Understanding the stable isotopic signature in the tap water system of the Kathmandu valley facing intermittent water supply

Abstract:

Worldwide, intermittent water supply (IWS) has been a common practice in coping with water shortage. The intrusion and mixing of the alien water origin from sewage and groundwater during IWS is yet a topic for discussion. The current study focuses on using stable isotopes of water (𝛿D and 𝛿¹⁸O-H₂O) in evaluating the drinking water network sources, their pathways, interaction, or mixing through leakage within the water networks depending upon regional water flow system, demographic parameter, and the seasonal impact on the supply sources. To incorporate these objectives, a stable isotope of water aided by water quality parameters is used as a tracer in the urban settlement of the Kathmandu Valley, where daily water supply is 1-4 hours/week. However, during no water supply, intrusion from leakage could be an active local process.

Spatially distributed 9 drinking water reservoirs and their successive 90 municipal tap water were sampled in two consecutive seasons; wet (Jun-Sept) and dry (Nov-Feb). The value of 𝛿D and 𝛿¹⁸O-H₂O ranged from -66 to -51‰ and -9.5 to -7.8‰, respectively during the wet season and the d-excess values were within the range of 7.4 to 14.2‰. Similarly, the value of 𝛿D and 𝛿¹⁸O-H₂O ranged from -68 to -46‰ and -9.4 to -7.3‰, respectively during the dry season, and the d-excess values were within the range of 6.0 to 12.9‰ in the same season. The tap water and reservoir samples were found to be isotopically distinct from precipitation falling within the local hydrological basin reflecting the variation in the supply source and points out the climatic control on the tap water network. Meanwhile, the variation in the d-excess also reflects the climate challenges for the sustainability of the individual sources. Additionally, the variation in the isotopic signature between some of the reservoirs and their successive tap water were also observed. The variations in the isotopic signature from reservoir water sample and tap water indicates the intermixing in the municipal water network. Although, the source of some tap waters are from the surface water and deep groundwater, the similarities in stable isotopes of water compared from shallow groundwater and tap water network shows the need of further researches in understanding the intrusion during IWS.