Ya-wei Zhang, Yun-tao Liu, Zi-wen Wang, Yu Cao, Xiao-ran Tu, Di Cao, Shuai Yuan, Xiao-man Cheng,Lian-sheng Zhang

1 School of Environmental Studies, China University of Geosciences, Wuhan 430074, China.2 Henan Fifth Geological Exploration Institute Co., Ltd., Zhengzhou 450001, China.3 Henan Second Geological Survey Institute Co., Ltd., Zhengzhou 450001, China.4 Henan Rock and Mineral Testing Center, Zhengzhou 450001, China.

Abstract: Over the years, the Shaying River Basin has experienced frequent instances of river pollution.The presence of numerous critical pollutant discharge enterprises and sewage-treatment plants in the vicinity of the Shaying River has transformed it a major tributary with relatively serious pollution challenge within the upper reaches of Huaihe River Basin.To study the sources of manganese (Mn), chromium (Cr),nickel (Ni), arsenic (As), cadmium (Cd) and lead (Pb) in Shaying River water, 123 sets of surface water samples were collected from 41 sampling points across the entire basin during three distinct phases from 2019 to 2020, encompassing normal water period, dry season and wet season.The primary origins of heavy metals in river water were determined by analyzing the heavy metal contents in urban sewage wastewater,industrial sewage wastewater, groundwater, mine water, and the heavy metal contributions from agricultural non-point source pollution.The analytical findings reveal that Mn primarily originates from shallow groundwater used for agricultural irrigation, While Cr mainly is primarily sourced from urban sewage treatment plant effluents, coal washing wastewater, tannery wastewater, and industrial discharge related to metal processing and manufacturing.Ni is mainly contributed by urban sewage treatment plant effluents and industrial wastewater streams associated with machinery manufacturing and metal processing.Cd primarily linked to industrial wastewater, particularly from machinery manufacturing and metal processing facilities,while Pb is predominantly associated with urban sewage treatment plant effluents and wastewater generated in Pb processing and recycling wastewater.These research provides a crucial foundation for addressing the prevention and control of dissolved heavy metals at their sources in the Shaying River.

Keywords: Industrial waste; Urban sewage; Mine water; Agricultural non-point source pollution

Introduction

Heavy metals possess distinct characteristics, such as long-lasting persistence and propensity for bioaccumulation, rendering them main pollutants in the ecological environment (Bhupal et al.2014;Zamora-Ledezma et al.2021; Dai et al.2015).Compared to other water ecosystems, rivers are particularly susceptible to heavy metal pollution due to their open ecological nature (Chen et al.2014).Extensive research, both domestically and internationally, has been conducted on the distribution patterns, temporal dynamics, and pollution source analysis of soluble heavy metals in water.In most river basins, the temporal distribution of heavy metal concentrations follows the sequence of dry season ＞ wet season, as observed in the Pearl River Basin (Wang et al.2012), the middle and lower reaches of the Le'an River (Yu et al.2020), the Tigris River (Varol et al.2013) , and the San Pedro River (Gómez-Alvarez et al.2014).This pattern is primarily attributed to the dilution effect caused by heavy rainfall during the wet season.However, some rivers exhibit the opposite trend, as seen in Kones River where heavy metal concentrations are higher in wet season than those in dry season, due to mineral resources weathering and dissolution into the river, particularly lead and zinc(Li et al.2009).The Mahanadi estuary experiences elevated concentrations of heavy metals in monsoon season, potentially related to agricultural runoff (Sundaray et al.2012).The high concentration of heavy metals in certain sections of rivers,such as the Rocha River, is related to industrial pollution (Terrazas-Salvatierra et al.2020).In terms of spatial distribution, heavy metal concentrations typically decrease along the course of the river, possibly due to the self-purification effect of rivers (Zhang et al.2021).

As the largest tributary in the upper reaches of the Huaihe River, the Shaying River plays a pivotal economic role in Henan Province and hosts two major coal production regions in the upper reaches, while its middle and lower reaches support densely populated and traditionally irrigated agricultural areas.The pollution load in the Shaying River has a significant impact on the water quality of the Huaihe River (Chu, 2001; Ding et al.2019).In recent years, there has been a noticeable upward trend in soluble heavy metal concentrations within the Shaying River.Understanding the spatial-temporal distribution and sources of these pollutants is crucial for the sustainable development and prudent management of water resources,as well as pollution transport in the basin.

Currently, studies on soluble heavy metals in the Shaying River Basin have mainly focused on concentration determination, their potential impact on human health, sediment pollution characteristics, and etc.(Ding et al.2019; Wu et al.2021).However, there is a dearth of comprehensive research in regarding the source of soluble heavy metals.This study addresses the gap by analyzing the spatial-temporal variations in soluble heavy metal concentrations in the Shaying River Basin during the normal, dry and wet seasons from December 2019 to August 2022 through combining with the data of wastewater discharge from industrial and agricultural activities to conduct a detailed analysis of the origins of soluble heavy metals in the basin.

1 Study area

The Shaying River's upstream tributaries, including Jialu River, Shahe River, Beiru River and Yinghe River, converge with the main stream at Zhoukou City.To the northwest of the river basin,the terrain is characterized by mountains and hills,with ground elevations ranging from 500 m to 2,100 m.Predominant land use types in this region consist of forested land and cultivated land.In Henan Province, the Shaying River Basin primarily features irrigated farmland, while in Anhui non-irrigated farmland prevails.Towns, villages, as well as industrial zones and mining areas are widely distributed in Shaying River Basin (Fig.1a).The geological composition of the upstream region is complex, with various mineral deposits.The cross section representation of Songji area geological structure (Fig.1b) encompasses Quaternary silty mudstone, Cambrian strata, Jurassic granite,Archaean granite and diorite, and etc.In the plains area, the predominant soil type is mainly alluvial soil, followed by protosol, denatured soil, alfisol and planosol.The vadose zone is typically composed of silt or silty clay.Shallow groundwater primarily comprises pore water within loose rock formations, occurring in the sand, gravel, and gravel layers of the Quaternary sediments.

The Shaying River Basin is located in the transitional climatic zone between the warm temperate and subtropical zones.Average annual rainfall typically ranges from 650 mm to 1,400 mm (Chu,2001; Wu et al.2021), with significant inter-annual changes.The climate is characterized by dry winters with minimal precipitation and hot, rainy summers.Rainstorms mainly occur from June to August.In 2019, the region experienced a lowwater year, while the year of 2020 witnessed highwater conditions.During the flood season (June to September) in Zhoukou region in 2019, the total precipitation was merely 260.7 mm, representing 52.7% of the historical average precipitation for the same period.In contrast, the precipitation in the Huaihe River Basin was 1,007.8 mm from January to August in 2020, 41.8% higher than the historical records in same period.The primary source of river water in the study area originates from the upstream area of the Shaying River, with an average annual runoff of 5.1 billion m3(Wu et al.2021), and the annual runoff of Zhoukou Station accounts for 4 billion m3.Statistics of rainfall and river runoff (Fig.1c) show that there are significant river runoff differences between high-water and low-water years, as well as during high-water and low-water periods.

2 Materials and methods

2.1 Layout of sampling points

Fig.1 a.Distribution of sampling points in the Shaying River Basin; b.Geological profile; c.Rainfall and river flow chart from Fuyang hydrologic station

Water samples were collected during three distinct seasons: The normal season (December 28, 2019 -January 2, 2020), dry season (May 25-29, 2020)and wet season (August 24-28, 2020).The locations of the sampling points are shown in Fig.1.A total of 41 sampling points were strategically positioned to collect 123 sample sets from the Shaying River, ensuring a comprehensive coverage of the entire river basin.When important tributaries converge into the main river, water samples from the main river were collected 500 m upstream and 2,000 m downstream from the confluence point.

2.2 Sampling method and water quality test

Water samples were obtained by collecting the water from a depth 5 cm below the surface within the flowing water area.On-site filtration was conducted using 0.22-micron PES filter membranes.Subsequently, all samples were added with 2 drops of nitric acid (GR), after which they were tightly sealed and stored in freezers maintained at 4°C for subsequent analysis (Feng et al.2012; Zhang et al.2019).The concentration of heavy metal elements in water samples was determined via inductively coupled plasma mass spectrometry (ICP-MS).The detection limits for Cr, Mn, Cd, Ni, As and Pb were 0.11 μg·L-1, 0.5 μg·L-1, 0.05 μg·L-1, 0.06 μg·L-1, 0.09 μg·L-1and 0.07 μg·L-1, respectively.The test was conducted at the Institute of Geochemistry Chinese Academy of Sciences.

3 Results

3.1 Test results

The summary of heavy metal concentration test results in the Shaying River Basin are shown in Table 1 and Table 2.

3.2 Basic characteristics of soluble heavy metals

Table 1 and Table 2 show that, with the exception of Mn, the average concentration of soluble heavy metals in the river water of the Shaying River basin is low.This observation is basically consistent with the findings of Ding et al.(2019).During the normal season, the average soluble heavy metals concentrations in the Shaying River follows the sequence of Mn ＞ Ni ＞ As ＞ Cr ＞ Pb ＞ Cd.In the dry season, the order is Mn ＞ As ＞ Ni ＞ Cr ＞Pb ＞ Cd, while during the wet season, it is As ＞Mn ＞ Ni ＞ Cr ＞ Pb ＞ Cd.It's worth noting that the maximum concentration of Mn during the dry season exceeds the limits specified in Category III of the Environmental Quality Standard for Surface Water.As demonstrated in Table 3, heavy metal concentrations in the study area are notably lower than those in economically developed basins such as Chaohu Lake (Wu et al.2018), Taihu Lake(Wang, 2016) and the Jiulongjiang River (Chen et al.2018).This difference may be attributed to the low industrial discharges in the Shaying River Basin.Compared to the basins with similar economic development level in the middle and lower reaches of the Han River (Chen et al.2018), the concentrations of As and Cd are higher in the Shaying River basin, while the concentrations of Mn and Pb are lower.These variations can be attributed to different patterns of industrial and agricultural development in these regions.

3.3 Heavy metal inputs in the basin

(1) Soluble heavy metal inputs from sewage treatment plants and industrial enterprises

Table 4 summarizes self-monitoring data obtained from sewage treatment plants and several enterprises located in proximity of the upstream tributaries of the Shaying River Basin.The results indicate noteworthy findings regarding the presence of specific metals.For example, Mn concentrations at coal mining-washing enterprises and sewage treatment plants in the tributaries, specifically Yinghe River and Shahe River, range from 80-210 μg·L-1and 4-92 μg·L-1, respectively.Ni concentrations observed in sewage treatment plants and some enterprises near the tributary Jialu River and Shahe River vary from 15-230 μg·L-1and 20-90 μg·L-1, respectively.Moreover, Cr concentrations exhibit notable diversity in sewage treat-ment plants and some enterprises situated at the areas close to the Jialu River, Ying River, Beiru River and Shahe River, with values ranging from 0.24-950 μg·L-1, 0.01-910 μg·L-1, 36-53 μg·L-1and 0.1-149 μg·L-1, respectively.Similarly, As concentrations in the area range from 0.05-30 μg·L-1, 0.021-350 μg·L-1, 0.3-30 μg·L-1and 0.1-30 μg·L-1, respectively.Cd concentrations are in the range of 0.05-6 μg·L-1, 0.1-30 μg·L-1, 0.1-5 μg·L-1and 0.025-18 μg·L-1, respectively.Pb concentrations are 0.09-50 μg·L-1, 1-200 μg·L-1,1-20 μg·L-1and 0.2-90 μg·L-1, respectively.Notably, leather production enterprises near the Jialu River and Shahe River Basin contribute relatively high Cr content in the water, while coal mining and washing enterprises exert a significant impact on the contents of Mn, Cr, As, and Pb in the waters of the Yinghe River Basin.

Table 1 The average heavy metal concentrations in river water of the Shaying River Basin at different time periods

Table 3 Concentrations of heavy metals and relevant water environment quality standards in different river basins

(2) Soluble heavy metal inputs in groundwater

Dissolved heavy metal inputs in groundwater include Karst water drained by coal mining and pore water extracted in plain areas.

Coal mine water: The upper reaches of the Shaying River Basin are the main coal production area in Henan Province, housing two major coalfields:Zhengzhou and Pingdingshan coalfields.Within this region, there are a total of 158 coal mines in Pingdingshan, 69 in Xuchang, 20 in Zhengzhou and 1 in Luohe.A large amount of mine water is generated during mining and coal washing processes, with most of this groundwater eventually entering the surface water system.Notably, the concentrations of certain heavy metals in mine water are significantly higher than those in river water.For example, based on the measurement of He et al.(2009), the concentrations of Pb, Cd and Mn are 18 μg·L-1, 6 μg·L-1and 62.9 μg·L-1, respectively.

Other groundwater inputs: The concentration of heavy metals in groundwater is different from that observed in the Shaying River.In the upper reaches of the Shaying River, groundwater is primarily exploited for rural water supply and agricultural irrigation.In the middle and lower reaches, it is mainly used for domestic, industrial and irrigation purposes.According to data from 1 475 groups of groundwater samples collected by the Henan Provincial Geological Survey Institute in the Henan plain area of the Huaihe River Basin,Mn concentrations range from 1 μg·L-1to 594 μg·L-1with a mean value of 345 μg·L-1; As levels span from 0-150 μg·L-1, with a mean value of 3.26 μg·L-1; Pb concentrations vary from 0-192 μg·L-1,with a mean value of 4.77 μg·L-1; Cd levels range from 0-880 μg·L-1, with a mean value of 1.46 μg·L-1.The Cr concentration exceeds the detection limit only in 34 groups, with a detection rate of 2.3% and low detection concentrations.In a separate study by Zhang et al.(2013), encompassing 59 groups of the groundwater samples collected in areas irrigated by the Yellow River,the average concentration of Ni was recorded at 2 μg·L-1.Generally, the concentrations of Cr and Ni in groundwater are slightly lower than those in the Shaying River, and As concentration is approximately twice as high as that observed in river water.In contrast, Mn, Pb and Cd concentrations are significantly higher in groundwater than those in river water.

4 Discussion

4.1 Spatial-temporal distribution and source analysis of Mn

The overall Mn concentration in the river water ofthe Shaying River Basin is consistently low, generally remaining below 2 μg·L-1.It has relatively obvious variation with the season change.At certain sampling locations, particularly at Jieshou to Taihe in the middle and lower reaches of the Shaying River mainstream, Mn concentrations are slightly higher during the dry season.Notably,sampling point No.31 situated at Qianping Reservoir in Ruyang, a tributary of the Shaying River,Mn concentration is 78.21 μg·L-1during the normal season.At point No.37 (Baisha Reservoir in Ying River), Mn concentrations in normal, dry and wet seasons are 74.59 μg·L-1, 112.32 μg·L-1,43.90 μg·L-1, respectively.Mn concentration at point No.24 (Baiguoshan Reservoir in the Sha River) in the normal season is 24.10 μg·L-1.Mn concentration at point No.25 (under the dam of Zhaotai Reservoir in the Sha River) in the wet season is 20.93 μg·L-1.Mn concentrations at points No.6 and No.8 (Jieshou - Taihe section of the Shaying River) in the dry season are 8.96 μg·L-1and 8.48 μg·L-1, respectively.

Table 4 List of soluble metal concentrations monitored by enterprises in the Shaying River Basin

Table 4 (continued)

The Shaying River Basin is a typical high-manganese groundwater area.However, the introduction of high-manganese groundwater into the river system does not lead to a significant increase in Mn concentration in the river water.This can be attributed to the manganese removal processes applied during the domestic and industrial use of groundwater, as well as the utilization of coal mine drainage as a resource.These factors are also the primary contributors to the low Mn concentration levels (Table 4) at sewage outlets of wastewater treatment plants.As depicted in the concentration distribution diagram in Fig.2, samples collected from points No.31, 37, 24, 25 are all collected at near large or medium-sized reservoirs or under adjacent dams.The relatively high Mn contents at these sampling point cab ne attributed to the enrichment effect typically associated with reservoirs(Christian et al.2015).Notably, the abnormal high Mn content in the river water is primarily characterized by higher Mn levels in dry season compared to in normal season and higher levels in normal season compared to the rainy season.This trend indicates that the dilution effect of rainwater plays a crucial role in reducing Mn content.Furthermore, the slightly elevated manganese concentrations observed in the river water of the middle and lower reaches of the mainstream can be attributed to increased irrigation in May, particularly during the wheat filling and maturation period (Liu et al.2016).The extensive flood irrigation during this period allows a small amount of high-manganese groundwater to enter the river system.In addition,the lack of both dilution of upstream runoff and atmospheric precipitation (Fig.1c) contributes to the elevated manganese concentration in the river water.

4.2 Spatial-temporal distribution and source analysis of Cr

The concentration of Cr in the river water of the Shaying River's upper reaches and its tributary, the Beiru River, exhibits a seasonal pattern, with the highest concentrations observed during the dry season, followed by the normal season and wet season, and similar levels during the wet season.Spatially, Cr concentration in the Shaying River mainstream increases downstream during the wet and normal seasons, with levels at the confluence of the Shaying River and the Huaihe River surpassing those in the mainstream of the Huaihe River.Notable concentrations during the normal season are 0.47 μg·L-1at No.9 (Jieshou), 0.43 μg·L-1at No.5 (Fuyang).In the wet season, ehe highest values are 0.43 μg·L-1at No.5 (Fuyang)and No.15 (Zhoukou).In the dry season, Cr concentrations are higher in the upper reaches and lower in the lower reaches, with a maximum value of 0.52 μg·L-1at No.24 (the Baiguishan Reservoir in the Shahe River).In the tributary Yinghe River,Cr concentrations increase downstream during the wet season, decrease during the normal season, and then increase again after decreasing during the dry season.The maximum value is 0.81 μg·L-1at No.34 in the normal season (Xihua).In contrast, Cr concentration in the tributary Beiru River changes little along its course, with a maximum value of 0.52 μg·L-1at No.31 (Ruyang) in dry season.In the Jialu River, the concentrations of Cr decrease downstream during the normal and dry seasons,but increases during the wet season, with a maximum value of 0.43 μg·L-1at No.40 in the normal season (Weishi).

Table 4 shows that Cr concentrations in urban sewage treatment plants are much higher than those in the Shaying River.The drainage from the Sewage treatment plants exhibits high Cr concentrations in Zhengzhou municipal district (16-80 μg·L-1) and Xingyang (4-16 μg·L-1) in the upper reaches of the Jialu River, Dengfeng (4-30 μg·L-1)and Xinmi (12-24 μg·L-1) in the upper reaches of the Yinghe River, Ruyang (38-53 μg·L-1) and Xiangxian (36-51 μg·L-1) in the Beiru River basin,as well as in Shangshui located at the mainstream of the Shaying River (20-100 μg·L-1).The coal washing industry is another significant source of high Cr concentration, with Cr levels of 15-910 μg·L-1in Jian'an and 50-90 μg·L-1in Yuzhou.There are 248 coal mines and numerous coal washing plants in the upper reaches of the Shaying River.Five tannery enterprises in Xiangcheng,downstream of the Shaying River, discharge wastewater with Cr concentrations ranging from 30-149 μg·L-1, while in Changge, the tannery wastewater contains Cr in the range of 70-102 μg·L-1.Jieshou City houses seven lead processing and recycling enterprises, which release wastewater with Cr concentrations of 39-44 μg·L-1.These aforementioned wastewater sources constitute the contributors to Cr levels in the Shaying River.Urban sewage and coal washing wastewater are the main sources in the middle and upper reaches, While tanning wastewater and sewage from metal processing and manufacturing enterprises are the main factors driving the rise of Cr concentrations in the lower reaches of the river.During the dry season,Cr concentrations in the lower reaches are significantly lower than those in the middle and upper reaches, which is due to the impact of COVID-19 epidemic, resulting in reduced production and sewage discharge of tannery enterprises, as well as metal processing and manufacturing enterprises.

Fig.2 Spatial-temporal distribution of heavy metal concentrations in the Shaying River Basin

4.3 Spatial-temporal distribution and source analysis of Ni

The temporal distribution of Ni concentration in the Shaying River mainstream follows the pattern of normal season ＞ dry season ＞ wet season.At the confluence of Shaying River and Huaihe River, Ni concentrations are higher than those in the mainstream of the Huaihe River.Regarding spatial distribution in the Shaying River mainstream, Ni concentration is highest in the midstream, followed by the downstream and upstream sections.The maximum Ni concentration is 4.83 μg·L-1at sampling point No.12 (Zhoukou) during the normal season.In tributary Ying River, Ni concentration exhibits a temporal-spatial pattern of normal season ＞ dry season ＞ wet season, with higher concentrations downstream compared to upstream.The maximum Ni concentration is 5.90 μg·L-1at point No.32 (Xihua), 5.63 μg·L-1at point No.34 in normal season and 4.26 μg·L-1at point No.34(Xihua) in dry season.Ni concentration in the Jialu River demonstrates a temporal-spatial distribution pattern of wet season ≈ normal season ＞ dry season.Concentrations increase along the route in normal and dry season but decrease along the route in the wet season.The highest Ni concentration in the entire river basin is 4.59 μg·L-1at point No.40(Weishi) during the wet season.Ni concentration in the tributary Beiru River remains relatively consistent across all seasons and increases gradually along the route.

Urban sewage discharge is one of the sources of Ni in the Shaying River.Sections of the Jialu River and Ying River exhibit the highest Ni concentrations.In the middle reaches of the mainstream, Ni concentrations are typically higher than in the upper and lower reaches, owing to presence of densely populated prefecture-level cities such as Zhengzhou, Xuchang, Pingdingshan, Luohe and Zhoukou.Wastewater discharged from their sewage treatment plants contains elevated Ni levels.For example, the Ni concentration in the drainage monitored by Luohe reaches 20 μg·L-1, approximately ten times the average concentration in the river water.Ni concentration in the mainstream peaks in the Zhoukou section during various seasons.This is attributed to the confluence of Sha River and Ying River tributaries, with no prefecture-level cities downstream of Zhoukou.In comparison to the mainstream and other tributaries, the Jialu River consistently exhibits higher Ni concentration in different seasons, indicating that the pollution discharge of industrial enterprises, such as machinery manufacturing and electroplating, contributes to elevated Ni levels in river water.For example, wastewater discharged by Xingyang printing equipment manufacturing enterprise contains Ni concentrations ranging from 15-86 μg·L-1, while wastewater discharged from Zhongmu auto parts manufacturing enterprise contains Ni concentrations ranging from 60-230 μg·L-1.The relatively lower Ni concentration in the Jialu River during the dry season is related to the COVID-19 epidemic that caused temporary production shutdowns in the enterprise.

4.4 Spatial-temporal distribution and source analysis of As

As concentration characteristic in the mainstream of the Shaying River shows a pattern of wet season＞ dry season ＞ normal season, with a consistent increase along the river's course in all seasons.In the dry season and wet seasons, As concentrations at No.21 (Xiangcheng) in the upper reaches of the mainstream reach 4.17 μg·L-1and 4.68 μg·L-1,respectively.In the dry and wet seasons, the concentrations at No.8 in the lower reaches (boundary)are 5.47 μg·L-1and 8.96 μg·L-1, respectively, all significantly higher than the adjacent sampling points.In the Yinghe River, As concentrations increase along the river course but decrease in the dry season.In the Jialu River and Beiru River,there is no significant change along the route in all seasons.

The variation characteristics of As concentration in the Shaying River differ significantly from other soluble heavy metal ions, as it consistently increase along the river course.Some cities within the study area, including Pingdingshan, obtain their water supply from mine water that contains a higher average concentration of As in the discharged wastewater than that in river water.In most other cities, urban sewage contains lower As concentrations than river water, suggesting that urban sewage is not the primary source of As.Coal washing wastewater is a notable contributor to elevated As levels, with concentrations reaching 0.5-80 μg·L-1in Jian'an and 0.3-350 μg·L-1in Yuzhou.The discharge of coal washing wastewater primarily contributes to the slow rise of As levels in the river's middle and upper reaches.The alluvial plain in the middle and lower reaches of the Shaying River is a traditional irrigated agricultural area, and As in the river primarily originates from agricultural non-point source pollution:

(1) Irrigation: The average concentration of As in groundwater is approximately twice that of river water, and the extensive irrigation causes a portion of groundwater to enter the river system.

(2) Chemical fertilizer application: Commonly used chemical fertilizers, including compound fertilizer, diammonium phosphate, urea and ammonium bicarbonate, contain average As concentrations of 1.27 g/t, 20.21 g/t, 0.48 g/t, and 0.34 g/t(Bao et al.2022), respectively.Compound fertilizer is predominant fertilizer used, and in 2017,Henan Province first became an ultra-high-intensity area of fertilizer application, with an average application rate of 462 kg/hm2.As a result, the input of As from fertilizer application in farmland in the Shaying River Basin is about 0.5 g/hm2.Some of arsenic input from farmland enters the soil and groundwater environment, while a portion enters the river system (Ross et al.2016), contributing to the rising As concentration along the river's course.

4.5 Spatial-temporal distribution and source analysis of Cd

The concentration of Cd in the mainstream of the Shaying River exhibits a pattern of wet season ＞dry season ≈ normal season, with a relatively small variation.Elevated Cd levels are observed in certain river sections with mining activities.The maximum value at sampling point No.15 (Zhoukou) in the mainstream in the normal season is 0.11 μg·L-1.In the dry season, it peaks at 0.06 μg·L-1at point No.8 (Jieshou section of the Shaying River), while in the wet season it reaches 0.35 μg·L-1.Cd concentration in the Jialu River shows relatively consistent levels across all seasons, with variation observed at point No.41 (Zhongmou) in the upstream, where it reaches 0.22 μg·L-1in the dry season.The maximum value is 0.07 μg·L-1at No.39 (Fugou) in the normal season, 0.11 μg·L-1at No.38 (Xihua) in the wet season, and 0.06 μg·L-1at No.39 in the dry season.

The source of Cd in the river is complex, originating from municipal wastewater treatment plants,mine water, coal washing wastewater, tannery wastewater and other industrial wastewater, all of which contain higher Cd levels than the Shaying River water itself.Several key observations can be made:

(1) Cd concentration in the Shahe River, Yinghe River and Jialu River in all seasons is much higher downstream than that in the upstream coal mine distribution area.This suggests that mine water and coal washing wastewater are not the main sources of Cd in river water.

(2) The Luohe-Shangshui-Zhoukou section of the mainstream hosts two large, seven medium and one small sewage treatment plants.Cd concentration in the river notably increases in the normal and wet seasons, but during the dry season when enterprises were generally shut down due to the COVID-19 epidemic, there was no increase in Cd concentration.This can be attributed to the controlled release of water through dams and gates in the Shaying River Basin, which diluted the retained water (Wu et al.2021).Additionally, the river flow during the dry year of 2019 and the dry period during the survey (Fig.1c), played a role in preventing a rise in Cd concentration.

(3) In some river sections, such as points No.38 and No.39 at Fugou and their downstream section in the Jialu River, wastewater from urban sewage treatment plants consistently contains Cd concentrations above 2 μg·L-1.This could contribute to the increase of Cd concentration in river water.

(4) Industrial enterprises are a significant source of Cd pollution in the river.For example, Jieshou City hosts seven lead processing and recycling enterprises, discharging sewage with Cd concentrations ranging from 7.1 μg·L-1to 8.92 μg·L-1,significantly higher than the average concentration of river water.As a result, Cd concentration in corresponding river water is notably elevated, with the highest value occurring at this point.

These factors collectively contribute to the complex spatial and seasonal variation in Cd concentration in the Shaying River, with industrial pollution being a primary driver of elevated Cd levels in the river.

4.6 Spatial-temporal distribution and source analysis of Pb

The concentration of Pb in the mainstream of the Shaying River and Shahe River follow a pattern of dry season ＞ normal season ≈ wet season, with relatively minor variation overall.However, the Pb concentrations at certain sampling points and river sections substantially fluctuate between dry and wet seasons.Specifically:

(1) In the mainstream, Pb concentration increases first and then decreases along the route in the dry season, with a maximum value of 0.06 μg·L-1at No.6 (Fuyang).

(2) In the Shahe River, Pb concentration increases along the route in the wet season, reaching a maximum value of 0.19 μg·L-1at No.24 (the Baiguishan Reservoir of the Shahe River), while it peaks at 0.26 μg·L-1in the dry season.

(3) Sampling point No.34 (Xihua) in the Yinghe River exhibits the highest Pb contents, with values of 0.16 μg·L-1in the normal season and 0.49 μg·L-1in the wet season, respectively.

(4) In Jialuhe River, Pb concentration decreases along the route in the dry season, with the maximum value of 0.43 μg·L-1at No.41 (Zhongmu) in the upper reaches.

Pb concentration in the middle and upper reaches of the Shaying River is primarily attributed to urban sewage discharge.Notably:

(1) The Baiguishan Reservoir (No.24) receives the urban sewage discharge from Pingdingshan City, with a maximum Pb concentration of 844 μg·L-1.

(2) Zhongmu (No.41 in the Jaru River) receives urban sewage with Pb concentrations of 20-21 μg·L-1.

(3) Zhengzhou in the upper reaches shows a Pb concentration of 1-50 μg·L-1.

In contrast, Pb contamination in the lower reaches of the Shaying River primarily results from the discharge of wastewater from metal processing and manufacturing industry.For example, the Pb concentration in Anhui section of the mainstream is much higher than that in the Henan section in the dry season.This discrepancy can be attributed to the Fuyang in Anhui, which serves as a prominent Pb processing and recycling hub in China, housing numerous related enterprises.Wastewater discharged from these enterprises contains Pb concentrations as high as 3-80 μg·L-1, with some consistently exceeding 60 μg·L-1.

4.7 Correlation analysis of sources of soluble heavy metals

Pearson correlation analysis chart (Fig.3) reveals several noteworthy patterns:

(1) Mn and As are highly positively correlated in the dry season.These two elements primarily originate from agricultural non-point source pollution, and show limited correlation with other four soluble heavy metals.Furthermore their sources are also significantly different.

(2) In the normal season, Pb has highly positive correlations with Cd and Cr.Similarly, in the wet season, Pb exhibits a positive correlation with Cd.These findings suggest that Pb has a common source with Cd and Cr, respectively.For example,both Pb and Cd come from industrial enterprises mainly involved in metal processing and manufacturing, while both Pb and Cr come from urban sewage.However, it's worth noting that the correlation between Cd and Cr is relatively weak, indicating that the primary sources differ.For example,Cd is not associated with urban sewage discharge,whereas Cr predominantly originates from urban sewage discharge and leather manufacturing.

(3) Ni displays a strong positive correlation with Cr in both normal and wet seasons.Both Ni and Cr arise from urban sewage discharge sources.The Pearson correlation is basically consistent with the previous source analysis conclusions, although some discrepancies exist.These variations suggest that the spatial distribution of heavy metals within the Shaying River basin during different seasons is not solely determined by their sources but may also be affected by factors such as the migration and transformation of heavy metal ions in the water body (Bhardwaj et al.2017).

5 Conclusions

5.1 Key findings

(1) The seasonal and spatial distributions of Mn and As concentrations in the Shaying River are significantly different from other soluble heavy metals, both of which originate from sources related to agriculture.Mn primarily emanates from shallow groundwater used for agricultural irrigation, while As stems from a combination of agricultural irrigation and fertilizer application.

(2) Urban domestic sewage treatment plant effluents play a pivotal role in introducing Cr, Ni,and Pb into the river water, significantly impacting Cr concentrations in the middle and upper reaches of the river.Coal washing wastewater predominantly influences Cr levels in the middle and upper river reaches, with more pronounced effects upstream of the Yinghe River tributary.Tanning wastewater and metal processing industry have a significant influence on downstream river water's Cr content, while the heavy metal processing sector emerges as an prominent source of Cd within the river basin.The mechanical manufacturing industry notably impacts Ni and Cd contents in river water, while the lead processing and recycling industry in the middle and lower reaches significant contributes to elevated Pb levels in the river.

(3) The spatial-temporal distribution of heavy metal concentrations in the Shaying River primarily results from industrial and agricultural production activities, leading to and the presence of complex and variable sources of contamination.Similar to findings from other studies, the temporary industrial activity shut down during the COVID-19 epidemic resulted in lower heavy metal concentration in the Shaying River water's dry season compared to other seasons and historical averages.In addition, the increase in Cd concentration in Luohe-Zhoukou section lacks sufficient data support and necessitates further investigation.

5.2 Research limitations

This research primarily establishe the source of heavy metals in the river by collecting significant pollutant discharge information area adjacent to the river basin.As a result, it can only conduct preliminary qualitative analyses on the sources of heavy metals, making it challenging to provide detailed insights into the specific reasons for variations in heavy metal contents across different river sections.Furthermore, the increase in Cd concentration in the river water of the Luohe-Zhoukou section lacks substantial pollution discharge information to support a conclusive analysis.Future research efforts should aim to comprehensively explore the sources of heavy metals in river water,incorporating data such as pollution discharge volume and river flow to provide a more nuanced understanding of these dynamics

Acknowledgements

This study was funded and supported by the Youth Science and Technology Project of Henan Provincial Bureau of Geology and Mineral Resources,YDKQKC [2008] No.8.