3.1. Water Quality Index Analysis (WQI)
The analysis of the river Ćehotina was conducted at the measuring stations Rabitlja, Below Pljevlja, Below firth of Vezišnica and Gradac, while on its left tributary Vezišnica the analysis was conducted at Firth Vezišnica, HS located at the mouth of Ćehotina. At the measuring station Rabitlja, which is located upstream from Pljevlja, outside the urban zone and the impact of TPP ‘‘Pljevlja’’, water quality in the entire observed period ranged from very good, WQI = 86–89 (2012, 2016 and 2017) to excellent, WQI = 90–92 (2011, 2013–2015 and 2018). Downstream from HS Rabitlja, the river Ćehotina flows through the city area of Pljevlja and collects water from its tributaries Vezišnica, so the water quality is deteriorating. Thus, at HS Below Pljevlja, the water quality in the entire observed period had values ranging from 46 to 68 WQI (bad quality). At the mouth of the Vezišnica in Ćehotina (HS Firth Vezišnica), the average annual WQI values changed almost alternately from bad to good quality class: bad quality classes belong to 2013, 2015 and 2017 (WQI = 64–71), and good 2011, 2012, 2014, 2016 and 2018 (WQI = 73–79). On the next HS, below firth of Vezišnica, which is located on the river Ćehotina downstream from the mouth of Vezišnica, almost in the whole observed period the water was of bad quality (WQI = 44–68), and only 2018 belonged to the class of good (WQI = 74).
The previously mentioned 4 HS (Rabitlja, below Pljevlja, firth Vezišnica and below firth of Vezišnica) are located in the urban zone of Pljevlja or in the immediate vicinity of the city, i.e., they are located in the sector of the river Ćehotina for a length of about 8 km. Downstream, there was only one other measuring point in operation, and that was on the profile of Gradac, HS, which is about 15 km away in relation to the previously analyzed (below firth of Vezišnica). During the observed period (2011–2018), HS Gradac recorded variations in water quality. For 2011, good river water quality was obtained, and for 2012, bad (Table 1
). In the period 2013–2015, water quality again belonged to the class of good (WQI = 73–82), and then in 2016 and 2017 it deteriorated and was classified in the class of bad (WQI = 53–71). At the end of the observed period, in 2018, there was a re-improvement of water quality at HS Gradac, because the water of the river Ćehotina was assessed as good quality (WQI = 72).
The obtained results indicated that the state of water quality of the river Ćehotina and its tributaries Vezišnica was alarming, especially in the part of the flow through the urban zone of Pljevlja and downstream from the mouth of Vezišnica. In almost all observed profiles there were year-on-year variations in water quality, which were most likely related to increased/decreased amount of wastewater from settlements, agricultural sources (e.g., livestock and poultry mini farms) and illegal disposal of garbage and other waste both along the stream and in the forest and in the river itself. Year-on-year variations in WQI also occurred in part due to changes in hydrological conditions. In years with an unfavorable hydrological situation (less rainfall and lower runoff than average), river water was of poorer quality (2011 and 2015), in general. On the other hand, the state of water quality in 2018 was slightly better, which can be related to higher water levels, i.e., more favorable meteorological conditions (higher precipitation and higher temperature compared to the average). It should also be mentioned that in recent years, certain measures have been taken to prevent pollution (e.g., the penal policy of illegal waste disposal), and this may be one of the reasons for improving the quality of river water for 2018. Of all the observed HS, the worst quality is the water of the river Ćehotina on the profile of below Pljevlja, and this is the result of an increased amount of municipal wastewater (mostly untreated) from the urban area of Pljevlja. Wastewater from TPP “Pljevlja”, low water levels and human activities along the stream, are the primary causes of water pollution in Vezišnica (a tributary of Ćehotina).
With the aim of analyzing WQI in more detail, the trend was calculated and its significance for the period 2011–2018 was examined. Calculations of the correlation coefficient between WQI and precipitation, i.e., temperature (see Table 1
) were also performed. Nevertheless, the value of the WQI trend is minor and has shown no significant bias in the data used. The lack of significance of the trend is likely related to the short record lengths (only 8 years, 2011–2018). The absolute values of the correlation coefficients ranged from 0.01 to 0.60. We are also sure that the reason for the weak correlation (lack of significance) between WQI and precipitation (temperature) is because the coefficient was calculated for annual values, thus canceling the relationship between these two climate elements and WQI. There is no doubt that this connection would be noticed on a monthly basis (probably also seasonally), but, unfortunately, we only had the annual WQI values.
Observing the average annual WQI values for the whole observed period (calculated as the arithmetic mean of annual WQI from 2011 to 2018), the calculation results showed that the water quality of the river Ćehotina was excellent (WQI = 90) and good (WQI = 73) at one HS, and bad at 3 HS (Figure 3
). Downstream from the city of Pljevlja and the mouth of the river Vezišnica, the water quality of the river Ćehotina was deteriorating, and this was a consequence of the discharge of municipal city water and wastewater from TPP ‘‘Pljevlja’’. We should mention other factors that had a negative impact on the water quality of the river Ćehotina: anthropogenic impact along its course (large amount of various wastes in its bed and along the banks, agricultural activities, wastewater from mini farms—livestock and poultry, etc.) and low water level (especially tributaries of Vezišnica).
Comparing the data for the 10 mentioned physicochemical and microbiological parameters used to obtain WQI, it is noticeable that their values at HS are located in the part of the flow through the city area of Pljevlja and at the mouth of Vezišnica, which is in line with the above. Thus, at HS Below Pljevlja and HS Firth Vezišnica there were higher concentrations, e.g., BOD5
and ammonium ions relative to the other 3 HS, in general. In addition, increased concentrations of BOD5
and ammonium ions were observed on almost all profiles for the years with the lowest WQI. This was also logical, because the value of BOD5
was an indicator of the biological activity of wastewater i.e., the degree of pollution with organic substances. Ammonium ion concentrations are an indicator of pollution from agricultural sources and industrial facilities [51
During hydrologically unfavorable years, such as 2011 and 2015, the highest average annual concentrations of BOD5
and ammonium ions (up to 7.2 mg/L and up to 1.33 mg/L, respectively) were registered at HS Below Pljevlja and HS Firth Vezišnica (see Table A1
in Appendix A
). Therefore, the quality of the river water quality of Ćehotina was worrying, especially in the part of the flow that flows through the urban zone of Pljevlja and downstream from the mouth of Vezišnica. In the south of Montenegro, the Morača river basin, the situation was much better, except in the part of the flow through the capital Podgorica [24
There are two important benefits of using the WQI method. Firstly several variables are included in one number and it gives the possibility to compare water quality of one water body in time and secondly we can compare several water objects in space. The main disadvantages of this methodology are that it does not take into account data on some important parameters, such as inorganic pollution (e.g., heavy metals) and that WQI can be calculated even if not all of the mentioned parameters are available [52
]. One of the parameters which indicates increased pollution of watercourses is the disturbed natural Ca/Mg ion ratio, which is not taken into account when calculating WQI. Therefore, in future research, the WQI method should be used in combination with other methods to assess water quality.
3.2. Analysis of PM10 Concentration of Suspended Particles
In the observed period (2011–2018), the lowest annual mean value of PM10 particles was in 2014 (77.7 µg/m3), and the highest in 2015 (101.5 µg/m3). It is a known fact that the air is cleaner after precipitation (rain, snow). The highest precipitation in Pljevlja (972.9 mm) was in 2014 while the lowest (672.5 mm) was noticed in 2015. This implies that there should have been a significant correlation between these two parameters (PM10 particle concentration and precipitation amount). However, a relatively low and statistically insignificant correlation coefficient (−0.48) was obtained, most likely due to the fact that it was calculated between annual values.
Air quality protection in Montenegro is regulated by new legislation. For the purposes of this paper, the lower thresholds for the concentration of PM10
particles were the values adopted by the State CETRM [47
] based on the recommendations of EU Directives [53
] and WHO [54
], which are 40 µg/m3
on an annual and seasonal level and 50 µg/m3
on a daily basis. CETRM has adopted 35 days per year as a tolerance limit, which means that exceedances above 35 times a year with a mean daily concentration >50 µg/m3
are not desirable (Table 2
It is important to point out that the annual mean concentrations in the entire observed period (2011–2018) were above the prescribed limit value (50 µg/m3
), so it can be concluded that at the annual level the air in Pljevlja is significantly polluted with PM10
particles (Figure 4
, left). When it comes to seasons (Figure 4
, right), the winter (December–January–February) mean concentration of PM10
particles ranged from 73.0 µg/m3
(2013) to 196.5 µg/m3
(2016). It should be noted that the winter of 2013 had an extreme amount of precipitation (274 mm), or 1.6 times higher than the average (170 mm). Regarding spring (March–April–May) and summer (June–July–August), 2012 with the highest mean concentration of PM10
particles (spring = 67.6 µg/m3
, summer = 40.4 µg/m3
) and 2018 with the lowest concentration (spring = 41.1 µg/m3
, summer = 20.7 µg/m3
). In the autumn season, 2018 also had the lowest average concentration of PM10
), while the highest average concentration was registered in 2011 (94.1 µg/m3
Previous results showed that the highest mean concentrations of PM10 particles were recorded during winter and autumn, i.e., in the colder part of the year, and the lowest were recorded in summer. This was to be expected, because during the heating period of houses and flats (in winter, and generally in the colder part of the year) the emission of pollutants (from individual fireboxes) is much higher. Additionally, the meteorological factor (temperature and precipitation) is important. In Pljevlja, there is less precipitation in winter, temperatures are often below 0 °C (average winter temperature is −1.2 °C, the absolute minimum is −29.4 °C, recorded on 26 January 1954), and cold air is heavier, so it settles in the valley, because poor air purification (frequent silences and days with light wind). Though, in summer the amount and frequency of precipitation is higher, the warm air is lighter, so it rises.
The average concentration of PM10 particles of 40 µg/m3 was accepted as a limit value and at the level of seasons (up to 40 µg/m3 allowed (acceptable) concentration, and above 40 µg/m3 dangerous concentration for human health). It was clear that during the winter, autumn and spring, the average concentration of PM10 particles in Pljevlja was above the allowed limit. It could be argued that the situation was alarming in winter, because the average values of PM10 particles were higher than 1.8 to as much as 4.9 times (2016) than allowed (40 µg/m3). In summer, the mean concentration of PM10 particles was within the permitted values. The only exception is 2012, when the summer mean concentration of PM10 particles was 40.4 µg/m3, i.e., slightly above the permitted limit.
The analysis of the number of days with the average concentration of PM10
particles in the air in the urban zone of Pljevlja indicates that the situation was more than worrying. Specifically, if we take the value of 35 times per year as the tolerance limit, it could be argued that the situation was alarming, because every year in the observed period (2011–2018) this threshold was exceeded from 3.7 to 5.4 times. For instance, in 2018 it had 129 days with an average daily concentration of PM10
particles greater than 50 µg/m3
, and in 2011 it had as many as 189 such days (Figure 5
). As already mentioned, the biggest air pollution is in winter, especially in January, which is the coldest month of the year. In the period 2011–2018, daily mean concentrations of PM10
particles in January ranged up to an enormous 793.9 µg/m3
(absolute daily maximum, registered on 10.01.2015). Rarely has a day in January been without a high value. In other words, out of 248 January days for the 8 mentioned years, only 37 times (or 37 days) the daily mean concentration of PM10
particles was below the prescribed limit value of 50 µg/m3
In the previous part, the influence of the meteorological factor (precipitation and temperature) on the air quality was mentioned several times. Therefore, the results of the calculation of the correlation coefficients of PM10
particles with precipitation and temperature are given below, and the trend of PM10
concentration for the period 2011–2010 was also calculated. It should be noted that the obtained results should be accepted with a certain dose of caution, especially when it comes to the trend, because it was too short a period. Only in the winter season did the average concentration of PM10
particles increase (0.45 µg/m3
/year), but the trend was insignificant. In other seasons, as well as on an annual basis, the linear trend was negative (Table 3
Consequently there was a decrease in the concentration of PM10 particles in the air. This was an encouraging fact, before the trend of decreasing PM10 particles meets the conditions of significance at the level of acceptance of the hypothesis of 95% (risk level p < 0.05) during the spring and annually (trend = −2.82 µg/m3/year and trend = −3.88 µg/m3/year), i.e., by 99% (p < 0.01) in the autumn season (trend = −4.56 µg/m3/year). When it comes to correlation, in most cases negative values of the coefficients (C) were obtained, the higher the amount of precipitation (higher temperature) the lower the concentration of PM10 particles, and vice versa. The inverse correlation between PM10 particles and precipitation was significant for the summer season (C = −0.76), while in other cases it was insignificant. A significant relationship was obtained with temperature for spring −0.89) and on an annual basis (C = −0.87).
The WHO has not defined any value of the concentration of PM10
particles as a lower threshold below which the impact of these substances on human health would be completely eliminated. The limit values for the concentration of PM particles mentioned in the WHO document [54
] are given as lower thresholds, i.e., concentrations that can be reached in order to minimize the effects on human health.
The previous analysis showed that during most of the year there was a lot of air pollution in the urban zone of Pljevlja. Three groups of factors had a major impact on the excessive concentration of PM10
particles in this city, especially in the colder part of the year: economic, morphological and meteorological. In addition to the usual economic elements, such as exhaust gases from cars and small and medium enterprises that consume fossil fuels, it can be concluded that the major impact had a huge combustion of coal for the needs of TPP “Pljevlja” and heating of individual households (houses). When it comes to the configuration of the terrain (morphological factor), Pljevlja is located at the bottom of the depression (valley), which is surrounded and quite closed by the sides of high mountains. This form of relief makes natural air ventilation very difficult. The influence of morphological factors was reflected through temperature, precipitation and frequency and wind speed. Pljevlja is the city with the lowest amount of precipitation, the highest cloudiness [55
] and with the highest percentage of silences (days without wind or with wind of negligible speed) in Montenegro. Winters are long and cold, i.e., negative temperatures are frequent. Higher amount and frequency of precipitation purifies the air, and the same goes for higher frequency and wind speed. Cold air is heavier, so in conditions of lack of precipitation and wind, it settles along the bottom of the Pljevlja valley. Consequently temperature inversions were of a common occurrence. Along with the cold air at the bottom of the valley (in the ground layer of the atmosphere), pollutants were also deposited.
In addition to all the above, Pljevlja is the city with the largest number of foggy days in Montenegro (it is not uncommon to have about 200 days a year with low fog). All three groups of the mentioned factors contributed to the increase of the concentration of pollutants in Pljevlja and frequent fogs of smog and smoke in the colder part of the year. The primary sources of emission of suspended PM10
particles in Pljevlja were those related to coal exploitation and fossil fuel combustion (TPP “Pljevlja”, individual combustion plants, motor vehicles and dust emitted from the surrounding coal mines). In the MORTA document [56
], agriculture, i.e., synthetic N-fertilizers, was mentioned among the main sources of PM10