The project entailed both field and laboratory components. Field studies evaluated routine sampling procedures for determination of aqueous inorganic geochemistry and assessment of contaminant transport by colloidal mobility. Research at three different metal-contaminated sites has shown that 0.45 micrometer filtration has not removed potentially mobile colloids, when samples have been collected using low pumping flow rates (approximately 0.2-0.3 L/min). However, when pumping velocities greatly exceed formation groundwater flow velocities, large differences between filtered and unfiltered samples are observed, and neither are representative of values obtaine with the llow flow-rate pumped samples. In controlled laboratory experiments, the stability and transport of radio-labeled Fe2O3 model colloids were studied using batch and column techniques. Variables in the study included flow rate, pH, ionic strength, electrolyte composition (anion/cation), colloid concentration, and colloid size. Iron oxide colloids in the 100-900 nm particle diameter range were not only mobile to a significant extent, but under some hydrogeochemical conditions were transported faster than a conservative tracer, tritium. Particle size and anionic composition together with particle stability provided the highest statistical correlation governing extent of colloidal transport. The rate of colloid-arsenate transport was over 21 times that of the dissolved arsenate.

Caption title. Shipping list no.: 91-586-P. "July 1991." Includes bibliographical references (pages 11-12). "EPA/600-M-91-040."