United States Environmental Protection Agency Air and Energy Engineering Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S8-91/050 Sept. 1991 Project Summary An Assessment of Soil-Gas Measurement Technologies Harry E. Rector This report reviews the technologies for measuring radon In soil gas. The review addresses methodologies Involv- ing In-srtu detection, sample extraction, and surface flux, focusing on identifying the range of options for measuring ra- don in the soil. The following aspects of each measurement approach are evalu- ated: • Measurement objectives—the spe- cific parameter(s) that each technol- ogy Is designed to measure (e.g., soil gas concentration, flux density, permeability). • Equipment needs—commercial availability of systems and/or com- ponents, and specifications for fab- ricated components. • Procedural information—docu- mented elements of field and labo- ratory methodology and quality as- surance. • Underlying assumptions—concep- tual and mathematical models uti- lized to convert analytical outcomes to estimators of radon potential. Basic technologies and field data are examined from a generic perspective (e.g., the common denominators of pas- sive detectors, hollow sampling probes, flux monitors) as well as specific con- figurations developed by individual in- vestigators (e.g., sample volume, depth) to develop the basis for separating ana- lytical uncertainties from sampling un- certainties. Available technologies are also reviewed in terms of theoretical and practical utility as well as cost effective- ness. This Project Summary was developed by EPA's Air and Energy Engineering Research Laboratory, Research Triangle Park, NC, to announce key findings of the research project that Is fully docu- mented In a separate report of the same title (see Project Report ordering Infor- mation at back). Introduction Afairly wide rangeof methods forcharac- terizing the radon potential of land areas has evolved over the last decade through research programs in this country and abroad. This reviews published technolo- gies that could support soil-based estima- tors of radon potential. Basic technologies concentrate on measuring (1) radon in soil gas, (2) radon flux from the surface, or (3) radium content. Approaches may also in- clude attendant measures of soil character- istics and otherfactors to support predefined indexes of radon potential. Fundamental Considerations Soil and rock are the main source of radon in buildings. Although broad spatial trends of indoor radon are in rough propor- tion to soil radium concentrations, the ema- nation and subsequent migration in the soil and ultimately into buildings is determined by processes and characteristics at work in the soil, in the building, and in the surround- ing environment. Quantitative estimates of radon potential for soils are predicated on a volume of soil in flow communication to a building, a sup- ply of radon to the pore spaces of the soil, and transport mechanisms to convey radon into the building. The situation is compli- cated by a number of factors. The soil volume of interest is not defined by physical boundaries; rather, the strength of the trans- port mechanisms coupling the building to the soil defines the basic limits of migration. Radon emanation rates to the soil pores are Printed on Recycled Paper ------- controlled by the radium content of the soil grains, and are further tempered by soil moisture. In most soils, the pore space contains both air and water, providing the opportunity for radon to partition between the air and water phases. If the volume portion of the pore space occupied by water is small, radon emanation is directed primarily to the gas phase. As the volume fraction of water grows, however, it does so at the expense of the gas phase. The gas phase vanishes at complete saturation. Radon delivered to the soil pores can migrate through the ground by: (1) diffusion, in which the radon moves with respect to the pore fluid in order to equalize concentration gradients; and (2) forced convection, in which the pore fluid moves under the influ- ence of external forces, carrying the radon along with it. Diffusion can occur with or without forced convection. For soil systems exposed to the air, the large concentration differences between the soil pores and the overlying atmosphere create a concentrat ion profile in the soil that increases with depth. While the production, migration, and exhalation of radon in undis- turbed soils is well-approximated by diffu- sion, the presence of a building dramatically changesthe system. First, excavation, grad- ing, and fill modify the soil environment. Second, the building interrupts the commu- nication between the soil and the atmo- sphere. Third, operation of building sys- tems and environmental influences on the building create pressure differences that supply the basis for forced convective trans- port through cracks, joints, and service pen- etrations connecting the building to the soil. Pressure-driven transport of radon-bear- ing soil gas into the building through cracks, joints, and service penetrations is favored over diffusion if the building is depressur- ized. Pressure-driven flows dominate trans- port in the soil at higher permeabilities, while diffusion is probably the dominant transport mechanism in the soil for situa- tions of low permeabilities. Measurement Technologies While the basis for judging radon poten- tial is still evolving, measurement strategies have converged along basic themes ad- dressing (1) radium content, (2) soil gas, and (3) radon flux. Other types of measure- ments have been developed to quantify moisture, bulk density, permeability, poros- ity, and other soil properties that relate to the production and migration of radon in the soil. Concerns have been raised about rep- resentative sampling. While measurements of radon potential based on invariant soil properties could alleviate some of these concerns, representative soil conditions would still need to be defined for this ap- proach and model relationships would still be required to adjust measured values. Measurement strategies for estimating radon potential hinge on detecting the ra- dioactivity in a known sample volume (or mass) whose history has been controlled to represent one or more processes germane to the production and migration of radon in the soil. Radium content is measured by isolating a defined volume of soil to retain the emanating fraction. At radioactive equi- librium, the activity concentration of radon and radon progeny is equated with the radium concentration. Soil-gas measure- ments, on the other hand, seek to isolate radon in the pore spaces without affecting emanation or transport. Flux-based mea- surements rely on natural or induced trans- port through the soil column to deliver the radon to a sampling volume defined over a specified area of the soil. Basic approaches for measuring the ra- dium content of soils involve sealing a soil sample in a leak-proof container, storing the sealed sample for a long enough period of time to establish radioactive equilibrium, and analyzing for radionuclides of interest using gamma spectroscopy. Protocols fre- quently accommodate concurrent analysis of moisture content, laboratory estimates of radon emanation, and other analyses by subdividing field samples. Variations in pro- cedure include repeated analyses to evalu- ate the secular equilibrium between radium and radon. Basic technologies for measuring radon concentrations in soil gas have evolved along three complementary pathways: (1) gas extraction from depth using holbwtubes, (2) analysis of bulk soil samples, and (3) in situ detection. The reconnaissance probe for soil-gas extraction is a relatively simple system consisting of a small-diameter (6- to 9-mm) thick-walled carbon steel tube that is driven to sampling depth (75 cm, nominal) using a slide hammer. While the reconnais- sance probe is intended for collecting grab samples of soil gas, it has been suggested that the system can be used for determining soil permeability. The permeameter probe is further equipped for controlled flow ex- traction to allow for estimates of soil perme- ability from pressure/flow relationships as well as radon concentration. The packer probe is a more complex apparatus that features inflatable packers to intercept sur- face air. Basic approaches for determining soil- gas concentrations from bulk samples of soil generally involve sealing the sample under known conditions and measuring the evolution of radon in the sample with time. Three basic patterns can be recognized: (1) emanation, a variation of the standard labo- ratory test for radium that infers pore gas radon from time-related changes in a sample at controlled dryness, (2) prompt bismuth, a second variation that monitors time evolu- tion from field conditions, and (3) exhala- tion, involving analysis of radon escaping from the sample to a headspace. Both the emanation and the prompt bis- muth techniques monitor the ingrowth of radon in the soil sample, producing data to readily estimate undepleted soil gas con- centrations. The exhalation technique, on the other hand, is used primarily to deter- mine the time rate of release of radon, and requires additional information to estimate undepleted soil gas concentrations. In situ detection involves direct burial of detectors to estimate radon concentrations in the soil. The main avenue of development entails forming a suitable detection volume in the soil and detecting alpha activity from radon diffusing into the cavity and subse- quent decays of the short-lived progeny. Two basic techniques are evident: passive and active detection. Passive in situ detection is probably the most widely used. While the alpha track detector is the system most closely identi- fied with in situ passive measurements in the soil, the basis can be extended to other technologies. The second approach, involv- ing an active detection system, presents an opportunity to study short-term effects but has not been used widely. While buried alpha track detectors have been used widely, generalized criteria with regard to placement have not emerged. A recent theoretical analysis indicates that passive in situ detectors could significantly underestimate soil gas concentrations at high moisture levels because diffusive trans- port is reduced as the pores fill with water. For cavities of the approximate size for alpha track detectors, however, significant departures may not appear until water satu- ration is fairly high (e.g., 80%). Measurement systems for radonflux seek to determine the net transfer from the soil to the atmosphere. Basic approaches have focused on capturing radon leaving the soil using (1) closed accumulators, (2) flow- through accumulators, and (3) adsorption. Each of these approaches involves isolat- ing an areaof soil and measuring the amount of radon captured over a defined period of time. A fourth method, induced flux, in- volves applying controlled suction to the surface of the soil. This technology has not been applied to soil gas radon, but could directly simulate flow coupling of a building to the soil. ------- The closed accumulation approach in- volves direct accumulation of radon into a volume defined by the soil surface and a vessel whose open face is affixed to the soil. The radon concentration in the accumulator begins to increase as soon as the vessel is emplaced because dispersion to the atmo- sphere is eliminated. To more closely simu- late natural conditions in the collection vol- ume, flow-through accumulation can be used to sweep radon out of the accumulator and replace it with radon-free (or nearly so) air. If radon concentrations in the accumulator are maintained low enough to suppress back diffusion, radon flux into the accumu- lator is proportional to the radon content of the exiting air stream. The basic method for adsorption involves placing a charcoal can- ister in contact with the surface for a period that may range from a few hours to a few days. Technical Considerations Currently, there are no hard and fast criteria to provide an unambiguous refer- ence for judging the performance of mea- surement technologies for radon potential. While there is little doubt that site-specific measurements can be used to determine the radon potential of land areas, interpreta- tions are driven by empirical correlations and theoretical considerations. A broad consensus, however, highlights the impor- tance of examining the abundance of radon in the soil and its propensity to migrate into buildings. Ideally, then, methods would pro- vide information on the undepleted soil con- centration, diffusion coefficient, and perme- ability through various combinations of di- rect measurements and model assump- tions. Each measurement approach reviewed in this report can provide useful information to evaluate radon potential. Technologies geared to measuring (1) radium concentra- tions in bulk soil samples or (2) soil gas concentrations are readily applied to the problem of estimating the undepleted radon concentration in soil gas. Measurements of unattenuated flux provide estimates of dif- fusive transport which, in turn, could be used to estimate soil-gas concentrations at depth. The induced flux method, although untested, may provide the means to directly simulate radon entry for slab-on-grade and crawl space construction. Laboratory mea- surements of exhalation, on the other hand, while not readily extrapolated to the soil environment, may provide clues to the rela- tive strength of radon sources through com- parative tests. Radium-based measurements have the distinct advantage of being suited to testing water-saturated soils. Soil-gas-based mea- surements (extraction probes, in situ detec- tion, flux), on the other hand, generally fail to obtain samples from saturated soils be- cause the gas volume is nearly zero. Rec- ognition factors to avoid generally saturated conditions can be built into protocols, as can rules to invalidate samples from saturated layers encountered at depth. Material that is permanently saturated in the native state but likely to reach varying degrees of dry ness after construction, how- ever, is best characterized using radium- based measurements. These circumstances are likely to occur with fill material and may occur in areas with a shallow water table that could recede as property development alters drainage patterns. Quality assurance is a vexing question for soil-gas measurements. Although ana- lytical proficiency can be deemed accept- able, there is little information at hand to evaluate system-level performance because relatively few studies have explicitly com- pared technologies. A number of studies have included more than one soil measure- ment technique, but additional analysis would be required to formally compare methods. Limited comparisons to date provide a fair degree of reassurance that the different methods are comparable, but the test con- ditions incompletely reflect current practice. It would be useful to address intermethod comparability by reanalyzing data bases from completed multicomponent studies as well as studies that are nearing completion. Staged intercomparisons are also recom- mended; it is generally known that such intercomparisons have been conducted on an informal basis, but the results have not been published as yet. Excepting the induced flux technique, the measurement strategies summarized in this report represent stable technologies sup- ported by operational experience. The ba- sis for assembling generalized protocols exists and needs to be evaluated in detail to develop method-specific protocols that can be circulated for consensus review. Practical Considerations Practical decisions are likely to be guided by two absolutes: (1) avoidance of clearly inappropriate technologies, and (2) meet- ing the schedule demands of the situation. For the radium-based measurements, the all-weather capability must be judged against the lengthy time period necessary to achieve radioactive equilibrium. Delays could be shortened by taking more counts during the ingrowth period to extrapolate data to equi- librium levels. For soils with a low emana- tion fraction, a number of days may need to elapse to resolve the trend, but turnaround time could, in concept, be reduced to a matter of days. Further, initial count data offer information to provide a rough esti- mate without extended waits. While the soil-gas extraction techniques are not suited to testing under saturated conditions, the simplicity of equipment and field operations for the hand-driven probes can deliver prompt results, making the re- connaissance probe and the permeameter probe likely candidates for widespread use. The packer probe is a bit more complex and requires an augered hole, but delivers data in a short timeframe. In situ detectors offer possibly the least expensive approach. Emplacing detectors at a satisfactory depth (1 m) and retrieving them may present a problem. The main disadvantages, however, could arise from the need to sample for relatively long peri- ods of time and from unreliable results in the presence of high moisture levels. As noted earlier, measurements of unattenuated flux can be converted to esti- mates of soil-gas radon at depth. This con- version, however, is predicated on model assumptions that may go unverified in the field. Similarly, laboratory exhalation can- not be readily extrapolated to quantitative estimators of radon potential. The induced flux technique may prove to be a useful test apparatus for soils receiving slab on grade or crawl space construction. At the present time, however, it is an untested technology. Conclusions and Recommendations Each available technique for measuring radon in the soil provides some useful and informative data, but the means to apply these data are still evolving. How this evo- lution will affect future strategies and proto- cols for soil measurements remains to be seen. In the end, the utility of soil-based measurements is probably more sensitive to the interpretive framework than to the technologies employed to collect the data. A firm and quantitative basis needs to be established for formally comparing different soil-measurement technologies. Achieving this basis would most likely require repli- cated testing of the various technologies across a range of soil types and sampling conditions. Allied to this, existing multicom- ponent data bases should be analyzed to place different measurement parameters on a common basis. The virtues of mea- surements directed toward invariant soil properties warrant further investigation from technical and practical standpoints, and the current range of accepted practice for soil- gas measurements needs to be assembled and compiled in a manner suited to consen- sus review. •6U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40061 ------- Harry E. Rector is with GEOMET Technologies, Inc., Germantown, MD 20874. David C. Sanchez is the EPA Project Officer (see below). The complete report, entitled "An Assessment of Soil-Gas Measurement Technolo- gies, " (Order No. PB91- 219 568; Cost: $17.00, subject to change) will be available only from: National Technical Information Se/v/ce 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 Official Business Penalty for Private Use $300 EPA/600/S8-91/050 ------- |