3.1. A Decade of Rainfall Intensity and Pattern
The study area has a tropical climate. The annual rainfall in Paringin pit lake for 2022 was 3,228.8 mm/year, higher than the average annual rainfall from 2011 to 2022, which was 2,646.7 mm/year (Fig. 5.). These findings confirm the observed rainfall patterns in Banjar Regency, South Kalimantan [50]. Over the last 11 years, there has been an increasing trend in rainfall in Paringin, South Kalimantan, with an annual increase of 68,56 mm. It was consistent with the previous findings of an annual increase of rainfall by 25 mm in different locations within South Kalimantan Province [51]. Peak rainfall occurred in November 2022, which is also consistent with the average from 2011 to 2022, showing a difference from the findings of Arini et al., 2015, which stated the peak rainfall in December to February. However, the dry season in 2022 and the average from 2011 to 2022 were in line with the findings of Arini et al., 2015, occurring in June to August. The December 2022 rainfall of 155.7 mm/month was far below the average December rainfall from 2011 to 2022, which was 294.6 mm/month (Fig. 4.). This finding confirms the changing rainfall intensity and patterns of rainy and dry seasons due to global climate change [52, 53] and locally in South Kalimantan [51]. Furthermore, these changes will have an impact on the availability of freshwater resources [54, 55].
3.2. Rainfall vs Evaporation 2022
In 2022, there were 297 rainy days and 68 days without rain. Meanwhile, evaporation is a natural factor that causes the loss of surface water. The total annual evaporation rate was 1054.1 mm/year. When compared to the total rainfall in 2022, which was 3,228.8 mm/year, the 2022 evaporation rate of 1054.1 mm/year accounted for 32% of the annual rainfall. This evaporation rate represents an external factor causing water loss from Paringin pit lake. This finding confirms the results of previous research on water losses due to evaporation, especially in small water reservoirs, and highlights the need for continuous monitoring to develop water management practices [56–59]. However, this research has not calculated and evaluated the amount of evapotranspiration within the Paringin Lake catchment area. Based on research by Chen, Liu [60], evapotranspiration plays an important role in ecosystem water consumption.
Figure 6. shows that the amount of evaporation rate fluctuates throughout the year, with a trend of increased evaporation rate during the dry season, especially from August to October 2022. The highest daily evaporation rate in 2022 occurred on 13th October 2022 at 7.05 mm/day, while the lowest rate occurred in February 2023 at 0.32 mm/day. Evaporation rate correlates with surface air temperature and wind speed [61].
The complete monthly rainfall and evaporation data of Paringin pit lake is presented in Table 1. This data represents the first annual data taken directly from the coal mine pit lake using an Automatic Weather Station in South Kalimantan. The annual water loss of Paringin pit lake from evaporation was 184,677 m3/year, which accounts for 6% of the total effective volume of the pit lake, amounting to 2,095,084.54 m3. This value is slightly higher than what has been reported by Tuheteru, Gautama [56] in East Kalimantan, which is 4%.
Table 1
Monthly evaporation and rainfall of Paringin pit lake in 2022
3.3. Land Cover Vegetation
Many previous studies have concluded that land use type and morphological characteristics significantly affect the water balance and water quality of receiving waters [62, 63]. However, in relation to pit lakes, the catchment areas are generally small and characterized by sparse vegetation [25].
The results of the high-resolution drone image and NDVI map were shown in Fig. 7. Figure 7a shows the multispectral image taken using a drone in May 2023, while Fig. 7b shows the NDVI map representing high vegetation density (green color), low vegetation density (yellow), areas with no vegetation (red), and the pit lake surface water (blue). This result has been confirmed through ground check. Figure 7c shows an area with low vegetation density, where only grass is found in the field. Figure 7d shows dense trees of Senna siamea and Albizia chinensis tree species, represented by the green color on the NDVI map. Meanwhile, the haulage road and an open area with no vegetation are represented by red color and pit lake is represented by blue color.
The highest elevation of the study area was located at the upper part of Paringin pit lake catchment, at 112.5 meters above sea level (m a.s.l.). The critical surface water level of Paringin pit lake was at 64.8 m a.s.l. There was an elevation difference of 47.7 meters and a slope degree of 60. The elevation of the compliance point, where water inflows into Nungkaran River, was at 59.5 m a.s.l., while the estuary of Nungkaran River at Balangangan River was at 48 m a.s.l. Based on fauna monitoring in 2022, it has been observed that the vegetation at the southern perimeter of Paringin pit lake serves as a habitat for the Proboscis monkey (Nasalis larvatus), an endangered and endemic species found on Kalimantan Island. Therefore, mine reclamation could provide wildlife habitat for fauna as found by previous study from Beale and Boyce [64] and Gerwing, Hawkes [65].
The proportion of the Paringin pit lake and its catchment area is shown in Table 2. The vegetation types in the catchment area and their respective percentages from largest to smallest were high vegetation density (trees) with an area of 91.81 Ha (68%), no vegetation (haulage roads) with an area of 22.21 Ha (17%), and low vegetation density (grass) with an area of 22.21 Ha (15%). Meanwhile, the total surface area of Paringin pit lake was 17.52 Ha. The proportion between Paringin pit lake and its catchment area was 17.52 Ha : 134.3 Ha, or approximately 1:7. This proportion is considered favorable, as other pit lakes around the world generally have small catchment areas [66, 67]. A large catchment area could generate sufficient surface and sub-surface inflow into the pit lake, resulting from rainfall on the catchment [28, 68]. Vegetation on the land plays an important role in the hydrological function by reducing erosion, soil compaction, and soil bulk density through its root systems. Additionally, it contributes to organic matter and increases soil fertility [69, 70].
The highest proportion of high vegetation density within the Paringin pit lake area could stabilize the catchment in terms of hydrological services, such as increasing water infiltration and reducing the slowing down time of direct runoff. This finding confirms the results of Clément, Ruiz [71], which suggest that land with forest vegetation cover of at least 47% could have a positive impact on the water quality of receiving water bodies. However, the haulage road mainly affects the increase of Total Suspended Solids (TSS) concentration within the water inflow to the pit lake.
Table 2
Proportion area of Paringin pit lake and its catchment area
3.4. Infiltration Rate
Waste dumps are known have high soil bulk density due to compaction from heavy equipment operation during dumping process. In relation with hydrological parameters, the important parameter needs to be investigated are soil bulk density and infiltration capacity [72]. Figure 8. shows graph of infiltration rate (y-axis) with time (x-axis) based on the measurement results are then plotted on a graph relating the infiltration rate per time. It shows that during 90 minutes of measurement the infiltration rate changed in three stages. From these three stages, it was evident that the decrease in the infiltration rate becomes more gradual. This was caused by the soil layer becoming increasingly saturated during the testing. In Stage 3, it can be observed that the infiltration rate has leveled off and become relatively constant, and the value at the end of this stage can be considered as the infiltration rate value, which is 5.46E-06.
In the testing area, soil layer sampling was also conducted, followed by laboratory testing, including density, sieve analysis, and Atterberg limits tests, with the following results. Wet density, dry density and moisture content of samples taken at lower part of Paringin pit lake were 1.531 t/m3, 1.122 t/m3 and 36.3% respectively. Meanwhile the infiltration capacity constant is 5.46 x 10− 6 m/s which is classified based on FAO land suitability classification is classified in moderate class [73].
Soil layer sampling was also carried out in the testing area, which was then subjected to laboratory testing, including density testing, sieve analysis, and Atterberg limits testing, with the following results presented in Table 3:
Table 3
Laboratory Testing Result
Based on the results of the Sieve Analysis and Atterberg Limit tests, the soil type of Paringin Catchment Area samples was determined by referring to the Unified Soil Classification System (USCS). According to the USCS, the soil type is classified as Clay Intermediate (CI).
3.5. Daily Rainfall and Water Inflow to Paringin Pit Lake from Its Catchment Area in 2022
Daily rainfall data and the results of measuring water inflow entering Paringin pit lake in 2022 were shown in Fig. 9. In general, the water inflow rate is affected by the intensity of rainfall within the catchment area. From the graph, it can be observed that after the rain intensity increases, there is a subsequent increase in the inflow rate, and vice versa. This finding confirms that the main source of water inflow into Paringin pit lake originates from rainfall within its catchment area, either flowing as surface runoff or sub-surface flow.
The longest consecutive days without rain were 4 (four) days in August 2022. On the other hand, the maximum inflow rate was 1.15 m3/s on 5 September 2022, one day after the highest rainfall of 97.23 mm/day occurred. The average inflow rate was 0.6 m3/s, and the lowest water inflow was 0.08 m3/s. Interestingly, even when there was no rainfall for 4 (four) consecutive days, there was still an average inflow rate of 0.1 m3/s. This observation was further confirmed during field observations in June 2023, where there was no rainfall for 5 (five) days, yet the water inflow was still flowing through main inflow 2 channel, amounting to 0.06 m3/s (Fig. 10.).
3.6. Paringin Pit Lake Morphometry and Bathymetry
Natural and artificial lakes/reservoirs have important functions within terrestrial landscape [23]. One of the most crucial aspects to investigate in relation to lake limnology is the estimation of the lake's volume, which requires conducting a bathymetry survey [74]. The basic pit lake morphometry data that need to be collected include total surface area, mean depth, water balance, and mean lake temperature [75].
The pit lake morphometry of Paringin pit lake resulted from backfilling activity at the Northeast towards the edge of Paringin Pit at the Southwest. The bathymetry map was generated based on the bathymetric survey conducted in February and September 2022 (Fig. 11). Paringin pit lake covers an area of 17.52 Ha, with a trapezium-shaped shoreline and steep pit walls. The slopes in Paringin pit lake were relatively low in the Northeast and Southwest, up to a depth of 2 m, after which the slope became steeper. On the other hand, in the Northwest and Southeast, the pit lake's slopes were steep from the lake's shoreline. This finding confirms previous research related to the steepness characteristics of pit lakes [7, 38].
The surface water level elevation was at 64.8 m a.s.l, while the deepest depth at the bottom of Paringin pit lake was at 46.1 m a.s.l. Therefore, the deepest water column of Paringin pit lake was 18.7 m. The total effective water capacity based on the bathymetry survey of Paringin pit lake was 2,095,084.5 m3. The bottom of the pit lake was built up from the accumulation of sediment materials carried by water inflow from the catchment area into the pit lake through years of erosion processes. The average increase in sediment due to sediment accumulation during February 2021 to September 2022 was 31 cm. It means that an additional 23,491.2 m3 of sediment settled at the bottom of Paringin pit lake during this period.
3.7. Water Balance
In general, the 2022 daily water inflow rate chart pattern was like the outflow rate that enters the Nungkaran River (Fig. 12.). The daily inflow rate to Paringin pit lake was greater than the daily outflow rate which is 0.2 m3/s and 0.1 m3/s respectively. However, in September 2022, the daily inflow rate recorded 1.15 m3/s made it the highest daily rate due to the highest daily rainfall intensity and contributed to the highest average monthly inflow rate of 0.37 m3/s. In November and December 2022, the average outflow rate is higher than inflow rate which is 0.23 m3/s and 0.24 m3/s in November and 0.01 m3/s and 0.05 m3/s respectively.
The outflow from Paringin pit lake occurs through an overflow mechanism from an elevation of 64.8 m a.s.l and underflow pipes system at elevation 63.8 m a.s.l. The outflow rate can be regulated whether through overflow or underflow. If the water level drops below 64.8 m a.s.l, the outflow from Paringin pit lake only flows from underflow pipes. As a result, a continuous flow into Nungkaran River could be managed throughout the year.
Water balance simulation of Paringin pit lake based on monthly rainfall 2021 has been studied by Suhernomo [76]. However, there was no data calibration from field measurement of hydrological data inflow and outflow of the lake and did not calculate water loss from evaporation. The complete water balance of Paringin pit lake in 2022 is presented in Table 4. The pit lake's maximum capacity, determined through a bathymetric survey, is 2.095 million cubic meters, with a critical surface water level at 64.8 meters above sea level. The total inflow volume of 4.95 million cubic meters measured from daily monitoring with water losses from the pit lake amounted to 3.12 million cubic meters through outflow and 184,676 cubic meters through evaporation. It can be concluded that with the proportion of pit lake and catchment area of 1:7 in tropical climate with rainfall more than 3,000 mm/year and evaporation of 1,000 mm/year, the lake will become flow-through lake.
Table 4
Water balance of Paringin pit lake 2022
3.8. Nungkaran River and Riparian Zone
Three hydraulic surveys of Nungkaran River have been conducted at upper, middle, and lower part of the river along 1,385 m length of Nungkaran River. The width of the river valley was only 9 m at the upstream and increased to 17 m at the middle and lower parts of the Nungkaran River. This shows an increasing river valley from upstream to downstream. At the estuary of the Nungkaran River, the river was affected by the Balangan River during the rainy season. The wet cross-sectional area during the survey from upstream to downstream was 2.1 m2, 1.8 m2, and 2 m2 with a water depth of 0.75 m, 0.5 m, and 0.5 m, respectively. Therefore, the average velocity of water during the survey was 0.05 m/s.
The flora and fauna observations have been also conducted in 2022 identified 102 plant species along the riparian zone of Nungkaran River, including 5 types of ferns (5 families), 17 types of grasses (2 families), 50 types of shrubs/herbs (21 families), and 30 types of trees (14 families). Additionally, 23 bird species, 4 mammal species, and 2 reptile species were found. Two mammal species, namely the Proboscis monkey (Nasalis larvatus) and Gray langur (Trachypithecus cristata), are classified as protected animals according to the Indonesian Regulation of the Minister of Environment and Forestry Number P.106/Menlhk/Setjen/Kum.1/8/2018. Both species can adapt well to the human environment. According to IUCN's red list (2021), the Proboscis monkey is categorized as endangered in the wild, while the gray langur is categorized as near threatened.
3.9. Geochemical Analysis
The results of the geochemical analysis are shown in Tables 5 and 6. Overall, from a total of 18 samples taken from 2 (two) locations in the Paringin pit lake catchment area, all were classified as Non-Acid Forming (NAF) category. This means that the material does not have the potential to produce Acid Mine Water (AMD) due to the small sulfide content and moderate Acid Neutralizing Capacity (ANC) content, preventing the formation of Acid Mine Water when in contact with water and air/oxygen [6, 77].
However, interesting findings were observed in the samples in Upper Paringin pit lake (Table 5), where up to a depth of 45 cm, the Paste pH values were < 4, indicating a potential to produce acid when reacted with water. Nevertheless, due to the high ANC value and the negative Net Acid Producing Potential (NAPP) value, the samples were still classified as NAF mathematically. The Total Sulfur value ranged from 0.07 to 0.18%, resulting in Maximum Potential Acidity (MPA) values ranging from 1.8 to 5.5 kgH2SO4/ton. The NAPP value is calculated by subtracting the ANC value from the MPA and falls within the range of -7.1 to -29.2 kgH2SO4/ton. Since the NAPP values are negative, the samples do not produce acid mathematically and are categorized as NAF. If the NAPP values were positive, the samples would be classified as acid-generating materials or Potentially Acid Forming (PAF).
On the other hand, the results of the soil and overburden samples analysis in the Lower Paringin pit lake Catchment Area (Table 6) show that the Paste pH values ranged from 6.9 to 7.4, indicating that the current condition of the samples does not produce acid when soaked with water. The Total Sulfur content ranges from 0.07 to 0.1%, resulting in MPA values ranging from 2.1 to 3.1 kgH2SO4/ton. Meanwhile, the ANC content varied at each depth, ranging from 12.1 to 30.1 kgH2SO4/ton, and all the NAPP values were negative, falling within the range of -9.1 to -27.0 kgH2SO4/ton.
Table 5
Geochemical characteristic of samples collected at upper Paringin pit lake Catchment Area
Table 6
Geochemical characteristic of samples collected at Lower Paringin pit lake Catchment Area
Total Sulphur(TS); Acid Neutralizing Capacity (ANC); Maximum Potential Acidity (MPA); Net Acid Producing Potential (NAPP); Net Acid Generation (NAG); Non Acid Forming (NAF).
Figures 13. and 14. show the variation of important geochemical parameters such as Total Sulfur (TS), Acid Neutralizing Capacity (ANC), Net Acid Producing Potential (NAPP), and Paste pH vertically through depth, along with a comparison between samples taken in the Upper Paringin pit lake and those in the Lower Pit Lake catchment area. The Total Sulfur content for both locations show low levels of % sulfur, with the upper catchment area tending to have higher values than the lower catchment area. Moreover, in terms of depth, both locations show an increase in Total Sulfur content with increasing depth (Fig. 13a). The Paste pH for samples in the upper catchment, at a depth of up to 30 cm, shows a pH < 4 and increases with increasing depth (Fig. 13b). As for the lower catchment samples, the pH was relatively consistent and remained > 6 (Fig. 13b). Even though the Paste pH for the upper catchment samples is < 4, the high ANC content (Fig. 14a) leads to a negative NAPP value, resulting in the classification of NAF material (Fig. 14b).
3.10. Daily pH, TSS Inflow and Outflow Concentration correlation
There is interesting feature of observed daily pH, Total Suspended Solids (TSS) inflow, and TSS outflow concentration of Paringin pit lake water. Figure 15. shows a graph of daily pH fluctuations and TSS concentration in Paringin pit lake during 2022. The average pH is 6.6, indicating a neutral pH level. However, there were fluctuations recorded throughout the year. In June 2022, the pH dropped to 5.81, which was the lowest pH value observed. On the other hand, the highest pH value of 7.25 was recorded in January 2022. In general, relatively low pH levels occurred during the dry season, specifically between June and August 2022.
The elevated TSS inflow values are mainly contributed by fine sediment carried by surface runoff originating from active haulage roads, which cover an area of 22.21 Ha or 17% of the total catchment area. When materials from the road surface receive the load from the double vessel truck of coal, they break into small particle-sized fragments. Consequently, during rainfall, these fragments were easily carried by run off and inflows into Paringin pit lake. The daily TSS concentration of water outflow consistently lower than TSS concentration of inflow during 2022 period. It means that most majority of sediments were deposited within the Paringin pit lake.
3.11. Quarterly Water Quality parameters
In total, four water samples were taken every quarter from the surface of Paringin pit lake to analyze the physical and chemical parameters of the water, in accordance with Indonesian Regulation Number 22 Year 2021, attachment VI, which sets the National Standard of Freshwater Lake. As shown in Table 7, almost all water quality parameters of Paringin pit lake meet the Water Quality Standards set for class 1, which was designated for raw drinking water supply, and class 2, which is suitable for freshwater fishery, irrigation, and other utilizations. There is no indication of Acid Mine Water or a spike in dissolved metal content. However, there was a slightly higher dissolved Iron (Fe) content, which exceeded the class 1 water quality standard of 0.3 mg/l, with a value of 0.71 mg/l.
It is confirmed that the water quality, especially regarding acid drainage in the pit lake, is affected by the geochemical condition of materials within the catchment area and the pit lake's wall, as studied by [78], Rybnikova and Rybnikov [79] and [80], particularly when there is a high content of sulfidic material [81].
Meanwhile, the ammonia level was slightly above class 1 (0.1 mg/l) and class 2 (0.2 mg/l) standards, measuring 0.31 mg/l and 0.28 mg/l during the rainy season and dry season, respectively. The spike in ammonia level in Paringin pit lake water was due to the sampling location being near the floating net fishery, where excessive fish pellets with high protein content were deposited and decomposed, producing ammonia, as confirmed by a previous study by Mondal, Mukherjee [82] (Fig. 16.). BOD5 and COD levels slightly exceeded class 1 and class 2 standards, potentially due to increasing organic content from fishery activity. The high COD and BOD5 levels mainly originated from leachate resulting from waste sanitary landfills [83, 84], and there were no sanitary landfills within Paringin pit lake catchment area. The COD level was higher than BOD5, with an average BOD5/COD ratio of 0.33 and 0.26 for the rainy season and dry season, respectively. This was consistent with previous research findings [85–87].
Meanwhile, even though the TSS data of the quarterly samples show very low TSS, the fact that there was increased sedimentation in Paringin pit lake indicates the need for mitigation of erosion and transport of sediment from the haulage road to maintain the effective water capacity of Paringin pit lake. Steep slopes can also increase erosion during heavy rainfall, leading to increased sediment inputs into the lake [88]. Another important point was, as in previous studies, the potential for water stratification and, therefore, the potential for water quality variation along the depth of the pit lake [82].
Table 7. Result of Paringin pit lake Water Quality Analysis and Standard
3.12. Hydrological Connectivity
Connectivity in the geomorphic context refers to the effectiveness of material transfer between different components of a system [89, 90]. There are three types of geomorphic connectivity within landscape scale: landscape connectivity, hydrological connectivity and sediment connectivity [90, 91]. Artificial landscapes and climatic conditions contribute to new landscape connectivity [88]. Understanding hydrological connectivity is crucial for managing water resources and ecosystem conservation and restoration [90, 92, 93]. In each catchment area, hydrological connectivity can be analyzed through the linkage between natural and artificial compartments within the landscape that form a particular river system using Remote Sensing (RS) and Geographic Information System (GIS) [43].
Paringin pit lake is located in the middle of a landscape with its catchment area being the mine rehabilitation area and the Nungkaran River being the water body that receives water from its outflow (Fig. 17.). The hydrologically connected area starts with rainfall within the rehabilitation area, which becomes the catchment area providing surface and subsurface flows into Paringin pit lake. Water overflows from Paringin pit lake and then flows into the Nungkaran River. The runoff or surface flow from the Paringin pit lake catchment area also carries sediments within the flow, indicated by the Total Suspended Solids (TSS) concentration of the inflow water. Most of the sediments are deposited at the bottom of the lake, while the water flowing into the Nungkaran River has a lower TSS concentration. This indicates the sediment connectivity of Paringin pit lake and its catchment area, implying two things: relatively good water quality of the Nungkaran River's water flow and the reduction of the effective capacity of Paringin pit lake due to increasing sedimentation. Good river water quality was crucial for public health and socioeconomic sustainability [94].
Another finding was man-made landform features resulted from open pit coal mining operations at the landscape level will always be connected hydrologically with natural landform features downstream. It confirms previous study of Huang, Zhang [95], that anthrophogenic landscape changing could affect the river water quality. Therefore, mine rehabilitation and mine closure programs should be carefully assessed not only to meet the successful criteria of mine rehabilitation and mine closure but also to consider the impacts on downstream areas. In the case of Paringin pit lake, it is crucial to maintain its healthy catchment area so that it can provide hydrological functions by maintaining the balance of surface and subsurface flow into Paringin pit lake. Maintaining a healthy catchment means ensuring the vegetation cover on a long-term basis, as most of the vegetation is categorized as fast-growing species with a short lifespan. From a regulatory perspective, these findings could be considered as a basis to review and revise the mine closure successful criterion, focusing on the sustainable benefits of mine rehabilitation and mine closure ecologically and socially.
Topographic evaluation was carried out using ArcGIS version 10, which included slope, elevation, drainage pattern, and distinctive landform features (Fig. 18.). The analysis revealed that most of the catchment area consists of gently sloping to sloping terrain. Both the Paringin pit lake catchment area and Nungkaran River topography exhibited mostly sloping characteristics, with a slope degree ranging from 6 to 13%. The highest elevation observed was 109 m a.s.l in the upper Paringin pit lake catchment area, while the surface water level of the pit lake was recorded at 65.8 m a.s.l. The compliance point, where water flows out into the Nungkaran River, is situated at 60 m a.s.l. On the other hand, the lowest elevation at the Nungkaran River estuary into the Balangan River is 45 m a.s.l. This resulted in height differences of 43.2 m from the highest elevation to the pit lake's surface water and 15 m from the compliance point to the lowest elevation at the Balangan River. These height differences contribute to the temporary drainage pattern observed in the Paringin pit lake catchment area and the permanent flow through the Nungkaran River, from the compliance point to the estuary of the Balangan River. The temporary flow drainage from the Paringin pit lake catchment area into the pit lake itself brings more sediments compared to the permanent flow in the Nungkaran River, mainly due to the overall steeper slopes in the catchment area (topographic cross section Fig. 18). Additionally, most of the sediments are deposited within the Paring pit lake before flowing into Nungkaran River.
From the topographical cross-section, Paringin pit lake stands out as a distinct morphological feature in the landscape, with an original bottom pit lake elevation of 35 m a.s.l. This makes it a man-made reservoir for water and sediment accumulation from its catchment area, thereby playing a crucial role in the water course from the catchment area at the upper catchment boundary into the Balangan River. However, the role of groundwater flows has not been identified yet. In terms of sediment transport, Paringin pit lake, due to its shape and depth, acts as the depositional area for sediments resulting from the erosion of its catchment area, and allows relatively good water with low Total Suspended Solids (TSS) concentration to flow into Nungkaran River. However, in the long run, erosion and sedimentation into Paringin pit lake should be mitigated to ensure the effective water capacity of the lake.
The inflow within the Paringin pit lake catchment area is mostly generated during rainfall through the drainage network. The drainage near the inflow to Paringin pit lake flows throughout the year with a minimum flow rate when there is no rainfall, due to the absence of subsurface flow. On the other hand, from Paringin pit lake to the Nungkaran River, the water continuously flows throughout the year, facilitated by a floating outlet mechanism that maintains a minimum flow rate into the Nungkaran River. In conclusion, there is hydrological connectivity from Paringin pit lake to Nungkaran River, down to Balangan River, throughout the year, with intermittent hydrological connectivity from the upper Paringin pit lake catchment to the Nungkaran River during the rainy season. Maintaining a minimum water outflow from Paringin pit lake to the Nungkaran River is essential to preserve the ecological condition of the river, allowing it to function sustainably both environmentally and socially.
As this study identified fauna both in the Paringin pit lake perimeter and the Nungkaran River riparian zone, it can be concluded that there is landscape and vegetation suitability e.g. water, food, shelter for supporting wildlife habitat, such as the Proboscis monkey. Further study needs to be conducted to identify whether there is a habitat connection within the Paringin pit lake landscape.