Analysis of Saltwater Intrusion and Recirculation of Seawater in a Vertically Anisotropic Coastal Aquifer

At the seacoast in a coastal aquifer, freshwater from inland terrestrial sources discharges to the sea, flowing over denser seawater from marine sources that intrudes inland along the base of the aquifer. As the seawater flows inland, it mixes with the seaward flowing freshwater, is recirculated, and discharges back to the sea. The sum of the recirculated seawater and fresh groundwater discharge is called submarine groundwater discharge (SGD). Recirculated seawater can be a major component of SGD, affecting the salinity of the SGD discharged into coastal and estuarine systems. Based on dimensional analysis, three dimensionless ratios (L_D, R_D, and P_R) that represent saltwater intrusion, the recirculation of seawater, and the percent of recirculated seawater that is contained in the SGD can be considered dependent variables in this problem. Two additional dimensionless ratios (a_z and r_K) that represent the ratio of the freshwater inflow relative to the vertical density-driven buoyancy flux and the ratio of the vertical and horizontal hydraulic conductivities in the aquifer cross-section can be considered independent variables in this problem. Numerical experiments were conducted using SEAWAT to determine how L_D, R_D, and P_R are related to a_z and r_K over a wide range of values of a_z = 0.1 to 10.0 and r_K = 0.01, 0.1, and 1.0. Based on the results of this investigation, L_D, R_D, and P_R all increase as a_z and r_K are decreased, and L_D, R_D, and P_R all decrease to zero or near zero as a_z and r_K are increased. Maximum values of L_D, R_D, and P_R occur at a_z = 0.1 for r_K = 0.01, and minimum values of L_D, R_D, and P_R occur at a_z = 10.0 for r_K = 1.0. The maximum value of P_R = 74.1 % at a_z = 0.1 and r_K = 0.01 confirms the important contribution that recirculated seawater can make to SGD and its effect on increasing the salinity of SGD in a vertically anisotropic aquifer, particularly at relatively small values of a_z ~ 0.1. The minimum values for L_D, R_D, and P_R, which approach zero and occur at the larger values of a_z, indicate that saltwater intrusion and the recirculation of seawater are significantly reduced in both isotropic and anisotropic aquifers at relatively large values of a_z ~ 10.0. In natural settings, aquifer properties such as horizontal and vertical hydraulic conductivities, aquifer thickness, freshwater inflows, and other properties can be quite variable. However, based on the results of this investigation, saltwater intrusion, the recirculation of seawater, and the percent of recirculated seawater in submarine groundwater discharge in such field-scale aquifers can be investigated and even predicted by quantifying the two independent dimensionless ratios a_z and r_K.