3.1 Microclimatic Condition in the Mining Complex
To better understand the microclimate of the Singrauli mining complex, meteorological characteristics such as minimum and maximum temperatures, wind speed, relative humidity, solar radiation, wind direction, and rainy days are analyzed for each mine (Table 5). Meteorological data collected from stations within each mine show that the yearly average rainfall for the mine complex is 0.0091 mm/day, with approximately 38 rainy days. The mining complex has an annual minimum temperature of 7.17 ºC and a maximum temperature of 45.08 ºC, with a significant variance. The low annual mean precipitation and high-temperature range indicate that the region has a hot and arid climate, primarily during the monsoon season (Li et al. 2010; Garg et al. 2016; Raja & Reddy 2019). The area experiences long periods of drought and water scarcity, which can have severe consequences for the local ecosystem, agriculture, and water resources, implying that the region is prone to desertification, which can exacerbate the arid conditions (Li et al. 2010; Chowdari et al. 2015).
Table 5
Meteorological Characteristics of the Studied Area
Sl. No | Meteorological Parameters | Amlohri | Blockb | Dudhichua | Jayant | Jhingurda | Nigahi | Bina | Kakri | Khadia |
1 | Minimum temperature (oC) | 6.95 (Jan) | 7.34 (Jan) | 6.73 (Jan) | 8.02 (Jan) | 3.94 (Feb) | 7.78 (Jan) | 8.73 (Jan) | 7.15 (Jan) | 7.87 (Jan) |
2 | Maximum temperature (oC) | 44.72 (May) | 45.91 (May) | 44.16 (May) | 45.52 (May) | 44.63 (May) | 45.63 (May) | 44.36 (May) | 45.8 (May) | 44.99 (May) |
3 | Minimum Wind Speed (m/s) | 0.01 (Oct) | 0.01 (Oct) | 0.01 (July) | 0.01 (Aug) | 0.01 (Aug) | 0.01 (Oct) | 0.01 (Mar) | 0.02 (Jan) | 0.01 (Mar) |
4 | Maximum Wind Speed (m/s) | 6.68 (April) | 0.9 (Jan) | 3.51 (Nov) | 2.92 (May) | 2.14 (Aug) | 3.48 (May) | 3.60 (May) | 2.16 (Mar) | 5.19 (Jan) |
5 | Wind Direction (Degree) | 0-360 | 0-360 | 0-360 | 0-360 | 0-360 | 0-360 | 0-360 | 0-360 | 0-360 |
6 | Minimum Relative Humidity (%) | 14.67% (May) | 10.4% (May) | 10.03% (May) | 4.85% (May) | 4.6% (April) | 10.67% (May) | 12.84 (May) | 13.24 (May) | 8.11 (May) |
7 | Maximum Relative Humidity (%) | 100% (June) | 100% (July) | 100% (Sept) | 100% (Sept) | 100% (July) | 100% (Oct) | 100% (June) | 100% (June) | 97.21% (Sept) |
8 | Number Of Rainy Days | 37 | 42 | 41 | 36 | 31 | 31 | 33 | 48 | 39 |
9 | Minimum Global Horizontal Radiation (W/m2) | 11.25 (July) | 19.14 (June) | 15.07 (April) | 17 (June) | 3.67 (Jan) | 18.1 (June) | 18.5 (Mar) | 7.69 (June) | 2.57 (Dec) |
10 | Maximum Global Horizontal Radiation (W/m2) | 971 (May) | 357 (June) | 435.53 (Sept) | 387.6 (Sept) | 499.87 (March) | 502.26 (May) | 502.4 (May) | 404.76 (June) | 878.08 (Aug) |
The monsoon season has the highest relative humidity and the lowest global horizontal radiation. The mining complex's yearly mean relative humidity is around 69.23%, and the minimum and maximum global horizontal radiation are 12.55 W/m2 and 548.72 W/m2, respectively. As a result, the inference of a hot and arid environment with high moisture content in the mining complex's air is strengthened. The maximum and minimum wind speeds are 3.40 m/s and 0.01 m/s, respectively. The wind direction is predominantly northwest (Fig. 4). The region's high atmospheric moisture can be ascribed to wind movement from the surrounding largest artificial lake, Govind Ballabh Pant Sagar (GBPS) (Beersma & Buishand 2003; Panda & Sahu 2019; Lone et al. 2022). The GBPS is positioned in the southeast, which explains the major wind movement in the northwest direction caused by the lake breeze from the water body to the arid mine complex.
Jhingurda mine has the lowest temperature (~ 4ºC) in February, while Block B has the highest temperature (~ 46ºC) in May. Amlohri experiences the highest wind speed (6.68 m/s in April), highest global horizontal radiation (971 W/m2 in May), and lowest relative humidity (14.67% in May). In contrast, Khadia has the lowest global horizontal radiation (2.57 W/m2 in December) and the rainiest days (48). The Jhingurda and Block B mines are located at higher elevations and farther away from the water body, resulting in extreme temperatures caused by external forces. On the contrary, since Khadia and Kakri are located near water bodies, aerosols (including cloud condensation nuclei) are projected to be highest above these mines, enhancing cloud cover and decreasing global horizontal radiation (Shrestha 2019). Amlohri is located in the southwest portion of the mine complex. The wind direction is the least prominent, while the wind speed is the highest. As a result, moisture-laden air does not enter the mine, reducing relative humidity, aerosols, and cloud cover. At the same time, the clear sky permits the location to receive the highest global horizontal radiation (Shrestha 2019).
Wind speed and direction are the main influences on complex microclimatic conditions in mining. As a result, the mining complex is separated into four quadrants for further examination of these variables. Jhingurda and Kakri, Block B and Dudhichua, Jayant, Nigahi, Amlohri, and Bina and Khadia mines are located in quadrants I, II, III, and IV, respectively. Quadrants I and IV are located near the lake, with Quadrant I's mines at higher elevations. As a result, the breeze from the lake is primarily directed towards the dry land in quadrant IV. However, the land breeze towards the lake can also be seen blowing down the slope. Quadrant IV is located at depths, and the wind generally moves south and southwest after returning from higher elevations from the north. During chilly weather, the land breeze flows in an easterly direction. Quadrat II is the farthest part from the lake and is located at a substantial depth. As a result, the wind moves primarily in the north, northwest, and western directions. The coal mines in quadrat III are located at higher elevations and steadily increase towards the south. As a result, the lake breeze movement is primarily northwest, whereas the land breeze flow is eastward. As a result, the wind's overall direction of flow is northwest.
3.2 Particulate Pollution Status of the Mining Complex
The annual PM10 and PM2.5 concentrations at the Singrauli mining complex are depicted in Fig. 5. The mining complex's PM10 concentration varies from 14.18 µg/m3 in August to 204.58 µg/m3 in March, with an annual average of 91.18 µg/m3. The average annual PM10 concentration in Amlohri, Dudhichua, Jhingurda, and Nigahi exceeds the Central Pollution Control Board's (CPCB) permissible limits of 100 µg/m3, while all mines, except Bina, exceed the WHO standard limits of 45 µg/m3 (CPCB 2010; World Health Organisation 2021). PM2.5 concentrations at the mining complex vary from 5.57 µg/m3 in July to 67.67 µg/m3 in February, with an annual average of 27.89 µg/m3. All mines, except Dudhichua, have PM2.5 concentrations below the CPCB's permitted limit of 60 µg/m3 (CPCB 2010). However, PM2.5 concentrations in all sites surpass the WHO recommended limits of 15 µg/m3 (World Health Organisation, 2021). Amlohri has the most significant annual mean PM10 concentration (152.75 µg/m3), whereas Dudhichua has the highest annual mean PM2.5 concentration (49.64 µg/m3) across all sites.
Amlohri and Dudhichua are in quadrats III and II, respectively. Both places are on the far side of the lake. Because PM10 particles are heavier than PM2.5 particles, the breeze from the lake moving south and southwest is more likely to carry and deposit them near the Amlohri mining site. However, the lighter PM2.5 particles are expected to move farther than the PM10 particles in the north and northwest and deposited at lower elevations in Dudhichua. The higher altitudes in the Block B region cause the wind to deposit PMs in the Dudhichua region before further moving to the northwest (Seastedt et al. 2004).
The seasonal variation in PM10 and PM2.5 concentrations (Fig. 5) demonstrates that PM concentrations in the region are very high during dry months, summer, and winter, confirming transboundary movement of atmospheric particulates under the influence of sea breeze and land breeze conditions (Varaprasad et al. 2024). Summer PM10 concentrations can reach 210 µg/m3, while winter PM2.5 concentrations peak at 80 µg/m3. The monsoon season has the lowest PM concentration due to the wet deposition process, while the concentration gradually increases after the monsoon season (Wu et al., 2018). The post-monsoon PM2.5/PM10 concentration ratio is very high. The high moisture content and low gust limit the smooth transport of larger air particulates (Jones & Harrison 2004). PM10 particulates tend to agglomerate and create larger particles, which settle down or act as cloud condensation nuclei, forming aerosols (Wu et al. 2018). As a result, the atmospheric particulates transported by the air during the post-monsoon season are typically PM2.5 particles.
3.3 AERMOD Dispersion Modeling
AERMOD simulates the effects of ground-level and elevated industrial sources on flat or rather complex terrain (ul Haq et al. 2019). AERMOD has been utilized in Indian geo-mining circumstances to assess PM concentrations in opencast mines and estimate dust concentrations at receptor sites (Kundu & Pal 2018; Sahu et al. 2018; Srivastava et al. 2021; Srivastava & Elumalai 2021). AERMOD predicts wind-driven PM dispersion by combining air dispersion based on planetary boundary layer turbulence structure and scaling ideas, including consideration of surface and elevated sources and simple and complicated topography (ul Haq et al. 2019). However, AERMOD has been shown to overestimate fugitive dust concentrations when applied, particularly in low wind speed conditions, and to underestimate in non-Gaussian vertical dispersion conditions (Huertas et al. 2012; Kundu & Pal 2018; Sahu et al. 2018; Srivastava et al. 2021; Srivastava & Elumalai 2021). The height of mine benches influences PM dispersion at higher depths in opencast mines. Particulate matter formed on a mine's lower benches moves up to the higher benches before exiting the mine (Gautam & Patra 2015). AERMOD describes non-Gaussian vertical dispersion under convective conditions, distinguished by updraft and downdraft motions with varying probabilities (Snoun et al. 2023).
The current AERMOD dispersion model forecasts yearly and seasonal PM10 and PM2.5 dispersion from all Singrauli mining sources (Figs. 6 and 7). The grid size is 100m × 100m, with nine distinct receptors at each CAAQMS station. Exactly 129 sources of PM were found. Jayant mine has the highest yearly AERMOD predicted PM10 concentration of 1461 µg/m3, whereas Jhingurda mine has the lowest value (< 300 µg/m3). Amlohri, Nigahi, and Jayant mining sites have the most significant annual PM10 concentration (> 1400 µg/m3), followed by Khadia, Block-B, and Kakri mines (~ 1000 µg/m3).
In mining locations, the winter season has the highest AERMOD predicted PM10 levels (> 1200 µg/m3). The cold weather conditions cause air inversion, which reduces PM10 dispersion. In contrast, PM10 concentrations around mining projects are less than 800 µg/m3 throughout the summer. The concentration of PM10 over residential sites is also much lower in the summer than in the winter, demonstrating that particulates are widely dispersed. The monsoon season has the lowest PM10 concentrations in the region, with levels below 600 µg/m3 for mining sites and 400 µg/m3 at residential locations. Wet deposition through rainfall appears to reduce PM10 concentrations in the atmosphere considerably. When the rain stops, the concentration of PM10 rises rapidly during the post-monsoon season. PM10 concentrations can reach over 1000 µg/m3 over mining sites and 400–800 µg/m3 over residential areas.
In contrast, the annual PM2.5 contamination is evenly spread throughout all mines except Jhingurda. Khadia mine has the highest yearly PM2.5 concentration (863 µg/m3), whereas Jhingurda mine has the lowest annual PM2.5 pollution (< 60 µg/m3). The Dudhichua project appears to have considerably impacted the region's PM2.5 concentration. However, the receptors at the project's residential site are oblivious to the contamination. PM2.5 accumulates extensively above mining projects throughout the winter season and dissipates away during the summer season, similar to PM10. During winter, PM2.5 levels over the Khadia project can exceed 800 µg/m3. In winter, residential areas near the Khadia mine experience high levels of PM2.5 contamination, reaching up to 500 µg/m3. Other mining sites have high PM2.5 concentrations ranging from 100 to 400 µg/m3. The decreased concentration of PM2.5 during the summer months also reflects the pollutant's dispersion in warm air conditions.
During the monsoon season, PM2.5 concentrations decrease significantly in all locations, including mining sites, where they can dip below 100 µg/m3. However, the post-monsoon season displays a significant increase in PM2.5 concentration. PM2.5 pollution levels in Khadia, Dudhichua, and Jayant projects range from 300 to 600 µg/m3, whereas Amlohri and Nigahi projects range from 200 to 300 µg/m3.
The mining projects are primarily located in quadrants III and IV, and the PMs typically travel south to them. The sloping topography south of the mining projects likely aids in the downhill migration of particles. The predominant wind movement in the northwest direction causes the flow of PM dispersion; however, the air appears to dispose of the PMs over the mining projects before going further northwest and carries only a tiny fraction of the PMs. As a result, PMs moved from the residential areas south to the mining projects. The PM, concentration over GBPS, is similarly notably high, demonstrating dust deposition over the water body caused by the land and lake breeze phenomenon.
The wind's south and south-western flow in quadrant IV prevents pollution from spreading over the Jhingurda and Dudhichua project residential sites, making these areas among the least contaminated in the research area. PM2.5 particles can travel farther northwest than PM10. In quadrant IV, wind movement in the east, south, and southwest directions disperses PM2.5 over the GBPS and residential areas south of the mining complex. Wind movement in quadrant III's northwest direction and north, west, and northwest directions in quadrant II spreads PM2.5 particles in these directions. The wind blows eastward in quadrants III and IV, sweeping the PMs over the GBPS on the southeast side of the mining complex.
The high PM levels during the winter and post-monsoon season demonstrate that the land breeze is the more effective phenomenon in the region, resulting in widespread dispersion of particulates in the southeast direction of the mining complex during cold weather conditions. The dispersion of PMs over the GBPS is seen in cold weather. As a result, there is an urgent need to perform suspended and dissolved particulate analysis on the GBPS lake. Airborne particles are also known to transport hazardous metals (Mishra et al. 2024). As a result, the water body is likely to have high concentrations of these harmful elements. Soler et al. (2011), Mavrakou et al. (2012), and Reddy et al. (2023) have also demonstrated that the land breeze-sea breeze phenomena contribute significantly to air particulate dispersion over a region.
3.4 Statistical validation
The AERMOD dispersion models for PM10 and PM2.5 were validated using statistical methods. Model bias (MB), normalized mean square error (NMSE), and fractional bias (FB). Table 6 shows the values of the statistical validation techniques for PM10 and PM2.5. The index of agreement (d) can detect additive and proportional differences in observed and simulated means and variances (Duveiller et al. 2016). In contrast, the Coefficient of determination (R2) measures a model's goodness of fit and provides information about how well the data fits the model (Chicco et al. 2021). Model bias (MB) is the systematic departure of model predictions from actual values, indicating overestimation or underestimation, and reflects the degree of prediction error (Ato García et al. 2008; Duveiller et al. 2016; Valbuena et al. 2019). Fractional bias (FB) refers to the model's tendency to overestimate or underestimate observed values (Webster & Thomson 2022). The normalized mean square error (NMSE) is a model's accuracy as a function of data variation (Moradi et al. 2019). These statistical techniques assist in determining the model's performance and correctness.
Table 6
Statistical validation performance computed for the AERMOD model
Duration | d | MB | FB | NMSE | R2 |
PM10 |
Winter | 0.84 | 207.90 | -0.95 | 3.36 | 0.46 |
Summer | 0.94 | 136.38 | -0.70 | 2.98 | 0.25 |
Monsoon | 0.89 | 115.16 | -1.08 | 4.19 | 0.59 |
Post-Monsoon | 0.98 | 19.26 | -0.17 | 2.01 | 0.45 |
Year | 0.58 | 131.11 | -0.77 | 2.90 | 0.37 |
PM2.5 |
Winter | 0.99 | 19.72 | -0.34 | 0.59 | 0.70 |
Summer | 0.98 | 18.53 | -0.40 | 1.21 | 0.50 |
Monsoon | 0.98 | 17.57 | -0.57 | 1.46 | 0.65 |
Post-Monsoon | 0.99 | 2.67 | -0.07 | 1.52 | 0.46 |
Year | 0.81 | 17.21 | -0.38 | 1.04 | 0.56 |
The d (0.58) and R2 (0.37) values of the yearly PM10 model are shallow, while the MB (131.11) is significantly high. The FB value is -0.77, and the NMSE is 2.90. As a result, the AERMOD model has low prediction accuracy and a substantial negative error when estimating annual dispersion. As a result, the annual model severely underestimates the area's PM10 dispersion. In contrast, the yearly PM2.5 models show a low R2 value (0.56) and a high d (0.81). The MB (17.21) and NMSE (1.04) are relatively low. However, FB is less than the permitted value (-0.38). Consequently, the AERMOD model offers moderate forecast accuracy for annual PM2.5 dispersion. The AERMOD model underestimates the yearly PM2.5 concentration with a significant margin of error for the entire mining complex.
The seasonal dispersion models of PM10 and PM2.5 show very high d values, ranging from 0.84 in winter to 0.98 in post-monsoon and 0.98 in summer and monsoon to 0.99 in winter and post-monsoon. However, the R2 values of the PM10 dispersion models vary from 0.25 in the summer to 0.59 during the monsoon. The models' NMSEs are also greater than 2.00, and except for the post-monsoon season, the PM10 dispersion models show extremely high MB and FB values throughout the year. As a result, except for the post-monsoon period, the AERMOD model has low accuracy in estimating the seasonal dispersion of PM10. The error variance is substantial throughout the year, and the models consistently underestimate PM10 concentrations.
On the contrary, the AERMOD seasonal model's MB and NMSE values for PM2.5 are significantly low, ranging from 2.67 in post-monsoon to 19.72 in summer and 0.59 in winter to 1.52 in post-monsoon, respectively. The dispersion model for winter has an R2 value of 0.70. However, R2 values in other seasons are much lower (0.46–0.65). In contrast, the MB for post-monsoon is relatively low (2.67), and for other seasons is likewise significantly low (< 20.00). The FB for post-monsoon is extremely low (-0.07), well within the optimal range. However, the FB during other seasons ranges from − 0.34 in winter to -0.57 in monsoon, which is outside the optimal range. Hence, the PM2.5 dispersion modeling is relatively accurate during the winter and post-monsoon seasons. The post-monsoon error frequency is relatively low, while the error variance is lowest during winter. The dispersion model is more reliable during the winter than during the other seasons. As a result, PM2.5 models outperform PM10 dispersion models, while AERMOD dispersion models outperform thresholds set by Teggi et al. (2018) and Borrego et al. (2016).
The AERMOD dispersion models of PMs from Jharia coalfields and Kulda surface mine in India, Sungun copper mine in Iran, and opencast coal mines in Colombia have also shown similar d, R2, FB, and NMSE values (Huertas et al. 2012; Sahu et al. 2018; Kundu & Pal 2018; Srivastava et al. 2021; Srivastava & Elumalai 2021; Khazini et al. 2021). The d, R2, FB, and NMSE values of the AERMOD models over Indian mining settings are 0.34 to 0.93, 0.37 to 0.94, -0.08 to 0.63, and 0.07 to 0.67, respectively. The results are more accurate with a more minor modeling source and scope. Compared to AERMOD models outside of Indian mining circumstances, the PM dispersion models appear slightly better (Huertas et al., 2012). Outside of mining contexts, PM dispersion models demonstrate even higher forecast accuracy (Peter & Nagendra 2021).
The AERMOD predicted PMs were tested against data from the area's CAAQMS data, as shown in Fig. 8. The predicted vs. observed graph allows us to evaluate model performance across an unknown dataset in real-time (Sergeev et al. 2024). The R2 value from the plot aids in measuring prediction accuracy and precision. The R2 for the observed vs expected graphs of PM10 and PM2.5 is 37.63% and 56.36%, respectively. As a result, the PM10 model fails to predict PM10 concentrations, while the PM2.5 model has poor prediction accuracy for the overall mine complex.