Ashes (residues), coal

Sediment toxicity

Description of key information

Toxicity to sediment organisms is considered to be low.

Additional information

End of 2008 4.1 million m³coal ash were released into the Emory, Tennessee and Clinch rivers from an accident in the Tennessee Valley Authority (TVA) Kingston Fossil Plant. This unfortunate event provided an opportunity to directly study the impact of coal fly ash in a large lotic system since more than one rivers were affected. A variety of studies were initiated by multiple actors to assess ecological risks to different organisms. Since fly ash contains a diversity of metals such as As, Cr, Cu, Pb, Hg, Ni, Se, Tl, V, Zn, it is of ecological concern. Among others the toxicity of ash and especially its comprised metals towards sediment organisms was investigated.

Stojak et al. (2014) applied a tiered approach to evaluate short and long term effects to Hyalella azteca and Chironomus dilutes. Sediment samples were collected within the river system that encompassed a spatial gradient of ranging ash content and analyzed. The ash content of the Clinch River sediment samples ranged from 20% to 41% and of the Emory River from 1% and 88%, allowing to perform studies with different ash concentrations for a dose - respond experiment. The short-term toxicity test was conducted according to the EPA methods 100.1 and 100.2, for H. azteca and C. dilutes, respectively (USEPA 2000, whole‐sediment acute tests). C. dilutus and H. azteca larvae were exposed to undiluted site sediment with site water as well as 4 different kind of controls for 10 days. The test parameters analysed were survival, growth and biomass.

The long-term partial life study was designed based on the results of the short-term study. 4 high priority sites per river were chosen for further testing. 10 different ash concentrations were tested (4 original samples and a dilution series in which the selected sediment was diluted with their reference sediment at a volume-based ratio of 10 20, 40, 60, 80 and 100%). The two test organsms were exposed to the ash-sediment concentrations for 28 d. Survival and growth were recorded at the test end.

Statistical analysis of the results demonstrated that percent ash, As, Se, and Sr were most highly correlated with statistically significant toxic effects and with each other. As and Se were found to be the most likely toxic constituents. Effects to sediment organisms were however low and mostly attributed to sediments with an ash content of greater than 40%, in which As was significantly elevated and exceeding recommended probable effect concentrations. Furthermore, the statistical analysis results showed that toxicity observed did not correlate with residues other than ashes, like PCBs, pesticides or PAHs that were detected in the sediments studied, therefore the statistic and discussion of this paper focused on the ash associated metals and physical properties of sediment. It was however not clear if effects were caused due to mixture toxicity or due to toxicity by specific compounds.

A further study on the same site is available that investigated concentrations of ash-related metals and compared to potential effects or contaminant advisory levels. The focus of following study was the bioaccumulation of metals in mussels but health condition of the organisms was additionally studied.

The first available publication intended to in situ investigate the bioaccumulation in three different mussel species (black sandshell, elephant ear, purple wartyback) during and after dredging activities in the ash affected sites (Otter et al. 2015). Mussels were caged for 1 year in ash affected and unaffected river sites for both in the dredging period and the post-dredging period. Additionally mussels of each species were kept under laboratory conditions as a control. Mussel health (mussel condition index) and concentrations of 23 metals in soft tissues were analyzed. Potential differences based on location, dredging activities and species were recorded. Metal mussel concentrations were compared with contaminant advisory levels given mainly by the Food and Drug Administration in order to identify potential threats. Results revealed that from all metals measured, only mean lead concentrations were observed in two cases exceeding the respective contaminant advisory level of 1.7 mg/kg given by the Food and Drug Administration. This was observed in the post-dredging period in purple wartyback, (3.2 mg/kg, at Lower Clinch) and in elephant ear (1.9 mg/kg, at Upper Clinch). Dredging activities seemed to have effects on mussel health. At post dredging period however the condition indexes remained constant giving evidence that dredging conditions might be an extreme situation. Therefore these results give further evidence of a low hazard potential caused by the ash spill to sediment organisms (here mussels) even at areas with high ash concentrations.

In conclusion, evidence from long and short-term studies show a low potential of toxic hazard of ashes to sediment organisms. The study conducted by Stojak et al. indicated a concern in case of elevated As and Se through ash contamination. However, toxic effects were found only when ash concentrations were over 40% in sediment. The field study by Otter et al. (2015) showed, a negative impact on mussel health only during the exceptional conditions of dredging activities while the condition indexes were constant or increased at post dredging period. Based on these studies the toxicity of ash to sediment organisms is considered low.