Previous studies have documented a weathering-limited regime in the
upper reaches of the Ganges River Basin. Chemical weathering and element
mobility at six sites in the lower reaches of the Ganges in the tidal
floodplain of Southwest Bangladesh were investigated by comparing
compositions of rice paddy soils, precursor tidal channel sediments,
surface waters, and extract solutions, which represent the soluble
fraction of solids. Little spatial variation in water and solid
compositions is observed in each season, indicating similar processes
are acting to transport elements across this region. Roughly one to
several decades after deposition, rice paddy soils are not significantly
different in mineralogy or composition from precursor tidal channel
sediments, and both are similar to the composition of average upper
continental crust. There is no detectable change in composition of tidal
channel water between upstream and downstream sites. Rice paddy and
tidal channel waters are saturated in the dominant minerals present in
the silt-sized soils and sediments, including quartz and clay minerals.
Together, these observations indicate the dominance of weathered
material and weak chemical weathering in the tidal floodplain,
consistent with a transport-limited regime. Multiple lines of evidence
indicate a lack of exchange equilibrium between surface waters and
coexisting solids, which may be a common feature in tidal river deltas
where transport-limited regimes likely dominate.