The assumptions necessary for successful application are that (1) there is no source of chloride in the soil water or groundwater other than that from precipitation, (2) chloride is conservative in the system, (3) steady-state conditions are maintained with respect to long-term precipitation and chloride concentration in that precipitation, and in the case of the unsaturated zone, (4) a piston flow regime, which is defined as downward vertical diffuse flow of soil moisture, is assumed. However, this assumption may be invalidated if the flow through the unsaturated zone is along preferred pathways.