Drivers of groundwater salinization and ecosystem change on Mid-Atlantic barrier islands
| Author(s) | Frederiks, Ryan | |
| Date Accessioned | 2024-01-24T15:03:43Z | |
| Date Available | 2024-01-24T15:03:43Z | |
| Publication Date | 2023 | |
| SWORD Update | 2024-01-22T20:12:57Z | |
| Abstract | Coastal groundwater is an important natural resource for a large number of people living in coastal zones where surface water is often saline. Similarly, fresh coastal groundwater is critical for the survival of coastal ecosystems. Climate change will cause deterioration of coastal groundwater resources by salinization through two processes: sea-level rise and more frequent storm surge events. Predicting groundwater vulnerability to each of these processes is complicated by high spatial and temporal variability in hydrological forcings and subsurface properties. In this dissertation, the influence of each of these processes was evaluated using field data and numerical models. ☐ To evaluate the influence of both storm surge and sea-level rise, wells were installed at 3 sites in the mid-Atlantic and equipped with conductivity, temperature, and pressure (CTD) loggers. The chosen sites were Fire Island, NY; Sandy Hook, NJ; and Assateague Island, MD. In Chapter 2, the data were analyzed with time series models to determine the primary driver of both water level and salinity changes. Additional data were collected on potential drivers including bay and ocean water levels, precipitation, evapotranspiration, and wave height. These drivers were used to calibrate a series of transfer function noise models using a Python package called Pastas. Because the specific conductance models were not able to be calibrated, additional analysis was conducted using cross correlation on the specific conductance time series. Each site displayed spatial and temporal variability in driving factors caused by periods of frequent overwash or high precipitation. Between sites there were substantial differences in the importance of different drivers. At Assateague, waves played an important role on head fluctuations, while overwash was more dominant at Sandy Hook and bay water levels were most dominant at Fire Island. Similarly, specific conductance was predominantly driven by overwash events at all sites, but movement of the freshwater-saltwater interface likely occurred on Assateague in response to storm events. Different island characteristics including vegetation, topography, and subsurface properties impacted the extent to which different drivers impacted the salinity and water levels. ☐ To test the relative influence of sea-level rise versus storm-surge overwash, Assateague Island was used as a test case in Chapter 3. A two-dimensional numerical model was calibrated to both hydraulic head and specific conductance data and a set of simulations was run to estimate salt mass and salinized volume under present and future conditions. Future simulations included both sea-level rise and storm-surge overwash. I demonstrated that changes in the 2-year return levels from present day to 2080 will result in substantially more salinization than sea-level rise over the same period. This difference persists even in a low-hydraulic conductivity test case in which sea-level rise would be expected to have a greater impact due to a smaller unsaturated zone for infiltrating saltwater to fill during overwash events. Finally, sea-level rise is expected to decrease the island area at Assateague by 40% while the area above future 2-year storms is predicted to decline by 75%. ☐ Finally, in Chapter 4, I investigated the impact of both sea-level rise and storm-surge overwash on the ecosystem at Fire island using a historical vegetation dataset and a three-dimensional numerical groundwater model. I found that substantial change has occurred in the number of trees across the island from 1967 to 2013. Using the calibrated numerical model, I correlated depth to water to ecosystem composition and found that depth to water explains 22% of the variance in community composition. On the other hand, storm-surge overwash explains 8% of the variance in community composition. Additionally, I tested the impact of erosion on water levels and found that erosion likely decreased water levels across the island likely counteracting some of the water-table rise due to rising sea levels. ☐ These studies show that that the impacts of sea-level rise and storm-surge overwash vary spatially across the mid-Atlantic with Assateague showing greater vulnerability to storm-surge overwash than to sea-level rise while ecosystems at Fire Island demonstrated greater vulnerability to sea-level rise. The insights from these studies can help inform coastal management and be used to predict how coastal environments will change due to climate change. | |
| Advisor | Michael, Holly A. | |
| Degree | Ph.D. | |
| Department | University of Delaware, Department of Earth Sciences | |
| DOI | https://doi.org/10.58088/cpz7-7224 | |
| Unique Identifier | 1445626445 | |
| URL | https://udspace.udel.edu/handle/19716/33873 | |
| Language | en | |
| Publisher | University of Delaware | |
| URI | https://www.proquest.com/pqdtlocal1006271/dissertations-theses/drivers-groundwater-salinization-ecosystem-change/docview/2917438389/sem-2?accountid=10457 | |
| Keywords | Coastal groundwater | |
| Keywords | Salinization | |
| Keywords | Saltwater intrusion | |
| Keywords | Sea-level rise | |
| Keywords | Storm surge | |
| Keywords | Vulnerability | |
| Title | Drivers of groundwater salinization and ecosystem change on Mid-Atlantic barrier islands | |
| Type | Thesis |