Historic Mussel Shells Illuminate Legacy Contaminant Patterns Over the Past 1000 Years

Oral Presentation

Prepared by W. Shoults-Wilson1, A. Fritts2, M. Fritts2, A. Casper2, J. Unrine3
1 - Roosevelt University, 430 S Michigan Ave, WB 914B, Chicago, IL, 60630, United States
2 - Illinois Natural History Survey, 1816 South Oak Street, Champaign, IL, 61820, United States
3 - University of Kentucky, 105 Plant Science Bldg, 1405 Veterans Drive, Lexington, KY, 40546, United States

Contact Information: [email protected]; 312-322-7163


Potentially toxic trace elements (PTTEs) provide a unique challenge in monitoring due to their natural distribution, long residence times in benthic systems and the fact that only a fraction of the environmental load is bioavailable to organisms. The calcareous shells of freshwater mussels provide a unique opportunity to enhance our understanding of the spatial and temporal variability of PTTE concentrations. Not only are bivalves considered ideal biomonitors of bioavailable contaminants, but divalent metals can be metabolically substituted into the shell matrix in place of calcium. We examined historic mussel shells that date back to the 1870s for the lower Illinois River and back to the 1950s for the Chicago region. Shells from the modern era and from museum collections were compared against archaeological specimens from 1000-1200 A.D., which provided a pre-industrial environmental baseline of metal concentrations prior to large-scale human alterations of aquatic environments. Mussel shells were thin-sectioned along the axis of growth, aged and then analyzed for trace element concentration. Concentrations of As, Co, Cu, Fe, Mn, Ni and Zn were readily detected in most shells, while concentrations of Al, Cr, Hg, Pb, Se and V were below detection in most shells. Cd and U were occasionally detectable. Shells collected at time points ranging from 1897-2013 had significantly higher concentrations of As, Co, and Cu than archaeological shells but significantly lower concentrations of Mn. Samples from 1897 to 2013 also showed a significant positive correlation between concentration and time for Co, Cu, and Fe. Arsenic concentrations appear to have decreased in 2013 samples relative to earlier samples, although this change is not significant. These results indicate the presence of anthropogenic PTTEs and can be used to produce a past baseline of contaminants currently missing.