Evaluation of size-distribution effects and laboratory precision in the analysis of bottom materials

Bottom materials may be useful in evaluating areal and temporal changes in concentrations of certain chemical constituents in the overlying waters. They are particularly useful for detecting hydrophobic compounds that may be present in the stream waters at low concentrations or at intermittent intervals. The particle-size distribution in the bottom materials must be considered when areal or temporal changes in sediment chemistry are interpreted. 

Bottom-material samples were collected at two sites on the Cuyahoga River in Cuyahoga and Portage Counties, Ohio. Multiple samples were collected from one cross section at each site and separated into three fractions based on particle size (less than 2 millimeters to 63 micrometers, less than 63 micrometers to 20 micrometers, and less than 20 micrometers). Each fraction was then separately analyzed for six trace metals (chromium, copper, iron, lead, manganese, and zinc). 

Significant differences in metal concentrations were observed within cross sections as well as between sites. The coefficient of variability within the cross sections decreased as the particle size decreased in the analyses for chromium, copper, iron, and lead, whereas the variability remained constant for manganese and zinc. Metals concentrations, however, generally increased as particle size decreased. An "index of discrimination" indicated that the less than 20-micrometer fraction was superior for detecting between-site differences in concentrations of metals in the bottom materials; the less than 63 to 20-micrometer fraction ranked second, and the less than 2 millimeter to 63-micrometer fraction ranked third. Replicate laboratory analyses were generally found to be unnecessary because differences in metal concentrations due to laboratory extraction and analysis were small.