Stochastic evaluation of Radon zoning
Based on the mean error values and RMSE, the kriging method was selected as the most optimal method. In this study, Radon levels were measured in 145 neighborhoods of the city. Then, Radon zoning was performed by four interpolation methods based on the measured values. As expected, the kriging method was selected as the most optimal method. The population density map of the city, the zoning map by kriging method, and the map of places and streets of the study area were used to evaluate the zoning results on an urban scale. The results are shown in Table 1. The column FRHI in Table 1 denotes the amounts of fuzzy Radon Hazard Index contrasted by predefined colors. As it is derived from FRHI columns for 159 stations, only 9 stations (about 5.6%) are categorized in Very Hazardous Level by Purple color, 89 stations (about 55.9%) are categorized in Non-Hazardous Level by Green color, 45 stations (about 28.3%) are categorized in Less-Hazardous Level by Yellow color, 7 stations (about 4.4%) are categorized in Rather Hazardous Level by Orange color, and nine stations (about 5.6%) are categorized in Hazardous Level by Red color. Fortunately, the majority of stations, 55.9%, receive non-hazardous exhibits; the city has good radon environmental health assessment conditions. For hazardous and very hazardous stations, environmental and health management methods should efficiently be employed in order to improve the radon hazard index. Table 2 also shows the measured quantities of Radon gas in each of them.
The most polluted areas of Radon in the city are region 8 in areas 1, 2, region 12 in area 2, region 12 in area 3, region 6 in area 3, and region 5 in area 3. The cause of pollution regions 8 in areas1 and two are being within the shrine's traditional texture and not observing urban planning principles and poor air conditioning in these areas. Region 12 in area 3 includes the Asian Highway, which is the busiest entrance road to the city from Tehran. Also, in this zone, geological formations naturally emit high amounts of Radon. Region 12in area 2 is located in near Industrial Town, which is likely to have high radon levels due to many industrial units in the area. Region 6 in area 3 is located in the adjacent of power plant, where the Radon released from its activity is the primary possibility for high amounts of Radon and can be the main reason for observing higher Radon levels in this area. The rest of the study area is within the Radon limit. The cleanest areas are region 9 in area 2, region 11 in area 2, region 12 in area 1, region 7 in area 3, region 2 in area 5, area 6, and region 2 in area 6. Also, in area 2 and region11 in area 2, there is the Great Nation Park, which has caused the Radon values in this zone to remain much lower than the permissible limit. Radon values are very high around region 12 in area 1, but in the middle of this area, due to green space (Vakilabad Park), Radon values are within the permissible range. In region 7 in area 3, Tough Forest Park is the main factor in the permissibility of Radon gas. In region 2 in areas 5, 6, and region 6 in area 2, due to very low population density and its geological structure, Radon values are within the allowable range (Fig. 4).
Four interpolation methods, such as areal, IDW, Kriging, and Co-Kriging, were used to model spatial variations (Table 3). Kriging and co-kriging methods are based on the definition of the variogram, and the success of the method depends on choosing the appropriate or optimal variogram model. In this method (IDW), the weights are determined only according to the distance of each known point from the unknown point, without considering the distribution of points around the estimated point. Nearby points are given more weight, and farther points are given less weight. R2, ME, and RMSE indices are used to compare the accuracy of kriging, co-kriging, IDW, and Areal interpolation methods (Warrick et al. 2021).
Table 3
Comparison of interpolation methods for Radon zoning measured at the urban scale
Interpolation Method
|
Average Error (%)
|
RMSE (%)
|
Number of Samples
|
Areal
|
13.28
|
12.70
|
145
|
IDW
|
11.25
|
24.23
|
145
|
CO-Kriging
|
10.49
|
14.40
|
145
|
Kriging
|
5.46
|
17.45
|
145
|
The Results Of The Implementation Of The Method In The Selected Residential Unit
After repairing the cracks, and absorption well was dug in the building area at a distance of 1.5 meters from the sidewall of the foundation, with a depth of 3 meters and a diameter of 30 cm. Used to hold the fan steady. For this purpose, a 20-watt blowing shaft fan with an airflow rate of 100 cubic meters per hour was used. The good chamber was covered with aluminum foil, and a hole with a diameter of 7.5 cm was passed in the center to expel the air. The area around the well was covered with concrete adhesive, and the perimeter of the PVC pipe was covered with glass adhesive to prevent air leakage. Then, using a Hilti device, a hole was dug to a depth of 25 cm below the slab and a diameter of 7.5 cm in the corner of the main corridor of the basement. Then it was connected to the air transfer pipe installed outside the building by an elbow pipe. In the first 30 cm of the buried pipe, holes were made under the slab at intervals of one centimeter to facilitate air passage. The PVC pipe used continued to the roof level, then was connected to the suction system installed in the roof by a flexible plastic pipe, and thus the air containing Radon was transferred out of the building. The suction system includes two 15-watt axial suction fans with an efficiency of 85 and a percentage placed in parallel (Appendix 2).
The measurement time at each harvest point was about ten minutes every twelve hours and up to 48 hours after installing the Radon disposal system. In total, 50 measurements of Radon gas have been taken in this residential unit. The maximum registered amount is equal to 219 Becquerel’s per cubic meter for the basement, and the minimum amount is equal to 86 Becquerel’s per cubic meter for the attic. As in previous sections, the Radon limit is set at 147 Becquerel’s per cubic meter (EPA). Table 4 shows the measured values.
Table 4
Natural Radon measured values in different parts of the residential unit Bq/m3.
Sampling location
|
Floor
|
Initial value
|
12 hours after installation
|
24 hours after installation
|
36 hours after installation
|
48 hours after installation
|
Hallway
|
Basement
|
127
|
119
|
112
|
108
|
101
|
WC No.2
|
Basement
|
219
|
189
|
168
|
139
|
112
|
The main room
|
Basement
|
179
|
167
|
159
|
146
|
132
|
Living room
|
1st floor
|
168
|
153
|
149
|
135
|
121
|
kitchen
|
1st floor
|
156
|
147
|
138
|
127
|
113
|
Attic
|
1st floor
|
86
|
85
|
83
|
82
|
79
|
Bedroom
|
1st floor
|
119
|
113
|
106
|
100
|
97
|
Child room
|
1st floor
|
138
|
124
|
119
|
115
|
110
|
WC
|
1st floor
|
163
|
157
|
138
|
121
|
119
|
Guest room
|
1st floor
|
114
|
110
|
108
|
105
|
98
|
Average
|
Both
|
146.9
|
136.4
|
128
|
117.8
|
108.2
|
In Table 4, natural Radon emission was measured at known points, and its values were assumed to be the initial value. After installing the Radon reduction system, it was measured continuously and every 12 hours. The Radon values were reduced alternately in all measurements, reflecting the positive result of this corrective method.
As expected, the toilets had the highest, and the gables had the lowest recorded value due to their adequate ventilation and the physical nature of the Radon accumulation in the lower floors and heights, as below Eq. (1).
y = 150.84 X (−0.182) (1)
Where y, is the amount of Radon over time X (one unit is half a day or 12 hours).
The cost of implementing this correction method is estimated at 100 USD, which is very small compared to Radon-related therapeutic costs. To further reduce the amount of Radon, the building owner uses a package and a wall-mounted radiator instead of a heating system.
In the bathroom and toilet area, use more robust artificial ventilation and seal the doors and windows in winter, improving natural ventilation. It is better to open all the doors and windows and change the air inside the building in some cases.
The diagrams in Figs. 5a, b and c show that the efficiency of the Radon ventilation system will reach its maximum value after about one week. It is better to use auxiliary sensors to optimize power consumption. For this purpose, a system can be used that starts working from a specific limit if the Radon values rise and stay in standby mode when unnecessary.
It is important to note that the initial fuzzy level for the mean value of FRHI is Hazardous (for FRHI value equal to 60.1) determined by Red color. This is while the Maximum FRHI level for 48 hours after installation is Rather Hazardous (for FRHI value equal to 44.8) determined with orange color. This matter expresses that the maximum statistical environmental health risk after steady-state installation would fall into an improved category exhibiting hazards instead, however, for mean values, this level would fall into less hazardous, which meets all three recommendations of ICRP, USEPA, and WHO.
The interesting point, in this case, is that the owner of the building initially opposed the installation of the system. Still, after learning about Radon gas's very dangerous harms and the relatively high amounts of Radon in his home, he agreed to adopt this system and cooperated in all stages. It can be concluded that most people, even the educated, are not sufficiently aware of Radon gas and its harms, and it is better to cultivate this issue first. When this issue is implemented, the people themselves tend to adopt corrective methods.