Browsing by Subject "Submarine groundwater discharge"

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Groundwater flow controls on coastal water quality and global groundwater ages
(2015-05) Befus, Kevin Martin; Cardenas, Meinhard Bayani, 1977-; Gleeson, Thomas P; Hesse, Marc A; Paine, Jeffrey G; Sharp, John M
Humanity relies on groundwater. But, current consumption may be outpacing groundwater renewal rates, and anthropogenic activities are altering its quality. This dissertation advances the state of knowledge of how local and regional groundwater dynamics affect its quality and quantity. First, I investigate groundwater discharge patterns and fluxes in three lakes in the Nebraska Sand Hills region and on the island of Rarotonga, Cook Islands, to understand the hydrologic connection between groundwater and surface water in these lacustrine and coastal settings. In Nebraska, I use electrical geophysical methods to characterize the spatial signature of groundwater recharge and discharge to and from the lakes using groundwater salinity patterns. On Rarotonga, a detailed field study of groundwater flow at the intertidal zone shows how groundwater flow influences the thermal regimes of nearshore environments, affecting the biota that live and chemical processes that occur near and below this dynamic interface. Next, a dense network of geophysical surveys across the coastal plain and into the lagoon on Rarotonga constrains multiple features of the larger-scale hydrologic system that are primarily controlled by the local carbonate and volcanic geology on the island. Finally, I give the first estimate of the global storage and spatial distribution of groundwater with a mean age since recharge of less than fifty years. I use several thousand two-dimensional groundwater flow and age-as-mass transport simulations parameterized by the best available hydrologic and geologic datasets. This global analysis suggested that ~6% of the groundwater stored in the upper 2 km of the Earth’s crust is younger than 50 years. Comparing this young groundwater storage to current groundwater depletion rates indicates that more than half of the irrigated areas depending significantly on groundwater could have already used up all of the young groundwater and are using groundwater more quickly than the storage is replenished. Together, these studies advance how to quantify groundwater as a renewable resource through the global estimation of groundwater storage associated with certain timespans and by analyzing the implications of groundwater flow on water quantity and quality in field settings.
Multi-scale thermal mapping of submarine groundwater discharge in coastal ecosystems of volcanic islands
(2023-08-10) Williams, Ebony Leah; Cardenas, Meinhard Bayani, 1977-; Goudge, Timothy A; Rempe, Daniella M
Submarine groundwater discharge (SGD), or the flow of groundwater from land to sea, transports different constituents to the coast, such as nutrients, chemicals, and dissolved metals. In volcanic regimes, groundwater might have especially high concentrations of these constituents when, it is hydrothermal in origin. The Verde Island Passage, which lies within the Coral Triangle in the western Pacific, has been designated a marine protected area, as it is home to over thousands of different species of fish and coral reefs. Interactions of this constituent-rich groundwater with coastal waters through fractured volcanic rock can either pose a threat to or promote coastal ecosystem productivity and health. Because of this setting, it is known that SGD is a prevalent source of constituents, but how much and specifically where it is occurring is unknown. Thermal Infrared (TIR) remote sensing has gained popularity as a method of detecting SGD as it helps to resolve spatial variability issues that are inevitably accompanied by this phenomenon. Here, we investigate the use of TIR satellite and airborne remote sensing, coupled with ground-based field measurements including thermistor rods and fiber-optic distributed temperature sensing to identify and quantify submarine groundwater discharge in a coastal area of the Verde Island Passage. This study utilizes USGS Landsat 8 TIR sensor to regionally denote SGD plumes via temperature measurements of ocean water, as well as local airborne TIR remote sensing which provides a finer spatial resolution than its satellite counterpart. This study also employs a 1 km-long fiber optic cable to continuously measure temperature along the coast for the very first time in this region, and thermistor rods buried in the seabed under 20-30 feet of water to measure vertical temperature flux. Results from these approaches reveal multiple SGD signals in both the intertidal and subtidal zones, and tens of meters offshore, with temperatures reaching up to 80°C and estimated fluxes reaching as high as 1.5 m/d. From these approaches, we conclude that airborne TIR remote sensing and vertical thermal profiling are the most feasible thermal sensing methods employed in the study area, as they capture regional to local and point scaled SGD, which yield plume location, morphology, and flux. This study serves as a foundation for mapped SGD locations which can be used in conjunction with flux estimations in future temperature and flow modeling of SGD in the region. This study also has implications on potential outflow points of natural hydrothermal constituents and/or polluted groundwater, which can be of benefit to ecosystem and resource management.