Comparison and correlation of lithostratigraphic and hydrostratigraphic units of southwest area 20, Pahute Mesa, Nevada test site

Traditionally, the boundaries of hydrogeologic units are restricted to the lithologic boundaries of the rock in which the hydrogeologic unit is recognized. This type of identification places emphasis on the composition and lithic characteristic of the unit rather than the material properties of the rock that affect potential ground-water flow. Hydrostratigraphic units represent an improved method of defining potential ground-water flow by identifying and grouping hydrologic units based upon the porosity and permeability of the rocks--not on geologic contacts, thus removing the limitations of presently defined lithostratigraphic boundaries; Six major and four minor hydrostratigraphic units were identified in and correlated between exploratory wells UE-20f and UE-20d located in Pahute Mesa of the Nevada Test Site based upon relative changes in porosity and permeability as recorded by geophysical logs. A comparison of hydrostratigraphic and lithostratigraphic units within these wells revealed sufficient variation in the unit-boundary locations to justify separation of these two types in future hydrogeologic investigations