Surface water quality is a matter of critical concern in developing countries because of growing population, rapid industrialization, urbanization, and agricultural modernization [1
]. Of all water bodies, rivers are the most vulnerable to pollution because of their role in carrying agricultural run-off and municipal and industrial wastewater [2
]. Water quality experts and decision makers are confronted with significant challenges in their efforts to manage surface water resources due to these complex issues [3
]. Spatial variation and source apportionment characterization of water quality parameters can provide a detailed understanding of environmental conditions and help researchers to establish priorities for sustainable water management [4
In recent years, the national income and standard of living in Taiwan have considerably improved following the nation’s focus on economic development [5
]. Rapid industrial development in Taiwan, including increased vehicle use, electrical power generation, and manufacturing of food, beverages, textiles, plastic, and metal, has affected pollution levels and other environmental problems, specifically water pollution [6
]. In 1998, the Taiwan Environmental Protection Administration (EPA) reported that 16% (2088 km) of the total length of Taiwan’s 21 major rivers was ranked as severely polluted, while another 22% were considered lightly and moderately polluted [7
The Taiwan EPA uses the river pollution index (RPI) to explore monitoring trends for both long-term planning and day-to-day management of surface water quality. The RPI involves four parameters: dissolved oxygen (DO), biochemical oxygen demand (BOD), suspended solids (SS), and ammonia nitrogen (NH3
-N). The overall index is divided into four pollution levels as follow: non-polluted, lightly polluted, moderately polluted, and severely polluted [8
]. Previous research has used the RPI to evaluate the pollution levels of the following rivers: the Tanshui River [9
], Kaoping River [11
], Chuo-shui River, Beigang River, Jishui River, Agongdian River, and Sichong River [12
] in Taiwan, and the Mahmoudia Canal in Egypt [13
The application of multivariate statistical analysis for cluster analysis, principal component analysis (PCA), and source apportionment by multiple regression on principal components provides a detailed understanding of water quality and the ecological status of the studied systems for improved interpretation of these complex data matrices [14
]. Such analyses also facilitate the identification of possible pollution sources that affect the water systems and offers a valuable tool for reliable management of water resources and the determination of potential solutions to pollution problems.
As shown in Figure 1
, this study was conducted in three phases with three main objectives: (1) evaluate and compare water quality and heavy metal data of 14 major Taiwan rivers with the Taiwan EPA standards; (2) classify the contamination level of those 14 major Taiwan rivers then determine the most polluted river; and (3) identify the major possible pollution source apportionment affecting water quality in the most polluted river.
The increasing population density in Taiwan is a significant source of domestic water pollution. Wastewater from agriculture, farming, and urban activities can also be major pollution sources causing diverse problems, such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity (including species important for commerce and recreation), and loss of aquatic plant beds and coral reefs [26
]. In addition, despite the decreasing number of domestic swine farms in Taiwan after it joined the World Trade Organization, approximately 7 million swine are still being raised in the country and their waste must be disposed of. Aside from domestic pollution sources, industrial wastewater is a major water pollution source as well. During the last three decades, Taiwan developed into a large trading economy with nearly 11,000 manufacturing plants disposing various contaminants [29
]. Moreover, the location of industrial area and high population density in Taiwan is scattered. In other words, industrial and high population areas around Taiwan rivers are not only located in the downstream areas, but also in the upstream areas (see Supplementary Figure S1
). Thus, water pollution in Taiwan rivers is spread along the river (see Supplementary Figure S2
). We interpreted the water quality and heavy metals characteristics for each river that could be used by the Taiwan government to plan proper river management strategy.
Our study used cluster analysis on large-scale data in one country to classify the pollution level of major rivers. Previous research has shown that cluster analysis is useful for classifying rivers that have similar water quality characteristics. For example, Shrestha and Kazama 2007 [18
] reported that cluster analysis results represent the influence of land use, residential sewage, agricultural activities, and industrialization, which can have major impacts on water quality. Another study grouped monitoring sites in rivers in South Florida into three groups (low, moderate, and high pollution) on the basis of their similar water quality characteristics [30
In the current study, Erren River was determined to be the most polluted river among the other major rivers in Taiwan. The most significant water pollutants were identified to have originated from industrial activity, domestic black water, runoff from other rivers, and natural sources, including climate conditions. Given that the Erren River is located in an industrialized and urbanized area [31
], the level of water pollutants in the river is very high due to huge amounts of nutrient salts, including organic pollutants, ammonia, and total phosphate. These pollutants are associated with possible pollution sources in Factor 1 of our study. Aneja et al. 2008 [14
] reported that ammonia is found in industrial gas emissions or natural sources that evaporate and become particulate matter, and then descend with precipitation and enter surface water. Urbanized areas with high population density in Taiwan also show that domestic wastewater is a major contributor to river water pollution because of the levels of BOD and DO, which show a strong correlation with coliform levels that are associated with domestic wastewater [27
]. Runoff from other rivers can be due to flash floods that often occur in Southern Taiwan following typhoons throughout the year. For example, in August 2009, Taiwan experienced the worst floods in 50 years after Typhoon Morakot struck almost the entire southern region. Yang et al. 2012 [32
] analyzed the impact of climate change on river water quality in the southern area of Taiwan. High amounts of sediments and debris flowed into the Erren River basin because of the high concentration of suspended sediments in the river, which in turn caused the failure of wastewater treatment plants. Therefore, the river received significantly higher SS, BOD, and ammonia loads from farms and domestic wastewaters. During the dry season, the evapotranspiration rate increase, which may contribute to the increased water salinity. However, during the wet season, precipitation increases and runoff from other tributaries brings SS or nitrate content to the river. Therefore, we assume that climate conditions are one of the factors affecting water quality in rivers.
This study only explored pollution sources that were identified and considered using multivariate statistical analysis of water quality data for all seasons. However, pollution levels vary every season. Therefore, further study is necessary to analyze in detail how different seasons can affect the water quality level. In addition, some water quality variables might be affected by soil types, geological conditions, terrain, and anthropogenic pollution sources [33
]. Further work is necessary to determine if these potential sources do significantly impact the rivers in Taiwan.
Our study found that the levels of heavy metal contamination in the Erren River are classified as among the highest in Taiwan. Since the 1970s, the development of a scrap metal industry along the Sanyegong River (a tributary of the Erren River) has severely polluted the river sediment with metals [34
]. Chen et al. 2004 [35
] determined that concentrations of Fe, As, Cd, Zn, Hg, and Cu in the Erren River were higher than those in other rivers, and that Cu levels exceeded the standard limit. This high heavy metal contamination problem has affected the river ecology and biota. They reported that the highest concentrations of Fe, Zn, Cu, and Mn in muscles were found in tilapia, striped mullet, large-scaled mullet, and milkfish. The highest concentrations of As and Hg were found in striped mullet and Indo-Pacific tarpon. The highest concentrations of Fe, Hg, and Cd were found in the livers of large-scaled mullet, while striped mullet had the highest concentrations of Zn, Cu, and As. Our data in 2002 revealed that As, Cu, Hg, and Zn levels in the Erren River were the highest compared to the other years, indicating that the high levels of the mentioned heavy metals may have affected the biota. However, the trend of heavy metals in the Erren River has been decreasing since the Taiwanese government started a river restoration program in 2002. The restoration program formed an implementation team that united the Water Resources Agency, Industrial Development Bureau, Construction, and Planning Agency, Council of Agriculture, Tainan City Government, Kaohsiung City Government, the river patrol team, and other units to make joint efforts toward improving the water quality of the Erren River. Through the combined efforts of the government and private entities over the long term, the Erren River’s water quality is continuing to improve.
In this study, cluster analysis was successfully utilized to classify the water quality of 14 Taiwan rivers and PCA–MLR was conducted to determine the possible pollution sources for the most polluted river in Taiwan. According to the cluster analysis, the most severe water quality pollution problem can be found in the Erren River in Southern Taiwan. According to the PCA–MLR results, 62.3% of water pollutants in the Erren River were contributed by ammonia and TP as the first factor; DO, BOD, COD, nitrite, and coliform as the second factor; and conductivity, nitrate, and SS as the third factor. An estimated 72% of the first factor was found to be from industrial emission, 16% from domestic black water, and 12% from natural sources and runoff from another tributary.
Water quality monitoring programs generate complex multidimensional data that require multivariate statistical treatment for analysis and interpretation to obtain better information about the quality of surface water. Such information can help environmental managers make better decisions regarding action plans. The management of domestic and industrial wastes should strive for low accumulation in rivers to minimize environmental degradation. This objective can be achieved by installing proper treatment methods for municipal and industrial wastewater before being released to the environment.