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
