9526714 Banner The proposed research will investigate how a limestone aquifer evolves through time. Barbados's uplifted coral-reef terraces comprise a 10-km scale meteoric groundwater flow systems with unique stratigraphic, lithologic, and hydrologic properties that are ideally suited to understanding the pathways and time scales of groundwater flow and fluid-rock interaction. These unique properties include (I) variations in aquifer-rock composition, mineralogy and age, soil composition, and porosity-permeability that all vary in predictable patterns within and between tectonically-uplifed groundwater catchments: (ii) access to water, rock, mineral, and soil samples from densely-spaced wells, springs and caves; and (iii) the 0 to 1 million year time scale within which the aquifer's evolution can be studied using newly-developed isotope techniques. To reconstruct the aquifer's Pleistocene-Holocene-modern evolution, samples will be taken of groundwater, limestones, and soils from the modern flow system, and carbonate minerals that grew as speleothems and cements in the pore spaces of the limestones during groundwater flow in the Pleistocene and Holocene epochs. Recently developed and conventional isotopic techniques (e.g., 238U-234U-230Th, 87Sr/86 Sr, 13C/12C, 18O/16O, 15N/14N) will be used in order to determine 1) the ages of growth zones in the speleothems and cements, 2) the sources of dissolved ions in the modern groundwaters and in the groundwaters from which the cements grew, and 3) temporal variations in the sources of dissolved ions for the groundwaters during the past million years. In determining high-resolution temporal variations in the sources of dissolved ion in an active, well-constrained groundwater flow systems, this study will be unprecedented. An initial study of a stalagmite provides a time series record for the past 5500 years. Sr isotope variations in groundwater recorded in the calcite growth layers of this sample reflect changes in the amount of rainfall tha t recharged the Pleistocene limestone aquifer during the mid to late Holocene, Similar records will be constructed for other parts of the aquifer and over extended temporal scales. These will be integrated with a detailed study of the modern flow system. The petrologic, geochemical, geochronologic, and hydrologic analysis of this aquifer will improve our understanding of (I) the controls of karsification patterns, porosity, aquifer rock mineralogy, soil development, and climate change on groundwater migration and chemical evolution, and (ii) the impact of industrial and agricultural pollution on groundwater quality. Such data on the rates of groundwater evolution represent an important step toward developing predictive models for the development of carbonate aquifers.
