In-situ radiometric assessment of Ruru region in Gulmi,Nepal

Abstract

In this study, we investigated the distribution of natural radiation and dose rates in the Ruru and Satyawati 1 regions of Gulmi district along the Kaligandaki River in Nepal, known for its abundance of Shaligram (fossil stones). The assessment of radioactivity levels in this region is crucial for evaluating potential radiological hazards to human health. Utilising the portable gamma-ray spectrometer PGIS-2 with a NaI(Tl) detector, we calculated various radiological hazard parameters associated with radionuclides, including radium equivalent activity, absorbed gamma dose rates in air, annual effective dose rate, external hazard index, and internal hazard index. The comparison was made among three wards: Ruru 1, Ruru 2, and Satyawati 1. The average absorbed dose rates in air for Ruru 1, Ruru 2, and Satyawati were 103.9, 104.2, and 118.2 nGy h⁻¹, respectively, all surpassing the world average value of 59 nGy h⁻¹. External hazard index values for Ruru 1, Ruru 2, and Satyawati were 0.60, 0.59, and 0.68, respectively, within the world-recommended safety limit. Importantly, no significant health issues were identified in the studied region, emphasising the importance of ongoing monitoring and assessment of natural radiation levels for ensuring community well-being.

1. INTRODUCTION

The natural presence of ²³⁸U, ²³²Th, and ⁴⁰K in the Earth's crust constitutes a significant source of natural radiation, with both beneficial and detrimental effects on human health. Despite its importance, only a limited number of studies have explored natural radioactivity and its associated hazards within Nepal, focusing on soil, rock, and the environment, utilising sodium iodide detectors, gamma-ray spectrometry, and in-situ gamma-ray spectrometers (Wallova et al., 2010; Mishra and Khanal, 2023a, 2023b; Lamichhane et al., 2021). This study employs in-situ gamma-ray spectroscopy to assess the radioactivity levels in the Ruru and Satyawati regions of the Gulmi district in Nepal, along the Kaligandaki River. Notably, this area is known for its rich distribution of Shaligram, a sacred fossilised shell often identified as an ammonite, found predominantly in the Kaligandaki River of Nepal. Understanding natural radioactivity is essential for unravelling the geophysical and geochemical processes that have shaped the Earth's evolution.

2. METHODOLOGY

2.1. The study area

The Ruru region is situated along the Kaligandaki River, part of the world's deepest gorge. Our study starts from the trijunction of Gulmi, Palpa, and Syangja districts. The study area lies in the latitudes of 27°56′4″ to 27°58′58″ N and longitudes between 83°26′13″ to 83°30′55″ E (cf. Fig. 1). The Rishikesh Complex of the Ruru region is in UNESCO World Heritage tentative list, and the precious fossil stone Shaligram is found in this region.

Fig. 1.

Maps showing the location of the study area within Nepal.

2.2. Instrumentation

The portable gamma-ray spectrometer PGIS-2, a NaI(Tl) detector, used for the study provides a cost-effective and efficient method of in-situ measurements for quantitative determination of gamma-ray emitting radioisotopes (Caciolli et al., 2012). This system, featuring auto calibration through natural photo peaks, comprises a detector unit integrated with GPS and a data logger unit. The detectors use a thallium-activated sodium iodide (NaI(Tl)) crystal with a volume of 0.347 L capable of, detecting energies from 30 keV to 3 MeV. Data collection involved walking through the area while carrying the detector at speeds below 3 km h⁻¹. Live data could be viewed and stored on an Android device connected to the detector via Bluetooth. In contrast to the laboratory experiments, high-resolution in-situ gamma-ray spectrometry provides more realistic information on environmental radioactivity (Malczewski, 2004; Streffer, 2007; ICRP, 2012).

2.3. Estimation of radiological parameters

The radiological indices in this study were derived from the collected data, incorporating key parameters such as radium equivalent activity, internal hazard index, external hazard index, annual effective dose rate (AEDR), and excess lifetime cancer risk. These parameters were determined using established equations reported by UNSCEAR (UNSCEAR, 2000).

The radium equivalent activity is given by:

$$R{a}_{\mathrm{eq}}=A_{\mathrm{U}}+1.43A_{\mathrm{Th}}+0.077A_{\mathrm{K}}$$

(1)

The total air absorbed dose rate, ADR (nGy h⁻¹), attributable to the activity concentrations is given by:

$$\mathrm{ADR}=0.462A_{\mathrm{U}}+0.604A_{\mathrm{Th}}+0.042A_{\mathrm{K}}$$

(2)

The outdoor AEDR in mSv yr⁻¹ is calculated as:

$$\mathrm{AEDR}=\mathrm{ADR}(\mathrm{nGy}{\mathrm{h}}^{-1})\times 8760(\mathrm{h})\times 0.2\times 0.7(\mathrm{Sv}\mathrm{G}{\mathrm{y}}^{-1})\times {10}^{-6}$$

(3)

Here 0.2 is used considering outdoor occupancy of individual as 20%, and 0.7 is the conversion coefficient from absorbed dose in air to effective dose received by an individual.

The external hazard index (H_ex) is employed to restrict the external gamma-radiation dose from materials and is determined by:

$$H_{\mathrm{ex}}={\displaystyle \frac{A_{\mathrm{U}}}{370}+{\displaystyle \frac{A_{\mathrm{Th}}}{259}+{\displaystyle \frac{A_{\mathrm{K}}}{4810}}}}$$

(4)

Similarly, the internal hazard index (H_in) is utilised to limit the internal gamma-radiation dose from materials, calculated as:

$$H_{\mathrm{in}}={\displaystyle \frac{A_{\mathrm{U}}}{185}+{\displaystyle \frac{A_{\mathrm{Th}}}{259}+{\displaystyle \frac{A_{\mathrm{K}}}{4810}}}}$$

(5)

Excess lifetime cancer risk is determined using:

$$\mathrm{ELCR}=\mathrm{AEDR}\times \mathrm{LE}\times \mathrm{RF}$$

(6)

where LE represents life expectancy and RF represents the fatal risk factor. According to the ICRP recommendation, RF is 0.05 Sv⁻¹ for the public (ICRP, 1991).

3. RESULTS

The distributions of ⁴⁰K, ²³²Th, and ²³⁸U in the Ruru 1, Ruru 2, and Satyawati 1 are illustrated in Figs. 2, 3, and 4, respectively. The distribution of ⁴⁰K in Ruru 1 displays a nearly symmetric pattern, whereas the distributions of ²³²Th and ²³⁸U are right-skewed. In Ruru 2, the distribution of ⁴⁰K and ²³⁸U is right-skewed, whereas the distribution of ²³²Th is nearly symmetric. For Satyawati 1, all distributions of ⁴⁰K, ²³²Th, and ²³⁸U are right-skewed.

Fig. 2.

Activity concentration ⁴⁰K, ²³²Th, and ²³⁸U for Ruru 1.

Fig. 3.

Activity concentration ⁴⁰K, ²³²Th, and ²³⁸U for Ruru 2.

Fig. 4.

Activity concentration of ⁴⁰K, ²³²Th, and ²³⁸U for Satyawati 1.

Comparing the average dose rates, Ruru 1 and Ruru 2 exhibit similar values, measuring approximately 103.93 and 104.19 nGy h⁻¹, respectively (Fig. 5). However, Satyawati 1 shows a significantly higher average dose rate at 118.25 nGy h⁻¹, doubling the world average value.

Fig. 5.

Absorbed dose rate for Ruru 1, Ruru 2, and Satyawati 1.

The hazard indices in the region of Ruru 1, Ruru 2, and Satyawati 1 are tabulated in Tables 1, 2, and 3, respectively.

Table 1.

Hazard indices in the region Ruru 1.

Hazard index	Mean value	SD	Minimum value	Maximum value
$A_{\mathrm{K}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	817.07	237.06	226.92	1819.78
$A_{\mathrm{U}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	72.57	34.09	5.80	260.39
$A_{\mathrm{Th}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	59.75	23.26	8.06	174.74
$R{a}_{\mathrm{eq}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	220.93	50.32	91.85	494.77
$H_{\mathrm{ex}}$	0.60	0.14	0.25	1.34
$H_{\mathrm{in}}$	0.79	0.20	0.30	2.02
$\mathrm{ADR}(\mathrm{nGy}{\mathrm{h}}^{-1})$	103.93	23.23	44.68	230.35
$\mathrm{AED}{\mathrm{R}}_{\mathrm{out}}(\mathrm{mSv}\mathrm{y}{\mathrm{r}}^{-1})$	0.18	0.03	0.05	0.28
$\mathrm{ELCR}(\times {10}^{-3})$	0.46	0.10	0.20	1.01

Table 2.

Hazard indices in the region Ruru 2.

Hazard index	Mean value	SD	Minimum value	Maximum value
$A_{\mathrm{K}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	969.83	326.73	211.58	2264.87
$A_{\mathrm{U}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	67.19	31.99	6.32	232.89
$A_{\mathrm{Th}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	53.67	22.01	3.61	154.10
$R{a}_{\mathrm{eq}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	218.62	54.44	85.05	436.54
$H_{\mathrm{ex}}$	0.59	0.15	0.23	1.18
$H_{\mathrm{in}}$	0.77	0.20	0.30	1.76
$\mathrm{ADR}(\mathrm{nGy}{\mathrm{h}}^{-1})$	104.19	25.86	41.58	205.37
$\mathrm{AED}{\mathrm{R}}_{\mathrm{out}}(\mathrm{mSv}\mathrm{y}{\mathrm{r}}^{-1})$	0.13	0.03	0.05	0.25
$\mathrm{ELCR}(\times {10}^{-3})$	0.46	0.11	0.18	0.90

Table 3.

Hazard indices in the region Satyawati 1.

Hazard index	Mean value	SD	Minimum value	Maximum value
$A_{\mathrm{K}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	752.66	372.50	24.41	2549.07
$A_{\mathrm{U}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$	108.44	94.00	3.22	823.14
$A_{\mathrm{Th}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$ )	60.49	27.06	1.05	193.97
( $R{a}_{\mathrm{eq}}(\mathrm{Bq}\mathrm{k}{\mathrm{g}}^{-1})$ )	252.89	111.10	46.37	984.81
$H_{\mathrm{ex}}$	0.68	0.30	0.12	2.66
$H_{\mathrm{in}}$	0.97	0.53	0.17	4.79
$\mathrm{ADR}(\mathrm{nGy}{\mathrm{h}}^{-1})$	118.25	50.93	22.06	450.15
$\mathrm{AED}{\mathrm{R}}_{\mathrm{out}}(\mathrm{mSv}\mathrm{y}{\mathrm{r}}^{-1})$	0.14	0.06	0.03	0.55
$\mathrm{ELCR}(\times {10}^{-3})$	0.51	0.22	0.09	1.97

Mean activity concentrations for ²³⁸U, ²³²Th, and ⁴⁰K in Ruru 1 were 72.6, 59.8, and 817.1 Bq kg⁻¹; in Ruru 2, 67.2, 53.7, and 969.8 Bq kg⁻¹; and in Satyawati, 108.4, 60.5, and 752.7 Bq kg⁻¹, respectively.

Examining mean activity concentrations, ⁴⁰K is most concentrated in Ruru 2, ²³²Th in Satyawati 1, and ²³⁸U in Satyawati 1, with the latter measuring 3.3 times higher than the world average value. These findings highlight notable variations in radioactivity levels across the studied regions, emphasising the importance of understanding and monitoring natural radiation for assessing potential risks to human health.

4. CONCLUSION

The assessment of external hazard indices for Ruru 1, Ruru 2, and Satyawati 1 reveals that all average values adhere to the world-recommended safety limits. Furthermore, the radium equivalent activity for all three wards remains within the reference range of 370 Bq kg ⁻¹. It is crucial to emphasise that, notably, no significant health issues were identified in the studied region. These findings collectively underscore the satisfactory radiological safety profile of the examined areas, providing reassurance regarding potential external gamma-radiation doses and contributing to the overall understanding of natural radioactivity in the region. The adherence to established safety limits and the absence of identified health issues contribute positively to the comprehensive assessment of radiological hazards in the studied locales.