Effect of atmospheric temperature on underground radon: A laboratory experiment.

The effect of atmospheric temperature on underground radon flow was investigated in a customized climate-controlled laboratory (CCL) system, which enabled us to isolate the impact of ambient atmospheric temperature variations on underground radon transport. The soil thermal gradients that developed, following atmospheric warming, acted as the driving force for the diffusive radon flow, resulting in a decrease in the radon concentration along the experimental column setup at a rate of ∼70 Bq∙m-3 per oC∙m-1 (∼0.4% of the radon concentration). When the ambient temperature decreased, compared to the soil temperature, an air-soil temperature difference developed along the column, which acted as a driving force for radon to flow along the column and promptly increased the radon concentration at a rate of ∼140 Bq∙m-3 per oC (∼0.8% of the radon concentration). The overall radon concentration changes under the experimental conditions were up to 30%. The changes in the molecular diffusion coefficient in the experimental temperature range were ∼7%, with thermal diffusion as a possible additional mechanism contributing to radon transport due to temperature. The cyclic changes in ambient temperature in the forced conditions experiments were found to be directly correlated with underground radon oscillations. The same frequency for ambient temperature and radon concentration, along the experimental column in low frequency warming-cooling cycles (i.e., 4-8 days), was found. This good correlation was lost at higher frequencies (two days or more), due to the asymmetrical response of radon to atmospheric warming and cooling. The results of this study explain some of the field observations in underground radon monitoring.

Monte Carlo modeling of scintillation detectors for continuous underground radon monitoring.

Nuclear simulation methods were applied to two systems that investigate radon transport within geological porous media: a) a laboratory system built to test, under controlled climate conditions, the effect of temperature on radon transport, and b) a field monitoring system comprising gamma and alpha detectors in an abandoned water well. The use of Monte Carlo simulations of NaI and BGO scintillation detectors in continuous underground radon measurements by gamma counting, to estimate the photon flux in the detector volume, is presented. The advantages of shielding side-view NaI detectors were demonstrated for a laboratory system containing ground phosphate rock, including avoiding high counting rates and reducing the effective source volume in radon transport studies. The gross gamma counting procedure was shown to result in a lower uncertainty than spectrometric measurement, by at least a factor of two, despite it being a simpler and more suitable procedure for field measurements. The calculation of simulated source volumes for a BGO detector in a borehole and the measurements in the field support the assumption that the gamma signal comes primarily from radon flowing in the bedrock's air-filled pores. As a practical outcome of this study, positioning the detector a few cm off-center from the borehole's axis increased the gamma counting efficiency; however, measurements in groundwater taken too close to the iron casing had a lower detection efficiency. The conversion factor from the scintillator signal to the radon activity concentration, for the laboratory system, was calculated. Monte Carlo simulations demonstrated the advantages of the gross counting procedures using gamma scintillation detectors in field underground high-frequency radon monitoring.

Detailed effects of particle size and surface area on 222Rn emanation of a phosphate rock.

The dependency of radon emanation on soil texture was investigated using the closed chamber method. Ground phosphate rock with a large specific surface area was analyzed, and the presence of inner pores, as well as a high degree of roughness and heterogeneity in the phosphate particles, was found. The average radon emanation of the dry phosphate was 0.145 ± 0.016. The emanation coefficient was highest (0.169 ± 0.019) for the smallest particles (<25 µm), decreasing to a constant value (0.091 ± 0.014) for the larger particles (>210 µm). The reduction rate followed an inverse power law. As expected, a linear dependence between the emanation coefficient and the specific surface area was found, being lower than predicted for the large specific surface area. This was most likely due to an increase in the embedding effect of radon atoms in adjacent grains separated by micropores. Results indicate that knowledge of grain radium distribution is crucial to making accurate emanation predictions.

Sustainable agricultural use of natural water sources containing elevated radium activity.

Relatively elevated concentrations of naturally occurring radium isotopes ((226)Ra, (228)Ra and (224)Ra) are found in two main aquifers in the arid southern part of Israel, in activity concentrations frequently exceeding the limits set in the drinking water quality regulations. We aimed to explore the environmental implications of using water containing Ra for irrigation. Several crops (cucumbers, melons, radish, lettuce, alfalfa and wheat), grown in weighing lysimeters were irrigated at 3 levels of (226)Ra activity concentration: Low Radium Water (LRW)<0.04 Bq L(-1); High Radium Water (HRW) at 1.8 Bq L(-1) and (3) Radium Enriched Water (REW) at 50 times the concentration in HRW. The HYDRUS 1-D software package was used to simulate the long-term (226)Ra distribution in a soil irrigated with HRW for 15 years. Radium uptake by plants was found to be controlled by its activity in the irrigation water and in the soil solution, the physical properties of the soil and the potential evapotranspiration. The (226)Ra apeared to accumulate mainly in the leaves of crops following the evapotranspiration current, while its accumulation in the edible parts (fruits and roots) was minimal. The simulation of 15 years of crop irrigation by HYDERUS 1-D, showed a low Ra activity concentration in the soil solution of the root zone and a limited downward mobility. It was therefore concluded that the crops investigated in this study can be irrigated with the natural occurring activity concentration of (226)Ra of 0.6-1.6 Bq L(-1). This should be accompanied by a continuous monitoring of radium in the edible parts of the crops.

High naturally occurring radioactivity in fossil groundwater from the Middle East.

High levels of naturally occurring and carcinogenic radium isotopes have been measured in low-saline and oxic groundwater from the Rum Group of the Disi sandstone aquifer in Jordan. The combined 228Ra and 226Ra activities are up to 2000% higher than international drinking water standards. Analyses of the host sandstone aquifer rocks show 228Ra and 226Ra activities and ratios that are consistent with previous reports of sandstone rocks from different parts of the world. A compilation of previous data in groundwater from worldwide sandstone aquifers shows large variations in Ra activities regardless of the groundwater salinity. On the basis of the distribution of the four Ra isotopes and the ratios of the short- to long-lived Ra isotopes, we postulate that Ra activity in groundwater is controlled by the balance of radioactive decay of parent Th isotopes on aquifer solids, decay of the dissolved radium isotopes, and adsorption of dissolved Ra on solid surfaces. The availability of surface adsorption sites, which depends on the clay content in the aquifer rocks, is therefore an important constraint for Ra activity in sandstone aquifers. These findings raise concerns about the safety of this and similar nonrenewable groundwater reservoirs, exacerbating the already severe water crisis in the Middle East.

Empirical compensation for the precipitation effect in the measurement of gross alpha content in drinking water.

In this experimental work the co-precipitation procedure is used to prepare samples of water for measurement of their gross alpha concentration. It has been suspected that total dissolved solids (TDS) in the water influence the results of radioactivity counting. The dissolved solids may influence the chemical procedures of sample preparation or the detection and counting of the alpha particles emitted by the radionuclides in the drinking water. The aim of this study was to check the hypothesized influence of the TDS on the counting procedure and offer countermeasures to compensate for this influence. In the frame of this study more than six hundred measurements were conducted on samples to determine their gross alpha concentration. The variety of samples represents different controlled dilutions with known TDS values. Each sample was measured several times repeatedly and independently, and also three runs of experiments were conducted, each run subjected to independent application of the co-precipitation procedure. The analysis of the accumulated experimental data revealed an effect of the TDS values on the corresponding number of counts from the samples which may be due to self-absorption in the co-precipitated layer. An empirical correction function for the TDS effect is presented in this work. Data analyses pointed out the experimental and procedural conditions required to allow an acceptable level of sensitivity in terms of the minimum detectable activity (MDA) to ensure that the water under inspection meets the standards.