A New Approach in Determining the Potential Ranges of Vertical Hydraulic Gradients through an Aquitard of a Leaky Aquifer System

In the system of leaky aquifers, aquitards play a key role in controlling the migration of contaminants from shallow aquifers to deep aquifers. The quality of water in a deep aquifer is mainly dependent on the hydrogeologic characteristics of the overlain and/or underlain aquitards. Regardless of how permeable an aquitard is, downward and upward flows can potentially be developed through the aquitard from one aquifer to another depending on its hydraulic conductivity and the vertical hydraulic gradient across the aquitard. The vertical velocity through an aquitard; which is a function of the vertical hydraulic gradient, effective porosity, and the hydraulic conductivity of the aquitard; is used in estimating the travel times of contaminant constituents as well as their concentrations in the underlain and overlain aquifers. Of these three parameters, especially the vertical hydraulic gradient is difficult to estimate. Therefore, the vertical hydraulic gradient is a critically important parameter in establishing strategies for protection of aquifers. Regarding this, an approach is developed in estimating the potential ranges of the vertical hydraulic gradient in aquitards. The approach is developed for an aquitard segment (L) which is underlain by a confined aquifer and overlain by an unconfined aquifer. Using the leaky aquifer theory, it has been shown that the vertical hydraulic gradient (I_v) is dependent on the horizontal hydraulic gradient (I_h) in the aquifer, leakage factor (B), thickness of the aquitard (b’), horizontal distance, leakage aspect ratio (B/L), and the hydraulic heads in the aquifers. Application of the method is shown by generating both dimensionless and dimensional curves. It has been shown that I_v can potentially be greater than unity (i.e., > 1 L/L), but in natural aquifers this situation is less likely depending on especially I_h, B/b’, and B/L values.