The impact of sewage-contaminated river water on groundwater ammonium and arsenic concentrations at a riverbank filtration site in central 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 x 10(-3) m/s, as opposed to 3.5 x 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.