Streams, sewage, and shallow groundwater: stream-aquifer interactions in the Kathmandu Valley, Nepal

Rajaram Prajapati,Jeffrey C Davids,Kate Happee,Rick Van Bentem,Surabhi Upadhyay,Vishnu Prasad Pandey,Nischal Devkota,Bhesh Raj Thapa,Bikesh Manandhar,Amber Bahadur Thapa,Anusha Danegulu,Niek Moesker,Rabin Malla,Nick N Overkamp,Rocky Talchabhadel

doi:10.1007/s40899-021-00542-8

Abstract

The Kathmandu Valley in Nepal is facing a water quantity and quality crisis due to rapid urbanization and haphazard water and wastewater planning and management. Annually, groundwater extractions in the Kathmandu Valley exceed capture, resulting in groundwater table declines. Streams are often important sources of recharge to (or destination of discharges from) aquifers. However, stream-aquifer interactions in the Kathmandu Valley are poorly understood. To improve this understanding, we performed topographic surveys of water levels, and measured water quality, in streams and adjacent hand-dug wells (shallow aquifer). In pre-monsoon, 12% (2018) and 44% (2019) of wells had water levels higher than adjacent streams, indicating mostly a loss of stream water to the aquifer. However, in post-monsoon, 69% (2018) and 70% (2019) of wells had water levels higher than adjacent streams, indicating that monsoon rainfall contributes to shallow aquifer recharge which, at least temporarily, causes streams to transition from losing to gaining. Concentrations of all water quality parameters (electrical conductivity, ammonia, alkalinity, and hardness) were higher in the pre-monsoon compared to post-monsoon in both streams and wells. There was no recurring trend in water level difference longitudinally from upstream to downstream. However, water quality in streams and wells depleted from upstream to downstream. While we clearly observed seasonal refilling of the shallow aquifer, the role of the deep aquifer in seasonal storage processes deserve future research attention.

Highlights

Stream‐aquifer interactionsWater, both groundwater and surface water, is fundamental to human life (Singh et al 2019; Mishra et al 2016)
The exchange between streams and aquifers may happen in three different ways: the stream is either (1) losing—stream water infiltrates into the aquifer, (2) gaining—groundwater flows into the stream, or (3) disconnected—losing stream that is disconnected from the aquifer by an unsaturated zone (Winter et al 1998)
Groundwater levels increased from pre-monsoon to post-monsoon; the average increase from the 16 wells monitored in both seasons was 1.99 m in 2018, and the average increase from the 33 wells monitored in both seasons of 2019 was 0.99 m

Summary

Introduction

Stream‐aquifer interactionsWater, both groundwater and surface water, is fundamental to human life (Singh et al 2019; Mishra et al 2016). A proper understanding of groundwater and surface water interaction is crucial for effective and sustainable water resource management (Oxtobee and Novakowski 2002; Sophocleus 2002; Brenot et al 2015). Knowledge of stream-aquifer interactions is a crucial component of developing effective and sustainable water management plans that integrate the issues of water quantity and quality (Brenot et al 2015). These stream-aquifer interactions are often characterized by high spatial and temporal variability, directly impacting the water balance and stream discharge (Krause et al 2007). Several factors like topography, geology, local aquifer system, etc., affect these interactions (Oxtobee and Novakowski 2002)

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