Karst is widely distributed in China, with a total area of approximately 3440 × 103
, which accounts for about one third of the land area [1
]. The exposed karst with an area of about 907 × 103
is mainly concentrated in Southwest China. Karst aquifers are composed of soluble rocks, often with large pathways and caves, which can store and transport a large amount of groundwater due to their strong permeability. Therefore, karst has a large potential for providing fresh water [2
]. Karst areas generally develop in Southwest China due to its rich rainfall. Spring water is one of the most important water sources and its quantity and quality are always as good as the discharge of underground water. However, the spring water quality is vulnerable to pollution due to the complexity of karst aquifers [4
In recent years, many coal mines have been abandoned in karst areas in China, and groundwater in the goaves has merged in the underground water system. The water in the goaves always contains high concentrations of Fe, Mn, and sulfate, with a high hardness and a low pH value, thus causing the pollution of spring water [5
]. The number of abandoned coal mines has gradually increased, and the mine water has polluted and damaged the surrounding water system, thus affecting the production and life of local residents. For example, there are 298 coal mines distributed in the Jinci Spring area, Shanxi Province, China. The amount of the mine water discharge in this area can reach 24.68 million m3
each year, which seriously affects the health of residents [6
]. After the Hongshan and Zhaili coal mines were abandoned, the groundwater in goaves could not be drained, causing a rise in the water table. The accumulated water in the goaves then polluted the karst water in this region [7
]. Around the famous Niangziguan Spring area, groundwater in abandoned coal mines increased the concentration of sulfate ions and increased water hardness [8
]. The Yudong River in Kaili City, Guizhou Province, was seriously polluted by acid mine drainage from abandoned mines [9
]. In central Montana, the Madison aquifer feeds two very large natural springs at either end of the Great Falls-Lewistown Coal Field, named Giant Spring and Big Spring, and it has been documented that the abandoned coal mine has caused localized contamination in the Madison aquifer from AMD water [10
In order to protect the karst water from pollution, different options have been adopted to eliminate pollution sources and pathways. The karst springs within the influence range of abandoned coal mines have been scientifically investigated to understand the flow characteristics of spring water and to delimit the protection area. Kakaroglu proposed the establishment of a karst water environment protection zone through scientific planning, investigation, and analysis [11
]. Hao et al. divided the Niangziguan Spring into 3 areas with 11 intersections and discharge areas as first-grade protection zones, the supply basin as a second-grade protection zone, and the dry area with a less supply as a third-grade protection zone [12
]. Giudici established a flow model in a karst aquifer, and Javier Valdes-Abellan established a karst aquifer model (KAGIS model) based on GIS to simulate the movement and influence of pollution [13
]. Jiang et al. established an environment impact model of water in goaves, taking the Jinci Spring as an example, and proved that the water near the coal mine was easily affected by the water from goaves [15
The pollution source for a polluted water area should be cut off first, and then all kinds of scientific methods can be used to purify the pollution. At present, mine wastewater treatments mainly comprise chemical, physical, and biological approaches. The most common method in China is the use of neutralization technology with limestone, fly ash, and red mud as materials. In addition, there are microbial, electrochemical, membrane separation, chemical precipitation, and artificial wetland technologies [16
In order to stop coal mine water from recharging karst water, it is necessary to set a protective coal pillar with a certain thickness [21
]. The mechanism of pollution should be analyzed and studied through geological investigation. In order to reduce the discharge of the polluted water to complete source control, the pathway between the karst and goaf water should be blocked. Some methods, such as pathway blocking, vertical curtain grouting, and goaf filling, can be carried out in regional treatments. Source control combined with grouting is another recent mode of treatment [22
A monitoring network of groundwater quality and pollution has been established to evaluate the dynamic changes in water quality and quantity in order to provide references for protecting karst water. Researchers have put forward many recommendations and measures, including water resource management, reasonable water resource planning, optimal water resource allocation, groundwater control, pollution source control, and dynamic groundwater monitoring [23
The geological conditions and groundwater runoff mode in the karst areas of Southwest China are special, so it is of great importance to protect karst spring water through study. Peng et al. stated that it is necessary to take an underground basin as a unit instead of a karst basin as a unit in order to investigate the characteristics of karst aquifers [24
]. Various methods need to be comprehensively adopted in karst areas, such as hydrogeological surveys, geophysical explorations, hydrogeological drillings, tracer tests, cave detection, and dynamic monitoring. Geophysical explorations are always used to detect complex karst caves, and common techniques for this detection include the electromagnetic, electrical sounding, and multi-electrode resistivity methods. Hydrogeological drilling is used to understand lithology, geological structure, and groundwater resources. A tracer test can trace and determine the source of pollutants and flow rate in these systems before estimating the recovery time. The cave detection method can be adopted to investigate the features of caves, to analyze the relationship between cave shape and geological structure, and to finally learn the direction of groundwater flow and the connection between an underground pathway and karst water [25
]. Dynamic monitoring can be used to analyze the hydraulic connection between boreholes through the accurate, real-time monitoring of water levels in mined-out areas. Ultimately, different methods should be selected by considering geological and natural conditions.
The study area is located in Qingganglin Village, about 2.5 km southwest of Dafengdong Town, Kaili City, Guizhou Province. There are more than 100 abandoned coal mines in Kaili City, Guizhou Province, and acid mine drainage has seriously polluted Yudong River [9
]. The Longdong Spring, which is an important source of drinking water in the village, was polluted after heavy rainfall on 27 June 2017. After that, pollution and recovery phenomena appeared several times. The Longdong Spring had become a “red spring” in the rainy season, which has seriously affected normal production and life.
In order to understand why Longdong Spring was polluted, investigations have been carried out since July 2017 with hydrogeological surveys, geophysical surveys, and borehole drilling to characterize the lithologies, water levels, and water quality of the karst aquifers. The authors of this study investigated the vertical and horizontal distributions of water abundance to identify the main source of pollution. Following the hydrogeological survey, the authors of this work analyzed pollution sources and pathways; then, they formulated recommendations for pollution control. The main goal of this research was to investigate the reasons for spring pollution in a karst area. As the pollution of the Longdong Spring is typical and representative in Guizhou Province, this investigation can provide reference for the study of spring pollution in karst areas.
2. Materials and Methods
The authors of this investigation focused on the source and the pathway of pollution based on the characteristics of the study area. First, a preliminary investigation of the study area was carried out based on data collection and analysis. Second, special methods, such as hydrogeological surveys, geophysical surveys, drilling, cave detection, and tracer tests, were used to explore the aforementioned pollution sources and pathway.
2.1. Study Area
The study area is dominated by a karst landform, with high terrain in the northeast, low terrain in the southwest and a slope of 5–60°. The highest elevation is +973 m, and the lowest elevation is +807 m. The terrain is steep due to the uplift of the Yunnan-Kweichow Plateau and river cutting. The vegetation in the area includes evergreen broad-leaved forests, coniferous forests, shrubs and grass, with a vegetation coverage rate of 53.28%. This area belongs to the subtropical humid climate zone with an annual average temperature of 13.6–16.2 °C July is the hottest period with an average temperature of 23.2–25.8 °C. (http://www.kaili.gov.cn
, assessed on 24 August 2021) January is the coldest month with an average temperature of 2.6–5.2 °C. There are four distinct seasons, mild climate, abundant rainfall, no severe cold in winter, no severe heat in summer, and a long frost-free period—thus showing obvious monsoon climate characteristics. The average annual rainfall is 1240.1 mm, which is suitable for the development of agriculture, forestry and animal husbandry. The surface rivers are well developed and abundant in the study area. The major surface creeks include Dafengdong gully, Xiaohe River, and Gebaochong gully. The Longdong Spring is the main supply source of the Xiaohe River, which originates in the southeast of the mining area and flows into the Baishui River. Figure 1
shows the location of the Longdong Spring. It is in a terrace slope, higher in the northwest and southeast with the highest point in the northwest of +1137 m asl. The southwest, middle and northeast areas are relatively low, with the highest altitude of +1100 m and the lowest altitude +855 m.
The strata sequences in the study area include the lower Permian Liangshan Formation, the middle Qixia Formation, the Maokou Formation and the Quaternary.
The Quaternary (Q) consists of clay, sand and gravel with an average thickness of 14.22 m and a poor hydraulic conductivity. The Permian Middle Maokou Formation (P2m
) has a thickness ranging from 20.8 to 91 m and an average of 52.6 m. It is composed of gray and white, thick, layered massive cryptocrystalline limestone that is filled with a few flint nodules. Dissolved pores, caves, and fissures have developed and are partially filled with calcite. According to borehole drillings, three layers of karst caves have developed. Two layers of karst caves are in the middle part, with a height of less than 1 m, and they are filled with fine sand. Another karst cave layer has developed at the bottom with a height of 1.2 m, and it is filled with clay mud. The Permian Qixia Formation (P2q
) has a thickness ranging from 44.8 to 72.5 m and the average thickness is 56.65 m. It is composed of dark gray, gray black medium-thick limestone, containing clayey, organic matter and flint nodules, with clayey bands in some parts. A small amount of brown calcareous shale is distributed in the lower part. In this formation, dissolved pores and fissures only developed at the bottom, most of them are closed and filled with calcites, and oily matter is partially distributed. The Lower Permian Liangshan Formation (P1l
) consists of clayey limestone, gray clayey shale, coal seams and white bauxite shale. Though our borehole drilling did not completely expose the entire formation, the drilling thickness in this formation was found to reach 16.60 m. The thickness of the coal seam was found to vary from 0.6 to 3.3 m. The CK-1 (Figure 2
) and CK-2 boreholes exposed goaves with cutting heights of 1.7 and 2.2 m, respectively.
The study area is geologically located in the east-middle part of the Guiding North-South structural transformation zone in the southern Qiannan platform of the Yangtze quasi platform. It forms a groove-shaped fold with a wide anticline and a tight syncline. The Dafengdong syncline is located in the northwest side of the study area, strikes N19°E, and extends about 22 km. The occurrence of the north west limb is 118°∠29° while the south east limb is 290°∠10°. There are four faults in this area. F1 is located in the northwest of the Feilong coal mine. It is a normal fault with an extension of 2.9 km in the mine, dipping to 105° with a dip angle of 77°. F2, developed in the northwest of the mining area, is a strike slip fault with a dip direction of 275°, a dip angle of 82°, and an extension of about 700 m. F3 passes through the Qingganglin Village, Dafengdong Town in the southeast of Feilong Coal Mine, extending 4.4 km. It is a reverse fault with a dip angle of 70°. F4 is located in the north of Qingganglin Village, strikes NNW-SSE, and has an extension of 670 m with unknown occurrence.
Hydrogeological features of the study area were investigated in this study. The principal karst forms mainly include stone teeth, karst trenches (troughs), and karst caves. The hollows are distributed in a beaded shape in the watershed area. The bottoms are covered with clay and often embedded with sinkholes and funnels (Figure 3
a). It is a recharge zone for underground karst pathway.
2.2. Hydrogeological Survey
Geological and geomorphic features, surface water types, vegetation coverage, site types, and human activity were observed in the study area. The hydrogeological conditions in the study area were analyzed based on previous hydrogeological information, and a further survey was carried out in this study [26
In order to investigate the reason for the pollution of the Longdong Spring, this survey started in July 2017 to further study the underground karst water in the research area. Nine hydrological boreholes (seven for groundwater level observation in the goaves and two for karst groundwater level observation), with a total of drilling length of 937 m, were created. In addition, we conducted 4 tracer tests, 2 cave detections, a special hydrogeological survey in a 3.84 km2 area, and 18 engineering measurement points, as well as corresponding water quality monitoring, geological cataloging, and well logging.
2.3. Hydrological and Geophysical Exploration
The transient electromagnetic method combined with the direct current electric method was used to detect water-rich areas, the distribution of the coal mine goaf, and polluted pathways. A V8 multi-function electrical instrument produced by Canada Phoenix Company and the Large Fixed Loop Source Transient Electromagnetic Method were used in this exploration.
2.4. Hydrogeological Borehole Drilling
Drilling provided the basis to characterize the distribution of aquifers or aquifuges, as well as the recharge and discharge and migration of groundwater in the study area through the characterization of geological structures, strata consequences, aquifer characteristics, and coal seam roof and floor thicknesses. A total of nine boreholes were drilled, with a total area of 1037.4 m. Among them, seven boreholes were for goaf groundwater surveys, and two were for karst groundwater surveys. A water level monitoring system with five sub-stations was established. In each borehole, geological logging and a downhole TV were used to obtain the strata data. Figure 4
a, b shows the water-conducting fracture development in borehole CK-1 in the depths from 123.6 to 124.2 m and from 139.1 to 139.7 m, respectively.
2.5. Tracer Test
A tracer test was used to investigate the hydraulic connection among sinkholes, boreholes, goaves, and the Longdong Spring.
When the flow in the sinkhole and the Longdong Spring was stable and the water was clear, the test could start. NaCl and fluorescein (C20H12O5) were selected as tracers. Then, 50 kg of salt, 2 kg of fluorescein, several sampling bottles, several labels, 4 portable pH/TDS/EC detection instruments, 4 BEHRINGER 7500 photometers, 200 pieces of 7500 chloride quantitative reagents, a stopwatch, and a flowmeter were prepared. Next, 50 kg of dissolved edible salt and 2 kg of fluorescein were placed at the launch point within 10 min. The flow rate was tested in the launch point of sinkhole.
The observation started at the same time at each observation point. Before the tracer was placed, the portable pH/TDS/EC detector was inserted into the water to record the initial data. The original water sample was collected and the chloride concentration in the original sample was detected by a BEHRINGER 7500 photometer. After the tracer was placed, the electrical conductivity (EC) in the water was monitored in real time. The water samples were collected and the chloride concentration in the water samples was detected by a BEHRINGER 7500 photometer with a sampling interval of 20 min. The observation interval decreased to 10 min when the water color changed. The interval returned to 20 min when the water started to be clear again and the EC and chloride concentration continued decreasing. The observation stopped when the water became clear again and the EC and chloride concentration declined to the initial values and stabilize for 3 h. Flow rate was recorded again at the end of the test. Springs 2, 3, and 4 were auxiliary observation points. If no tracer was observed or the spring cut off during testing, the observation could be extended. A total of 4 tracer tests were performed. The tracer-released points were Sinkhole No.5 and boreholes CK-1, CK-2, and YR-2. The observation lasted for 50 days.
2.6. Cave Detection
Cave detection in the mouth of the Longdong Spring showed that water flows out from the Longdong Spring mouth all year round. The opening is narrow but has a large flow rate. The water level in the cave is high and the space inside is unknown. At the same time, it has been speculated that there are many large reservoirs in the cave. Therefore, the diving and boat methods were used to detect the direction of water flow and karst cave development to track the pollution source and pathway (Figure 3
Cave detection for Sinkhole No.5 was divided into two parts. First, we tracked the direction of underground river flow to detect whether it eventually flows into the Longdong Spring. If it was found to eventually merge into the Longdong Spring, it would be necessary to clarify the merge position. If it was not found to merge into the Longdong Spring, it would be necessary to determine the direction of the underground river and the development of the karst cave, as well as to analyze the discharge location. Second, we traced karst development and the recharge source of the underground river (Figure 3
The detection of two caves near the main shaft of the Feilong coal mine showed that water at the entrance of cave is shallow. There is no water at the exit, and the space in the cave is open. Downstream detection was used to track the flow direction and determine the position of the cut-off. Based on this, whether the cave is connected to the Feilong coal mine and whether it supplies the Feilong coal mine goaves were determined.
2.7. Groundwater and Rainfall Monitoring
Automatic groundwater level monitoring equipment was installed in 5 boreholes: CK-1, CK-2, CK-3, YR-1, and YR-2. Boreholes CK-1 and CK-2 were in the goaf in the middle of the mine, CK-3 was in the mine sumps in the southeast of the mine, and YR-1 and YR-2 were in the footwall of the F3 fault in northwest of the Longdong Spring. The monitoring data could be transmitted to the internet in real time. The monitoring frequency was 4–5 times per hour or adjusted according to needs. Real-time monitoring and comparative analysis could indicate the hydraulic connection among boreholes and goaves.
Water quality monitoring was carried out a total of 326 times from May 2018 to March 2019, including 297 instances of the on-site monitoring of polluted water in the Longdong Spring, 8 instances for sinkholes, 5 instances for boreholes, and 12 instances for caves. The type of pollution in the Longdong Spring was clarified by the monitoring.
The rainfall monitoring was carried out with a rainfall gauge from May 2018 to investigate the relationship between Longdong Spring water pollution and precipitation. Rain was monitored for a total of 51 times in this work. The beginning and ending times and rainfall were recorded. The relationship between the flow rate and pollution of the Longdong Spring was analyzed to investigate the pollution source.
Continuous monitoring was carried out for a total of 326 times during the rainy season from May 2018 to March 2019. The flow rate and concentration of Fe in the Longdong Spring were monitored with a frequency of one time per day normally and twice per day after rainfall. The monitoring frequency returned to once per day and even once per three days when the flow rate and pollution index returned to the initial level.
4.1. Water Quality
According to preliminary investigations and water quality analysis, it was judged that the pollution is from close-range because the water quality of the spring was found to quickly improve after pollution and the polluted water was found to be similar to the drainage in the Feilong coal mine about 1 km north of the Longdong Spring. Continuous monitoring of the Longdong Spring showed that the spring water was always in a polluted state from May 2018 to 29 August 2018, as the concentration of Fe and Mn ions in the spring water were found to exceed the Standards for Drinking Water Quality (China), which states that the concentration of Fe and Mn cannot exceed 0.3 and 0.1 mg/L, respectively. After rainfall in 2017, the concentration of Fe was 33.83 mg/L; it was 62.70 mg/L in the same period in 2018, and it was as high as 94.20 mg/L on 28 June. This comparison shows that the pollution of the Longdong Spring in 2018 tended to increase during this period from May to August in the wet season.
We also found that SO42− and Fe are associated. In addition, the trend of the change of Mn ion was found to be roughly consistent with that of Fe/SO42−. That is, when the concentration of Fe/SO42− is high, the Mn concentration is relatively high. For example, the concentration of Mn was 0.62 mg/L and the concentration of Fe was 0.3 mg/L on 23 June; the concentration of Mn and Fe increased to 20.1 and 19.1 mg/L, respectively, on 24 June.
4.2. Influence of Mine Water Level on the Spring Pollution
The goaves stopped discharging after the coal mine was abandoned in April 2017. A heavy rain on 30 June 2017 caused the pollution of the Longdong Spring. This implies that when the goaf water accumulates to such an elevation, the water overflows to the karst pathways and then mixes into the Longdong Spring. The continuous monitoring of rainfall quantity and water quality also showed that the pollution of the Longdong Spring is controlled by rainfall. Rain recharges the goaf water, and the goaf water overflows when exceeding a certain elevation. The water in the Longdong Spring became clear again and the Fe concentration was less than 0.3 mg/L at the end of August 2018 (Figure 8
b) when the water level was found to reach the lowest in borehole CK-2. It can be inferred that the water in the goaf will no longer overflow and mix with spring water when the water level in the goaf is lower than +909 m asl.
4.3. Comprehensive Analysis of Pathway with Geophysical Survey
shows a suspected pathway with bright gray colors from three-dimensional transient electromagnetic survey. There is an obvious pathway between the Bailachong Sinkholes group in the southwest and the Qingganglin Spring group in the southeast, which is indicated by the blue arrow. In addition, there is a suspected pathway, with a bright gray color, that is distributed from the northwest to the southeast in the northern area (red arrow 1). Here, the Dafengdong karst cave group exists, which is distributed from the southwest to northeast according to the survey. A water-conducting fractured zone near the F4 fault leads from the suspected goaves of the Feilong coal mine to the polluted Longdong Spring, extending from the northwest to southeast (red arrow 2).
4.4. Mechanism of the Pollution
The above-mentioned analysis shows that there is a positive correlation among the Longdong Spring water pollution, flow rate, and rainfall. When the rainfall and the flow rate are small, the spring water is not polluted, but when the rainfall and spring water flow rate in the Longdong Spring increase to certain values, the Longdong Spring becomes polluted. Secondly, the concentration of pollutants decreases due to the increased flow on the first day after heavy rain in the Longdong Spring. Third, in the dry season the pollution risk is relatively low and the flow is clear. Fourth, when the groundwater level rises to +908 m in the goaves, the spring water begins to be polluted, and when it drops below +909 m, the spring gradually reaches a stable clean state.
The pollution source of the Longdong Spring is mine water in the goaves of the Feilong coal mine, which leads to unsustainable pathway pollution under special conditions. The water above the +908 m asl in the goaf was determined as the probable source of pollution according to the geological drilling survey and results analysis. The scope of treatment can be delineated according to the analysis of hydrological and geophysical survey results. There are three suspected pathways from the south to the north: a pathway from Bailachong to Sinkhole No.5 (clear water pathway), a pathway in the southwest direction of CK-1 parallel to F4 (inferred to be a polluted pathway but not verified), and a pathway near the CK-1 borehole (a polluted pathway that has been verified as goaf) with a main direction towards the east.
Our hydrogeological investigation and survey identified the source of the pollution of the Longdong Spring. The continuous monitoring of the water quality in the Longdong Spring after April 2018 showed that the pollution of the Longdong Spring is mainly controlled by rainfall. The rainfall quantity determines the duration of pollution in the Longdong Spring. The maximum concentration of Fe and Mn in the Longdong Spring was found to exceed that in the Standard for Drinking Water Quality in China by more than 300 times. It was also concluded from hydrogeological survey that the source of pollution in the Longdong Spring is the mine water from the abandoned Feilong coal mine to the northwest. We found that the groundwater levels in the three goaves fluctuate with change in rainfall. Rainfall induces a rise in the groundwater level, which fluctuates from +908 to +914 m. It was judged that the elevation for mine water drainage to the spring in this area is approximately +908 m. Therefore, the pollution treatment range can be set at above +908 m asl.
The authors of this work ascertained the development and distribution of the pollution pathway to spring water, and they analyzed the mechanism of spring water pollution. Correlations among the Longdong Spring flow and rainfall, the groundwater level in the goaf, and spring water pollution factors show that when the mine water level in the goaf rises to +908 m after being recharged by rainfall, the pollution of spring occurs 24 h later. Two recharge pathways between the mine water and the Longdong Spring were identified. The first one is a clean groundwater pathway from the Bailachong Sinkholes to Sinkhole No.5 from the south to the north, and the second one is a polluted pathway parallel to F4 in the southwest of borehole CK-1 (judged but not verified).
Pollution control with grouting treatment is recommended to be implemented in the dry season when the flow rate of the Longdong Spring is small. The project may include grouting in the mined-out area and fan-shaped curtain grouting in the two pathways around the Longdong Spring and Sinkhole No.5, the pollution source.