Defining the Effect of Stratification in Coastal Aquifers Using a New Parameter

This study, building on the comprehensive discharge potential theory presented by Strack and Ausk (2015) https://doi.org/10.1002/2015WR016887, discovered that the effects of layer arrangements on steady state seawater intrusion and groundwater discharge in stratified confined coastal aquifers can be approximated via transmissivity centroid elevation (TCE), defined as the summation of the product of the transmissivity and elevation (above the aquifer base) of each layer divided by the total aquifer transmissivity. Specifically, a higher TCE in aquifers with high transmissivity layers at high elevations results in a more landward interface-toe position in both flux- and head-controlled coastal aquifer systems, as well as a higher freshwater discharge rate in head-controlled systems. Furthermore, the toe position in head-controlled stratified systems is only a function of the TCE and independent of the total transmissivity. Therefore, we can estimate the upper limit of seawater intrusion, that is, the furthest inland toe position, in stratified aquifers by letting the TCE equal to the elevation of the aquifer top. Another important implication of our results is that the interface toe position in coastal aquifers containing a preferential flow layer is controlled by the elevation of the preferential flow layer. Additionally, effective parameters and homogenization of the interface flow in stratified aquifers were developed to approximate the toe position and discharge rate. The results developed in this study provide significant advances in understanding the effect of aquifer stratification on groundwater flow and seawater intrusion in coastal aquifers.