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Water-levels and salinity were measured in seven shallow (ca. 2 m deep) wells installed at distances proximal, medial, and distal to the source of tidal flooding between 2017 and 2019 in a warm-season grass meadow adjacent to a salt marsh. Water-table fluctuations greater than 10-cm were associated with seawater, precipitation, or a combination of the two. When the field was flooded by tides (> 0.5 m above predicted), groundwater salinity increased; when the field was flooded by precipitation (> 2.5 cm), the salinity of the groundwater decreased. The increased head gradient that accompanied the rise in the water table appeared to be sufficient to allow the freshwater from precipitation to push the salt water down and towards the marsh creek, resulting in a freshening of the groundwater that persisted until the next saltwater flooding event. Thus, the relative frequency between saltwater flooding, salinization, freshwater flooding, and flushing controlled the groundwater salinity. These findings indicate the importance of high-tide events in the process of salinization of the groundwater and the ameliorating effects of rainfall events whose magnitude is sufficient to increase groundwater elevation at least ten centimeters. Further, they contribute to a growing body of evidence in support of the interaction between fresh- and saltwater flooding events to enhance the salinity of groundwater and drive ecosystem transition from uplands to salt marshes.
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Effects of Future Increases in Tidal Flooding on Salinity and Groundwater Dynamics in Coastal Aquifers
Abstract 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.
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- Award ID(s):
- 1933058
- PAR ID:
- 10402708
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 58
- Issue:
- 12
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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