As mentioned earlier, Burhi Gandak originates from the West Champaran District and meets Ganga in the Khagaria District. Considering this, five places were selected to collect the water quality data of the Burhi Gandak River. These places are near Lauria, Sugauli, Muzaffarpur, Samastipur, and Khagaria. Figure 1 shows the map of the places from where water quality data were collected. The coordinates of the places are shown in Table 1. Burhi Gandak originates from the district of West Champaran. So, Lauria, a small town from West Champaran, which is on the bank of the Burhi Gandak River, was selected for the study. It is approximately 28 km from the district headquarters and is one of the important Buddhist archaeological sites. The next location selected on the downstream side was Sugauli. It is situated in East Champaran. Both Lauria and Sugauli are famous for the sugar industry.

The next sites selected for study were Muzaffarpur and Samastipur. Both are significant cities in North Bihar. As per the 2011 census of India, the population of Muzaffarpur City is more than three lakh, and the population density is 13,285 persons per sq km. Both Muzaffarpur and Samastipur are important from the industrial activity point of view. The last location selected in the study is Khagaria. Burhi Gandak meets the Ganga River in the district of Khagaria. Considering this, the last location was selected. Figure 2 illustrates the delineated watershed of the Burhi Gandak River, incorporating the hierarchical stream ordering and designated data collection points. The stream order classification effectively highlights the hydrological network’s structure, while the marked data collection sites provide critical reference points for subsequent spatial and hydrological analyses within the watershed.

The Burhi Gandak River displays various geomorphic features, including point bars, oxbow lakes, meander scars, and abandoned channels. In the middle and downstream sections, notable features include river terraces, point bars, floodplain deposits, and natural levee deposits. The river channel becomes shallower as it flows upstream but deepens downstream. The Burhi Gandak River exhibits pronounced meandering characteristics, particularly in Samastipur and Begusarai districts. Throughout these areas, the river channel frequently divides and reunites downstream, creating an anastomosing channel pattern. In the proximal, middle, and distal sections, the slope gradients of the Burhi Gandak River measure 69.0, 12.0, and 8.60 cm/km, respectively (Singh and Singh, 2005). The valley width ranges from 20 to 800 m, being narrowest in the proximal part and widest in the distal part.

The annual rainfall data of the study regions considered in this study is presented in Figure 2. The rainfall data presented in this study are from 1971 to 2022. It can be observed that the average annual rainfall for all the locations considered in this study is more than 1000 mm. It indicates that a sufficient amount of rainfall is obtained in the catchment area of Burhi Gandak. Most of the rainfall in this region occurs during the monsoon season. The annual average rainfall data from 1971 to 2020 show that West Champaran receives the maximum rainfall among all the locations selected. West Champaran is located in the Tarai region of the Himalayas. This increases the rainfall in this region. These data further show that Samastipur is getting a relatively lesser amount of rainfall among the sites considered. However, rainfall data from 2019 to 2022 show that the average annual has decreased for all the locations. However, Muzaffarpur has the highest reduction in rainfall. It has become a place with the lowest rainfall among all the considered locations. It is difficult to estimate the possible cause of it with limited data. However, recently, the highest growth in industrialization and urbanization has taken place in Muzaffarpur City among all the selected cities. This might be one of the major causes of the reduction in rainfall.

The land use land cover (LULC) map for the study area was generated using ArcGIS, leveraging satellite imagery and a digital elevation model (DEM) for enhanced spatial accuracy. After preprocessing the imagery, supervised classification was employed to delineate key land cover types, including agricultural, forest, urban, and water bodies. The integration of DEM data provided critical topographical context, refining land classification, particularly in areas with varied elevations. This synergy of LULC and DEM in ArcGIS enabled precise spatial analysis, which is essential for environmental and topographical assessments. Figure 3 shows the LULC map of Bihar. The LULC map indicates the land and its utilization, like forest area, crop area, and urban area. It can be observed in Figure 4 that most of the regions have similar rainfall patterns. Table 2 shows the area of the different LULC classes. It shows that agricultural cropland covers the highest area. The district, except for West Champaran, through which Burhi Gandak flows, has mainly cropland or buildup area. The northern part of West Champaran is covered with forest areas.

To understand the impact of urbanization, industrial activities, and location on the water quality of the Burhi Gandak River, data related to geomorphology and hydrology were collected from the available literature and the official government websites of the government of Bihar and India after this water quality-related data were collected. For this purpose, results from the laboratory tests and data from the literature were used. The water quality data of Lauria, Sugauli, and Muzaffarpur were obtained from the laboratory test results, while data of Samastipur and Khagaria were obtained from the available literature. The data obtained from the literature were confirmed through the laboratory test performed on the sample obtained from the mentioned location through random sampling.

The variations in the pH, total dissolved solids (TDSs), total hardness (TH), biological oxygen demand (BOD), and dissolved oxygen (DO) for different locations are presented in Figure 5. It can be observed that all these water quality parameters vary significantly from one location to another. The pH value of the water is 7.1, while it is 7.8 for Lauria and Khagaria (Fig, 4a), with Samastipur exhibiting the highest pH levels. Furthermore, TDS and BOD are also highest in Samastipur (4b, 4d). For Lauria and Khagaria, TDS values are almost in a similar range, but Sugauli and Muzaffarpur show relatively high TDS values. TH for all locations is in the range of 113–144 mg/L (4d). The variation in the DO value, as presented in Figure 5E, shows that the DO of water in Lauria, Sugauli, and Muzaffarpur is around 8. However, in Samastipur and Khagaria, it decreases to around 7. All the places have good DO levels. The presence of DO is important for aquatic life. So, the present condition of DO indicates that Burhi Gandak is satisfactory from the aquatic life point of view. Furthermore, we can conclude that the quality of the river water heavily depends on the local conditions. So, there is always a requirement for local control of human activities to improve or maintain the quality of water.

The impact of the industry on the water quality of rivers can be understood by comparing the results shown in Figures 6, 7. Figure 6 shows the result of the water quality of Sugauli, and Figure 7 shows the water quality results of Lauria. Both of the places have sugar plants near the banks of the Burhi Gandak River. Sugar plants do not operate for the full year. They start in November and close in March. So, to understand the impact of the industry on the water quality of the river, water samples were collected and tested during the operational and shutdown seasons of the industry. It can be observed for both places that degradation in water quality took place when the industry was running. The quality parameters of water are within permissible limits in all cases. However, it can be concluded from Figures 6, 7 that due to industrial activity, water quality is affected. There is a requirement for proper supervision of these industries so that they maintain the quality of the water coming from their effluent treatment plants.

The impact of urbanization can be understood by comparing the results of water quality presented in Figures 8–10. Figure 8 indicates the water quality of the river for all the seasons collected from the entry point of Samastipur City. Figure 9 shows the water quality for all the seasons of the river collected from the midpoint of the city, while Figure 10 shows the water quality for all the seasons at the endpoint of the city. It can be observed that the water quality is degraded at the end of the city to the entry point. The BOD of the water at the entry point in summer was 7.5 and has increased to 9.5 at the exit point of the city. A similar trend can be observed for the monsoon and winter seasons. Currently, water quality at the entry and exit points of the city is satisfactory. However, Samastipur and other cities in Bihar are growing rapidly. Different industrial activities are also increasing in these cities. So, there is a requirement for proper supervision of the quality of water and human activities in such cities or towns.

Table 3 shows the quality of water collected from Burhi Gandak near Sugauli in different months. In North Bihar, heavy rainfall occurs mainly during the monsoon season, which starts from mid-June and remains till mid-September. During this time, flood conditions occur in all the rivers of North Bihar, and it remains till September. Considering this, the results of post- and pre-monsoon are presented in Table 3. It can be observed that the other quality of water is not changing except the amount of the TSSs. During flood conditions, huge amounts of sediments get mixed in the water. Due to this, the total suspended solids increased during the monsoon season, i.e., in September.

The quality of the Burhi Gandak River is influenced by several key pollutants, including the following:

a. Organic matter: the high BOD levels, especially around urban areas like Samastipur, suggest the presence of organic pollutants. These may originate from untreated or poorly treated domestic sewage, agricultural runoff, and effluent discharges from sugar plants.