Effects of Future Increases in Tidal Flooding on Salinity and Groundwater Dynamics in Coastal Aquifers

• Published in: Water resources research 2022-12, Vol.58 (12), p.n/a
• Main Authors: Heiss, James W., Mase, Bryce, Shen, Chengji
• Format: Article
• Language: English
• Subjects: Aquifers; coastal aquifer; Coastal aquifers; Coastal dynamics; coastal groundwater‐surface water interactions; Coastal waters; Dynamics; Extreme values; Flood frequency; Flooding; Floods; Freshwater; Groundwater; Groundwater flow; Groundwater resources; High water levels; Inland water environment; Intertidal environment; Intertidal zone; Pore water; Saline water; Saline water intrusion; Saline-freshwater interfaces; Salinity; Salinity effects; Salinization; Salt advection; Salt water intrusion; Saltwater intrusion; saltwater‐freshwater mixing; Saturated flow; Sea level changes; Sea level rise; Seawater; Spring tides; submarine groundwater discharge; System dynamics; Tidal constituents; Tidal flooding; Tidal floods; Tides; Water levels; Water resources
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2022WR033195

Future increases in the frequency of tidal flooding due to sea level rise (SLR) are likely to affect pore water salinities in coastal aquifers. In this study, we investigate the impact of increased tidal flooding frequency on salinity and flow dynamics in coastal aquifers using numerical variable‐density variably‐saturated groundwater flow and salt transport models. Short (sub‐daily) and long (decadal) period tides are combined with SLR projections to drive continuous 80‐year models of flow and salt transport. Results show that encroaching intertidal zones lead to both periodic and long‐term vertical salinization of the upper aquifer. Salinization of the upper aquifer due to tidal flooding forces the lower interface seaward, even under SLR. System dynamics are controlled by the interplay between SLR and long period tidal forcing associated with perigean spring tides and the 18.6‐year lunar nodal cycle. Periodic tidal flooding substantially enhances intertidal saltwater‐freshwater mixing, resulting in a 6‐ to 10‐fold expansion of the intertidal saltwater‐freshwater mixing area across SLR scenarios. The onset of the expansion coincides with extreme high water levels resulting from lunar nodal cycling of tidal constituent amplitudes. The findings are the first to demonstrate the combined effects of gradual SLR and short and long period tides on aquifer salinity distributions, and reveal competing influences of SLR on saltwater intrusion. The results are likely to have important implications for coastal ocean chemical fluxes and groundwater resources as tidal flooding intensifies worldwide. Key Points Future increases in tidal flooding enhance saltwater‐freshwater mixing in coastal aquifers due to greater seawater infiltration Salinity dynamics is controlled by the interplay between sea level rise and long period tidal forcing at semiannual to decadal time scales The lunar nodal cycle controls the onset of tidal flooding‐induced vertical salinization of coastal aquifers