Supplementary Data for "Impact of surficial lithology on arsenic mobility in riverbanks of tidally fluctuating rivers: The Hooghly River, West Bengal, India"

Arsenic (As) contamination in groundwater persists in South Asia. Precipitated amorphous Fe(III)-oxides regulate
the mobilization of aqueous As and iron (Fe) within the hyporheic zone (HZ). Depending on the chemical stability
of these Fe(III)-oxides, this so-called Natural Reactive Barrier (NRB) can function as a sink or source of
aqueous As and Fe within shallow alluvial aquifers under influences of tidal and seasonal fluctuations of river
stage. The extent to which surficial lithology influences the As mobility along a riverine (upstream) to tidal
(downstream) continuum is uncertain. To explore this process along a tidally fluctuating river, two new study
sites with contrasting surface lithology were characterized along the banks of Hooghly River. The upstream
sandy riverbank aquifer experiences robust mixing with oxygen-rich surface water under influences of tidal fluctuations
which maintain oxic conditions in the riverbank aquifer. Introduced riverine dissolved oxygen (DO) drives
the in-situ precipitation of crystalline Fe(III)-oxides which remove dissolved As and Fe from groundwater before
discharging to the river. Although sediment from the downstream silt-capped riverbank contains higher concentrations
of sedimentary As and Fe compared to the sandy site, lower proportions of crystalline Fe(III)-oxide
minerals were observed. Arsenic was more easily mobilized from the aluminosilicate clay minerals to which the
As was primarily bound at the silt-capped riverbank, compared to the As bound to Fe(III)-oxides at the sandy site.
Thus, aluminosilicates can be an important source of dissolved As. These findings demonstrate that the surficial
lithology of a riverbank along a tidally and seasonally fluctuating river regulates the mobility of As and its mineralogical
association within riverbank sediments in shallow riverbank aquifers.