Potential volumes of arsenic in Holocene clay plug sediments in Bihar, India

Abstract

Natural occurring arsenic contamination of shallow aquifer groundwater is a problem affecting millions of people worldwide. Long term exposure to high concentrations results in severe medical conditions. On-going research into the origin and spread of the problem, risk mitigation and problem solving is of great importance. Sediment eroded and transported from mountains adsorbs arsenic (As) from river water onto its iron oxyhydroxides coatings. The geomorphology of the river is related to the concentration of As in shallow aquifers. Helicoidal flow in a meandering river leads to erosion of the cut bank in the outer bend and accumulation of sediment at the point bar in the inner bend. The result is an asymmetrical depth profile. The process of meandering and avulsion sometimes leads to the complete abandonment of a part of the river’s channel. This still-standing water body is known as an oxbow lake. Fine sediment settles from suspension and the oxbow lake gradually fills up with silt and clay. A clay plug forms, surrounding the sands of the adjacent point bar. Clay filled oxbow lakes formed by meandering rivers are high in organic content and the anoxic conditions in the hypolimnion are considered the source for the release of the adsorbed arsenic. Under reducing conditions the As is released from its solid state by microbial respiration. In the geomorphological setting of meandering rivers, abandoned channels and point bar, this process of reductive dissolution is the generally accepted release mechanism for arsenic. This research aims to provide insight in the potential arsenic volume in Holocene clay plugs in Ganges River floodplains and to present ideas on the migration processes of arsenic from clay plug to adjacent point bar. Migration of dissolved As occurs by advection and by diffusion. Satellite data from Google Earth Pro was used to simulate clay plugs with a Matlab model. The simulated data was used for the calculations of the surface area of the clay plug, the volume of the clay plug, the potential volume of As and the contact area between the clay plug and adjacent point bar. These geometric properties and concentrations of As were used to apply Fick’s first law to estimate the initial diffusion flux and the initial discharge. The surface area of the twenty selected clay plugs vary from 10! to 10! m2. The corresponding volumes are in the order of magnitude of 10! to 10! m3. The calculated As volumes range within the orders of 10! to 10! kg. The initial diffusion flux was calculated and ranges approximately between 15-300 g/m2year. For the volume calculations the exact shape of the depth profile turned out to be of little influence. For calculations of the contact area and thus the diffusion flux estimations, the true profile is crucial. In-situ sampling would provide data to minimize uncertainties and improve results.