Simulation of transport of mobilized arsenic applying 3D ground water flow model

Abstract

The existence of widespread arsenic contamination in groundwater in Bangladesh has been known for decades though there is still significant uncertainty regarding the causes responsible for its mobilization. At a typical site in Bangladesh, where groundwater-irrigated rice fields and constructed ponds are the main sources of groundwater recharge, a new pond is constructed to trace the contribution of pond in arsenic contamination in groundwater. This hydro-geochemical study involved fieldwork, laboratory analyses, and modeling. The field research spanned over three and a half years and included the deployment of a sensor network to continually monitor soil moisture and water potential, soil and water samples to ascertain chemical characteristics. The large amount of generated data was synthesized with hydrologic, geochemical and mass-balance models. The study showed that physical and chemical influence of newly constructed pond explains the spatial and temporal patterns on shallow aquifer of the study area. Recharge from pond is both temporally and spatially heterogeneous. Flow from ponds is constant and uniform through the pond sediments. Shallow aquifer chemistry shows a considerable amount of the temporal variability where Arsenic concentration is changing during dry season or irrigation period while recharge from pond is occurring. Again, for DOC the concentration in the same shallow aquifer is increasing during the flood season and during irrigation period the concentration reduces further. It may be due to recharge from pond which causes transport of DOC from shallow aquifer to deeper aquifer. Also, the reason may be the release of organic carbon as DOC and associated As during Fe/Mn oxide reduction. A detailed three-dimensional hydrological model of the study area in Munshiganj indicates that: (1) the shallow aquifer acts primarily as a conduit for flow from ponds and rice fields to irrigation wells and rivers; (2) most inflow to the aquifer occurs during the dry season, and monsoon contributes relatively little to the inflow since the aquifer storage is small; (3) since the increase in irrigation pumping and pond construction have changed the groundwater flow dynamics, arsenic concentrations are unlikely to be at steady-state. These observations are consistent with those from the Groundwater Vistas model. Model simulations carried out for the current ground water flow condition continues which indicates that in general, the rice field water dominates at the shallowest depth while pond water dominates at the depth of irrigation well, and the contribution from river water increases with depth. Analysis of the average groundwater age distribution indicates that younger age dominates at shallower depths. More importantly, the age values at the monitoring locations can be explained by the relative contribution of recharge water from different sources. The stable water isotope values in the study area shows a similar profile of the dissolved arsenic concentration. Furthermore, comparison of aquifer chemistry and measured isotopic values at the study field site indicates that the observed isotopic profile results from the mixing of water from various recharge sources. More importantly, the lighter water at the depth of peak arsenic concentration can only be derived from lighter pond water recharge in dry period, whereas recharge from river and rainfall mainly occurs towards the end of dry period when those waters are actually heavier. Finally, the measured geochemistry of the study area, with the numerous modeling results and isotopic characterization, strongly supports the "Pond Hypothesis" for explaining the distinct profile as well as the spatial variability of dissolved arsenic concentration. The water chemistry beneath the newly constructed pond explains the changes of the concentration of dissolved arsenic at the shallowest depth which is an evidence of the "Pond Hypothesis".