Wetlands support a variety of biological diversity owing to their productive, adaptive, complex, and dynamic nature. These also provide a living for millions of people by performing critical ecosystem functions, which provide both direct and indirect benefits including clean water, climatic regulation, food provision, and other spiritual values (Merriman et al. 2018). The wetland covers approximately 12.8 million km2 (8.5%) of the Earth's land surface on a global scale, with the inland wetland covering approximately 9.5 million km2 (6.3%). Approximately, one-third of the world’s drinking water demands come from surface sources such as rivers, canals, and lakes. The quality of water in these wetlands depends on weathering, soil erosion and precipitation processes; and also on industrialization, haphazard urbanization and agricultural activities (Jarvie et al. 1998; Giridharan et al. 2009; Pejman 2009; Pant et al. 2018). Hence, surface water quality of the wetlands is often subjected to widespread deterioration due to both natural processes and anthropogenic activities. Therefore, the protection of the integrity of world freshwater has been given topmost priority in the 21st century (Ansari et al. 2000; Pant et al. 2018; Li et al. 2020).
Over the past few decades, many wetlands worldwide have been under pressure primarily due to drainage, industrial effluents, surface runoff, unsustainable resource harvesting, and other anthropogenic activities that affect water quality (Jha 2008; Gautam et al. 2014). Additionally, the construction of water infrastructures like river channels, dams, and hydropower, the introduction of non-native species, and nutrient enrichment affect water quality and nutrient availability (Carmignani and Roy 2017; Regmi et al. 2021). The cumulative effects could increase the rate of conversion of wetlands to land in the long run.
In Nepal, about 5% of the total land area is covered by wetlands (Kafle and Savillo 2009). Low-laying Tarai consists of a majority of the wetlands (68.2%), followed by the high mountain (31.6%) and mid-hill (1%) (GN/MFSC 2009). These wetlands have several species of rare and endangered flora and fauna (Pant et al. 2019). They provide a wide range of beneficial services (Gautam et al. 2014) and act as conservation and enhancement of water quality, recharging of drinking water supplies, agricultural development, prevention of erosion, maintenance of surface water flow, and prevention of floods. Wetlands preserve open spaces, landscapes, cultural values, provide recreation and tourism opportunities, and perform natural environmental monitoring (Jha 2008). Among the wetlands in Nepal, ten have been designated as the Ramsar sites namely Koshi Tappu, Jagadishpur Reservoir, Ghodaghodi Lake Area, Beeshazari and Associated Lakes, Rara Lake, Phoksundo Lake, Gosaikunda and Associated Lakes, Gokyo and Associated Lakes, Mai Pokhari and Lake Cluster of the Pokhara Valley (Pant, & Adhikari, 2015, Pant et al. 2020).
Very few hydrochemical investigations have been carried out in Nepal as compared to the Ramsar-listed wetland across the world. The present paper focuses on the Ramsar-listed Koshi Tappu wetland to assess the spatial variation in surface water quality. The Koshi Tappu wetland is the first site from Nepal to be listed in Ramsar in 1987 (Poudel 2009). It lies on the floodplain of Saptakoshi, a tributary of the Ganges and was established to protect the country's last wild water buffalo population. It is also one of the most important sites in the country for migratory and wintering water birds. There are, however, several threats to the wetland, including natural dynamics such as siltation and flash flooding, along with local activities such as overfishing, overgrazing, and agricultural run-off (Kafle et al. 2008; Heinen 1993). Besides these, touristic activities also contribute to the addition of the pollutants in the wetlands (Neupane et al. 2010). Changing climatic conditions, rapid urbanization, unmanaged settlement and accelerating alien invasion species are also adversely affecting the site. Climate change may have contributed to the alteration of water quality and quantity in especially in the wetlands of fragile Himalayan regions (Bishwakarma et al. 2019, Thapa et al. 2020).
The combined effects of human activities and natural processes are responsible for surface water pollution, endangering the Koshi Tappu wetland’s fragile ecology (Regmi et al. 2021). Hence, in the Koshi Tappu wetland water pollution became a visible criterion. As a result, insights on water quality are very essential for effective water management. Therefore, the study has been performed to evaluate the spatial variation in the water quality of the Koshi Tappu wetland. In this study, the multivariate statistical approaches combined with geochemical indices were used to characterize the water quality of the Koshi Tappu wetland. The study could provide an important scientific foundation for water quality and environmental management of the Ramsar-listed wetlands, especially in the Himalayas. Furthermore, the findings will be highly relevant to the researchers and policymakers including other concerned stakeholders for assisting the conservation of aquatic biodiversity and supporting the people’s livelihoods through improving the wetland services.