The impact of sewage-contaminated river water on groundwater ammonium and arsenic concentrations at a riverbank filtration site in central Delhi, India; [L’impact d’une eau fluviale contaminée par des eaux usées sur les concentrations en ammonium et arsenic des eaux souterraines au niveau d’un site de filtration par les berges dans le centre de Delhi, Inde]; [O impacto das águas de rio contaminadas por esgoto nas concentrações de amônia e arsênico das águas subterrâneas na área de filtração por bancadas em Delhi Central, Índia]; [El impacto del agua de ríos contaminados por aguas residuales en las concentraciones de amonio y arsénico del agua subterránea en un sitio de filtración en la margen del río en el centro de Delhi, India]

The groundwater abstracted at a well field near the Yamuna River in Central Delhi, India, has elevated ammonium (NH4 +) concentrations up to 35 mg/L and arsenic (As) concentrations up to 0.146 mg/L, constituting a problem with the provision of safe drinking and irrigation water. Infiltrating sewage-contaminated river water is the primary source of the NH4 + contamination in the aquifer, leading to reducing conditions which probably trigger the release of geogenic As. These conclusions are based on the evaluation of six 8–27-m deep drillings, and 13 surface-water and 69 groundwater samples collected during seven field campaigns (2012–2013). Results indicate that losing stream conditions prevail and the river water infiltrates into the shallow floodplain aquifer (up to 16 m thickness), which consists of a 1–2-m thick layer of calcareous nodules (locally known as kankar) overlain by medium sand. Because of its higher hydraulic conductivity (3.7 × 10−3 m/s, as opposed to 3.5 × 10−4 m/s in the sand), the kankar layer serves as the main pathway for the infiltrating water. However, the NH4 + plume front advances more rapidly in the sand layer because of its significantly lower cation exchange capacity. Elevated As concentrations were only observed within the NH4 + plume indicating a causal connection with the infiltrating reducing river water. © 2017, Springer-Verlag Berlin Heidelberg.