Measuring radon-222 in soil gas with high spatial and temporal resolution

Dispersion Simulations of Radon Discharges between Neighboring Buildings and Their Sensitivity to Meteorology, Discharge Rate, and Building Geometry

ABSTRACT

In northern climates, it is common to install the discharges of radon sub-slab depressurization systems near ground level. However, this also elevates the ground level outdoor radon concentrations and raises the possibility of radon re-entrainment into homes. The study aims to assess outdoor radon concentrations near above-ground-level discharges along the surfaces of an emitting building and its close neighbor and identify parameters that most influence the dilution. This study employs a series of computational fluid dynamics calculations to assess concentrations along the exhaust-facing and non-exhaust-facing surfaces of the buildings. Different meteorological, venting, and building geometry parameters are explored. Boundary conditions for the CFD calculations are based on field measurements of the ground-level wind speeds and seasonal air temperatures and atmospheric stabilities. Outdoor concentrations can be as high as 7% of the discharge gas, although these become smaller at greater distances from the vent. The direction of the prevailing wind is a particularly important parameter, as it influences the formation of circulating building cavities and building wakes where radon could accumulate. The wind speed, atmospheric stability, and season (plume buoyancy) also have important influences on the outdoor radon concentrations, as do the velocity of the vent system and the size of the buildings. The study has assessed the dilution of the radon-laden exhaust gas and determined the outdoor concentrations that can be expected under a variety of conditions. These results can be used to inform regulators about the potential for radon re-entrainment into homes.

A fast method for the simultaneous determination of soil radon (222Rn) and thoron (220Rn) concentrations by liquid scintillation counting

This paper presents a fast method dedicated to measurements of radon nuclides in the soil gas. The soil gas is sampled by a typical hollow tube probe by 10 min of sucking of about 3 dm³ of gas and passing it directly through a 16 cm³ of water-immiscible liquid scintillator placed in a typical 20 cm³ scintillation vials, where the radon and thoron nuclides are effectively absorbed. Most of the presently used active methods for radon isotopes determination (e.g., RAD7 or AlphaGuard) require the soil gas transfer to the measuring device. The serious limitation of such approach is the necessity to wait until the radon daughter isotopes decay, before counter is ready for the next measurement. In the proposed method, several samples can be simultaneously gathered from the examined areas in the form of the scintillation vials, which are ready for later measurements in the automatic liquid scintillation counters in the lab or directly in situ. For that purpose, the combined mathematical model for the simultaneous radon and thoron determination has been elaborated. The direct in situ measurements of the sample activity between 60 and 240 s after the end of sampling followed by a second activity measurement after 3 h allow for the determination of both ²²⁰Rn and ²²²Rn concentrations in the soil gas. The limit of detection for ²²²Rn isotope during 10 min counting is 25 Bq·m^-3, whereas for a 3 min counting of ²²⁰Rn just after sampling was found to be ca. 150 Bq·m^-3. The method was successfully verified and applied for the simultaneous radon and thoron concentrations measurements in the soil gas in Central Poland region.

Unperturbed, high spatial resolution measurement of Radon-222 in soil-gas depth profile

This work presents a method for measuring the depth distribution of ²²²Rn activity in soil gas. The method is based on the capacity of polycarbonates to absorb ²²²Rn and on the possibility of performing sensitive measurements of ²²²Rn absorbed by the polycarbonates via liquid scintillation counting (LSC). The method is the following: cylindrical holes are drilled along a metal rod and Makrofol^® N polycarbonate foils enclosed in polyethylene envelopes are placed in each hole. The rod is driven into the soil and kept for a certain time. As long as the rod is in the soil, the polycarbonate foils are exposed to the ²²²Rn concentration at their depth. At the end of the exposure the rod is pulled out and the foils are transferred to liquid scintillation (LS) vials filled with liquid scintillator. The ²²²Rn absorbed in the foils is then measured with a LS analyzer. The rod with the polycarbonate foils acts as a passive probe which senses the ²²²Rn concentration at different depths beneath the ground surface. The achievable minimum detectable ²²²Rn activity concentration with the equipment and conditions used in this study is around 12.5 kBq/m³. It can easily be lowered below 1 kBq/m³ if larger foils and low-background LS analyzers are used. Since the method does not require air sampling the depth distribution of ²²²Rn in the soil is unperturbed by the sampling. The spatial distribution and the maximum measurement depth are set by the distance between the holes and the depth to which the rod can be fixed into the ground. Results from in situ applications of the method in terrains with high ²²²Rn in soil-gas are reported, which demonstrate the feasibility and the usefulness of the proposed approach.

Methane transport in agricultural soil after injection of isotopically-enriched methane in the sub-surface

Small quantities of radioactive methane (¹⁴CH₄) may be released over prolonged periods from geological disposal facilities for radioactive waste. The impact of this release depends on the capacity of soil to oxidise ¹⁴CH₄ to ¹⁴CO₂ during transport from the sub-surface to the atmosphere. We investigated this capacity by pulse-injecting isotopically-enriched methane 50 cm below the surface of an agricultural soil in central England. Three sequential injections were made during growth of a spring wheat crop. Samples of gas were taken from the pore space throughout the soil profile at predetermined time points after injection, accompanied by samples of the atmosphere above the soil collected in sampling chambers, deployed at scheduled intervals. Methane and CO₂ were measured in soil and above-ground gas using gas chromatography; the isotopic composition of CH₄ and CO₂ was determined using gas chromatography with isotopic ratio mass spectrometry. Supporting measurements of environmental variables were made during the experiment. The data can be used to test mathematical models describing CH₄ and CO₂ transport and fate in temperate agricultural soils.