An Approach to the Field Study of Hydraulic Gradients in Variable‐Salinity Ground Water

Abstract

ABSTRACTUnderstanding what can and cannot be determined about hydraulic gradients in field studies of variable‐salinity ground water is becoming increasingly important because of the use of subsurface injection as a waste‐disposal method in saline aquifers throughout the country. A field study approach is proposed for reliably estimating hydraulic gradients in subregions within a region of variable‐salinity ground water. It is based upon Hubbert's concept about the kind of density distributions that are required for ground water to have a potential. The approach consists of dividing a region of variable‐salinity ground water into subregions with constant density, subregions with only vertical variations in density, and subregions with vertical and lateral variations in density before determining magnitude and direction of hydraulic gradients. Subregions with constant density and subregions with only vertical variations in density according to Hubbert can have a potential; because of this, hydraulic gradients in such subregions may be reliably estimated. Subregions with vertical and lateral variations in density according to Hubbert may not have a potential; because of this, hydraulic gradients in such subregions cannot be reliably estimated. The approach was applied to an unconfined coastal aquifer and also to a confined and layered coastal aquifer that is used for subsurface injection. As the two applications show, the analysis of water levels and pressures from subregions with constant or approximately constant density and the analysis of pressures from subregions with only vertical variations in density provide simple and direct means for deducing the characteristics of hydraulic gradients within a region of variable‐salinity ground water.