232-1 Tidal and Seasonal Groundwater-Surface Water Mixing Zones: Hot or Cold Spots for Arsenic Contamination?

Session: GSA Hydrogeology Division Birdsall-Dreiss and Lamoreaux International Distinguished Lecture Series

Presenting Author:

M. Bayani Cardenas

Authors:

Cardenas, M. Bayani¹, Varner, Thomas Scott², Kulkarni, Harshad Vijay³, Kwak, Kyungwon⁴, Nguyen, William David⁵, Hossain, Alamgir⁶, Shuai, Pin⁷, Wang, Lichun⁸, Ahmed, Kazi Matin⁹, Datta, Saugata¹⁰, Knappett, Peter S. K.¹¹

Abstract:

In lowland rivers, wet season flooding is compounded by tides going far inland. This complex flooding situation is common in fluvial-deltaic systems in Asia where many aquifers are contaminated with arsenic. This brings up the question of what happens to the arsenic when water moves back and forth between rivers and aquifers over flood cycles. The different time scales and magnitude of flooding can induce complex and highly dynamic hyporheic zone mixing. The mixing of oxic river water and anoxic groundwater controls the precipitation of iron oxides which traps arsenic and dissolution which releases arsenic. This presentation discusses investigations along the banks of the tidal and seasonally-flooding Meghna River in Bangladesh. Prior studies at some sites suggested that iron oxides accumulate in the Meghna’s mixing zones, and these are associated with high levels of sediment-bound arsenic and low levels of dissolved arsenic. However, our observations show this may not be true everywhere and that mixing zones may be spatially limited and may remain mostly anoxic. Flow and reactive transport simulations help explain and generalize the observations.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5764

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