Steady-state freshwater–seawater mixing zone in stratified coastal aquifers

• Both lab and numerical studies were conducted for interface distribution in stratified coastal aquifers.
• Thick mixing zone may occur in low-conductivity layers in stratified coastal aquifers.
• Impact of hydrogeological conditions on mixing zone distribution were investigated.

SummaryThe mixing zone between freshwater and intruding seawater, which controls regional groundwater flow dynamics and reactive transport processes, is the most important component in coastal aquifers. Laboratory experiments and numerical simulations were conducted to investigate the effects of aquifer stratification on the thickness of a steady-state freshwater–seawater mixing zone. A series of experiments were implemented in a laboratory tank packed with three layers of coastal sands and manipulated by two flow cells for maintaining constant-head boundary conditions. Packed aquifers included a homogenous aquifer, a high hydraulic conductivity (K)–low K–high K stratified aquifer, and a low K–high K–low K stratified aquifer. Both tank-scale and field-scale density-dependent models were developed to simulate the laboratory experiments and to systematically investigate aquifer stratification effects on the thickness of a steady mixing zone with varying layer configurations. Results showed that when an aquifer layer with a low K overlies a layer with a relatively high K, slanting upward flow of diluted saltwater and re-circulated seawater flow are refracted at the interface between the two layers, resulting in streamline separation and a broad mixing zone in the low-K layer. By contrast, the mixing zone in a high-K layer is slightly narrowed when it overlies a layer with a relatively low K because flow refraction squeezes the streamlines within the mixing zone. The mixing-zone profile in head-controlled systems is a function of K contrast, but independent of the magnitude of K. Our results demonstrated the significant impact of aquifer stratification on controlling groundwater flow and solute transport in coastal subsurface environments and would have significant implications for studying physical, chemical and biological processes in coastal subsurface environments, as aquifer stratification is naturally prevalent.