Tracking hydroclimatic and urbanization impacts on groundwater and stream water evolution: A tale of two carbonate hydrologic systems
Creators: name: Beal, Lakin Kathleen
name: 0000-0002-2091-6476
Publisher: name: The University of Texas at Austin
Year: 2019
Subjects: Hydrogeology, Geochemistry, Isotopes, Carbonate aquifer, Carbonate watershed

description: 1.0 Abstract Assessing the sensitivity of groundwater systems to hydroclimate variability is critical to sustainable management of the water resources of Guam, US territory. We assess spatial and temporal variability of isotopic and geochemical compositions of vadose and phreatic groundwater sampled from cave drip sites and groundwater wells, respectively, to better understand the vulnerability of the freshwater lens on Guam to variability in hydroclimate. We independently evaluate the existing conceptual model of the Northern Guam Lens Aquifer that is largely based on physical, as opposed to geochemical, observations. Sampling was conducted from 2008 to 2015, over which rainfall gradually increased. Major ion geochemistry and Sr isotope values of groundwater show varying influence from soil, limestone bedrock, and seawater. Geochemical modeling that explains spatial variability in groundwater Na+ and Mg2+ concentrations and Sr/Ca and 87Sr/86Sr values indicates that groundwater compositions are dominantly controlled by mixing of freshwater with seawater and water-rock interaction. Differences between amount-weighted annual average precipitation δ18O values and groundwater δ18O values indicate a recharge bias toward the wet season, consistent with other tropical carbonate island aquifer settings. Intra- and inter-annual variations in Na+ concentrations and δ18O values in groundwater reflect sensitivity of recharge to seasonal variations in rainfall amount and changes in annual rainfall amounts. Our results indicate the influence of multiple modes of recharge on groundwater compositions and spatial variability in the sensitivity of groundwater to seawater mixing. This sensitivity of the freshwater lens points to the vulnerability of groundwater resources to changes in recharge associated with climate, land-use change, and increases in population. 2.0 Abstract Quantifying urban development impacts on freshwater quality and quantity is critical, especially as growing populations concentrate in urban centers and with climate change projections of increased hydrologic extremes. We investigate geochemical processes through which municipal (supply and waste) water impacts stream and spring water compositions within the carbonate Bull Creek watershed (Austin, Texas), which exhibits a distinct geographic divide between urban development and rural land. 87Sr/86Sr and elemental variations are assessed for waters sampled from rural and urban sites to quantify relative influences of natural versus municipal water. Higher 87Sr/86Sr values for urban sites relative to rural sites can be accounted for by two models: (1) water leakage from municipal infrastructure and/or irrigation, or (2) ion exchange as precipitation infiltrates through soils with varying 87Sr/86Sr. Irrigated soils have higher 87Sr/86Sr values than unirrigated soils, indicating that municipal water resets soil compositions, and that process (1) is a dominant driver of urban stream and spring water evolution. Fluid mixing models indicate that urban waters consist of 50% - 95% municipal water. Water-rock interaction modeling documents the geochemical evolution of infiltrating municipal water, whereby municipal water infiltrates as groundwater and undergoes diagnostic extents of water-rock interaction with the carbonate bedrock. These results are compared with regional phreatic and vadose groundwater compositions to infer local flow pathways and relative groundwater residences times of both municipal and rural water. This study advances our understanding of the significance of municipal water influences on urban stream water and soil compositions, and provides a geochemical modeling framework that quantifies the evolution of infiltrating municipal water within carbonate watersheds and aquifers.