Combined Analysis of Tide- and Wave-induced Water Table Fluctuations for Structural Characterization of a Coastal Aquifer

Understanding of coastal hydrogeology is essential for the assessment, management, and protection of coastal groundwater resources. Coastal groundwater is often an important source of drinking water for coastal communities but can be contaminated by saltwater or human-derived contaminants. The groundwater table in coastal aquifers fluctuates in response to various oceanic pressure forces acting at the shoreline, including tides, individual waves, and offshore storms. Measurements of water table fluctuations in response to tides and waves can be used to characterize coastal aquifers and provide important insight into the hydraulic properties and structure, including the connectivity between the aquifer and ocean. Most studies use simple laboratory, analytical, or numerical experiments to study the effects of ocean forces on coastal groundwater table fluctuations rather than collecting data in real field settings. This study presents an improved approach of understanding coastal aquifers by evaluating groundwater level fluctuations in response to pressure forces from tides and waves due to offshore storms (i.e. storm pulse). Long-term continuous groundwater level data collected on a sand barrier island (Sable Island, NS, Canada) suggest heterogeneous propagation of pressure forces from tides and storm-induced waves through the aquifer system. Groundwater levels in isolated inland areas were found to be highly fluctuating in response to tides and waves suggesting that the coastal aquifer is not homogeneous and isotropic as previously reported. It is hypothesized that observations are due to the presence of a layered aquifer system with localized leakage of pressure forces from an underlying confined aquifer that is connected to the ocean slightly offshore of the coastline. Two-dimensional numerical groundwater flow simulations were conducted in MODFLOW-2000 to test if the leaky confined-unconfined aquifer conceptualization is able to explain the tide-induced inland groundwater level fluctuations observed. The effects of key aquifer parameters (e.g. aquifer storage, depth of buried confining layer, width of leak) were investigated through model simulations and the presented model setup is consistent with observations. This study shows that analysis of both tidal and storm pulse propagation may be a valuable and affordable approach to investigate complex coastal aquifers. Comparison of field data with existing analytical solutions, however, suggests more work is required to describe the effects of tides and offshore storms on groundwater table fluctuations in complex aquifer settings. Improved methods for coastal aquifer characterization will assist in the development of effective management strategies required for the many coastal aquifers worldwide that are impacted by human activities