coefficients for iron oxides were found to vary from about 0.004 L/mg at pH 7 to 0.13 L/mg at pH 9, corresponding to sorptive capacities of 20-650 pCi/g of Fe in equilibrium with 5 pCi/L zz6Rae Hydrous manganese oxides sorbed much more radium over the same conditions with distribution coefficients ranging from approximately 2 to 4 L/mg, corresponding to sorptive capacities of 20 000-40 000 pCi/g of Mn. Valentine (13) also noted that coatings on common filter sand used in the treatment of 226Ra containing water could be a significant reservoir of radium, with measured content in the range of 100-400 pCi/g of acid dissolvable coating when used to treat water containing approximately 6 pCi/L zz6Ra. Possible sources of these absorbing and coprecipitating materials would include not only those precipitating from the water but corrosion products as well (14).

I t is quite logical then to expect that radium would concentrate in a variety of environments within treatment plants and distribution systems. If radium has accumu- lated in deposits, then radon gas is also produced. In the absence of any mass transfer limitations, the radon concentration would be determined by secular equilibrium between 226Ra and 2z2Rn according to (15)

A,, = ARa(l - (1)

where, A R ~ is the activity of the radon produced per unit of water volume, A R ~ is the amount of 226Ra in the deposit per unit of water volume in contact with the deposit, t is contact time in days, and the number 0.18 is ln(2) divided by the half-life of z22Rn (3.82 day). At secular equilibrium, the activity of the radon produced would equal the activity of radium. Hence, the maximum possible radon formation can be calculated from knowledge of the radium content of the deposits.

Study Approach

We collected deposit material and water samples from five cities in Iowa (designated FC, MP, WL, OX, and WA)

0013-936X/94/092S-0534$04.50/0 0 1994 American Chemical Society

Table 1. Summary of zzeRa Content of Deposit Material and Radon Released at Secular Equilibrium

sample sample extractable radon from filter radon from whole % dissolvable

MP-1 HFD 6 0 f 4 4.6 f 0.4 39 f 13 MP-2 HFD 47 f 10 7.1 f 0.7 32 f 4 13f 1 MP-3 HFD 44 f 2 1.7 f 0.1 31 f 3 13f 1

ID description" zz6Ra (pCi/g) residue (pciig) deposit (pCi/g) of whole deposit

7.6 f 2.2

MP-4 HFD 66 f 5 4.3 f 0.3 3 5 f 6 21 f 4 MP-5 PD 33 f 1 3.5 f 0.3 26 f 15 27 f 2 FC-2 PD 1 3 f l 0.9 f 0.4 3.9 f 2.7 6 0 f 3 FC-4 PD 1 6 k 1 1.2 f 0.8 4.6 f 0.6 28f 3 FC-5 PD 25 4 4 0.4 f 0.3 6.1 f 1.2 50 f 7 FC-6 PD 3 2 k 2 0.5 f 0.8 7.2 f 7.2 3 1 f l WA PD 56f 8 7.2 f 5.4 1 6 f 12 57f 1

a PD = Pipe Deposit. HFD = Hydrant Flushing Deposit.

having groundwater supplies containing z26Ra ranging from approximately 2 to 10 pCi/L . Cities FC (population 43001, MP (population 7300), WL (population 2600), and OX (population 700) aerate their water to oxidize the iron found in the range of 0.2-1.0 mg/L and then sand filter the water to remove the precipitates. At the time of this study, WA (population 7500) added phosphate to sequester iron found at less than approximately 0.1 mg/L and then chlorinated their supply without filtration. Deposits ejected during hydrant flushing were collected from MP. Several pieces of actual pipe were obtained from MP, FC, and WA when maintenance requirements offered the opportunity.

We first determined the extractable radium content of collected deposit material as an indicator of the potential to release radon and then compared these values to actual radon released. Radium was extracted from triplicate samples by digesting several grams in an agitated solution of 0.1 M hydrochloric acid and 50 g/L hydroxylamine hydrochloride. zz6Ra was then measured using coprecip- itation with barium sulfate followed by a counting (16). Radon release from uncrushed deposits was determined by placing 1 g of deposit in a scintillation vial containing water and counting cocktail (Opti-fluor 0) and then subjecting the vial to scintillation counting (17) using a Beckman Model LS 6000 scintillation counter after approximately 21 days, time enough for secular equilibrium to be reached. The radon release potential was then calulated from the radon activity a t secular equilibrium normalized to the mass of deposit. Control samples showed that no significant scintillation was induced aside from that caused by radon. Precision was estimated to be approximately f25 pCi/L at a 50 pCi/L concentration. Non-dissolvable residues retained on the filter paper were also evaluated for radon release potential measured as described for the whole deposits. Large (up to 12-in. long, 8'". diameter) whole pipe sections were immersed in tightly sealed 5-gal polyethylene HDPE containers filled with water. After times ranging from 0.5 to 45 days, 10-mL water samples were carefully withdrawn for radon analysis and replaced with 10 mL of deionized water to eliminate air bubbles.

Water samples were taken at several locations within the distribution systems of the five cities, and radon was measured within 24 h. The number of sampling points was not great, ranging from 4 at OX, 6 a t MP, 7 a t WL, 8 at FC, to 15 at WA. The cities of MP and OX were sampled two different dates to verify increased radon levels while. WA was sampled six times because of the number of samples indicating increased radon. The water a t the

tap was allowed to flow at maximum output for approx- imately 5 min before three consecutive samples were taken (1-2 min apart) unless otherwise noted.

Results and Discussion The collected material varied considerably in appear-

ance. Material from hydrant flushings at MP consisted of irregular reddish chunks approximately 2-5 cm in length and about 1-2 cm thick. Pipe sections were generally coated on the inside with deposits ranging in thickness from about 0.1 cm to approximately 3 cm. These deposits ranged in appearance from the rough reddish material collected during flushing at MP to whitish-looking smooth deposits. The acid-hydroxylamine dissolvable fraction of deposits obtained from hydrant flushing ranged from approximately 7.6% to 27% with an average of 16.4%. After digestion, a black metallic magnetic material re- mained undissolved. No elemental analysis was performed although the red color is likely due to the presence of iron.

The extractable radium content and radon released from deposit material was variable but significant for all three locations from which material was collected (Table 1). The radium content of material from MP ranged from ap- proximately 32 to 66 pCi/g of whole deposit with an average of 49 pCi/g. Deposits obtained from FC contained less radium, ranging from about 13 to 32 pCi/g with an average of 21 pCi/g. Only one deposit a t WA was analyzed and found to contain 56 pCi/g of zz6Ra. When normalized to dissolvable fractions, the radium content overall ranged from approximately 21 to 800 pCi/g of deposit dissolved.

In general, the maximum amount of radon released at secular equilibrium was less than the extractable radium content, suggesting a limitation to radon release from the deposit matrix. Maximum release was observed for samples from MP with a value of 32 pCi/g representing about 65% of the average extractable radium content. The radium released from pipe deposits collected from FC was considerably lower with an average of 8 pCi/g or 37 % of the average 226Ra content. A comparatively low recovery was also measured for WA material with radon release of 16 pCi/g, representing 29 % of the 56 pCi/g226Ra content. The lower radon recovery at FC and WA must be due to the nature of the deposits, which appeared much smoother and more dense than the deposits obtained from MP.

These measured radon release rates are high enough to suggest their potential importance as a source of radon in treated water under the right circumstances. Based on an average radon release of 25 pCi/g, the maximum radon produced in the water contained in a 8-in. pipe with a

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Table 2. Completely Submerged Pipe Sections at Secular Equilibrium and after 0.5, 1.0, and 2 Daye

radon at secular caicd radon calcd radon calcd radon sample equilibrium after 0.5 day after 1.0 day after 2.0 day

MP-5 4040 zk 100 352 671 1229 FC-3 911 f 160 79 151 277 FC-7 340 f 160 29 56 102 w-1 109 f 90 95 181 332

Summary of Radon Concentrations Released from

ID (pCiiL) (PCiiL) (PCiiL) (pCiiL)

Concentrations normalized to water volume in pipe.

5000 , , , , I , , , , I , , ~ I I , , , , I , , , ,

2 1000 [; Theoretical Ingrowth

A IngrowthExp27

0 IngrowthExp39

0 10 20 30 40 50 Time (days)

Figure 1. Comparison of measured and predicted radon released from an 8-in. diameter pipe section from city MP (sample MP-5).

uniform coating of 1.0 cm of deposit (density 1.5 g/cm3) would be 8600 pCi/L at secular equilibrium. However, even shorter contact times of 0.5 and 1.0 day result in radon production of 750 and 1400 pCi/L, respectively, significant relative to the recently proposed MCL. These calculations support the hypothesis that this source of radon could be important if sufficient amounts of deposit were available.

Consistent with our analysis of deposit samples and possible impact on radon in drinking water, radon released from submerged whole pipe sections was also considerable when normalized to the volume of water that would be contained in that particular section of whole pipe. Values at secular equilibrium (Table 2) ranged from a maximum of 4000 pCi/L for pipe section MP-5 containing reddish deposits approximately 1.5 cm in thickness to a minimum of 340 pCi/L from FC-7, a pipe having a minimal coating of about 0.1 cm of whitish deposit. Furthermore, the temporal approach to secular equilibrium (Figure 1) for pipe MP-5 was consistent with what would be predicted assuming a radium content equal to the radon measured at secular equilibrium. Radon concentrations for pipe MP-5 exceeded the proposed MCL after only half a day. While radon release was less for the other pipe sections, significant increases are expected for contact times ex- ceeding approximately 1 day.

Radiological “hot spots”, defined as sampling points where radon was at least double the concentration at the point of entry to the distribution system, were found in three of the five distribution systems sampled. No trends in increasing or decreasing radon concentrations were observed for the three consecutive samples taken at each point after the initial 5-min flush, indicating that steady- state concentrations were being measured. However, a t several locations it was shown that initial radon levels could decrease dramatically with time during the 5-min flush period, pointing out a potential sampling problem.

Table 3. Radon Concentrations in Distribution System of City WAe

radon radon location (pCiiL) location (pCi/L)

raw water 80 f 5 residence no. 2 70 f 8 80 f 20

finished water 74 f 20 98 f 20 prior to storage 76 f 9 residence no. 3 340 f 50 point of entry 58 f 10 residence no. 4 1900 f 100

1400 k 200 630 f 4

160 f 20

68 f 7 72 f 20 90 f 6 residence no. 5 920 f 70

137 f 10 residence no. 6 870 f 30 waste treatment 290 f 10 residence no. 7 240 f 10

plant 93 f 30 residence no. 8 170 f 6 230 f 20 residence no. 9 330 f 10. 242 f 10 residence no. 10 290 f 30

70 f 20

Each entry was determined from three consecutive samples.

residence no. 1 120 f 20 discount store

For example, at residence no. 3 at city WA, the initial radon concentrations decreased from approximately 1200 to 300 pCi/L during one flushing period.

The results clearly indicate that contrary to conventional wisdom (18), radon concentrations can indeed increase in distribution systems. The most frequent and highest radon concentrations were measured in WA (Table 3) where 8 out of 13 different sampling points (excluding raw and treated water) showed radon above the influent value of approximately 84 pCi/L. The largest observed concentration was approximately 1900 pCi/L. Only one sampling point a t OX and WA significantly exceeded the influent radon concentration. At OX, an increase to 230 pCi/L from an influent value of 40 pCi/L was observed while at MP an increase from 80 to 240 pCi/L was measured. While no clear “hot spots” were observed at WL and FC where the radon concentrations at the point of entry were approximately 30 and 60 pCi/L, respectively, there was also no evidence that radon decreased in the distribution system except a t one location on a new main. In fact, small increases were generally observed.

Conclusions

The significance of radium bearing deposits in an actual distribution system would depend on a number of factors including the radium content of deposits, the amount and distribution of deposits, the type of deposit, and possibly most important, the time in which a given water volume is in contact with deposits. The quantity and formation of deposits would in turn be determined by the water quality, treatment, and age of the system. These factors could complicate assessing the overall extent of radon release from deposits without an extensive system-wide survey. The purpose of our exploratory survey was to simply determine if evidence of radon formation existed in systems exposed to radium-bearing water.

The increase in radon observed in actual distribution systems is consistent with our analysis of the radium content of distribution system deposits and measured radon release rates. However, radon entry into the system from the soil cannot be ruled out, although pressure gradients would hinder such transfer. We conclude that radon produced in radium-bearing deposits in distribution systems (and possibly from filter media) may cause radon to significantly increase in concentration.

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Whatever the final radionuclide rules, they should be based on a complete understanding of the sources of radon and processes that could limit the benefits of treatment to remove radon. Even though the source from within a distribution system may be small in comparison to a geological source and impact only a very small fraction of the population of a city, the importance must be gauged by the proposed MCL and rationale for requiring com- pliance at the point of entry. Any radon production within a distribution system could cause the radon content to exceed the MCL for some users, even if compliance at the point of entry is achieved. This finding also has impli- cations on the proposed increase in the 226Ra MCL and in treatment practices that may affect radium deposition and radon release such as those implemented for control of corrosion. The linkage between radium deposition and radon release may need to be considered in the final EPA radionuclide regulations. Lastly, it indicates that sampling procedures both in the system and at the tap need to be carefully considered to correctly assess exposure. Clearly, further work is needed to better evaluate the significance of this source of radon, which may prove to cause a rather large headache for regulatory agencies and purveyors of drinking water.

Acknowledgments

We would like to thank the Center for Health Effects of Environmental Contaminantion at the University of Iowa for providing seed funding for our pilot radon survey. We also thank the US. EPA Inorganics Research Division for additional support to complete our initial objectives. Funding critical to the development and completion of this work was also provided by the Iowa State Water Resources Research Institute and the American Water Works Association Research Foundation.

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Received for review July 14,1993. Revised manuscript received November 18, 1993. Accepted November 29, 1993.

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