There were many phosphorus chemical enterprises and phosphogypsum slag fields in the Fuquan section of the Chong-an River in the Guizhou Province of China; therefore, the dissolved phosphorus in the Fengshanqiaobian water quality monitoring section of the area has exceeded 0.2 mg/L for a long time. This study analyzed the monitoring data of dissolved phosphorus, fluoride, nitrate–nitrogen, and chemical oxygen demand in the main stream and key tributaries of the Fuquan section of the Chong-an River from 2015 to 2021, in which the main exceeding factor was dissolved phosphorus of more than 0.2 mg/L; determined the variation law of phosphorus pollution in the water body of the Fuquan section of the Chong-an River, in which the exceedance of phosphorus was mainly concentrated in May of flood season and in January and March of the drought season; evaluated the sources of phosphorus pollution, in which the main pollution sources were from phosphogypsum slag field leakage, phosphorus chemical industry plant leakage, agricultural nonpoint source pollution; and presented measures to reduce phosphorus pollutants in rivers, such as strengthening phosphogypsum slag field and phosphorus chemical enterprise seepage prevention measures, and reducing the amount of phosphate fertilizer application.

  • The monitoring data of dissolved phosphorus, fluoride, nitrate–nitrogen, and chemical oxygen demand were analyzed in the mainstream and key tributaries of the Fuquan section of the Chong-an River from 2015 to 2021.

  • The variation law of phosphorus pollution was determined in the water body of the Fuquan section of the Chong-an River.

Many phosphorus chemical enterprises and phosphogypsum slag fields located in the Fuquan section of the Chong-an River in the Guizhou Province of China discharged some pollutants into the river; therefore, the dissolved phosphorus (DP) in the Fengshanqiaobian water quality monitoring section of the area has exceeded 0.2 mg/L for a long time, the maximum beyond 0.8 mg/L, which needs to be treated seriously and has been criticized by environmental departments many times. The purpose of this study was to help solve the problem of the long-term increasing concentration of DP at the Fengshanqiaobian section of the Chong-an River in Fuquan, be of great benefit to the sustainable development of the local phosphorus chemical industry, and relieve the worries of the local government and environmental protection departments.

Phosphate mines, phosphorus chemical enterprises, and phosphogypsum slag fields in different regions of the world, which emit pollutants into rivers, have resulted in serious phosphorus pollution (Farmer 2018). To address these issues, the source of phosphorus pollution needs to be identified. Therefore, a lot of work has been carried out for the source analysis of phosphorus pollution in the past decade. Phosphorus pollution sources have been identified in key tributaries of the Tuojiang River, China (Sun et al. 2013), in the Yangtze River basin, China (Qin et al. 2018), in the Poyang Lake basin, Jiangxi Province, China (Yang et al. 2020), in the Jiaomen waterway of the Nansha District in Guangdong Province, China (Chen et al. 2021), in Gaoyou Lake in Jiangsu Province, China (Xie et al. 2021), in the Xiaojiang River of Dongchuan and its tributaries in Yunnan Province, China (Cai 2020), along the Siahroud River, located in Guilan Province (Latifi et al. 2018), in the surface sediments of the Anzali wetland of the Caspian Sea (Bastami et al. 2021), the Keban Dam Reservoir of Turkey (Varol 2020), and the hypereutrophic Krishnagiri Reservoir, Tamil Nadu, India (Saha et al. 2022). Of these pollution sources, phosphorus chemical companies have discharged the most phosphorus into rivers, accounting for the highest proportion, followed by agriculture, roads, livestock and poultry farming, rural domestic sewage, and rural household garbage (Fink et al. 2016; Cheng et al. 2018; Goyette et al. 2018; Lin et al. 2018; Hu et al. 2020).

After identifying the source of phosphorus pollution, it is necessary to treat the existing pollution. Effective measures to control regional phosphorus pollution include the following: (1) controlling agricultural pollution within a reasonable range, adopting scientific fertilization measures, and reducing phosphorus pollution caused by excessive fertilization (Zhang et al. 2016; Wang et al. 2018; Ngatia et al. 2019; Wen et al. 2020; Huang et al. 2021); (2) strengthening the construction and management of urban sewage treatment plants, ensuring the quality of sewage treatment, and reducing phosphorus pollution in urban sewage (Chen et al. 2016; Ren et al. 2016; Mekonnen & Hoekstra 2018; Wang et al. 2020; Yan et al. 2020); (3) adopting ecological restoration technologies such as wetland ecological restoration to increase the self-purification ability of water bodies and reduce phosphorus pollution in water bodies (Small et al. 2019; Sánchez-Colón et al. 2021; Ren et al. 2022); (4) strengthening environmental monitoring and management and promptly discovering and solving phosphorus pollution problems (Jefri et al. 2021; Kor et al. 2021; Oron et al. 2021); and (5) enhancing public's environmental awareness, encouraging people to take environmentally friendly actions, and working together to protect the environment and reduce the occurrence of phosphorus pollution.

In the past decades, many studies on phosphorus pollution, both domestic and foreign, have focused on flat areas located at low altitudes, without underground rivers, moderate rainfall, and where underground phosphogypsum has been treated to prevent seepage, over a shorter time span of 1–3 years. However, few studies have been conducted on phosphorus pollution in rugged mountainous areas located at high altitudes, with underground rivers, and increasing rainfall, and where some underground phosphogypsum has not been treated to prevent seepage, over a longer time span of seven years, just like the Fuquan section of the Chong-an River in Guizhou Province of China, which has reference significance for the sustainable development of phosphorus chemical industry in the world. In addition, TP alone is not a good indicator of pollution because in most cases it is dominated by particulate matter fraction that can change on an hourly basis due to suspended solids transport. Therefore, we chose DP as an indicator. This study analyzed the DP data of each monitoring section, determined the variation law of phosphorus pollution, and analyzed the sources of phosphorus pollution in the mainstream and key tributaries of the Fuquan section of the Chong-an River from 2015 to 2021. Measures were proposed to reduce phosphorus pollutants in rivers, which has theoretical guiding significance for phosphorus pollution control of local and other places with similar geographical structures.

Sampling site description

The Fengshanqiaobian monitoring section of the Fuquan section of the Chong-an River is located in Fuquan City, Guizhou Province, China. Fuquan City is one of the important phosphorus chemical bases in China. The Fuquan section of the Chong-an River is 131 km long and has a drainage area of 2,774 km2. There are many representative phosphorous chemical enterprises in the basin, which have a great impact on the water quality of the river.

Figure 1 shows that the main first-level tributaries on the left bank of the Chong-an River are the Pilong River, the Weizu River, and the Houhe River, and on the right bank are the Alipu River and Pingpu River.
Figure 1

Sketch map of the monitoring sections and enterprises distribution of the upstream of the Fengshanqiaobian section.

Figure 1

Sketch map of the monitoring sections and enterprises distribution of the upstream of the Fengshanqiaobian section.

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In Figure 1, there are two slag yards in the upstream of the Fengshanqiaobian section, namely, the phosphogypsum slag field of Wengfu and the phosphogypsum slag field of Chuanheng, both of which have been leaking sewage into an underground river. In addition, there are seven urban wastewater treatment plants.

Figure 1 shows that there are a total of 68 industrial enterprises in the basin, which mostly involve phosphate-containing minerals, phosphorous chemicals, and phosphate fertilizers and are mainly distributed along the Pilong River, Houhe River, Alipu River, and the mainstream of the Chong-an River.

In Figure 1, a total of nine monitoring sections of water quality have been set up in the upstream basin of the Fengshanqiaobian section, of which there are four monitoring sections on the mainstream of the Chong-an River. From upstream to downstream, they are the Heitangqiao, Wujiaqiao, Yacaoba, and Fengshanqiaobian monitoring sections. A total of five monitoring sections of water quality have been set up on the tributaries, namely, the City Fertilizer Factory, the Chuanheng Company discharge upstream, the Chuanheng Company discharge downstream along the Houhe River, and the Yueduqushuikou and the Wuliqiao along the Pilong River.

Analysis of water samples

Reagents and instruments

The main parameters of reagents and instruments for the determination of water quality in this study are listed in Table 1.

Table 1

Main parameters of reagents and instruments for the determination of water quality

No.ItemsSpecification
Silver sulfate (Ag2SO4A.R. 
Mercuric sulfate (HgSO4A.R. 
Potassium dichromate (K2Cr2O7A.R. 
Ammonium iron(Ⅱ) sulfate hexahydrate ((NH4)2Fe(SO4)2.6H2O) A.R. 
Iron(II) sulfate heptahydrate (FeSO4.7H2O) A.R. 
Potassium hydrogen phthalate (KC8H5O4A.R. 
Magnesium oxide (MgO) A.R. 
Potassium iodide (KI) A.R. 
Potassium hydroxide (KOH) A.R. 
10 Sodium thiosulfate (Na2S2O3A.R. 
11 Zinc sulfate heptahydrate (ZnSO4.7H2O) A.R. 
12 Sodium hydroxide (NaOH) A.R. 
13 Boric acid (H3BO3A.R. 
14 Sodium carbonate anhydrous (Na2CO3A.R. 
15 Ammonium chloride (NH4Cl) A.R. 
16 Amylum ((C6H10O5)n) A.R. 
17 Potassium persulfate (K2S2O8A.R. 
18 Ascorbic acid (C6H8O6A.R. 
19 Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) A.R. 
20 Potassium phosphate monobasic (KH2PO4A.R. 
21 Sodium fluoride (NaF) A.R. 
22 Sodium acetate trihydrate (CH3COONa·3H2O) A.R. 
23 Lanthanum nitrate (La (NO3)3·6H2O) A.R. 
24 Phenolphthalein (C20H14O4A.R. 
25 Perchloric acid (HClO4A.R. 
26 Potassium antimony tartrate monohydrate (KSbC4H4O7. H2O) A.R. 
27 Acetone (CH3COCH3A.R. 
28 Acetic acid (CH3COOH) A.R. 
29 Sulfuric acid (H2SO4G.R. 
30 Hydrochloric acid (HCl) A.R. 
31 Mercury chloride (HgCl2A.R. 
32 Potassium sodium tartrate (KNaC4H6O6.4H2O) A.R. 
33 Nitric acid (HNO3A.R. 
34 3-Methylamine-alizarin-diacetic acid (C14H7O4·CH2N(CH2COOH)2A.R. 
35 Bromthymol blue (C27H28O5SBr2A.R. 
36 1,10-Phenanathroline monohydrate (C12H8N2·H2O) A.R. 
37 Multiparameter water quality analyzer LH-3B 
38 Fluoride analyzer LH-F3H 
No.ItemsSpecification
Silver sulfate (Ag2SO4A.R. 
Mercuric sulfate (HgSO4A.R. 
Potassium dichromate (K2Cr2O7A.R. 
Ammonium iron(Ⅱ) sulfate hexahydrate ((NH4)2Fe(SO4)2.6H2O) A.R. 
Iron(II) sulfate heptahydrate (FeSO4.7H2O) A.R. 
Potassium hydrogen phthalate (KC8H5O4A.R. 
Magnesium oxide (MgO) A.R. 
Potassium iodide (KI) A.R. 
Potassium hydroxide (KOH) A.R. 
10 Sodium thiosulfate (Na2S2O3A.R. 
11 Zinc sulfate heptahydrate (ZnSO4.7H2O) A.R. 
12 Sodium hydroxide (NaOH) A.R. 
13 Boric acid (H3BO3A.R. 
14 Sodium carbonate anhydrous (Na2CO3A.R. 
15 Ammonium chloride (NH4Cl) A.R. 
16 Amylum ((C6H10O5)n) A.R. 
17 Potassium persulfate (K2S2O8A.R. 
18 Ascorbic acid (C6H8O6A.R. 
19 Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) A.R. 
20 Potassium phosphate monobasic (KH2PO4A.R. 
21 Sodium fluoride (NaF) A.R. 
22 Sodium acetate trihydrate (CH3COONa·3H2O) A.R. 
23 Lanthanum nitrate (La (NO3)3·6H2O) A.R. 
24 Phenolphthalein (C20H14O4A.R. 
25 Perchloric acid (HClO4A.R. 
26 Potassium antimony tartrate monohydrate (KSbC4H4O7. H2O) A.R. 
27 Acetone (CH3COCH3A.R. 
28 Acetic acid (CH3COOH) A.R. 
29 Sulfuric acid (H2SO4G.R. 
30 Hydrochloric acid (HCl) A.R. 
31 Mercury chloride (HgCl2A.R. 
32 Potassium sodium tartrate (KNaC4H6O6.4H2O) A.R. 
33 Nitric acid (HNO3A.R. 
34 3-Methylamine-alizarin-diacetic acid (C14H7O4·CH2N(CH2COOH)2A.R. 
35 Bromthymol blue (C27H28O5SBr2A.R. 
36 1,10-Phenanathroline monohydrate (C12H8N2·H2O) A.R. 
37 Multiparameter water quality analyzer LH-3B 
38 Fluoride analyzer LH-F3H 

Analysis methods

Chemical oxygen demand (COD) was analyzed according to the potassium dichromate method (Patnaik 2010a). Nitrate–nitrogen (NO3-N) was analyzed according to the colorimetric Nesslerization method (Patnaik 2010b). DP was analyzed according to the colorimetric analysis (Patnaik 2010c). The water sample should be filtered with quantitative filter paper or 0.45 μm filter membrane immediately after its collection and tested soon. Fluoride (F) was analyzed according to the ion-selective electrodes analysis (Patnaik 2010d).

Sampling description

Each monitoring section was sampled once a week, for a total of four times a month and 48 times a year. Over the course of seven years, a total of 336 water samples were collected. The preservation and handling of water samples strictly followed the guidelines outlined in the studies by Patnaik (2010a, 2010b, 2010c, 2010d). Water samples were collected at approximately 50 cm below the surface of the river center.

Sample dilution and preservation are carried out as follows: A 100 mL sample aliquot is acidified with the phenolphthalein indicator and then processed with 1–2 mL H2SO4 (∼5 mol/L) before heating. After the sample is hydrolyzed, it is neutralized to a faint pink color using the NaOH solution. The final volume is restored to 100 mL using distilled water. Aqueous samples are taken in polypropylene bottles. Samples should be stored below 4 °C, protected from light, and analyzed within 2 weeks.

Pollution definition

The statement about pollution is defined as polluted with DP, fluoride, nitrate nitrogen, and COD when their concentrations exceed 0.2, 1.0, 1.0, and 20 mg/L, respectively.

Method validation and quality control measures

Method validation. The accuracy and precision of the analytical methods adopted by us were evaluated by comparison with the ones by the reference laboratory (the environmental monitoring organization working for the local government). The analytical approaches verified by the reference laboratory can be used to confirm the reliability of our methods by comparative test. That is, obtained data by our laboratory and the reference one with the same method for the same batch of samples were compared (such as at least five batches of samples and repeated measurements five times for each batch), to determine the reliability of the analytical procedures used by our laboratory.

Quality control measures. The quality control requirements consist of an initial laboratory demonstration capability, subsequently, analyzing a laboratory reagent blank and conducting calibration check standards and instrument performance-check plans in each analysis batch. Other major parts of quality control requirements should comprise running a duplicate analysis in each batch of samples to evaluate the precision of analysis and estimating the accuracy from the repeatability of the known amount of analytes added into the sample.

Water quality of tributary sections of Chong-an River

Water quality of the Houhe River

Industrial enterprises along the Houhe River are densely distributed, as shown in Figure 1; therefore, the water quality was generally poor. From the upstream City Fertilizer Factory section, the midstream Chuanheng Company discharge upstream section to the downstream Chuanheng Company discharge downstream section, with the influx of enterprise discharge and nonpoint source pollution, DP and F generally showed an increasing trend, as shown in Figure 2. Each point in Figure 2 represents the average value from each monitoring section for that specific year. It is calculated by taking the average of 48 water samples collected each year (four water samples are collected each month, totaling 12 months in a year). Since there are a total of 336 water samples collected for each monitoring section over seven years, plotting all of them on the graph would appear crowded and unclear. Therefore, to maintain clarity and facilitate comparison, annual averages are taken instead. The error bars in the graph represent the range of distribution for the 48 data points. Unless otherwise specified, the data presented in Figures 3 and 4 are processed using the same mathematical treatment.
Figure 2

Changes in DP, F, NO3-N, and COD in three monitoring sections of the Houhe River from 2015 to 2021.

Figure 2

Changes in DP, F, NO3-N, and COD in three monitoring sections of the Houhe River from 2015 to 2021.

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Figure 3

Changes in DP, F, NO3-N, and COD in two monitoring sections of the Pilong River from 2015 to 2021.

Figure 3

Changes in DP, F, NO3-N, and COD in two monitoring sections of the Pilong River from 2015 to 2021.

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Figure 4

Changes in DP, F, NO3-N, and COD of four monitoring sections of the main stream of Chong-an River from 2015 to 2021.

Figure 4

Changes in DP, F, NO3-N, and COD of four monitoring sections of the main stream of Chong-an River from 2015 to 2021.

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City Fertilizer Factory section

The section is located at the uppermost part of the Houhe River in Figure 1. According to the monitoring results from 2015 to 2021 in Figure 2, the F, NO3-N, and COD of the City Fertilizer Factory section did not exceed 1.0, 1.0, and 20 mg/L, respectively. However, DP appeared at 0.24 mg/L in 2015, and the main reason was that there were two phosphorus chemical companies nearby whose wastewater did not meet discharge standards.

Table 2, row 3, presents data describing the shape of the hydrological system in the area of the monitored station.

Table 2

Main parameters for describing the shape of the hydrological system in the area of the monitored stations of Houhe River

Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
City Fertilizer Factory section 16.8 0.20 17.1 0.35 17.3 0.48 16.7 0.20 11.6 0.27 17.0 0.34 13.0 0.45 
Chuanheng Company discharge upstream section 17.2 0.39 16.9 0.52 18.2 0.61 16.9 0.27 12.1 0.29 17.2 0.42 12.7 0.63 
Chuanheng Company discharge downstream section 18.3 0.54 17.2 0.56 18.3 0.78 17.2 0.41 12.3 0.43 17.2 0.56 12.7 0.78 
Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
City Fertilizer Factory section 16.8 0.20 17.1 0.35 17.3 0.48 16.7 0.20 11.6 0.27 17.0 0.34 13.0 0.45 
Chuanheng Company discharge upstream section 17.2 0.39 16.9 0.52 18.2 0.61 16.9 0.27 12.1 0.29 17.2 0.42 12.7 0.63 
Chuanheng Company discharge downstream section 18.3 0.54 17.2 0.56 18.3 0.78 17.2 0.41 12.3 0.43 17.2 0.56 12.7 0.78 

Note: There are two columns below each year, the first representing water temperature (°C) and the second representing water flow rate (m3/s).

In the case of the City Fertilizer Factory section, by discussing the observations of the City Fertilizer Factory section, we traced back to the source of the abnormally higher DP concentration in 2015. Then, improvements were made to the wastewater treatment processes of the two chemical companies nearby, resulting in the reduction of phosphorus pollutants in the river.

Chuanheng Company discharge upstream section

The section is located at the downstream side of the City Fertilizer Factory section and adjacent to it (Figure 1). According to the monitoring results from 2015 to 2021 shown in Figure 2, the COD of the Chuanheng Company discharge upstream section did not exceed 20 mg/L. However, DP indicated 0.22, 0.21, 0.24, and 0.50 mg/L in 2015, 2016, 2017, and 2018, respectively, in which the DP concentration reached the highest value in 2018. At that time, there were 10 phosphorus chemical enterprises upstream, and there were leaks in their production areas. There was one domestic sewage treatment plant upstream that discharged treated water into the river. In 2018, there was decreasing rainfall, and the phosphorus chemical industry had a large production volume. From 2015 to 2021, F and NO3-N represented less than 1.0 and 1.0 mg/L, respectively.

Table 2, row 4 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of the Chuanheng Company discharge upstream section, from the analysis of the earlier observations, it is clear that the quantity of rainfall was an important factor in the phosphorus concentration in the river. In addition, the more phosphorus chemical enterprises produced, the more phosphorus pollutants were discharged into the river.

Chuanheng Company discharge downstream section

This section is located at the downstream side of the Chuanheng Company discharge upstream section and next to it (Figure 1). According to the monitoring results from 2015 to 2021 in Figure 2, the COD of the Chuanheng Company discharge downstream section did not exceed 20 mg/L. DP displayed 0.70, 0.30, 0.34, 1.50, and 0.40 mg/L from 2015 to 2019, respectively (Saoudi et al. 2022). From 2015 to 2019, pollutants mainly came from emissions of Chuanheng phosphorus chemical companies and other 20 phosphorus chemical companies upstream, and the reason was that the wastewater from the equipment and pipelines of these enterprises leaked. The production was increasing in 2018, which was why the emissions were also the maximum. Another reason might be the leakage of the Chuanheng phosphogypsum slag field. In addition, there was one domestic sewage treatment plant upstream discharging treated water into the river. F reached less than 1.0 mg/L in 2017, 2018, 2019, 2020, and 2021 and was 1.2 and 1.5 mg/L in 2015 and 2016, respectively. NO3-N was 6.6 mg/L only in 2016. The reasons were as follows: (1) at that time, there was a significant application of nitrogen fertilizers in the nearby farmland and (2) there were three livestock and poultry farms in the vicinity.

Table 2, row 5 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of Chuanheng Company discharge downstream section, it is known from the aforementioned analysis that phosphorus pollutants mainly came from the leakage in the production process of phosphorus chemical enterprises, and the leakage of phosphorus gypsum residue field, and therefore strengthening the seepage and leakage prevention of their plants and the phosphorus gypsum residue fields could give rise to the reduction of phosphorus and fluorine into the water body. Reducing the application of nitrogen fertilizer and keeping livestock breeding enterprises away from the riverbanks can lead to a decrease in NO3-N in the river.

Water quality of the Pilong River

From upstream to downstream, the Pilong River has two monitoring sections, Yueduqushuikou and Wuliqiao, as shown in Figure 1. From the upstream Yueduqushuikou section to the downstream Wuliqiao section of the Pilong River, with the influx of enterprise discharge and nonpoint source pollution, DP and F showed an increasing trend (Zhu et al. 2021).

Yueduqushuikou section

The section is located on the upstream side of the Pilong River in Figure 1. According to the monitoring results from 2015 to 2021 in Figure 3, the DP, F, NO3-N, and COD of the Yueduqushuikou section did not exceed 0.2, 1.0, 1.0, and 20 mg/L, respectively, and showed a slight downward trend, and the water quality was relatively stable. At that time, the six phosphorus chemical-related companies upstream had already ceased production or relocated, and there was no phosphogypsum waste yard. In addition, the agricultural lands along both banks of the Pilong River had a relatively small area, and therefore, the amount of phosphate fertilizer applied was also relatively small.

Table 3, row 3 presents data describing the shape of the hydrological system in the area of the monitored station.

Table 3

Main parameters for describing the shape of the hydrological system in the area of the monitored stations of Pilong River

Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
Yueduqushuikou section 17.6 0.63 18.5 0.38 18.6 0.14 18.3 0.19 13.0 0.38 17.5 0.42 18.0 0.53 
Wuliqiao section 18.7 0.96 18.0 0.45 18.4 0.24 18.0 0.24 13.3 0.58 17.0 0.66 17.9 0.76 
Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
Yueduqushuikou section 17.6 0.63 18.5 0.38 18.6 0.14 18.3 0.19 13.0 0.38 17.5 0.42 18.0 0.53 
Wuliqiao section 18.7 0.96 18.0 0.45 18.4 0.24 18.0 0.24 13.3 0.58 17.0 0.66 17.9 0.76 

Note: There are two columns below each year, the first representing water temperature (°C) and the second representing water flow rate (m3/s).

In the case of the Yueduqushuikou section, reducing emissions from phosphorus chemical enterprises and controlling leakage from phosphogypsum residue sites contributed to the decrease in phosphorus concentrations. Moreover, agricultural nonpoint source pollutants were an important factor in phosphorus concentration in the river; therefore, limiting the area of farmland along both sides of the river might play a vital role in bringing about reducing the amount of phosphorus fertilizer applied.

Wuliqiao section

This section is located on the downstream side of the Yueduqushuikou section in Figure 1. According to the monitoring results from 2015 to 2021 in Figure 3, the NO3-N and COD of the Wuliqiao section did not exceed 1.0 and 20 mg/L, respectively. The DP showed 1.42, 0.62, 0.60, and 0.24 mg/L in 2015, 2016, 2017, and 2018, respectively, which reached the maximum value in 2015 and then showed a decreasing trend from 2019 to 2021 up to the DP of less than 0.2 mg/L. F appeared 0.12 mg/L in 2015. At that time, the main sources of pollution were the leakage from the nearby Wengfu Phosphorus Chemical Company and the other three phosphorus chemical company production areas upstream. There was also leakage from Chuanheng's phosphogypsum waste yard into an underground river. In addition, there were large areas of farmland near the banks with significant application of phosphate fertilizers.

Table 3, row 4 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of Wuliqiao section, from the evaluation of the aforementioned data, it can be seen that the improvement of seepage control measures by the phosphorus chemical enterprises (Wengfu Company, etc.) and the phosphorus gypsum residue field of Chuanheng Company had a significant impact on the reduction of phosphorus concentration in 2019–2021. Furthermore, reducing the amount of phosphorus fertilizer applied or replacing solid phosphorus fertilizer with a liquid one in nearby farmlands along both sides of the river had a positive effect on it.

Water quality of the monitoring sections of the mainstream of the Chong-an River

Heitangqiao section

This section is located at the upstream side of the mainstream of the Chong-an River and is the uppermost one of the four monitoring sections on the mainstream, as shown in Figure 1. As shown in Figure 4, according to the monitoring results from 2015 to 2021, the DP, F, NO3-N, and COD of the Heitangqiao section did not exceed 0.2, 1.0, 1.0, and 20 mg/L, respectively. Among them, DP, F, and NO3-N did not change significantly, COD showed a slight upward trend, and the water quality was relatively stable. This was because there were no phosphorus chemical enterprises and phosphogypsum slag fields nearby.

Table 4, row 3 presents data describing the shape of the hydrological system in the area of the monitored station.

Table 4

Main parameters for describing the shape of the hydrological system in the area of the monitored stations of Chong-an River

Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
Heitangqiao section 15.9 4.39 18.3 3.33 18.2 2.24 18.3 2.66 17.8 2.82 17.3 3.02 17.2 3.44 
Wujiaqiao section 17.0 9.00 18.3 4.97 18.5 5.55 18.5 3.91 13.3 5.26 17.4 4.23 17.9 4.87 
Yacaoba section 17.8 15.53 18.2 12.65 18.1 14.93 17.7 8.52 12.8 6.91 17.2 11.67 18.3 7.34 
Fengshanqiaobian section 18.2 21.70 18.2 23.57 18.2 21.55 17.8 14.56 13.0 9.02 15.3 22.89 18.0 9.65 
Sections2015
2016
2017
2018
2019
2020
2021
°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s°Cm3/s
Heitangqiao section 15.9 4.39 18.3 3.33 18.2 2.24 18.3 2.66 17.8 2.82 17.3 3.02 17.2 3.44 
Wujiaqiao section 17.0 9.00 18.3 4.97 18.5 5.55 18.5 3.91 13.3 5.26 17.4 4.23 17.9 4.87 
Yacaoba section 17.8 15.53 18.2 12.65 18.1 14.93 17.7 8.52 12.8 6.91 17.2 11.67 18.3 7.34 
Fengshanqiaobian section 18.2 21.70 18.2 23.57 18.2 21.55 17.8 14.56 13.0 9.02 15.3 22.89 18.0 9.65 

Note: There are two columns below each year, the first representing water temperature (°C) and the second representing water flow rate (m3/s).

In the case of the Heitangqiao section, if there were no pollutants from phosphorus chemical enterprises and phosphogypsum residue sites discharging into the river, phosphorus pollution from the natural environment would be minimal.

Wujiaqiao section

The section is located on the downstream side of the Heitangqiao monitoring section and adjacent to it (Figure 1). As shown in Figure 4, according to the monitoring results from 2015 to 2021, the F and COD of the Wujiaqiao section did not exceed 1.0 and 20 mg/L, respectively. However, DP showed 0.6, 1.0, and 0.24 mg/L, respectively, from 2015 to 2017. The DP concentration was the highest in 2016 and then decreased year by year, from 2018 to 2021. Reasons for the increasing phosphorus concentration in 2015, 2016, and 2017 were as follows: (1) at that time, 26 phosphorus chemical companies were allowed to have direct discharge outlets; (2) leakage occurred from these 26 phosphorus chemical companies; (3) there were large areas of farmland along the coast with the significant use of phosphate fertilizers; (4) there was one urban sewage treatment plant nearby discharging treated water into the river. The reasons for the decrease in phosphorus concentration in 2018, 2019, 2020, and 2021 were as follows: (1) the prohibition of direct discharge outlets for the 26 phosphorus chemical companies; (2) reduction in the use of solid phosphate fertilizers in coastal farmland, with an increase in liquid phosphate fertilizer usage, leading to better phosphorus absorption and reduced runoff; (3) however, in 2020, there might have been occasional phosphorus concentrations exceeding 0.2 mg/L due to insufficient rainfall and increased production by the phosphorus chemical industry. In 2015, 2017, 2018, 2020, and 2021, the NO3-N exhibited 1.3, 2.6, 1.7, 1.6, and 1.5 mg/L, respectively. The reasons were as follows: (1) at that time, there was a significant application of nitrogen fertilizers in the nearby farmland; (2) untreated rural sewage was discharged into the river; (3) there was one urban sewage treatment plant nearby, discharging treated water into the river; (4) there were three livestock and poultry farms in the vicinity.

Table 4, row 4 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of the Wujiaqiao section, it shows that prohibiting the drainage outlet of phosphorus chemical enterprises, the seepage control of their production plant, reducing the area of farmland along the banks of the river, and limiting the concentration of the discharge from the urban domestic wastewater treatment plant had a significant impact on the phosphorus pollution. If the appropriate response measures were used, the impact of reduced rainfall and increased production from phosphorus chemical enterprises on phosphorus pollution could be attenuated.

In the case of this place, measures for controlling NO3-N can be categorized into strengthening the treatment of sewage from rural life and production, reducing the amount of nitrogen fertilizers or replacing solid nitrogen fertilizers with liquid ones, decreasing the discharge concentrations from sewage treatment plants, and keeping livestock and poultry farms away from riverbanks.

Yacaoba section

This section is located on the downstream side of the Wujiaqiao monitoring section and next to it (Figure 1). As shown in Figure 4, according to the monitoring results from 2015 to 2021, the F and COD of the Yacaoba monitored section did not exceed 1.0 and 20 mg/L, respectively. However, the DP levels were 0.80, 0.78, 0.26, and 0.28 mg/L in 2015, 2016, 2017, and 2020, respectively. The DP concentration was the highest in 2015 and then decreased year by year, satisfying less than 0.2 mg/L in 2018, 2019, and 2021. Reasons for the increasing phosphorus concentration in 2015, 2016, 2017, and 2020 were as follows: (1) 10 phosphorus chemical companies were allowed to have direct discharge outlets; (2) leakage occurred from these 10 phosphorus chemical companies; (3) leakage from the Wengfu phosphogypsum residue field; (4) discharges from three urban sewage treatment plants located in densely populated main city areas upstream; (5) there were large areas of farmland along the coast with significant use of phosphate fertilizers. In 2020, the reason for exceeding the standards might be excessive rainfall, causing the dissolution of unsealed phosphogypsum residue without proper antileakage measures. The reasons for the decrease in phosphorus concentration in 2018, 2019, and 2021 were as follows: (1) prohibition of direct discharge outlets for 10 phosphorus chemical companies; (2) strengthened measures to prevent leakage from the 10 chemical companies; (3) improved antileakage measures at the Wengfu phosphogypsum residue field; (4) upgraded water quality discharge standards for sewage treatment plants in the main city area; (5) reduction in the use of solid phosphate fertilizers in coastal farmland and increased usage of liquid phosphate fertilizers, leading to better phosphorus absorption and reduced runoff. In 2018 and 2020, the concentrations of NO3-N were 1.4 and 1.5 mg/L, respectively. The reasons for the increase in NO3-N were as follows: (1) nearby farmland had a large amount of nitrogen fertilizer application; (2) untreated sewage from nearby rural areas was discharged into rivers; (3) there were three urban sewage treatment plants in the vicinity that discharged treated water into rivers; (4) there were also five livestock and poultry farms nearby.

Table 4, row 5 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of the Yacaoba section, as discussed earlier, the measures adopted for the reduction of phosphorus pollution were effective and the approaches used for the reduction of NO3-N were useful.

Fengshanqiaobian section

This section is located at the downstream side of the Yacaoba section and adjacent to it (Figure 1), which is the border control point for the mainstream of the Chong-an River to flow out of the Fuquan city. As shown in Figure 4, from 2015 to 2021, with the continuous pollution control efforts adopted in the Chong-an River basin, the DP, F, and NO3-N concentrations in this section show a significant downward trend (Huang et al. 2017). Among them, F was less than 1.0 mg/L from 2016 to 2021 except in 2015. Because phosphate ore is a mixture, fluorine and phosphorus are associated substances that occur simultaneously in phosphate ore. Therefore, when the phosphorus concentration is increasing, the fluorine concentration is also increasing. The DP concentrations were 5.4, 0.6, 0.44, 0.3, 0.20, 0.22, and 0.21 mg/L in 2015, 2016, 2017, 2018, 2019, 2020, and 2021, respectively, in which they were beyond 0.4 mg/L from 2015 to 2017, whereas less than 0.4 mg/L from 2018 to 2021, respectively. The following are reasons for increasing phosphorus concentration in the first three years: (1) at that time, the 68 upstream phosphate chemical enterprises were allowed to have direct discharge outlets; (2) leakage occurred from the 68 upstream phosphate chemical enterprises; (3) leakage occurred from the Wengfu phosphogypsum residue yard; (4) one upstream leakage industrial wastewater treatment plant discharged treated water into the river; (5) there were a large number of farmlands along the coast where phosphate fertilizers were extensively used. Reasons for decreasing phosphorus concentration in the following four years are as follows: (1) direct discharge outlets for the 68 upstream phosphate chemical enterprises were prohibited; (2) the 68 upstream phosphate chemical enterprises strengthened leak prevention measures; (3) the Wengfu phosphogypsum residue yard strengthened leak prevention measures; (4) upgraded water quality emission standards were implemented by one industrial wastewater treatment plant; (5) farmland along the coast reduced the use of solid phosphate fertilizers, increased the use of liquid phosphate fertilizers, enhanced phosphorus absorption, and reduced runoff.

Each point in Figure 5 represents the average value for that specific month. It is calculated by taking the average of four water samples collected each month. Since there are a total of 48 water samples collected over one year, plotting all of them on the graph would appear crowded and unclear. Therefore, to maintain clarity and facilitate comparison, month averages are taken instead. The error bars in the graph represent the range of distribution for the four data points.
Figure 5

Variations in the monthly average DP of the Fengshanqiaobian section of the Chong-an River in 2021.

Figure 5

Variations in the monthly average DP of the Fengshanqiaobian section of the Chong-an River in 2021.

Close modal

In Figure 5, by the monitoring data of DP from January to December 2021, DP values are 0.24, 0.22, and 0.29 mg/L in January, March, and May, respectively. January and March were the dry season, and the decrease in water volume caused the increase of the phosphorus concentration in the river. May belonged to the wet season, and the groundwater level rose, resulting in an increase in the solubility of the unsealed phosphogypsum. Farmlands applying phosphorus fertilizer were washed by rain into the river, and phosphorus chemical enterprise plants were brushed by rain, leading to phosphorus into the river (Oehmen et al. 2007). Resuspension of within river channel constituents could indeed be a possible reason for elevated concentrations during wet periods. During periods of increased streamflow, such as the wet season, there can be increased turbulence and higher velocities within the river channel. This can lead to the resuspension of sediments and associated constituents that were previously deposited on the riverbed or banks.

Table 4, row 6 presents data describing the shape of the hydrological system in the area of the monitored station.

In the case of the Fengshanqiaobian section, this indicates that rainfall was a more important factor. Therefore, the decrease in rainfall would lead to an increase in phosphorus concentration, and the increase in rainfall would result in flushing more phosphorus into the water and more dissolutions of phosphogypsum buried in the subsurface.

The DP in the basin was more than 0.2 mg/L in the dry season, because the decrease in water volume caused the increase of the phosphorus concentration in the river, whereas in the wet season, the increase in water volume led to the increase of the phosphorus concentration for the dissolution of unsealed phosphogypsum slags buried underground, the washing by rain of the plant areas of the phosphorus-related enterprises, and the entering river of a lot of agricultural nonpoint source pollutants with rain. Thus, it is helpful to strengthen the comprehensive utilization of phosphogypsum slags, carry out the antiseepage engineering operations in the plant areas of phosphorus-related enterprises, and reduce agricultural nonpoint source pollutants such as phosphorus fertilizers, pesticides, and animal manure into the river to decrease the DP.

The strengths of this study focus on the research lasted seven years, and the data collected are large and representative, which is conducive to finding out the specific reasons. The limitation of this study is that the structure, distribution, and location of contaminated underground rivers cannot be determined, which makes it difficult to accurately trace the source of pollution. Moreover, it is impossible to accurately grasp the dissolution, transformation, and movement of a lot of phosphogypsum slags buried in the ground without antiseepage engineering adopted before 1997 caused by the variation in the groundwater level.

The authors are grateful to the staff of the local environmental department for facilitating the study and providing data.

Xiaobin Lu was accountable for sample analysis, data evaluation, writing, reviewing and editing.

This work was supported by the Guizhou Provincial Basic Research Program (Natural Science) (Qiankehejichu[2020]1Y047).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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