Dynamic Processes in the Coastal Zone Impact Saltwater Intrusion Under Future Sea Level Rise Scenario

The Science

Coastal saltwater intrusion (SWI), the movement of seawater into freshwater aquifers, is one key factor affecting how coastal ecosystems function. Previous simulations have predicted changes to saltwater concentration in coastal aquifers as sea-levels rise, but have always assumed the current coastal landscape elevation will remain the same. However, coastal landscapes are highly dynamic in response to sea level rise (SLR) due to sediment deposition and erosion. A multi-institutional team of researchers investigated how SWI would change under a dynamic coastal wetland system and found coastal marsh evolution plays an important role in controlling seawater inflow, thereby affecting saltwater distribution under SLR. 

The Impact

Intensified SLR is expected to trigger SWI into coastal freshwater aquifers more extensively. Results from a new simulation study provide the first suggestion that marsh topographic change affects coastal SWI. The study showed marsh accretion under SLR might significantly reduce surface seawater inflow and prolong surface seawater residence time. Future SWI on the evolved marsh landscape might increase sensitivity to upland groundwater inflows. These insights help improve understanding of coastal freshwater system vulnerability under SLR, marsh landscape dynamics, and changes in upland groundwater resources; these interconnections have not previously been considered.

Summary

This simulation study investigated the impact of coastal marsh evolution on predictions of SWI under future SLR by using the Advanced Terrestrial Simulator, a process-based coastal hydro-eco-geomorphologic model. Using a representative synthetic coastal marsh landscape, a multi-institutional team of researchers first predicted marsh landscape change with different upland slopes under two SLR scenarios. Results showed the coastal marsh landscape was dynamic, responding strongly to SLR. Marsh accretion was projected to cause a significant reduction of saltwater inflow at the ocean boundary due to the decrease in the hydraulic gradient between the land and ocean. Also, a topographic depression zone prolonged the residence time of surface ponding water, which affected surface saltwater infiltration, thereby increasing subsurface salinity under the depression zone.  

Using a simulated but evolved marsh landscape, the team also tested the impact of different upland groundwater conditions on SWI under SLR, reflecting the impact of future drier and wetter climate conditions and human groundwater extraction on fresh groundwater dynamics. With the future topographic change, SWI was found to be more sensitive to the upland fresh groundwater supply because of the intensified freshwater-saltwater interaction in the depression zone. Thus, this study revealed the importance of protecting upland freshwater resources. 

Principal Investigator

Yu Zhang
Los Alamos National Laboratory
[email protected]

Co-Principal Investigator

J. David Moulton
Los Alamos National Laboratory
[email protected]

Funding

The initial research was supported by the Laboratory Direct Research and Development program at Los Alamos National Laboratory (LANL) under project number 20180033DR, but additional efforts were supported by the Environmental System Science program of the U.S. Department of Energy (DOE), Office of Science, Biological and Environmental Research program as part of the Integrated Coastal Modeling (ICoM) project. This study was also partially supported by the Center for Space and Earth Science (CSES) Rapid Response effort at LANL under project number 20210528CR. This research used resources provided by the LANL Institutional Computing Program, which is supported by DOE’s National Nuclear Security Administration under Contract No. 89233218CNA000001.

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