Estuaries are not closed systems to act as sinks. They act as transition storage to accumulate some contaminants but also can be releasing previously stored contaminants under river and tidal fluxes. Thus they cannot be just sinks.

On relatively short time scales estuaries generally act as temporary sinks for contaminants, especially in the mud reach. However, the widespread occurrence of estuarine valley fills in the rock record would suggest that, under certain conditions and especially in transgressive situations, estuarine deposits can become permanent sinks to sediments and, hence, also contaminants.

Burg Flemming

Estuaries are sinks for both conservative and non-conservative pollutants, particularly their conc. are governed by salinity and pH

Estuaries act as a sink in their parts where sedimentation occurs, in particular where cohessive sediments dominate. But in most cases they are storaged only temporarily since they will be remobilzed with their "mother sediments" by higher current velocities due to mid-term natural variability or extreme events like storm surges or run-off or by dredging.

It also depends on the Hydrodynamics of the estuary. Probably during low discharge periods it can be, but during a flood, I certainly doubt it.

The tidal range is another variable to consider. Microtidal estuaries don't have much hydrodynamics, but high tidal ranges may induce very important fluxes during the day, and then is very unlikely to act as a sink.