While variation in equilibrium lead solubility probably explains at least some of the difference between October and February point-of-use lead levels, particle-generating mechanisms are also likely to be important, including partitioning of lead to particulate (>0.45 µm) or colloidal (<0.45 µm) aluminum. Particulate aluminum was seasonal in the distribution system we studied, with the median concentration in October less than half that in February (20 and 48 µg L -1 , respectively, Figure ). The particulate fraction of total aluminum ranged from 16% in August to 35% in February, as estimated from the cyclic cubic regression splines shown in Figure . The variation in particulate aluminum is consistent with turbidity in October being 66 and 26% of that in February in stagnant and flushed point-of-use samples, respectively (p <<0.001 and <<0.001, n = 134 and 32, signed-rank tests).