EFFECTS OF ACID VOLATILE SULFIDE ON BLACK SHALE SEDIMENT-METAL BIOAVAILABILITY AND TOXICITY TO CHIRONOMUS TENTANS

A better understanding of metal behavior in aquatic ecosystems is necessary for adequate prediction of their impacts upon resident organisms. Metal bioavailability and toxicity can significantly be altered by acid volatile sulfide (AVS). The AVS interacts with metals forming highly insoluble and stable sulfides that render metals biologically immobile. Few studies however, have determined spatial variability of AVS, and AVS-simultaneously extracted metal (SEM) interaction in sediments contaminated by metals from natural geochemical processes such as black shale weathering. Our study area consisted of four headwater bedrock streams; three black shale-streams, and one limestone-draining stream in north-central Arkansas. Sediment samples were collected and analyzed for AVS, total recoverable metals, and SEM. Additionally, we conducted a 10-day acute toxicity test using Chironomus tentans to evaluate relative toxicity among the sediments. Significant differences (F = 21.7; df = 3, 12; p < 0.001) in organic carbon (OC) among sediment samples were noted. Only two samples (BGA1 and BGB1) had moderate AVS concentrations, whereas the concentrations in the remaining samples were negligible. The differences between SEM and AVS exceeded 1 for all sediments except limestone-derived sediments. 73%, and 59% of the total variation in C. tentans mortality was explained by the difference between SEM and AVS. Chironomus tentans survival in black shale-derived sediments was significantly lower (F = 24.02; df = 4, 15; p < 0.001) than that of limestone-derived samples. Total metal concentration differences among sediments were attributed to differences in lithology of their watersheds. We ascribed the observed C. tentans toxicity to metal complexation by organic carbon, and Fe and Mn oxides. Based on the SEM – AVS values, and sediment toxicity data, only BGA2 and BGB2 would be described as toxic. Though limited in temporal variability, this study demonstrated that AVS has little effect on sediment-associated metals particularly during cold temperature periods. Finally, it was apparent from the study that even when SEM exceeds AVS, other sorption phases such as organic carbon, Fe and Mn oxides can bind metals leading to a considerable reduction in toxicity to sediment-dwelling organisms.