We use Monte Carlo simulations of flow and transport in two-dimensional random conductivity, porosity, and geochemistry fields to explore the influence of their spatial variability on flow and transport processes for both conservative and reactive chemicals. For conservative transport, results show that when the porosity is correlated to the hydraulic conductivity (which may be expected in geologic formations); the dispersion process is significantly affected. Positive cross correlation between the porosity and the conductivity decreases dispersion, while a negative correlation tends to increase dispersion in the longitudinal direction. For reactive transport in physically and chemically heterogeneous media, the geochemical variability alone yields results that are significantly different than when both geochemistry and porosity are random space variables correlated to the conductivity field. These results suggest that it is necessary to examine the role porosity variability and its correlation to the conductivity play in transport theories. The stochastic perturbation transport theory, previously developed by our group for conservative chemicals, is modified to account for the porosity variability and its correlation with the hydraulic conductivity. The results of the modified theory support the Monte Carlo simulations and indicate that porosity variability significantly affects the second longitudinal moment. The implication of the study for field problems depends on the existence of cross correlation between porosity and conductivity. ¿ 1998 American Geophysical Union