The Distribution of Chromium in Various Particle Size Fractions of Sediment and Soil.

Chromium (Cr) is one of the priority contaminants of major concern in the environment in Taiwan. Factory waste stream is the major source of Cr pollution. The Cr usually transported with factories wastewater into irrigation canal and soil system. Those pollutants might be transported in solution and particle form. However, the relative importance of suspension particles remains obscure. In some investigations, researchers found the distribution of heavy metal contaminants (include Cr) are not equal in farmland. The concentration of heavy metals decreases with increasing distance and depth from gateway. Those evidences indicate that Cr and other heavy metals might mainly transport with particles but not solution. In this study, we used ultrasonic dispersion to separate soil into three particle size fractions (sand, silt and clay), and measured Cr concentration of each fraction. In these three soil fractions, the highest Cr concentration is in clay. And the Cr relative percentage of sand, silt and clay are 6.7, 76.9 and 16.4% respectively at gateway. With increasing distance and depth the Cr percentage of sand and silt are decreased, but clay is increased. In sludge, the highest Cr concentration is in silt but relative percentage is in sand. This result shows the Cr in farmland should come from sludge in irrigation canal, and mainly be carried by silt. However, the larger particles (sand and silt) deposited at first around the gateway, and the smaller ones (clay) could be transported to longer and deeper distance. Therefore, this result might explain why the Cr concentration decreases rapidly with the increasing distance and depth from the entrance and how sediment effects the Cr transportation from irrigation into farmland.