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The Hydrogeochemistry of Pond and Rice Field Recharge:

..Implications for the Arsenic Contaminated Aquifers in Bangladesh Speakers: Rebecca B. Neumann, Ph.D., NOAA Climate and Global Change Postdoctoral Fellow at Harvard University Abstract: Researchers have puzzled over the origin of dissolved arsenic in the aquifers of the
Ganges Delta since widespread arsenic poisoning from groundwater was publicized two
decades ago. Previous work has concluded that biological oxidation of organic carbon
drives geochemical transformations that mobilize arsenic from sediments; however, the
source of the organic carbon that fuels these processes remains controversial. A
combined hydrologic and biogeochemical analysis of a typical site in Bangladesh, where
constructed ponds and groundwater-irrigated rice fields are the main sources of recharge,
shows that only recharge through pond sediments provides the biologically degradable
organic carbon that can drive arsenic mobilization. Chemical and isotopic indicators
suggest that contaminated groundwater originates from excavated ponds and that water
originating from rice fields is low in arsenic. In fact, rice fields act as an arsenic sink.
Irrigation moves arsenic-rich groundwater from the aquifers and deposits it on the rice
fields. Most of the deposited arsenic does not return to the aquifers; it is sorbed by the
field's surface soil and bunds, and is swept away in the monsoon floods. The findings
indicate that patterns of arsenic contamination in the shallow aquifer are due to rechargesource
variation and complex three-dimensional flow.

Location: Kaprielian Hall (KAP) - 209 (Available by Webex upon request)

Audiences: Everyone Is Invited

Contact: Evangeline Reyes