The role of agricultural activity for ammonium contamination at a riverbank filtration site in central Delhi (India)

At a riverbank filtration (RBF) site in central Delhi unusually high and strongly fluctuating ammonium (NH4+) concentrations were measured in the groundwater. Sewage contaminated river water has been identified as the main source of the NH4+ plume. However, the well field is located within an irrigated agricultural area and irrigation return flow might have an additional impact on the groundwater contamination. To quantify the role of irrigation return flow, sediments of the vadose zone were characterized concerning their hydraulic and sorption characteristics, and x-ray diffraction for clay mineral analyses and laboratory column studies were conducted with representative sediments. The sediments range between silt and fine-medium sand with calculated hydraulic conductivities between 2.1 x 10(-4) and 1.0 x 10(-7) m/s and a cation exchange capacity between 0.9 and 37.2 meq/100 g sediment. The column experiments show that NH4+ retardation through cation exchange is the main process occurring in the sediments. Contrary to the results of similar experiments conducted with sediments from the saturated zone, mass balances reveal that NH4+ fixation or degradation also takes place in significant amounts and up to 0.09 meq NH4+/100 g sediment were transformed this way. This indicates that irrigation return flow can be neglected as a major source of NH4+ at the field site. Instead the NH4+ fixation or degradation in the unsaturated zone might be one reason for the observed variations of NH4+ concentrations in the groundwater.