Date of Award

12-15-2020

Degree Name

Doctor of Philosophy

Department

Geological and Environmental Sciences

First Advisor

Dr. Mohamed Sutlan

Second Advisor

Dr. William Sauck

Third Advisor

Dr. Richard Becker

Fourth Advisor

Dr. Matt Reeves

Keywords

GRACE, groundwater, Nubian, lakes, remote sensing, precipitation

Abstract

Our recent analysis of Temporal Gravity Recovery and Climate Experiment (GRACE) solutions over the largest aquifer system in Africa (the Nubian Sandstone Aquifer System: NSAS) revealed that while the response of deep aquifers to climatic variations remains a relatively slow process that takes thousands to tens of thousands of years, there is a much faster response in aquifers that are characterized by dense networks of faults, fractures and karst as is the case with the NSAS. This rapid groundwater flow, when it occurs, is detected as an increase in GRACETWS over areas downgradient and distant (hundreds of km) from the source areas over which increased precipitation occurred in wet periods. The increase in GRACETWS over these distant areas cannot be accounted for by an increase in precipitation, soil moisture, or surface water flow given the hyperarid conditions even during periods of increased precipitation over the source areas. In this research we will accomplish the following using the NSAS in North Africa as test sites: (1) investigate whether the rapid response observed over the NSAS could be common to many of the aquifers worldwide, (2) identify the conditions under which such rapid flow occurs, and (3) apply groundwater flow models in fractured rocks and use the models to develop sustainable and/or optimum scenarios for groundwater management in areas showing evidence of rapid groundwater flow. These three tasks will be accomplished by conducting the following for each of the investigated aquifer systems: (1) use satellite-based precipitation mission data to identify temporal (short-term climate variability: wet and dry periods) and spatial variations of precipitation over areas of interest (source versus discharge areas), (2) identify the spatial and temporal response of GRACETWS to short climate variability over the areas of interest, (3) generate GRACETWS phase, difference, and amplitude images to investigate the direction and extent of GRACETWS variations that are indicative of mass movement in dry and wet periods and seasons; (4) delineate potential preferred pathways for groundwater flow by mapping fault traces from shaded relief maps, radar backscatter images, and geologic maps and their postulated extension in the subsurface from tilt derivative (TDR) product of a selected Gravity Field and Steady-State Ocean Circulation Explorer (GOCE)-based global geopotential model (GGM). Finally GRACE-based inferences will be validated using: (1) field data (temporal head data, isotopic and geochemical composition for groundwater samples) along the identified structures and satellite-based (SMOS) soil moisture content to test whether the identified structures represent preferred pathways for groundwater flow, and (2) conceptual flow models that simulate the rapid groundwater flow from the source areas towards discharge areas along preferred pathways and a much slower flow in the surrounding media. Findings will provide new insights into the response of large, deep aquifers to climate variability and address the sustainability of the NSAS and similar fossil aquifers worldwide.

Access Setting

Dissertation-Open Access

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