Seawater Circulation in Coastal Aquifers: Processes and ImpactsEPA Grant Number: F07B30483
Title: Seawater Circulation in Coastal Aquifers: Processes and Impacts
Investigators: Karam, Hanan N.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Lee, Sonja
Project Period: January 1, 2007 through January 1, 2010
RFA: STAR Graduate Fellowships (2007) RFA Text | Recipients Lists
Research Category: Aquatic Ecology and Ecosystems , Academic Fellowships , Fellowship - Groundwater Dynamics in Coastal Aquifers , Fellowship - Hydrology
In coastal aquifers, groundwater has two origins, inland meteoric recharge and seawater. Hence, subsurface flow dynamics are driven by unstable density variations and by hydraulic head gradients, which are subject to time-varying forcings at both the terrestrial (e.g. seasonally varying recharge) and marine (e.g. tides and waves) boundaries of these aquifers. These forcings drive a circulation of seawater in the subsurface of potentially great ecological significance: it increases subsurface mixing between fresh groundwater and seawater and provides a mechanism to continuously flush submarine sediments, enhancing chemical exchange between the aquifer and the overlying water column; additionally, it may play a role in the breakdown of marine-derived organic matter, and hence contribute fundamentally to the nutrient cycle of coastal ecosystems. However, seawater circulation in coastal aquifers is still poorly understood: its characteristic spatial and temporal scales and the relative importance of various forcings are unclear. The goal of this project is to quantitatively model the rates and patterns of this circulation, capturing its most important spatial and temporal scales and their relationship to various landward and seaward forcings, and elucidating its control on the extent of subsurface mixing between fresh groundwater and seawater.
High-resolution, long-term data will be collected describing the groundwater system at the head of Waquoit Bay, located on the south shore of Cape Cod, MA. The effects of forcings with different timescales (hours to seasons) on groundwater dynamics will be analyzed. Differential pressure sensors and a fiber-optic temperature sensing cable will be used to monitor exchanges between the subsurface and the water column. The pressure sensors will provide time series of head gradients across the bay floor, which can be used to infer the direction and rate of water flux across the water-sediment interface. The fiber-optic cable will provide time series of bay bottom temperatures, which will also be used to calculate water fluxes through heat transport modeling. Additionally, electrical resistance tomography (ERT) data on subsurface resistivity will be used to understand the evolution of salt distribution in the coastal aquifer over a year’s duration, particularly the mixing patterns of fresh groundwater and seawater and the movement of fresh-saline mixing zones in response to the various forcings. The field data will be used to develop and constrain a numerical model of the local groundwater dynamics, which will then be generalized to investigate other coastal systems.
To provide new understanding of patterns and rates of seawater circulation in coastal aquifers, which will contribute to research on seawater contamination of fresh coastal aquifers, groundwater-transported pollution of marine environments, and nutrient cycling in these environments.