Low-tide rainfall effects on metal content of suspended sediment in the Sacramento-San Joaquin Delta

Abstract

Rain falling at or near low tide is capable of eroding and transporting cohesive sediment from intertidal areas. Given that metals adsorb strongly to silt- and clay-sized particles, it is conceivable that low-tide rainfall may also liberate previously-deposited metals from storage in intertidal sediment. To investigate the potential for rainfall as an agent of remobilization, this study tested the hypothesis that suspended sediment in a tidal creek during low tide rainfall events contains different suites of adsorbed metals than during wind-only events and fair weather days. Water samples were collected during low-tide rain events in winter and wind resuspension events in summer. The concentrations of suspended sediment, particulate organic carbon and nitrogen, and the total adsorbed concentration (gL−1) of most metals were higher for a low-tide rainfall event than during wind-only and fair-weather conditions. Metal contents (gg−1) were also greater during the rain event for most metals. Principal components analysis and relationships between metal content and suspended sediment concentration suggest that rainfall during low tide can mobilize a different source of sediment than wind–wave resuspension and regular tidal action. The metal content of bulk sediment samples from around the study area could not be matched satisfactorily to the suspended sediment in any of the events. This implies that bulk sediment composition should not be used to extrapolate to suspended sediment composition in terms of adsorbed metal content.