Rapid growth in the industrial and agricultural sectors is one of the main contributors to environmental deterioration. Environmental pollutants from both anthropogenic and natural sources enter the environment through various processes and cause serious threats to the ecosystem and human health. Heavy metals are a group of environmental pollutants that are potentially toxic, persistent, nondegradable, and able to bioaccumulate ^[1][2][1,2]. Heavy metals enter the environment through several processes such as soil erosion of metal ions and leaching of heavy metals, metal corrosion, sediment resuspension, and atmospheric deposition. Their natural sources are dominated by parent rocks and metal-bearing minerals. The weathering of metal-bearing rocks and volcanic eruptions are significant processes contributing to heavy metal pollution. The major anthropogenic sources are industrial and metallurgical activities, agricultural activities, transportation, wastewater discharge, and waste disposal [3]. Like many other heavy metals, cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) can cause a combination of environmental and health problems. Major sources of these metals in agricultural applications are, for example, fertilizers, pesticides, and livestock manures [4]. Cu and Zn are biochemically classified as essential elements. However, they can be toxic to biota at certain threshold concentrations [4]. Cd and Pb are nonessential metals and are naturally toxic even at low concentrations. Cd is a naturally occurring environmental pollutant derived from both industrial and agricultural sources [5]. One of the major causes of Cd released into the environment is from excessive application of pesticides and phosphate fertilizers ^[6][7][6,7]. Chronic exposure to Cd can invariably cause health problems such as stress, hypertension, kidney dysfunction, proliferative lesions of the prostate, bone fractures, and lung cancer ^[5][8][5,8]. Major contributing sources of Pb in the environment are natural and anthropogenic activities. Much of it comes from burning fuel, production of batteries, ammunition, metal products, and pigment-related manufacturing [9]. Moderate exposure to Pb can significantly impair human reproductive function such as a decrease in human semen quality ^[1][10][1,10]. Concerning the vulnerable groups, i.e., pregnant women, infants, and children under the age of four, excessive intake of lead can cause long-term health problems associated with children’s behaviors, growth, and development of their hearing and learning abilities. In adults, lead poisoning impairs the stomach, kidneys, brain, and nervous system [11]. Among other heavy metals, Cu and Zn are classified as essential elements. Apart from being used in electrical systems, copper compounds are used in agriculture as fungicides, algaecides, and feed additives while Zn uses vary such as in fertilizers, batteries, medical drugs, and household applications. Major contributions of Zn and Cu contamination in the surface waters are both point and nonpoint sources. For the Chan Thnal reservoir, the possible sources of Cu and Zn contamination in the environment are improper disposal of household waste and wastewater, direct use of pesticides on paddy soils, burning of coal, or waste-burning operations. Their toxic effects on humans are relatively less than those of other nonessential metals [12]. Adults typically need 2 to 3 mg of Cu and 12 to 14 mg of Zn daily. Exposure to Cu above certain levels may lead to liver and kidney dysfunctions. The provisional tolerable weekly intake (PTWI) value of Cu is 3500 µg/kg body weight/week ^[13][14][13,14]. The oral reference dose (RfD) of Zn is 300 µg/kg body weight/day ^[13][15][13,15]. Effects of Zn toxicity can be diverse. Excess intake of Zn over the destroying capacity of the body can interfere with metabolic processes and stress development [16]. Indications of Zn overexposure are a combination of gastrointestinal effects and pancreatitis [17].

The Chan Thnal reservoir located in Cambodia was built in 1976 during the Pol Pot period. Rainwater is the main water source of this reservoir. With a capacity of approximately 3,000,000 m³, it is the major water source for approximately 2300 farmers in Krang Chek commune, Kampong Speu Province, which is known to be one of the lowest income provinces in Cambodia with a low landholding area of 0.3 ha per farmer household [18]. People use water from this reservoir for agriculture and daily consumption. During the wet season, the surplus water from the reservoir is discharged to supply the downstream area through irrigation gates and canals, as shown in Figure 1a. Generally, the presence of heavy metals in the aquatic environment comes from both natural and anthropogenic activities ^{[2][19][20][21][22][23][24][25][26]}[2,19,20,21,22,23,24,25,26]. Chan Thnal reservoir is surrounded by large agricultural areas, scattered farmer households, commercial and residential areas, poultry farms, schools, and community markets. In recent decades, an increase in population and urbanization has led to an increase in quantities of both non-hazardous and hazardous waste production ^[19][24][27][19,24,27]. It can be expected that the catchment of this reservoir has been affected by improper waste disposal since open dumping and open burning of household and agricultural hazardous wastes (i.e., batteries, empty bottles of pesticides, and herbicides) are common practices in this area. In addition, excessive use of fertilizers and pesticides in a vast agricultural area is responsible for producing heavy metals in the water and soil. These activities can cause an increase in the mobilization of heavy metals in the environment ^[1][6][28][1,6,28]. It is likely that the Chan Thnal reservoir receives drainage from this catchment via runoff and two road canals connected to the reservoir, as shown in Figure 1a. Road canal 1 runs from the northwest and road canal 2 runs from the northeast. The road canals carry residential, commercial, and agricultural discharges to the reservoir. Increasing heavy metal pollution may cause significant adverse effects on invertebrates, fish, and human health ^{[27][28][29][30][31][32][33][34][35][36]}[27,28,29,30,31,32,33,34,35,36]. Recently, water pollution and metal contamination in this reservoir have become major environmental and health concerns for the people and local authorities. Thus far, the quantitative analysis of heavy metals pollution in the Chan Thnal reservoir has not been undertaken. Our study is the first to provide insight and information on the heavy metals contamination in both the water and sediment of the Chan Thnal reservoir. Hence, this study aimed to: (1) measure the contamination levels and investigate the distribution of four heavy metals (Cd, Cu, Zn, and Pb) in both the water and sediment; (2) use various sediment indicators such as contamination factor, pollution load index, and enrichment factor to evaluate the pollution degree and ecological risk levels of the sediments based on Cd, Cu, Pb, and Zn; (3) use the health index to assess the potential non-carcinogenic health effects; and (4) perform the cluster and factor analysis to determine the possible sources of heavy metals contamination in the water of the Chan Thnal reservoir.