Relative size distributions of lead, aluminum, and iron were typically bimodal (Figure ), with two incompletely resolved peaks representing colloids with different apparent molecular weights. Aluminum co-occurred with lead (and iron) in at least one of these two fractions in all samples with detectable aluminum peaks (Figures and). This is consistent with previous work documenting adsorption of lead to aluminum hydroxides or mixed iron/aluminum (oxyhydr)oxides and with lead and aluminum occurring in a common colloid size fraction. The presence of aluminum, iron, and lead in distinct but overlapping colloid populations, however, cannot be ruled out completely. Moreover, these data do not provide a complete picture of colloid composition; the role of phosphorus, for example, is not clear.