Assessment of water quality status of Doyang River, Nagaland, India, using Water Quality Index

The Doyang River of Wokha district, Nagaland, NE India, has a strong economic and traditional attachment to the local people. It provides sufficient fertile plains and slopes for cultivation, good grounds for community fishing and hunting. It is not only important for the people of Wokha but also for the state of Nagaland because of the rich natural resources it provides. This study was conducted to assess the Water Quality Index (WQI) of the Doyang River from eight selected sampling stations. Maximum WQI values were recorded during monsoon season in all the stations followed by pre-monsoon and post-monsoon. Sampling stations located in the upstream of the river experience deteriorating WQI due to the presence of hydroelectric dam, changing landuse practices, increasing settlements and deforestation in the catchment and river banks. The overall WQI values showed good water quality status indicating suitability for different human uses. The present study points out that pH, DO and BOD played a central role in affecting the WQI of the river; however, in case of nutrient elements no such significant roles were observed in affecting the water quality of the river. The condition of water quality in our present study felt the necessity to adopt proper management policy and conservation efforts along the riparian zones of Doyang River.


Introduction
Rivers are an important source of freshwater but are also vulnerable to kinds of pollution to both point and nonpoint sources. Anthropogenic activities related to extensive urbanization, agricultural practices, industrialization and population expansion have led to water quality deterioration in many parts of the world. The adjacent landscapes that act as an interface between the aquatic and terrestrial ecosystem called the 'riparian zones play a significant role in controlling water and chemical exchange between surrounding land and stream systems (Burt and Pinay 2005). Disturbances in this landscape can lead to deterioration of water quality as they influence the flows of energy and material between the terrestrial and aquatic (Fausch et al. 2010) interface. Riparian zones form a unique ecosystem and act as 'buffer zones' between upland and streams (Hill 1996;Lowrance 1998) and are vital to the health of the watershed. The riparian forest along the river that receives and processes water, sediments and nutrients transports from upslope areas and effectively functions as sinks for sediment and nutrients, thus regulating the nutrient loading to the aquatic system (Luke et al. 2007;Mayer et al. 2007). Water quality of any specific area or source may be assessed using physical, chemical and biological parameters; it is considered harmful and unfit for different human usage and other agricultural activities once they occur more than the well-defined limits (ICMR 1975;BIS 2003). Accordingly, the suitability of water for its usage may be categorized or described in terms of Water Quality Index (WQI), which is one of the most effective ways to describe the status of water quality. It is calculated from the point of aptness of surface water for human consumption (Atulegwu and Njoku 2004).
WQI is a single number that expresses water quality by aggregating the measurements of water quality parameters (such as dissolved oxygen, pH, nitrate and total hardness). It reduces the bulk of information from the several water quality parameters into a single value and expresses the data in a simplified and logical form (Semiromi et al. 2011). Assessment of water quality could provide us the overall information on the quality of the water bodies and its potential threat to various uses. The application of WQI is a useful method in assessing the water quality of the river. It helps to understand the overall water quality status of individual sampling stations at a certain time (Yogendra and Puttaiah2008) and its suitability for various beneficial uses. The concept of indices to represent gradation in water quality was first proposed by Horton (1965), since then numerous water quality indices have been formulated that can easily evaluate the overall water quality of an area promptly and efficiently. The general WQI developed by Brown et al. (1970) has undergone much improved modification suitable for a different purpose. Many workers like Debels et al. (2005), Yisa and Jimoh (2010), Akoteyon et al. (2011), Othman et al. (2012, Naubi et al. (2016), Ewaid (2017) and Bouslah et al. (2017) have worked out the study of WQI of different rivers. Similarly, in India, Yogendra and Puttaiah (2008), Kumar et al. (2011), Sharma andKansal (2011), Singh andKamal (2014) and Shah and Joshi (2017) have also worked on WQI of rivers in different states of India. So far only a few studies on WQI from the northeastern part of India, mainly confined to Assam and Manipur (Singh et al. 2016, Bora andGoswami 2017), have been reported. Nagaland state is dissected by a number of seasonal and perennial rivers and rivulets. Major rivers that flow westward into Brahmaputra River of Assam are Dhansiri, Doyang and Dikhu. The Doyang River passes through a great part of Wokha district of Nagaland and is called 'POFU' by the local inhabitants (Lothas) which simply means 'encircle' because the river flows right through the middle of the district touching all the three ranges encircling the whole district. The Dam area of Doyang River is an important ecotourism spot for bird-watchers as it is a roosting place of a migratory bird Amur falcon (Falco amurensis). The falcons travel almost 22,000 km every year (October-November) from southeastern Siberia and Northern China in millions and spend nearly a month around the vicinity of the Dam. The river also has a strong economic and traditional attachment to the local people because of its sufficient fertile plains and slopes for cultivation. However, the changing landuse practices, increasing population and deforestation in the catchment and river banks, shifting cultivation along the river have threatened the riparian habitats as never before. This has drawn much attention in preserving the riparian vegetation along the streams and in other sensitive areas in order to protect the water quality and habitat value of these areas. Geomorphology and seasonal variation of physicochemical parameters of Doyang River had been worked out by Imnatoshi and Ahmed (2012); however, there has been no scientific investigation on water quality assessment of Doyang River till date. In the present study, the application of WQI would give us comparative results of the water quality status of Doyang River at different sampling stations in varying seasons. The main reason for using WQI in the present study is to test the hypothesis whether the riparian forest present along the stretch of Doyang River may help in improving the status of water quality besides several pockets of landuse practices being found. This study would provide us a comprehensive water quality status of the Doyang River. It would ultimately pave ways for future management and action plans so as to protect the riparian zones that face pressure from different landuse practices, and facilitate improvement of the water quality.

The study area
Nagaland has a total geographical area of 16,579 km 2 extending from 25°6′ N to 27°4′ N Latitude and 93°20′ E-95°15′ E Longitude. The state is bounded by Assam in the north and west, by Myanmar and Arunachal Pradesh in the east and by Manipur in the south. Nagaland experiences heavy rainfall, and the annual rainfall varies from 100 to 300 cm. The monsoon seasons last for a period of 5 months from May to September with June, July and August experiencing the highest rainfall. The Doyang River is one of the major rivers in Nagaland and runs along the southern boundary of the state. It originates from the Japfü Hill near the southern slope of Mao in Manipur and moves in a southwest direction passing through Kohima district and flows northward into Zunheboto and Wokha. The river has a length of 167 km (from Gariphema/Ghathashi area to Liphi) and a catchment area of 3283 km 2 (Laishram and Yumnam 2016). It passes through a great part of Wokha district of Nagaland and flows south westerly into Dhansiri in Sibsagar District of Assam and finally joins the mighty Brahmaputra River of Assam. The main tributaries of Doyang are Tsui, Tullo and Tishi. The present study was conducted within a stretch of 40-45 km of Doyang River under Wokha district, Nagaland. The Doyang hydroelectric project (DHEP) is located in this river at 26°14 N Latitude and 94°16 E Longitude of Wokha district. The large reservoir lake created for generating hydroelectric power is more than 20 km 2 , and it also comes under the present study area. There are several landuse practices around the catchment area of the Doyang hydroelectric dam and along the riparian zone of the river. Figure 1 shows the landuse/landcover (LULC) map of the present study area. The characteristics features of the selected sampling stations, their coordinates and elevation along the Doyang River are presented in Table 1.

Materials and methods
Along the stretch of Doyang River, surface water samples were collected from the eight sampling stations. Sampling was done during the first week of each month from June 2016 to May 2017 for a period of 1 year. The months were later categorized into three different seasons, namely pre-monsoon (PRM), monsoon (MON) and post-monsoon (POM) for interpretation of data. Figure 2 shows the sampling location selected along the river for the study of WQI. Water samples were collected from the first 20 cm of the water column using a bottom-weighted polyethylene flask, previously washed in the laboratory with lapoline, 10% HCl and then with a water sample from each spot. In this study, twelve physicochemical parameters of water were selected, namely pH, electrical conductivity (EC), total dissolved solids (TDS), total alkalinity (TA), total hardness (TH), calcium (Ca 2+ ), magnesium (Mg 2+ ), chloride (Cl − ), nitrate (NO 3 ), sulfate (SO 4 2− ), dissolved oxygen (DO) and biological oxygen demand (BOD) for generating the overall WQI of Doyang River. Parameters like pH and TDS were measured on the spot with the help of pen-type digital pH and TDS meter. Conductivity was analyzed with the help of a digital conductivity meter in the laboratory. Total alkalinity, total hardness, calcium, magnesium and chloride were analyzed by the titration method. For the measure of dissolved oxygen, fixatives were added on the spot and analyzed thereafter using Winkler's method. Separate samples for BOD were also collected, incubated in the dark at 20 °C for 5 days and analyzed thereafter. Parameters like nitrate and sulfate were analyzed using the double-beam UV-visible spectrophotometer. All the parameters were analyzed using standard methods as prescribed by Trivedy and Goel (1986) and APHA (2005). Finally, the WQI was calculated by employing the Weighted Arithmetic Index method developed by Brown et al. (1970) which is given in the following equation: The quality rating scale (Q i ) for each parameter was calculated by using the expression: where V i = concentration of ith parameter in the water sample analyzed. V o = ideal value of parameter in pure water, i.e., V o = 0 (except pH 7.0 and DO = 14.6 mg/l), S i = recommended standard value of ith parameter.
The unit weight (W i ) for each water quality parameter is calculated by using the following formula: where K = proportionality constant calculated by using the equation The WQI range, its status and possible usage (Brown et al. 1972) are presented in Table 2.

Water quality parameters
The values of water quality parameters obtained from all the sampling stations in three different seasons are presented in Table 3. The pH is a measure of the acidic or alkaline condition of water and serves as an important indicator of water quality and determines the suitability of water for various purposes. The experimental water bodies recorded approximately neutral or slightly alkaline in nature (Bouslah    (Yisa and Jimoh 2010). Dissolved oxygen (DO) is the measurement of the amount of oxygen dissolved in water and is a direct indicator of water quality. In a healthy water body that ensures good water quality, DO must be > 4 mg/l (Prasad and Bose 2001). DO along Doyang River was recorded significantly high from all the stations throughout the study period. The highest concentration of DO was observed during the PRM season range from 10.02 to 11.38 mg/l with a mean value of 10.56 ± 0.15 mg/l. The turbulent nature of the water bodies, photosynthesis and a decrease in temperature might have resulted in the increased concentration of DO (Bouslah et al. 2017). Biological oxygen demand (BOD) determines the strength of oxygen to stabilize domestic and industrial waste. A higher value of BOD levels represents a higher level of organic pollution (Patel et al. 1983) indicating higher organic pollution in a water sample. Observed BOD in PRM, MON and POM was 1.68 ± 0.08 mg/l, 2.18 ± 0.09 mg/l and 2.29 ± 0.08 mg/l, respectively. The low level of BOD in the present study indicates less organic matter in the water sample to be oxidized by microorganisms (Singh et al. 2016). All the twelve physicochemical parameters of water analyzed were well within the permissible limits of drinking water given by BIS (2003) and ICMR (1975).

Water Quality Index (WQI) calculation
The calculation of WQI using Weighted Arithmetic Index involves the estimation of 'unit weight' assigned to each physicochemical parameter selected. Different units and dimensions of the selected parameters are transformed into  a common scale using the assigning units. Table 4 shows the drinking water quality standards and the unit weights assigned to each parameter used for the calculation of WQI. Considering the significance of water quality assessment and their impact on the value of WQI, a maximum weightage of 0.366 is assigned to both DO and BOD. Tables 5,6,7,8,9,10,11 and 12 depict the values observed for the selected physicochemical parameters from the eight sampling stations during each season and their corresponding WQI values. pH, DO and BOD were found to be the most significant parameters in the WQI scores worked out.
The overall values of WQI of the water samples from all the eight sampling stations for each season are presented in Table 13. WQI were observed to have a positive relationship with the seasonal changes. Maximum WQI values were recorded during MON from all the eight stations followed by PRM and POM. A similar finding has also been reported by researchers like Singh and Kamal (2014), Bora and Goswami (2017) in their studies of assessment of surface water quality status. An average value of WQI for all the stations during PRM, MON and POM was 42.95,47.13 and 36.66,respectively,as presented in Table 14. This result indicates that the quality of the water samples from all the stations falls under the class of good water samples (25 < WQI < 50) suitable for drinking, irrigation and industrial purpose (Fig. 3). Ranges of WQI values from all the eight stations   during PRM, MON and POM were: 35.89 (S 6)-50.14 (S 2), 40.77 (S 7)-55.45 (S 4) and 33.00 (S 4)-44.35 (S 2), respectively (Table 13). In all the stations, both PRM and POM showed good water quality status. However, MON showed poor water quality status at stations 2, 3, 4 and 5 located around the vicinity of the hydroelectric dam.
The WQI value showed a mixed pattern of changes in all the seasons (Fig. 4). WQI of the upstream stations from 1 to 5 is higher than the downstream stations, i.e., 6-8 showing the decrease in pollution level while moving downstream of the river. Such observation was also made by Bora and Goswami (2017) in their studies of water quality assessment of Kolong River, Assam, where the water samples showed a decreasing pollution trend further downstream. Workers like Ewaid (2017) have observed better water quality status in upstream than downstream due to a decrease in water and accumulation of contaminants along the downstream of the river. However, the above case is not the same in the present study. This could be due to the absorption of contaminants by healthy riparian vegetation that is present along the downstream of the river. Despite witnessing several landuse practices along the riparian zones, there also observed abundant growth of riparian vegetation that might have positively mitigated in controlling pollution of the river. Workers like Othman et al. (2012) and Naubi et al. (2016) have   shown encouraging results in the improvement of water quality due to proper management policy and remedial measures. Stations 2-5 experience an abrupt rise in pollution level as all these stations are located near the vicinity of the hydroelectric dam. The stagnant condition of water bodies due to the presence of hydroelectric dam and different landuse activities around these stations could have contributed to the deteriorating condition of water quality. Particularly at station 1, runoff of bridge construction materials (concrete, asphalt, etc.) from the ongoing construction of national highway bridge (NH-02) across the river and cutting down of riparian hill slope for the same have contributed to the increased concentration of many of the water quality parameters analyzed. The presence of some residential homes in the adjoining areas of station 1 has also played a vital role in influencing the physicochemical parameters of water. Different landuse activities located in the upstream of the river like Jhumming (S3), residential area (S1 and S5) and monoculture like teak plantation (S4) have imposed a serious threat to water quality deterioration. Besides, burning of forest annually for shifting cultivation, felling and logging of   trees for timber, picnic spot along the river and fishing activities have also exerted much pressure in influencing the water quality of the river. Anthropogenic activities like sewage disposal by the communities residing in the catchment areas, agricultural runoff and unprotected river sites (Yisa and Jimoh 2010, Bouslah et al. 2017and Shah and Joshi 2017 have also been contributing agents in the deterioration of water quality.

Conclusion
The study provides us with valuable information about the overall water quality status of the Doyang River by calculating the WQI values. As per the observation, recorded WQI values fall in good water quality status during preand post-monsoon in all the sampling stations and poor water quality status during monsoon in some of the sampling stations that are located upstream of the river. No considerable changes in WQI were observed throughout the study period except in few sites, where a modest increase in WQI was observed during monsoon. The overall average WQI, however, indicated good water quality status. All the physicochemical parameters of water analyzed were within the permissible limit of drinking water quality, and at present, they do not pose a serious threat for different human usage. In the present study, pH, DO and BOD played a significant role in affecting the WQI of the river. Though in the case of nutrient parameters, no such significant roles were observed. Nevertheless, there are disturbances like Jhum cultivation, extensive teak plantation (monoculture) and increased settlements in the catchment area. Annual burning of the forest for shifting cultivation, logging of trees, eco-tourism, poisoning of rivers and use of explosives for fishing can impose a serious threat to the water quality. These activities, if not controlled, could lead to further deterioration of water quality in the near future. To further improve the water quality, proper management policy must be adopted on disposal of sewage by the communities residing in the catchment areas, agricultural runoff, unmanaged landuse practices and unprotected riparian areas. Special focus on community participation in conservation efforts could be helpful. Remedial measures along the riparian zones could play a positive role in future monitoring and improvement of Doyang River water quality.