An improved model for predicting hydrate phase equilibrium in marine sediment environment

Based on the models of hydrate phase equilibrium in bulk water and porous media, an improved model was proposed to predict the methane hydrate equilibrium in marine sediment environment. In the suggested model, mechanical equilibrium of force between the interfaces in hydrate-liquid-vapor system was considered. When electrolyte was present in pore water, interfacial energy between hydrate and liquid was corrected by an equation that is expressed as the function of temperature and electrolyte concentration. The activity of water is calculated based on the Pitzer model and the interfacial energy between liquid and gas is solved using the Li method. The prediction results show good agreement with the experimental data. By comparison with other models, it is proved that this model can improve the accuracy for predicting hydrate phase equilibrium in marine sediment environment.     

A multiple approach to the determination of radon fluxes from sediments

Determination of sedimentary fluxes of²²²Rn via diffusion was required as an input for a mass balance model of radon in a freshwater lake. We obtained these fluxes by: (1) direct measurement in the laboratory using a simulated sediment bed and water column; (2) a “sediment equilibration” technique; and (3) porewater modeling. The first method, analogous to an in situ benthic chamber approach, uses direct observation of the increasing²²²Rn activity in water overlying a sediment bed packed in plastic columns. This allows one to directly measure the fluxes and determine the effective wet bulk sediment diffusion coefficient (D _s). Radon flux estimates using these three techniques agreed to within approximately 10–15%.     

Determination of air-loop volume and radon partition coefficient for measuring radon in water sample

A simple method for the direct determination of the air-loop volume in a RAD7 system as well as the radon partition coefficient was developed allowing for an accurate measurement of the radon activity in any type of water. The air-loop volume may be measured directly using an external radon source and an empty bottle with a precisely measured volume. The partition coefficient and activity of radon in the water sample may then be determined via the RAD7 using the determined air-loop volume. Activity ratios instead of absolute activities were used to measure the air-loop volume and the radon partition coefficient. In order to verify this approach, we measured the radon partition coefficient in deionized water in the temperature range of 10–30 °C and compared the values to those calculated from the well-known Weigel equation. The results were within 5 % variance throughout the temperature range. We also applied the approach for measurement of the radon partition coefficient in synthetic saline water (0–75 ppt salinity) as well as tap water. The radon activity of the tap water sample was determined by this method as well as the standard RAD-H₂O and BigBottle RAD-H₂O. The results have shown good agreement between this method and the standard methods.     

A continuous monitor for assessment of 222Rn in the coastal ocean

Radon-222 is a good natural tracer of groundwater flow into the coastalocean. Unfortunately, its usefulness is limited by the time consuming natureof collecting individual samples and traditional analysis schemes. We demonstratehere an automated system which can determine, on a continuousbasis, the radon activity in coastal ocean waters. The system analyses ²²²Rn from a constant stream of water passing through an air-water exchangerthat distributes radon from the running flow of water to a closed air loop.The air stream is feed to a commercial radon-in-air monitor which determinesthe concentration of ²²²Rn by collection and measurement of theemitting daughters, ²¹⁴Po and ²¹⁸Po, via a charged semiconductordetector. Since the distribution of radon at equilibrium between the air andwater phases is governed by a well-known temperature dependence, the radonconcentration in the water is easily calculated.     

Characterization of airborne radon released by domestic showering and the use of a washing machine

Water originated from groundwater systems can have relatively high concentration of radon. In many situations, radon is released from the water and mixes with the indoor air. In the present study, laboratory experiments were conducted to characterize the airborne radon released by showering and the use of a washing machine. The followings were discussed: (1) a comparison between the loss of waterborne radon and the increase of airborne radon, (2) time-related changes in airborne radon after the water use, (3) changes in the equilibrium factor, and (4) the relationship between radon concentration and the distance from the source of radon release.     

Development of the DET technique for high-resolution determination of soluble reactive phosphate profiles in sediment pore waters

The DET (diffusive equilibrium in thin films) technique is developed to measure soluble reactive phosphate (SRP) profiles in sediment pore waters at a millimetre resolution. The analytical procedure includes equilibration of the gels in sediments, section of the gels after retrieval from the sediments, back elution of phosphorus in the gels, and analysis of SRP in the eluents. Recovery of phosphorus is improved from back elution with 0.25?M nitric acid relative to deionised water. SRP concentrations in pore waters of different sediments measured by DET probes agree well with those directly measured by the colorimetric method. Pore water profiles obtained simultaneously using gel probes and other techniques (including Rhizon and dialysis peeper) also show comparability at similar resolutions. The DET probes were used to investigate pore water SRP profiles in the sediments of two contrasting regions (algal-dominated and macrophyte-dominated) in Lake Taihu. An increasingly upward movement of SRP was observed in subsurface pore waters of the algal-dominated region coupled with an increase in water temperature from March to May. Peak-shape distribution of SRP and horizontal heterogeneity was observed in pore waters of the macrophyte-dominated region, which is most likely caused by the activity of submerged macrophyte roots.     

Comparative study of different types of granular activated carbon in removing medium level radon from water

Granular activated carbon (GAC) has proven its effectiveness in removing radon from water supplies. Laboratory and pilot plant studies were carried out using three different types of activated carbons (F-300, F-400, and HD-4000) to remove radon from water supply. From the experimental kinetic study, the data indicated that at least 6 h are needed to attain equilibrium between radon activity adsorbed onto carbon and its concentration in the aqueous phase. Also, it showed that HD-4000 has higher capacity for removing radon than the other two investigated carbons F-300 and F-400. The adsorption isotherms were satisfactorily explained by Freundlich equation. In the pilot plant study, the performance of the three activated carbons in removing radon at medium concentration (~111 Bq dm⁻³) was evaluated over 60 days of continuous water flow. Four empty-bed contact times (EBCTs) corresponding to four bed depths were continuously monitored and the corresponding steady state adsorption-decay constant values were calculated and the efficiency of each carbon was used to provide a facet for comparison. The γ-radiation exposure rate distribution throughout each GAC bed was measured and compared. This study, despite paucity of literature in this field, is useful for designing a GAC adsorption system for the removal of medium level radon concentration from water supplies.     

Release of137Cs from anoxic lacustrine sediment measurement and formulation

Low level measurements of¹³⁷Cx in lacustrine sediment and its pore water are possible to activity as low as 10 mBq. From this data, the flux of¹³⁷Cs through the sediment and to the bottom water can be estimated. The value of the diffusion coefficient in the pore water follows from separate radiotracer experiments.     

Temporal variation patterns of radon in mineral waters along the Cota Mil Highway, Caracas, Venezuela

Measurements of radon in potable mineral waters along the Cota Mil Highway at two sites, La Castellana with five sampling points within 20 meters of each other and two collection points at the San Jose site were carried out from November, 1997 to December, 1998. Temporal radon variation patterns will be presented for the seven sampling points, which had very different water flow rates and short-term variations. The extraction of the radon from mineral waters was accomplished in the laboratory a few hours after sampling and the measurements were performed employing a radiation monitor with a scintillation cell 18 hours latter to insure that the radon and its decay products were in equilibrium. The results suggest that the increase of radon from the middle of February to the middle of November can be related to the decrease in atmospheric temperature and rainfall. The large differences in the minimum and maximum values of radon and the short-term variations show the need for many measurements over a year to quantify a correct annual value to be employed in dose calculation for radiological impact studies on human health. Finally, we have also concluded that water sampling points with very low water flow rates are more suitable for investigating the possible relation between radon anomalies and seismic events.     

Radon concentration levels in dry CO<Subscript>2</Subscript> emanations from Harghita Băi,Romania, used for curative purposes

The aim of this work is to study the exposure due to the radon of the patients from the dry carbon dioxide baths (mofettas) from Harghita Băi, Romania. These mofettas consist of emanated gas with high carbon dioxide content, and proved curative effects. The vertical and horizontal distribution of the radon activity concentration was monitored indoor, the seasonal variation, as well as the soil radon and thoron activity concentration outdoor, in the vicinity of the bath to determine the origin of the radon and the location of the fault through which it propagates. The equilibrium factor of the baths and the effective dose received by the patients was also calculated.     

A study on the radon concentration in water in Coonoor, India

The gas collection measurement method was employed to determine radon activity concentrations in the water of Coonoor. Open well water, dam water and stream water have been investigated for their radon concentrations. It is observed that the highest radon concentration is in the open well water and the lowest in stream water. From these measurements, the corresponding annual effective ingestion dose is determined.     

High resolution characterization of uranium in sediments by DGT and DET techniques ACA-S-12-2197

Diffusive equilibrium (DET) and diffusive gradient in thin film (DGT) techniques with an inductively coupled plasma mass spectrometry detection of elements were applied to characterize uranium, manganese, iron and ²³⁸U/²³⁵U isotopic ratio depth profiles in sediment pore water at high spatial resolution and to monitor uranium uptake/remobilization processes in uranium spiked sediment core samples under laboratory, well controlled conditions. Modified constrained sediment DGT probes, packed with Spheron-Oxin^® resin gel, were employed for selective uranium measurements. Spatially resolved DET and DGT responses were indicative of local redistribution of uranium in naturally uranium poor and rich sediments.     

Direct and precise determination of environmental radionuclides in solid materials using a modified Marinelli beaker and a HPGe detector.

A simple but precise detection method was studied for the determination of natural radionuclides using a conventional HPGe detector. A new aluminium beaker instead of a plastic Marinelli beaker was constructed and examined to reach radioactive equilibrium conditions between radon and its daughter elements without the escape of gaseous radon. Using this beaker fifteen natural radionuclides from three natural decay series could be determined by direct gamma-ray measurement and sixteen radionuclides could be determined indirectly after radioactive equilibrium had been reached. Analytical results from ground water were compared with those from conventional alpha spectroscopy and the results agreed well within 12% difference. Nitrogen gas purge was used to replace the surrounding air of the detector to obtain a stable background and reducing the interference of radon daughter nuclides in the atmosphere. The use of nitrogen purging and the aluminium Marinelli beaker results in an approximately tenfold increase of sensitivity and a decrease of the detection limit of 226Ra to about 0.74 Bq kg(-1) in soil samples.     

P-T stability conditions of methane hydrate in sediment from South China Sea

For reasonable assessment and safe exploitation of marine gas hydrate resource, it is important to determine the stability conditions of gas hydrates in marine sediment. In this paper, the seafloor water sample and sediment sample (saturated with pore water) from Shenhu Area of South China Sea were used to synthesize methane hydrates, and the stability conditions of methane hydrates were investigated by multi-step heating dissociation method. Preliminary experimental results show that the dissociation temperature of methane hydrate both in seafloor water and marine sediment, under any given pressure, is depressed by approximately -1.4 K relative to the pure water system. This phenomenon indicates that hydrate stability in marine sediment is mainly affected by pore water ions.     

Radon measurements in water samples from the thermal springs of Yalova basin, Turkey

The radon concentration has been measured in thermal waters used for medical therapy and drinking purposes in Yalova basin, Turkey. Radon activity measurements in water samples were performed using RAD 7 radon detector equipped with RAD H₂O (radon in water) accessory and following a protocol proposed by the manufacturer. The results show that the concentration of ²²²Rn in thermal waters ranges from 0.21 to 5.82 Bql^?1 with an average value of 2.4 Bql^?1. In addition to radon concentration, physicochemical parameters of water such as temperature (T), electrical conductivity, pH and redox potential (E_h) were also measured. The annual effective doses from radon in water due to its ingestion and inhalation were also estimated. The annual effective doses range from 0.2 to 0.75 μSvy^?1 for ingestion of radon in water and from 2.44 to 9 μSvy^?1 for inhalation of radon released from the water.     

Measurement of 222Rn and 226Ra in municipal supply tap water to evaluate their radiological impacts

ABSTRACT

Radon (²²²Rn) and its parent radionuclide Radium (²²⁶Ra) are classified as carcinogen. Human exposes to radon in water via inhalation and ingestion, although ingestion is the only way for radium to enter the human body. In this research, tap water collected from Bornova distinct was studied to determine the concentration of radon (²²²Rn) and radium (²²⁶Ra) for evaluating their radiological impact. For this reason, the annual effective doses for ingestion and inhalation were estimated. The measurements were performed using a collector chamber method. The mean concentrations of ²²²Rn and ²²⁶Ra were determined as 0.85 and 0.76 Bq/L, respectively. It can be stated that the ²²²Rn and ²²⁶Ra concentrations of tap waters here are lower than the international reference levels. Obtained concentration levels were applied to estimate annual effective dose due to the inhalation and ingestion. The dose values are also found to be lower than the recommended maximum values. On the other hand, it should be considered that consumption of these waters (2 L) and average radon and radium concentrations of water are the significant factors for estimating doses.     

The effect of meteorological parameters on radon concentration in borehole air and water

Seasonal and short term variations of ²²²Rn activity concentration in borehole air and water of the borehole drilled in cracked quartzite were studied and possible response on meteorological parameters was examined. Seasonal change of radon concentration in borehole air due to atmospheric temperature was confirmed. Short term variation of radon concentration in borehole air coincided with the atmospheric pressure changes. The strong impact of rainfall on radon concentration values was observed both in air and water environments. The decrease of radon content in borehole air and water followed radioactive decay law exclusively in spring and summer month. Contrary to borehole water, rainfall increased radon concentration in borehole air during spring and summer months only. In this paper the results from two and half years of investigation are presented.     

Study of radon in ground water and physicochemical parameters in Khartoum state

This study was conducted primarily to measure and map radon activity concentration in wells within water supply network of Khartoum State. Ground water samples were collected before and after autumn and analysed using low level γ-spectrometry equipped with HPGe-detector. Radon activity concentration was found in the range of 1.58–345.10 Bq/L with an average value of 59.20 ± 6.60 Bq/L. Upon comparing the radon concentration values obtained with EPA it was found they were far below the maximum contaminant level of EPA with the exception five samples. Physicochemical water parameters were measured and no correlation was noted between radon concentration and these parameters. The overall annual effective dose for adults due to radon ingestion is less than WHO recommended reference dose level for most except 14 samples.     

Electric-field-controlled water and ion permeation of a hydrophobic nanopore

The permeation of hydrophobic, cylindrical nanopores by water molecules and ions is investigated under equilibrium and out-of-equilibrium conditions by extensive molecular-dynamics simulations. Neglecting the chemical structure of the confining pore surface, we focus on the effects of pore radius and electric field on permeation. The simulations confirm the intermittent filling of the pore by water, reported earlier under equilibrium conditions for pore radii larger than a critical radius R(c). Below this radius, water can still permeate the pore under the action of a strong electric field generated by an ion concentration imbalance at both ends of the pore embedded in a structureless membrane. The water driven into the channel undergoes considerable electrostriction characterized by a mean density up to twice the bulk density and by a dramatic drop in dielectric permittivity which can be traced back to a considerable distortion of the hydrogen-bond network inside the pore. The free-energy barrier to ion permeation is estimated by a variant of umbrella sampling for Na(+), K(+), Ca(2+), and Cl(-) ions, and correlates well with known solvation free energies in bulk water. Starting from an initial imbalance in ion concentration, equilibrium is gradually restored by successive ion passages through the water-filled pore. At each passage the electric field across the pore drops, reducing the initial electrostriction, until the pore, of radius less than R(c), closes to water and hence to ion transport, thus providing a possible mechanism for voltage-dependent gating of hydrophobic pores.     

Determination of radon partitioning between groundwater and atmosphere by liquid scintillation counter

A simple method for the determination of the air?Cwater partition coefficient (K_air/water) of radon (Rn-222) was studied using a liquid scintillation counter. In the present work, the radon activity of groundwater phase in a closed container was measured and used to calculate the partition coefficient instead of the radon activity of gaseous phase in other works. The partition coefficient was determined for four groundwater samples by using a modified equilibrium partition coefficient in closed system method. The effect of temperature on the partition coefficient was investigated at 0, 10, 20 and 30?°C. Within the temperature range, the partition coefficients were 1.72?C2.03, 2.11?C2.28, 2.78?C3.92 and 4.93?C5.61 at 0, 10, 20, and 30?°C, respectively. It was found that the effect of temperature on the air?Cwater partition coefficient of groundwater radon was agreed well with literature values.