Successful reduction of indoor radon activity concentration via cross-ventilation: experimental data and CFD simulations.

Advanced computational fluid dynamics (CFD) simulations are essential for predicting airflow in ventilated spaces and assessing indoor air quality. In this study, a focus was set on techniques for the reduction of indoor radon-222 activity concentration [Rn], and it is demonstrated how true-to-scale 3D CFD models can predict the evolution of complex ventilation experiments. A series of ventilation experiments in an unoccupied flat on the ground floor of a residential block in Bad Schlema (Saxony, Germany) were performed. Specifically, the 'Cross-ventilation 100 %' experiment resulted in room-specific [Rn] reductions from ∼3000 to ∼300 Bq m-3. We quantitatively interpreted the results of the ventilation experiment using a CFD model with a k-ϵ turbulent stationary flow model characterised by the used decentralised ventilation system. The model was coupled with a transient transport model simulating indoor [Rn]. In a first approach, the model overestimated the decrease in the starting of the experiment and the steady state. Adjusting the model parameters inflowing radon and inlet velocity the model results are in a good agreement with the experimental values. In conclusion, this paper demonstrates the potential of CFD modelling as a suitable tool in evaluating and optimising ventilation systems for an effective reduction of elevated [Rn].

Decentralised ventilation efficiency for indoor radon reduction considering different environmental parameters.

ABSTRACTRadon-222 contributes to half of the natural radiation exposure of humans and is one of the main causes of lung cancer. Of particular importance for humans is the exposure to radon-222 indoors, which enters living and working areas from the soil air, e.g. through cracks in the foundations of buildings. An easy and efficient way to minimise indoor radon in dwellings can be achieved through ventilation. How meteorological parameters and the geological background can influence ventilation efficiency in reducing indoor radon has not yet been fully investigated. Therefore, a decentralised ventilation system was installed in an unoccupied flat located in a former uranium mining region to analyse the effect of already existing ventilation modes on indoor radon activity concentration. It is aimed to assess 22 different ventilation experiments that were performed within the time period of one year. Even with a strong seasonal trend with significantly lower indoor radon activity concentrations in summer compared to winter, the decentralised ventilation system was able to reduce indoor radon by up to 83 %. Thereby, strong dependencies on the experimental parameters such as ventilation type or performance level of the fans were found.

Radon protection in apartments using a ventilation system wireless-controlled by radon activity concentration.

The new German Radiation Protection Act (StrlSchG) of 31 December 2018 established a reference value of 300 Bq m-3for the annual average radon activity concentration in buildings with recreation and living rooms, as well as in workplaces. It is expected that the reference value will be exceeded in a vast number of buildings throughout Germany and that radon protection measures will become indispensable. A simple and inexpensive radon protection measure for existing buildings is ventilation. In the scope of a joint project, ventilation systems with zone control and heat recovery are to be extended by the control parameter radon activity concentration. A highly sensitive, miniaturized radon monitor will be developed for this purpose, which can be integrated wirelessly into ventilation systems. Radon measurements were carried out in 13 apartments of an unoccupied heated apartment block in Germany over a period of three weeks in the wintertime. High radon activity concentrations were found on all three floors. The maximum values were 14000 Bq m-3on the first floor, 6000 Bq m-3on the second floor, and 2000 Bq m-3on the third floor. Ventilation experiments were carried out in an apartment with high radon activity concentration. Two decentralized ventilation systems with heat recovery were installed in each of the two opposite outside walls. The controlling device of the system was activated wirelessly depending on the radon activity concentration. The radon activity concentration was reduced from 8000 Bq m-3to 800 Bq m-3in a first experiment in the living room.

Benzene degradation in contaminated aquifers: Enhancing natural attenuation by injecting nitrate.

Natural attenuation processes depend on the availability of suitable electron acceptors. At the megasite Zeitz, concentrations of the main contaminant benzene were observed to increase constantly in the lower aquifer to levels of more than 2.5 mM. This was accompanied by decreasing concentrations of sulphate (SO42-), which has been previously shown to be the main electron acceptor for benzene oxidation at this site, resulting in an electron acceptor-limited, sulphidic benzene plume. Therefore, a field experiment was conducted to stimulate benzene biodegradation by injecting nitrate (NO3-) into the sulphidic benzene plume aiming (i) to recycle sulphate by nitrate-dependent sulphide oxidation, and (ii) to serve as direct electron acceptor for benzene oxidation. Within 60 days, 6.74 tons sodium nitrate (NaNO3) were injected into the lower aquifer, and the resulting biogeochemical effects within the benzene plume were monitored for more than one year by chemical and microbiological analyses of groundwater samples taken from various depths of ten monitoring wells located in three observation lines downstream of nitrate injection. Nitrate was microbiologically consumed, as shown by changes in δ15N-NO3- and δ18O-NO3- values, partial nitrite accumulation, and changing ratios of Na+/NO3-. Main electron donors for nitrate reduction were reduced sulphur compounds, verified by changing δ34S-SO42- and δ18O-SO42- values, partially increasing sulphate concentrations, and strongly increasing abundances of typical sulphur-oxidizing, nitrate-reducing bacterial taxa within the nitrate plume. The general absent hydrogen isotope fractionation of benzene, also in the sulphidic, nitrate-free part of the plume, indicates that benzene was not biodegraded by sulphate-reducing consortia. However, detected small carbon isotope fractionation of benzene points to in situ benzene biodegradation processes in the plume, probably supported by nitrate. In conclusion, nitrate injection resulted in changing redox conditions and recycling of sulphate in the sulphidic, sulphate-depleted benzene plume due to microbial oxidation of reduced sulphur species, leading to presumably favored conditions for in situ benzene biodegradation.

Compound Specific and Enantioselective Stable Isotope Analysis as Tools To Monitor Transformation of Hexachlorocyclohexane (HCH) in a Complex Aquifer System.

Technical hexachlorocyclohexane (HCH) mixtures and Lindane (γ-HCH) have been produced in Bitterfeld-Wolfen, Germany, for about 30 years until 1982. In the vicinity of the former dump sites and production facilities, large plumes of HCHs persist within two aquifer systems. We studied the natural attenuation of HCH in these groundwater systems through a combination of enantiomeric and carbon isotope fractionation to characterize the degradation of α-HCH in the areas downstream of a former disposal and production site in Bitterfeld-Wolfen. The concentration and isotope composition of α-HCH from the Quaternary and Tertiary aquifers were analyzed. The carbon isotope compositions were compared to the source signal of waste deposits for the dumpsite and highly contaminated areas. The average value of δ13C at dumpsite was -29.7 ± 0.3 ‰ and -29.0 ± 0.1 ‰ for (-) and (+)α-HCH, respectively, while those for the ß-, γ-, δ-HCH isomers were -29.0 ± 0.3 ‰, -29.5 ± 0.4 ‰, and -28.2 ± 0.2 ‰, respectively. In the plume, the enantiomer fraction shifted up to 0.35, from 0.50 at source area to 0.15 (well T1), and was found accompanied by a carbon isotope enrichment of 5 ‰ and 2.9 ‰ for (-) and (+)α-HCH, respectively. The established model for interpreting isotope and enantiomer fractionation patterns showed potential for analyzing the degradation process at a field site with a complex history with respect to contamination and fluctuating geochemical conditions.

Tracking in situ biodegradation of 1,2-dichloroethenes in a model wetland.

The spatial and temporal biogeochemical development of a model wetland loaded with cis- and trans-1,2-dichloroethene contaminated groundwater was characterized over 430 days by hydrogeochemical and compound-specific isotope analyses (CSIA). The hydrogeochemistry dramatically changed over time from oxic to strongly reducing conditions as emphasized by increasing concentrations of ferrous iron, sulfide, and methane since day 225. delta(13)C values for trans- and cis-DCE substantially changed over the flow path and correlated over time with DCE removal. The carbon enrichment factor values (epsilon) retrieved from the wetland became progressively larger over the investigation period, ranging from -1.7 +/- 0.3% per hundred to -32.6 +/- 2.2% per hundred. This indicated that less fractionating DCE oxidation was progressively replaced by reductive dechlorination, associated with a more pronounced isotopic effect and further confirmed by the detection of vinyl chloride and ethene since day 250. This study demonstrates the linkage between hydrogeochemical variability and intrinsic degradation processes and highlights the potential of CSIA to trace the temporal and spatial changes of the dominant degradation mechanism of DCE in natural or engineered systems.