Modeling Arsenic Mobilization in a Riverbank Aquifer under the Influence of Tidally Fluctuating River and Irrigation Pumping

Abstract:

The role of rivers in releasing or trapping As (Arsenic) in aquifer sediments has not been widely explored. In large deltas with flat topography, transient water levels, changes in flow directions, and redox conditions in riverbank aquifers are often observed. This creates a unique condition that may promote As release and trapping. Extremely high As concentrations in solid phase have been found in the sediments lining the Meghna River, Bangladesh. We hypothesize that this enrichment is the result of gaining river conditions; the originally high concentration of As present in groundwater sorbs to FeOOH as it comes into contact with oxic river sediments. The objective of the present study is to model the transport and geochemistry of groundwater and surface water within the riverbank aquifer under the influence of extensive local irrigation pumping using high-frequency water levels and water chemistry. Two years of hydraulic head observations from shallow monitoring wells within 100m from the river on both the west and east sides indicate the Meghna River is gaining throughout the year in this part of the delta. Under non-irrigation pumping times, lateral hydraulic gradients varied from 0 to 0.045 towards the river. Hydraulic gradients trending away from the river were only observed during short term irrigation pumping and varied from 0 to 0.053. Using hydraulic conductivities derived from slug tests these hydraulic gradients correspond to groundwater moving 0 to 1.53±0.10 m/d towards the river during non-irrigation pumping conditions, and 0 to 1.80±0.12 m/d away from the river during irrigation pumping. Water chemistry was analyzed on samples taken from the wells and the River. As concentrations range from 0 to 366 ppb in the shallow wells for the pre-irrigation sampling. Elevated chloride and specific conductivity in the riverbank aquifer inland were used to identify the mixing zone across the aquifer. A two-dimensional flow and reactive transport riverbank aquifer model was developed using COMSOL Multiphysics. This study has important implications on the potential risk of As concentration seasonally rising in shallow aquifers adjacent to rivers. Furthermore, our model helps understand redox processes in riverbank aquifers affected by natural and anthropogenic processes that affect groundwater quality.