Analysis of the features and causes of the contamination in the Shandi River basin

: This study examined the contamination of the surface water in the Shandi River basin in Yangquan, Shanxi province. A total of 11 sampling points (numbered S1-S2, S3-S4, S5–S11) were selected from the upstream to the downstream of the river, and 19 indexes were selected for inspection to identify the causes and features of contaminated water in the basin. The Nemerow Pollution Index was applied to evaluate the quality of the surface water and the overall extent of contamination in the basin. The result revealed that the content of Cl - , As, Cr 6+ , CN - , and volatile phenols met the grade III water quality standards in China, and the remaining indexes exceeded the standards by varying degrees. The five sources of contamination in the basin negatively affect the environment. The presence of contaminants at the sampling points was in the following order, from highest to lowest: SO 42- = Mn = NH 3 -N> Fe 2+ = Hg >Fe 3+ > Cd > Zn = F - >COD>NO 3- = NO 2- . The severity of the comprehensive contamination indexes decreased in the order of S6 > S9 > S7 > S11 > S10 > S3 > S8 > S5 >S4.


Introduction
Shanxi province has abundant coal resources; however, in recent years, coal mines have been consolidated and many of them been closed down because the coal resources were exhausted after continuous mining. In Yangquan, Shanxi province, a number of goaves were abandoned because of excessive mining. After mines are closed for a period of time, mine water accumulates in the mines and pockets of water develop that become trapped in the goaves [1] , yielding acid mine drainage. As drainage continues to gather, the water level may rise and eventually overflow a mine to the surface, resulting in severe environmental damage, negative effects on people's lives, and a high level of social and governmental concern. The Shandi River basin is a highly typical instance of this process.
The contaminated acid mine drainage within the Shandi River basin is mostly concentrated in the carbonate rock leakage sections near the Niangzi Pass spring field and greatly affects the karst underground water within the region. Liang's [2] research stated that the springs at Niangzi Pass have been seriously contaminated because of the leakage of the acid mine drainage in parts of the region, specifically the Taohe River, Wenhe river, and Nanchuan River leakage sections, which contaminate the karst underground water. Huo's [3] study indicated that the main routes of contamination in the Niangzi Pass spring field are the following two: Pollutants and atmospheric precipitation enter the river course and directly infiltrate the ground surface where carbonate rocks are exposed to supply the karst underground water; and precipitation indirectly seeps into the coverage zone of carbonate rocks and supplies the karst underground water.Mining is the most influential factor on the groundwater environment in the Niangzi Pass spring field. Wang [4] conducted an analysis and suggested that pyrite within coal seams and the surrounding strata oxidizes and hydrolyzes during coal mining. Therefore, this study focused on the surface water condition of the Shandi River and selected sampling points within the Shandi River basin on the basis of the geologic environment, mining activities, and the current condition of abandoned mines. The level, volume, and quality of the river water were monitored, and water samples were collected and analyzed to examine the actual sources and distribution of contamination along with the levels and features of the contaminants. In this study, the harm of contamination on Niangzi Pass was specified, and corresponding management measures are suggested.

The general situation of the studied region
The Shandi River basin is 58 km 2 in area, and the overall geology in this region is complex. The lithology of the stratum is relatively complete, with loess covering most of the area and bedrock partially exposed. The geological structure and the hydrogeological condition are simple. Ordovician carbonate karst aquifer is the main composition of the four types of groundwater aquifer groups that are found in the region. The aquiclude is mainly a combination of Carboniferous mudstone and sandstone, and mudstone predominates in the Upper Carboniferous Taiyuan Formation and Permian Shanxi Formation. The main source that contributes water to the aquifer is precipitation, and the water is primarily discharged from wells, springs, natural evaporation, and human activities. In 2009, the mines in the region were integrated and reorganized, and currently, only eight mines remain, with one of them being an open-pit mine.
After field studies and investigations were conducted, five sources of contamination were found to cause the contamination in the basin: shafts in the open-pit mine, and the sources of the gathered water are precipitation and groundwater. The gathered water is yellowish-brown and relatively turbid, and the water quality is extremely poor, with a pH value around 2.5. The gathered water infiltrates into the ground [5] directly and participates in groundwater circulation. When precipitation is heavy, the water volume in the ditch increases, gathers into a surface runoff, and flows into the Shandi River; accordingly, the acid mine drainage seriously contaminates the river [6] .

Placement of sampling points
According to the hydrogeological condition of the Shandi River basin and the principle of having an even and feasible arrangement of sampling points, 11 sampling points were placed from south to north along the river. The distribution of the sampling points is depicted in Figure 2. The upstream lies to the south of Yankan, the midstream extends from Yankan to Zhainao village, and the downstream is located to the south of Zhainao village. The upstream, midstream, and downstream areas received 2, 2, and 7 sampling points, respectively, with sampling points numbered S1-S2, S3-S4, and S5-S11 in order from upstream to downstream.  Subsequently, an analysis of the contamination sources was conducted on the basis of the level of correlation.

Method for evaluating the quality of surface water
The Nemerow Pollution Index method is commonly applied for evaluating the water environment of basins. This method can calculate a single pollutant index as well as the comprehensive indexes of different factors within the basin and reflect the comprehensive contamination situation [7][8] .
According to the environmental quality standards for surface water (GB3838-2002) III standard value [9] , the calculation formulas are as follows: In formula (1) and (2)

Analysis and assessments of the cations and
anions in the water sample rate with the movement of flowing water; therefore, the spatial heterogeneity of Fe 3+ was the most apparent [11][12] . The content of Fe 2+ ranged from 0 to 1364 mg/L, the average value was 590.86 mg/L, and the over-standard rate was 55%. Fe 2+ was evaluated at sampling points S6-S11; the content of Fe 2+at S6-S7 and S9 was lower than the minimum detection limit of the equipment and is marked as 0. This indicated that Fe 2+ did not exceed the standard in the upstream and midstream in general. Nonetheless, the content of Fe 2+ was extremely high at S6-S7 and S9, and the reason is that S6 is located near contamination source IV and S9 adjoins contamination source V. In addition to Fe 3+ , Fe 2+ is also abundant in acid mine drainage, and when the outflow increases, the content of Fe 2+ rises accordingly; in addition, Fe 2+ does not accumulate easily, so its content sampled at S7 remained high [13] . The content of SO4 2ranged from 71.40 to 19836 mg/L, the average value was 5883.09 mg/L, and the over-standard rate came to 81%. SO4 2was assessed at sampling points S1-S11; the content of SO4 2at S6, S7, and S9 was extremely high, which proved that the SO4 2content exceeded the standard at S6, S7, and sampling points S9. The reason is that plentiful SO4 2can be found in acid mine drainage, and with the abrupt increase of the outflow, the content of SO4 2became exceedingly high.
The content of NO3ranged from 0.33 to 28 mg/L, the average value was 10.22 mg/L, and the over-standard rate was 45%.
NO3was assessed at sampling points S1-S11, and the NO3content reached its peak at S5. The reason is that S5 is located within the range of influence of contamination source II. The secondary sewage used by the pig farm is discharged into the river course, and water quality deteriorates with the release of the highly concentrated organic sewage, which leads to the increase and exceedance of NO3 - [14] .   reached 40%. The content of Hg inspected at S1-S3 and in the S1 water sample was lower than the minimum detection limit of the equipment and is marked as 0. This indicated that Hg did not contaminate the river at sampling points S1-S3 and S1, but the content of Hgwas relatively high at S5, S6, and S9.Hg had a significant correlation and shared a similar source with the acid mine drainage feature Fe 2+ , which confirmed that the higher content of Hg at S5 was caused by acid mine drainage contamination source II, and S6-S7 were influenced by contamination sources IV and V. The content of Cd ranged from 0.0004 to 0.566 mg/L, the average value was 0.1283 mg/L, and the over-standard rate reached 55%. Sites S6, S7, and S9 were inspected and found to have a higher content of Cd than sampling points S1-11.    Table 4. The content of CNwas lower than the minimum detection limit of the equipment at sampling points S1-S11, which indicated that the CNcontent met the grade III water quality standards in China. The content of volatile phenols ranged from 0.002 to 0.005 mg/L, the average value was 0.0033 mg/L, and the over-standard rate was 0%. This also indicated that the content of volatile phenols within the Shandi River basin metthe grade III water quality standards in China. The content of H2S ranged from 0 to 0.35 mg/L, the average value was 0.1045 mg/L, and the over-standard rate was 18%. The H2S content inspected at S1, S2, S9, and S11 was relatively high.

3.1.2Analysis and Evaluation of the heavy metal factors in the water
According to Table 4, H2S had no significant correlation with the acid mine drainage ions, proving that their contamination sources were different and the reason for the exceedance remains to be identified. The content of NH3-N ranged from 6.22 to 63.8 mg/L, the average value was 22.0333 mg/L, and the over-standard rate reached 81%. The content of NH3-N inspected at S3, S6, S7, and S9 was relatively high. Based on Table 4, NH3-N had a positive correlation with the acid mine drainage feature ions Fe 2 + and SO4 2-. This indicated that the contamination sources were similar and confirmed that mine washery contamination source I was the cause of the higher NH3-N content at S3, whereas acid mine drainage contamination sources IV and V were the reason for the higher NH3-N content at S6, S7, and S9. The content of NH3-N ranged from 6.22 to 63.8 mg/L, the average value was 22.0333 mg/L, and the over-standard rate reached 81%. The content of COD ranged from 0.8 to 240 mg/L, the average value was 49.3785 mg/L, and the over-standard rate was 27%.
The concentration of COD inspected at S7 and S9 was relatively high. Based on Table 4, the COD concentration has no significant correlation with the acid mine drainage features; accordingly, the contamination sources were different, and the reason for the contamination remains to be known.

Summary
Based on the preceding analysis results, the content of Cl -,

Results and analysis of the comprehensive contamination indexes
The evaluation results concerning Shandi River water quality comprehensive contamination indexes were obtained through the calculation of the heavy metal comprehensive indexes at the sampling points (see Table 5). The results revealed that only the water quality at S1 and S2 was graded excellent, whereas that at the other nine sampling points was very poor.
The severity of the comprehensive contamination indexes decreased in the order of S6 > S9 > S7 > S11 > S10 > S3 > S8 > S5 > S4. Areas with severe and long-term contamination surrounded the Shandi River village, and the two areas with acid mine drainage contamination were in the most impaired condition.

Conclusion and suggestions
This study conducted field surveys and discovered the distribution of the contamination sources in the Shandi River basin; based on the results of the surveys, sampling points were placed at the contamination sources to obtain water sample data for water quality evaluation. The results are as follows: (  (2)The river was not contaminated by the content of Cl -,As,Cr 6+ ,Pb,CN -, volatile phenols, or H2S but was contaminated by Fe 3+ ,Fe 2+ ,SO4 2-,NO3 -,NO2 -,F -,Zn,Hg, Mn,Cd, NH3 -N, and COD by varying degrees. The level of contamination by each index decreased in the order of Fe 2+ >Fe 3+ >Mn>SO4 2 >Cd>NH3-N> Zn > F ->Hg >COD>NO3 ->NO2 -. Fe 2+ was responsible for the highest level of contamination within the Shandi River basin, and SO4 2-, Mn, and NH3-H exhibited the widest range of contamination.
(3)Within the Shandi River basin, only the water quality at S1 and S2 was graded excellent, whereas that at the remaining sampling points was very poor. The severity of the comprehensive contamination indexes decreased in the order of S6 > S9 > S7 > S11 > S10 > S3 > S8 > S5 > S4. Areas with serious and long-term contamination exist around the Shandi River village, and the two areas with acid mine drainage contamination were in the most severe condition.
Given the contamination within the Shandi River basin, upstream regulations are suggested to lower the harm to the Niangzi Pass spring field. Abandoned open-pit mines should be filled, and the garbage in the mines should be disposed of properly. The mine shafts should be sealed to prevent the oxidization of mine water. Additionally, on-site processing measures are recommended (e.g., permeable reactive barriers(PRB) or phytoremediation techniques) to deal with the contamination that has already been caused by acid mine drainage.