This paper hypothesizes that equations used by numerous screening models to calculate ground water mixing zone depth are conceptually incorrect. The models incorrectly omit contaminated infiltration volumes in calculating the resulting aquifer contaminant mixing and concentrations. As a consequence dilution factor, DF, and site cleanup criteria are also calculated incorrectly. The paper proposes a conservative, yet more reliable, method of estimating dilution factor which takes into account mixing by a receptor point pumping well and downward contaminant displacement due to natural infiltration. Aquifer contaminant concentrations are calculated at the down-gradient edge of the vadose zone source's planar projection on the water table. Processes addressed include advection, degradation, sorption, and volatilization. Using a spreadsheet setup, model equations allow the user to quickly evaluate multiple modeling scenarios with varying source geometry, vadose zone thickness, and soil heterogeneity.