Abstract
In the Indian metropolis of Delhi, the Yamuna River is highly influenced by sewage water, which has led to elevated ammonium (NH4+) concentrations up to 20 mg/L in the river water during 2012–2013. Large drinking water production wells located in the alluvial aquifer draw high shares of bank filtrate. Due to the infiltrating river water, the raw water NH4+ concentrations in some wells exceed the threshold value of 0.5 mg/L ammonia-N of the Indian drinking water specifications, making the water unfit for human consumption without prior treatment. However, to meet the city’s growing water demand, it might be advantageous to consider the long-term use of the well field. This requires the development of an adapted post-treatment unit in concert with an adjusted well field management. To better understand the groundwater dynamics and contamination and decontamination times at the well field, a theoretical modeling study has been conducted. The results of 2D numerical modeling reveal that the groundwater flux beneath the river is negligible because of the aquifer and river geometry, indicating that infiltrating river water is not diluted by the ambient groundwater. Increasing the water abstraction in the wells closest to the river would result in a larger share of bank filtrate and a decreasing groundwater table decline. Simplified 1D reactive transport models set up for a distance of 500 m (transect from the riverbank to the first production well) showed that the NH4+ contamination will prevail for the coming decades. Different lithological units of the aquifer (sand and kankar—a sediment containing calcareous nodules) have a strong influence on the respective contamination and decontamination periods, as the retardation of NH4+ is higher in the kankar than in the sand layer. Although this simplified approach does not allow for a quantification of processes, it can support decision-making about a possible future use of the well field and point to associated research needs.
Highlights
The urban agglomeration of Delhi, which comprises the National Capital Territory (NCT) of Delhi as well as the contiguous cities, is the second largest urban agglomeration in the world after Tokyo [1].It is expected to further grow from 22 million inhabitants in 2011 to 31 million inhabitants in the year2021 [2]
The results of 2D numerical modeling reveal that the groundwater flux beneath the river is negligible because of the aquifer and river geometry, indicating that infiltrating river water is not diluted by the ambient groundwater
The available data about the central Delhi floodplain aquifer at the studied well field are not sufficient to set up a meaningful groundwater flow and transport model of the well field
Summary
The urban agglomeration of Delhi, which comprises the National Capital Territory (NCT) of Delhi as well as the contiguous cities, is the second largest urban agglomeration in the world after Tokyo [1]. The enhanced use of bank filtration along the Yamuna River might be one measure to help infiltration of surface water into the aquifer by strategically constructing wells parallel to surface water ensure a sustainable water supply for the coming decades. Simple numerical models can help to estimate the influence of different groundwater flow conditions in the well field and identify the data gaps for further research The objectives of this modeling study, are (1) to investigate the possible inflow of groundwater from the western side beneath the river, and (2) to decide whether the production wells draw shares of ambient groundwater from the eastern areas of the floodplain where the groundwater NH4 + concentrations were reported to be low [15]. This might motivate scientists and stakeholders to collect and share the data necessary for setting up a calibrated 3D flow and transport model, which is necessary for more detailed planning of such measures
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