Interconnectivity and water quality of shallow aquifer and the river system in the Kathmandu valley

Abstract

The groundwater and surface water are connected systems of single water resources. The connection condition can be noticed in different lands such as ponds, lakes, seas, and reservoirs but is mainly investigated at the stream reach scale. Interconnection of river-groundwater is a natural process that exchanges water between the river channel and water in subsurface areas. The exchange flow of water is dependent on the hydraulic conductivities of the river bed and aquifer sediments; the difference in water level in the river channel and adjacent groundwater; and the geometry of the river channel within the alluvial plain. The flow direction of water exchange is dependent on the hydraulic head between the river channel and the aquifer. The exchange process can be affected by anthropogenic activities such as sewage load in rivers, and a decline in the water table, which can alter the exchange condition, reduce connectivity, and contaminate aquifers chemically or biologically. Thus, the research related to interconnection is very essential to develop effective water resource management and policy as it can change the water quality and quantity of both water systems. However, there is a lack of such research in the case of Nepal. Hence, this study is focused to identify spatial and temporal interconnectivity between contaminated rivers of the Kathmandu Valley with peripheral groundwater. The isotopic analysis of δD and δ18O, chemical analysis of cations and anions along with sediment distribution patterns on the surface and subsurface were major utilized methods of the research. Hierarchical cluster analyses were used for grouping water samples into clusters depending on isotopic and chemical composition (Na+ and Cl-). The combination of river and groundwater samples into a single cluster indicated the presence of interconnection. For this research, Water samples were collected in August 2017 (wet) and Feb 2018 (dry). A total of 165 and 162 samples were collected from rivers, dug wells and shallow tube wells in the wet and dry seasons respectively. The isotopic composition (δD and δ18O) of the river presents a meteoric source for river discharge in both seasons. Samples from the Bagmati River, and Hanumante and Godawari Khola with enriched isotopic composition exhibit the possibility of evaporation during the dry season. The isotopic composition of groundwater shows spatially variable. Compared to GMWL and LMWL, groundwater is recharged through precipitation with some evaporation effect on samples. Interconnection condition of the groundwater and river water has been identified using HCA. River-groundwater interconnection is spatially and temporally variable. Wet season analysis shows that about 68% of sites are non-connected with river water which is especially located at the center of core urban areas of the Kathmandu Valley. The percentage of non-connection sites is reduced to 11% in the dry season showing a dominant influent condition (54%) as the exchange process. Only 9% of sites which shows non-connected in both seasons imply that the rivers of the Kathmandu Valley are connected with adjacent shallow groundwater. Chemical analyses of river water classify wet season as Ca-HCO3 type. Except for Godawari Khola and a few other river sections; others are changed to Na-K-HCO3, Ca-SO4, and Na-Cl-SO4 type, which indicates an increment of contamination during the dry season. The presence of a significant positive correlation between chemical ions indicates the influence of anthropogenic activities such as untreated municipal and industrial sewage discharge and leachate of solid waste disposal in river water. Additionally, a strong positive correlation of PO4--P with SO42- suggests the effect of fertilizer and pesticides used in the river’s peripheral agricultural land. Cluster analysis of dry season river water signifies that the Godawari Khola is the least polluted and the Hanumante Khola is a seriously contaminated river of the valley. The quality status of groundwater is determined by comparing it with the limit of NDWQS. The percentage of dug wells exceeding NH4+-N and EC becomes doubled (60.9% and 8.5% respectively) in the dry season, whereas dug wells exceeding pH has become increased up to ten times (10.4%). About 80% of dug wells exceed the limit of NH4+-N from the Manahara River and Hanumante and Balkhu Khola in the dry season showing severe anthropogenic contamination in the shallow aquifers. The presence of river-groundwater interconnection as a dominant influent condition again indicates that the higher contamination in shallow groundwater is the result of groundwater recharge by heavily contaminated river water during the dry season.