Browsing by Subject "Coastal groundwater"
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Item 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 MHumanity 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.Item Mixing dynamics of groundwater-seawater systems at the land-ocean interface(2015-08) Zamora, Peter Basilio; Cardenas, Meinhard Bayani, 1977-; Sharp, John M; Bennett, Philip C; McClelland, James W; Cook, Perran LMSubterranean estuaries are important coastal features where dissolved materials from groundwater and seawater can react and transform. Hence, they affect the quantity and quality of fluid and chemical fluxes across the sediment-water interface. Local geologic and hydrologic heterogeneities in coastal systems can modify this interaction and fluxes. I investigate groundwater-seawater mixing dynamics and groundwater discharge in geographically relevant coastal systems where groundwater upwells through permeable sediments, under small river estuaries, and at a site susceptible to episodic rise in the water table with heavy rainfall. Using numerical simulations of variably-saturated density-dependent flow in porous media coupled with solute transport, I show that freshwater plumes and seawater recirculation cells develop where groundwater upwells from discrete freshwater outlets. Mixing zones that form along the freshwater plume-recirculation cell boundary change with tides and seasonal variations in the strength of the upwelling groundwater. Brackish fluxes increase with the waning strength of groundwater upwelling and sediment thickness but to a much lesser extent. Freshwater fluxes increase with stronger upwelling. Using electrical resistivity (ER) as salinity proxy complemented with groundwater head and temperature profile measurements, I find cut banks to be persistent groundwater discharge sites whereas point bars are predominant locations of hyporheic exchange underneath small river estuaries. These trends are consistent on an estuary-wide scale where sinuous reaches show high ER indicating groundwater discharge hotspots while straighter channel segments have resistivities similar to surface water suggesting the dominance of hyporheic exchange. Time-series porewater salinity profiles along a beach cross section directly showed the flushing and freshening of the intertidal subterranean estuary within a 24 hour period. Subsurface temperature profiles, hydraulic head measurements, and ²²²Rn in seawater concentration indicate consequent elevated groundwater discharge towards the coast. These demonstrate that episodic rainfall can rapidly elevate the groundwater table in permeable beach sediments which affects the chemical and thermal regime of the subterranean estuary. This can potentially deliver materials and energy cycled within the mixing zone in episodic doses to coastal areas akin to flooding of rivers and estuaries.