The study area consisted of Bagmati and Bishnumati River corridors of the Kathmandu Valley. The valley is a bowl-shaped intramontane basin located in the central Nepal. The valley covers an area of about 665 km² which is surrounded by mountain ranges with elevations up to 2800 m above mean sea level [3] . Kathmandu basin was formed by uplift of the surrounding mountain ranges and was filled by Mid-Pliocene to Holocene fluvio-lacu- strine deposits that overlay Precambrian to Devonian metamorphic basement rocks [15] [16] . The mineral composition of the basin is dominated by quartz, K-feldspar, plagioclase and mica with minor chlorite and calcite [17] .

Kathmandu Valley has humid sub-tropical climate, which is largely affected by the monsoon [17] . Average precipitation is about 1755 mm/year with 80% of rain falling during the summer monsoon between June and September [3] . Surface hydrology in Kathmandu Valley is governed by the Bagmati River system which originates in the Shivapuri Mountains to the north of the valley and discharges to the south of the watershed. This consists of two main groundwater aquifers, shallow and deep aquifers that are separated by a clay aquitard of up to 200 m thickness [5] [11] . Based on the hydrogeological similarities, the basin is classified into 3 major groundwater districts: northern, central and southern groundwater district [18] . The underlying deposits of northern groundwater district are composed of unconsolidated, permeable materials consisting of micaceous quartz, sand and gravel. The central groundwater district sediment is dominated with a thick black clay layer; which is overlain by fluvial originated fine to medium sand, silt, intercalated clay and fine gravels. The southern groundwater districts possess thick clay layer however parts of the zone (eastern area of this district) are covered with sand and gravel [3] .

Bagmati River is an important tributary of the Ganges and has catchment area of 3710 km² in Nepal. The river has religious and cultural significance which in fact is known as Bagmati Civilization. Bagmati River corridor is the principal resource base of municipal water in the valley, which constitutes water supply to almost 92% in wet season and 60% in the dry season. Main sources of water in Bagmati River are rainfall and natural springs. The recorded discharge at Jorpati during wet season is 3.44 m³/sec [9] .

Bishnumati River, one of the tributaries of the Bagmati River, is also part of the Bagamati Civilization. The river flows between the altitudes of 2481 m (the origin in the Shivapuri hills) and 1289 m (confluence with the Bagmati River at Teku). The recorded mean monthly flow of this river at Teku is 0.72 m³/sec in March (minimum) and 16.62 m³/sec in August (maximum) [8] .

A monitoring network was designed along the Bagmati and Bishnumati River corridors for two years (Figure 1). Three river water samples: R1, R2 and R3 were collected from three different sections (approximately 500 m apart) along the Bagmati River (at Jorpati reach). Three groundwater samples G1, G2 and G3 were collected from the adjacent shallow tube wells along this river. Similarly, three river water samples: R4, R5 and R6 were collected from Bishnumati River (at Manamaiju reach). Four groundwater samples: G4, G5, G6 and G7 were collected from the surrounding tube wells along this river (Table 1). Water samples from each source were collected monthly from May 2012 to June 2014.

The water samples collected in polyethylene bottles (250 mL) were rinsed 3 times with the sample water and kept immediately in an ice box. The global positioning system (GPS) was recorded for all the sampling locations.

Some of the physico-chemical parameters of the water samples were measured following standard methods and instruments as listed in the Table 2. Stable isotope analysis were performed using water equilibration system (Sercon, WES) and isotope mass spectrometer (Sercon, Hydra 20 - 20) in the laboratory of the Interdisciplinary

Note: SG―Shallow groundwater, R―River water.

Centre for River Basin Environment (ICRE), University of Yamanashi, Japan. The analyses were standardized with the international references Vienna Standard Mean Ocean Water (V-SMOW). For δ¹⁸O, 1 mL of each standard and samples were flushed for CO₂, then left to equilibrate at 35˚C on the hot block in the WES for 8 hours before being analyzed. For δD, 1 mL of each standard and samples were transferred into individual exetainers containing platinum bead and flushed by H₂. The samples were then left to equilibrate in a hot block WES for 3 hours at 35˚C analysis.

According to standard conventions, all isotopic compositions are given in per mill (‰) units and δ notation relative to the V-SMOW standard [19] .

δD or‰

where R indicates the ration of ²H/¹H or ¹⁸O/¹⁶O. Analytical uncertainties were ±0.1‰ for δ¹⁸O, and ±1‰ for δD.

The sum of Na⁺ and Cl⁻ concentrations of river waters at sampling locations R1, R2 and R3 were nearly similar to the concentrations of shallow groundwater at SG1 during the months November 2012 to May 2013 (Figure 2). Similarly, the concentration was found similar to the concentration at sampling locations SG1, SG2 and SG3 during the months January to May 2014. The sum of Na⁺ and Cl⁻ concentrations of river water was high which reached up to 60 mg/L during the month of May 2014. However, these ions were found higher in shallow groundwater compared to river water.

Similarly, the Na⁺ and Cl⁻ concentrations of shallow groundwater and river water were similar during several months along the Bishnumati River corridor (Figure 3). The river water collected from Bishnumati River had high Na⁺ and Cl⁻ concentrations. It could be due to the discharge of a large amount of domestic waste water. The banks of the Bishnumati River have become provisional landfill sites. The uncontrolled dumping has severely contaminated the groundwater as well as river water [8] .

The similar patterns of sum of Na⁺ and Cl⁻ concentrations in river waters and shallow groundwater collected from Bagmati and Bishnumati River corridors showed that there is possibility of interrelationship between these water sources.

The relationship of Na⁺ and Cl⁻ concentrations is used to identify groundwater origin and sources of the groundwater chemical constituents [30] . The ratio of Na⁺/Cl⁻ of river water and shallow groundwater samples

were plotted in Figure 4. The ratio of Na⁺ and Cl⁻ nearly followed 1:1 ratio in river water (R² = 0.983). The natural sources of these ions are rock-water interactions, saline seeps, and minor atmospheric inputs. Other anthropogenic sources include: agricultural chemicals, effluent from private and municipal septic systems, animal waste [31] . However, Na⁺ concentrations were comparatively higher than Cl⁻ concentrations in shallow groundwater. High concentration of Na⁺ irrespective to Cl⁻ concentrations in shallow groundwater probably resulted from rock-water interactions with Na-clays and Na-feldspars present as silt-and sand-size fragments in the soil zone and aquifers [31] .

The mean values of the isotopic composition (δ¹⁸O and δD) of shallow groundwater and river water are presented in Table 5. The δ¹⁸O value of shallow groundwater ranged from −7.7‰ to −4.8‰; and δD from −69.8‰ to −49‰. The δ¹⁸O value of river water ranged from −7.5‰ to −4.4‰ and δD ranged from −71.3‰ to −49.6‰ respectively. The isotopically lightest for δ¹⁸O and δD in sample were −7.7‰ and −71.3‰, and the heaviest were −4.4‰ and −49‰ respectively.

The Global Meteoric Water Line (GMWL) was drawn using the following equation described by [19]

The Local Meteoric Water Line (LMWL) published for Kathmandu Valley was used which was based on sampling of rain water during pre-monsoon and monsoon seasons [32] .

The hydrogen and oxygen isotope values of groundwater and river water were plotted on reported LMWL for Kathmandu valley (Figure 5). The isotopic composition of the shallow groundwater nearby the Bagmati River corridor plots close to the LMWL. It revealed the rainwater as the possible source of this shallow groundwater. While the isotopic compositions of Bagmati River, Bishnumati River and Bishnumati shallow groundwater plotted below the LMWL in the plot, this indicated possible evaporation or mixing line. The main causes of variations in the stable isotope signature of water sources were natural variations in the isotopic composition of rainfall, mixing with pre-existing waters, and evaporation through soil [33] [34] . In addition, isotopic composition of river water and shallow ground water clustered together (see Figure 5) reflected existence of certain degree of interrelationship between river water and groundwater [14] .

The Figure 6 and Figure 7 showed the seasonal isotopic compositions of shallow groundwater and river water during dry and wet seasons.

The isotopic compositions were clearly divided into three clusters.

Cluster 1: The isotopic composition of river water and shallow groundwater plotted on or nearby the LMWL.

Cluster 2: The isotopic composition of some of the shallow groundwater samples deviated from LMWL

Cluster 3: The isotopic composition of river water and shallow groundwater deviated from LMWL.

The deviation of isotopic compositions of river water and groundwater from the LMWL revealed water has undergone evaporation with systematic enrichment in stable isotope [29] [35] . In contrast to this, previous studies investigated that none of the Kathmandu Valley rivers showed evidence for evaporation except for minor

^*Please note that all the parameter values are the mean values of the monthly sampling of the 2 years.

catchments in the valley during monsoon [32] [36] . Such variation in isotope compositions of river water could be expected due to lack of natural river flow, whereas the river discharge in Kathmandu is usually influenced by anthropogenic activities such as discharge of domestic wastewater into river. It revealed that the sampling locations were more or less likely to be mixed with sewer water. Similar results were reported by [31] in which most of the river water samples were plotted along a similar evaporation trend. Also, landfill leachate, septic effluent and animal wastes which have elevated δ¹⁸O values were also plotted along the evaporation trend. However, such effects could be detected using stable isotope ratios such as, δ¹⁵N, δ¹³C and δ³⁴S [37] .

Similar pattern was seen in the isotopic composition of wet season of the river water and groundwater (Figure 7).

The relationship of chloride (Cl⁻) and stable isotope of oxygen (δ¹⁸O) in river water and groundwater was plotted in Figure 8. It showed a close range of δ¹⁸O and Cl⁻ concentrations in shallow groundwater and river water, which indicated the possible hydraulic interaction between river water and shallow groundwater [38] .

The physicochemical properties and isotopic compositions of the river water and groundwater revealed interrelationship existed between them. In addition, mass balance approach was used to find out the mixing contribution of river water into the shallow groundwater.