The Influence of Rice Field Water Management On Groundwater Arsenic Concentrations in Bangladesh.

Arsenic-contaminated groundwater in Bangladesh affects the drinking and irrigation water supplies of millions of its citizens. Current arsenic concentration patterns are related to groundwater flow and recharge chemistry. Constructed ponds and groundwater irrigated rice fields serve as the primary aquifer recharge sources, with pond recharge evolving into high-arsenic groundwater and rice field recharge evolving into low-arsenic groundwater. Recharge is largely controlled by the practice of groundwater irrigation, which removes water from the aquifer, creating a downward gradient that drives surface water into the subsurface. These dynamics suggest that rice field water management schemes that reduce irrigation water demand will necessarily change arsenic concentration patterns in the aquifer by reducing the total amount of surface recharge pulled into the aquifer and by altering the proportions of rice field and pond recharge. 

Here we present results from a study that experimentally tests the water savings potential of a specific field management scheme – sealing the bunds or raised boundaries around the edges of fields with plastic sheeting – and models the subsequent effects of this scheme on groundwater arsenic concentrations. Previous research efforts conducted at our site in Bangladesh demonstrated that over half of all irrigation water applied to fields is lost through bunds. We employ the collected irrigation usage data in a three-dimensional, seasonal groundwater flow and conservative transport model to track the proportion of recharge water reaching domestic wells from ponds and rice fields. We assume that an increase in the proportion of recharge originating from ponds represents the potential for a future increase in arsenic concentration. The results clarify the sensitivity of current arsenic concentration patterns to rice field management practices and highlight the potential for future contamination of currently clean, shallow domestic wells.