Abstract: The transport velocity of radioactive pollutants in groundwater is an important parameter for predicting whether they pose risks to the surrounding ecological environment at nuclear test sites. It has been widely reported that Pu(Ⅳ) adsorbed on the surface of natural colloids(i.e., formation of plutonium pseudo-colloids) has faster transport velocity than that of the porewater in porous media. However, how to accurately obtain the relative transport velocity between colloids and porewater remains unanswered. The previous single-point method(i.e., using only one data point on each of the two breakthrough curves), lack of the theoretical support, is considered as only a rough estimate. This method cannot even be used in many cases. For example, when there is little difference in the transport velocities between colloidal pollutants and tritiated water, the effluent concentration peaks both appear in the same test tube collecting the effluent(i.e., they have the same effluent volume). Note that the transport behavior of tritiated water(an electroneutrally inert tracer) is consistent with that of the porewater. In a long-pulse transport experiment, the breakthrough curves exhibit prolonged plateaus rather than showing concentration peaks. As there is no concentration peak, it is impossible to compare the transport velocity of the two pollutants. In this study, the mixed tracer sources containing plutonium pseudo-colloids, tritiated water, and Sr²⁺ were injected into a column packed with porous media(soils) to conduct a transport experiment. The column effluent was collected at regular intervals. The breakthrough curves for plutonium pseudo-colloids, tritiated water, and Sr²⁺ were obtained after their concentrations(or activity) in the effluent were measured. The results show that the transport velocity relationship between plutonium pseudo-colloids, tritiated water(TW), and Sr²⁺ is v(Pu)＞v(TW)＞v(Sr²⁺). The quantitative relationships between relative transport velocities still need to be accurately determined. The negatively charged plutonium pseudo-colloids were electrostatically repelled to the center of the pore channels. The electroneutral tritiated waters were uniformly distributed within the channels. In contrast, the positive Sr²⁺ ions were electrostatically attracted near the channel walls. Accordingly, the expressions for the relative transport velocities of v(Pu)/v(TW) and v(Sr²⁺)/v(TW) were deduced based on the parabolic porewater flow and the differences in the electrostatic interactions between plutonium pseudo-colloids, tritiated water, Sr²⁺, and the negatively charged pore-channel walls. The accurate relative transport velocities were thus determined by using all data from the breakthrough curves of plutonium pseudo-colloids, tritiated water and Sr²⁺: v(Pu)/v(TW)=1.4, v(Sr²⁺)/v(TW)=0.58.