Abstract
Simulations and effective-medium calculations were performed on the absorption of impurity particles by static sinks in three dimensions. The theory shows no qualitative effects due to two-sink clusters and agrees well with simulations. The conclusion is drawn that effective-medium theory is adequate for time-dependent impurity concentrations C(t) above the lower bound recently demonstrated by several authors. Crossing of the concentration curves occurs at a time such that C(t)C(0)=1067 or less.