A framework of cadmium speciation studies is described for flow-through mini-diluter bioassay systems which employs Lake Superior water containing added concentrations of calcium carbonate, Aldrich humic acid and/or less than 2 microns suspended Lake Superior red clay. Aspects of the flow-through diluter function were qualitatively related to major changes in the observed Cd(2+) concentration as a function of time and different water quality chemical variables. The most significant variable was the red clay, which markedly accelerated the rate of loss of Cd(2+) from Lake Superior water containing added CaCO3. This acceleration factor was highly sensitive to other variables such as pH and humic acid. In addition, ion selective electrode (ISE) potentiometry, dialysis, and membrane filtration were used to verify an ionic chemical equilibrium model designed to describe various dissolved cadmium specie concentrations.