A Spatial Scale Assessment of Depositional Areas in Mid-West Water Bodies: Implications for Ecological Risk of Hydrophobic Organic Compounds

Abstract

Lenwood W Hall and Ronald D Anderson

This study was designed to summarize grain size data (% sand, % silt and % clay) from mid-west United States water bodies to determine the percent of sites where sediment samples are primarily depositional sediment (more than 50% silt/clay) or non-depositional (less than 50% silt/clay). Sediment sampling protocols were reviewed for all the grain size measurements to determine if non-random or random sampling was conducted. Based on four studies with a total of 269 sites, the mean percent of sites dominated by the silt/clay fraction was approximately 5%. This value is very low and certainly shows that depositional areas are spatially limited in mid-west water bodies. The ecological relevance of this result is that silt/clay areas where Hydrophobic Organic Compounds (HOCs) may be found are spatially limited for mid-west water bodies. Before judging HOC ecological risk it is critical to know how sites for sediment collection were selected, using either a non- random or random approach, and this information also needs to be carefully considered when assessing potential ecological risk of HOCs in ambient mid-west sediment. A random sampling design on a watershed scale that includes spatially extensive sediment mapping is recommended in areas where HOC ecological risk is suspected.

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