Hydrological and geochemical controls of surface water and suspended sediment toxic metal fluxes from nearshore large coal ash pond

Toxic metals leached from ash coal ponds can pose a severe hazard to waterways and coastal areas. Observed toxic metal levels in surface waters near the largest ash pond in the Mobile-Tensaw Delta, Alabama, were the result of the interplay of multiple factors, including the specific chemical properties of individual metals and in situ environmental conditions driven by seasonal hydrological controls and flooding versus non-flooding conditions. We found that erosion and sediment resuspension after a significant rain event and flooding resulted in nearly double increase of major and trace metal concentrations in surface waters compared to non-flooding conditions. However, aluminium-iron (AlFe) co-precipitation and flocculation also controlled trace metal levels in surface water, especially during the dry season when seawater with higher pH and salinity from Mobile Bay propagated upstream. The highest arsenic (As) content in suspended sediments (44.6 mg/kg) was found near the Coal Power plant's discharge channel during the dry season. This level is similar to legacy contaminations found following the Kingston, TN ash spill (10 to 57 mg/kg). Higher river flow was associated with higher suspended sediment and suspended sediment-bound trace metal fluxes. However, when analyzing contaminated sediments near the ash pond, trace metal fluxes during the dry season exceeded the wet season, and ultimately, enrichment of As and Cd was observed near the discharge channel. These findings aim to promote research in similar environments impacted by coal ash and to more comprehensively understand the relationship between toxic metals' partitioning and seasonal environmental conditions within the Mobile-Tensaw Delta.