Pollution Characteristics, Sources and Assessment of Heavy Metals(Hms) in Surface Sediments of Ningyuan River in Hainan Province,

. Heavy metals are of wide concern due to their high toxicity, bioaccumulation and non-degradability. In this study, eight heavy metals (Cr, Mn, Ni, Cu, Zn, As, Cd, Pb) in the surface sediments of Ningyuan River were studied to investigate the distribution pattern, pollution status and potential pollution sources, and it was concluded that the concentration distribution of heavy metals in sediments is highly consistent with the population distribution pattern in space and there is a certain degree of risk of Cd pollution, and the heavy metals mainly come from mineral weathering, agricultural and shipping wastes.


introduction
Since the 20th century, with the high level of economic growth and industrialization, a large number of harmful chemical substances have been discharged into the water environment, among which heavy metal pollution is of great concern. Heavy metals(HMs) in this study are defined as metal elements with density greater than 4.5 g/cm³, such as chromium (Cr), copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), and metalloid arsenic (As) and their compounds, which are highly toxic, bioaccumulative, and nondegradable, with high ecological risk [1]. Excessive intake of HMs by humans may cause a variety of serious health problems such as acute poisoning, fetal malformations, malignant tumors, and even death [2]. Due to the harmful effects of HMs, China has been making the prevention and control of heavy metal pollution one of the priorities of ecological environmental protection.
HMs in the environment can be divided into two categories according to their sources: natural sources and anthropogenic sources, natural sources of HMs refer to those released into the environment in small amounts from the earth's crust under the action of internal or external forces, and their content is low and their nature is relatively stable, which usually hardly affects the environment [3]. anthropogenic sources of HMs refer to those that enter the environment due to human activities such as mining and smelting, fuel combustion, and industrial and domestic wastewater discharge [4]. HMs from anthropogenic sources are those that enter the environment due to human activities such as mining and smelting, fuel combustion, industrial and domestic wastewater discharge. HMs can enter the environment through surface runoff, sewage discharge, and atmospheric deposition, and the transport in rivers and streams is one of the most important sources, and HMs entering the aquatic environment eventually accumulate in sediments from water bodies or along the food chain in organisms [5]. HMs in sediments are not stable and can re-enter the water column when external conditions change, causing secondary pollution of the surrounding water column [6]. Therefore, it is important to study the distribution of HMs in sediments of rivers entering the sea for heavy metal pollution prevention and control.
Hainan province is located in the southernmost part of China, with two-thirds of the country's sea area and a coastline of 1823 km, which has rich sea resources and is also a national key development area. As a typical pollutant with high ecological risk, it is important for the environmental management and ecological protection of Hainan Province to explore its distribution characteristics and comprehensively evaluate its pollution status and potential sources.

Study area and sampling
Ningyuan River (18°19′30″N-18°39′30″N, 109°5′46″E-109°33′20″E) is located in the south of Hainan Island, the fourth largest river in Hainan Province, with an average annual runoff of 649 million m³, located in the tropical monsoon climate zone, June to September is the main flood season, 45-65% of precipitation recharge is consumed by evaporation. The main study area in this study is the section of the Ningyuan River from Dalong Reservoir to the estuary, which flows through areas such as Nanbin Farm and Yacheng Town, and is the main population distribution area along the Ningyuan River, passing through 11 villages from top to bottom with a resident population of about 70,000 people, which is dominated by agricultural production and is a large agricultural area in Sanya City, where the main cultivation includes mango, rubber, betel nut, etc. The South Propagation Science and Technology City, the Deep Sea Science and Technology City, and the Global Plant and Animal Germplasm Resources Introduction and Transit Base in the Free Trade Port of Hainan Province are located in this area.
The collection of sediment samples was carried out with reference to the Technical Guidance on Water Quality Sampling (HJ 494-2009): Through preliminary data collection and site investigation, a total of 12 sampling points were finally set up in Ningyuan River to collect sediment samples in February 2023. Due to technical reasons, surface sediment samples were not collected at point N-2. The Beidou Navigation Satellite System (BDS) was used to record the sampling site locations (Fig. 1).

Detection of samples and quality control
The extraction and determination of HMs (Cr, Mn, Ni, Cu, Zn, As, Cd and Pb) in sediment samples were based on electrothermal ablation aqua regia extraction and inductively coupled plasma mass spectrometry (ICP-MS, Jena PQ-MS), respectively, with internal standards using 45Sc, 72Ge, 103Rh and 186Re, and response signals ranging from 92.7 to 122.3%, and all calibration curves with Quality control was performed using offshore sediment standards (GBW07314), and the final values of sample spiked recoveries were 82.7%±5.1%~88.2%±7.1%, which were in accordance with the method requirements.

Statistical analysis
The extent of the Ningyuan River basin was extracted from digital elevation data (DEM) using arcGIS 10.7 and spatially interpolated using the inverse distance weighting (IDW) method to analyze the spatial distribution characteristics of HMs. Person correlation test and PCA analysis were performed using SPSS 25, and histogram plotting was completed using Origin 2023.
The contamination level of HMs in sediments was evaluated using the ground accumulation index method [7], calculated based on the following equation, I geo = log 2 (C n /1.5B n ), C n is the measured value, B n is the geochemical background value of HMs, and the mean value of Chinese crustal elements was used as the background value in this study, 1.5 is a correction index selected to consider rock differences, and each increase of 1 indicates an increase of one level of contamination. The enrichment factor (EF) was calculated based on the following equation: EF=(C n /M n ) sample /(C n /M n ) background , and Mn was used as the reference element in this study , EF>1 indicates the presence of enrichment of the metal [8].

distribution of HMs
The content of HMs in the sediments of the Ningyuan River is shown in Figure 2 N-5, and N-6, respectively, and the differences between the maximum and minimum values were 3.96, 17.49, 5.17, 12.66, 13.52, 6.08, 5.67, and 12.15 times, respectively, with coefficients of variation were 45%, 102%, 40%, 99%, 83%, 46%, 53%, and 100%, respectively, all of which were highly variable. HMs had large spatial variability, and the metal element with the smallest variation was Ni and the largest was Mn. Figure 2(right) shows the IDW interpolation results of sediments in the Ningyuan River basin. The darker the color, the higher the concentration of HMs in the area. It can be seen that the eight HMs show different distribution patterns: Cr, Mn, Zn and As all have higher values in the middle section of the Ningyuan River (points N-3 to N-6), which is the area of Nanbin Farm, where the Longtan River converges. Cr, Ni, Cu, As and Cd have high values at the mouth of the sea, and the sampling point at the mouth of the sea is the Yazhou central fishing port wharf, where the construction of the global plant and animal germplasm resources introduction transit base (Moon Island) is currently underway, which may have some influence on the sediment quality; Cd and Pb have high values at Dalong Reservoir, which is a first-class protected area for water sources, and the high values may be related to the activities of vessels on the reservoir. The high values of Cd and Pb in Dalong Reservoir, which is a first-class protected area of water sources, may be related to the activities of vessels on the reservoir.  Table 1 shows the results of the ground accumulation index, it can be seen that: Cr, Ni and As are all free of pollution, Cu is lightly polluted in N-11, Pb is lightly polluted in N-11 and partially polluted in N-1. The results of Zn show that there are a total of 2 points in Ningyuan River with moderate pollution in N-3 and N-4, accounting for 18.2%, 2 points in N-9 and N-12 with partial to medium pollution, accounting for 18.2%, N-8, N-11, 2 sampling points have light pollution, accounting for 18.2%, the total exceedance rate is 54.5%. The results of the enrichment factors are shown in Table 2. It can be seen that Ni is not enriched and the rest of the elements are enriched to different degrees, among which the enrichment of Zn and Cd is the most significant.

Assessment of HMs
In a comprehensive view, the pollution status of HMs in Ningyuan River is Cd＞Zn＞Pb, Cr, Ni, Cu and As are not polluted, and there is a risk of Pb pollution at points N-1 and N-11. N-1 is located in Dalong Reservoir, and small boats are often active near the sampling point, and N-11 is a wharf, so it can be presumed that the main source of Pb may be the fuel of boats. The results of enrichment factors show that Cr and Ni are almost not enriched, indicating that they mainly come from natural sources, while the enrichment degree of other elements is ranked as Cd > Zn > Pb > Cu > As, which indicates that these HMs are more related to human activities.

Potential Sources of HMs
In this study, the analysis was performed using the rotated component matrix, and the cumulative variance contribution of the three principal components (PC1, PC2, PC3) was 94.715%, which can reflect most of the information of the original monitoring data in the study area [9]. In PC1, Ni, Cu, As, and Cd have high loadings, and there is a significant correlation between these metals, which may have similar sources, and Ni, Cu, and As are almost free of contamination, indicating that the three HMs are less influenced by human activities and are mainly natural sources [10].The loadings of Cd are also relatively high, and from the evaluation results, the contamination of Cd is the most serious and has a greater anthropogenic influence, analyzed in the context of the actual situation in the study area, which has a large amount of farmland along the Ningyuan River and is dominated by agricultural production, Cd is the signature element used in fertilizers [11] and is closely related to agricultural production, and pesticides and insecticides are also important sources of Cu and As, so PC1 characterizes the natural sources and agricultural production sources.
In PC2, the loadings of Cr, Mn, Cu, Zn, and Pb are high, and the five metals are homologous; tire wear and exhaust emissions are important sources of Pb and Zn in the environment, fuel combustion and leakage are the main sources of Mn and Pb, Cr is widely used in the electroplating industry, and the wear of ship hulls leads to the alloy and plating layer of Cr, Cu and Zn into the aqueous environment. Therefore, PC2 characterizes shipping and traffic sources [12].
In PC3, Zn and Mn have high loadings, Zn is present in feed additives used in animal husbandry, Mn in aquaculture nets is often exceeded seasonally, and these farming wastewaters can discharge Mn and Zn into the water body, and villages along the Ningyuan River often use family free-range poultry farming, which discharges wastewater directly into the river, while The aquaculture process also lacks corresponding wastewater treatment facilities, which becomes a potential source of Mn and Zn. Therefore, PC3 characterizes the source of farming waste water [13].

Conclusion
The spatial distribution pattern of HMs in sediments is highly consistent with the population, and high values of concentrations exist in areas of intensive human activities. In terms of pollution level, the potential ecological risk of Cd is high, and the pollution status of Ningyuan River is Cd > Zn > Pb, while Cr, Ni, Cu, and As are not polluted. Cr and Ni come from natural sources such as weathering of minerals, Zn, Cu, Pb, and Cd come from weathering of minerals and discharge of domestic sewage and farming wastewater, and Zn, As, and Mn come from waste from ship activities and agricultural activities.
Overall, the environmental quality of the Ningyuan River does not seem to pose a very serious threat in terms of HMs pollution, indicating that the protection measures for the Ningyuan River have achieved positive results. However, considering the construction of projects such as the free trade port, the heavy metal pollution situation may become more serious in the future, and effective measures should be taken to control the anthropogenic discharge of heavy metals. The results of this study provide an important reference basis for the management of HMs pollution in Ningyuan River, which is of great significance for the protection and sustainable development of coastal ecological environment and can also provide some reference for other coastal areas in Hainan Province.