Integrated Physical and Chemical Observations of Water-rock Interactions and Coupled Matrix-conduit Flow in the Karstic Floridan Aquifer

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0510054MartinIntellectual Merit. It has been recognized recently that the flow field of karstaquifers includes conduits, fractures, and intergranular porosity of the matrix and that fullcharacterization of flow requires understanding flow in each component, couplingbetween the components, and how water recharges to the aquifer from the surface. Mostwell-studied karst aquifers have low intergranular porosity where diffuse flow is confinedto a network of fine fractures surrounding conduits. In contrast, this proposed workcenters on the Floridan aquifer, which retains high (up to ~20%) intergranular matrixporosity. Previous work on physical hydrogeology (discharge measurements,temperature tracing of flow rates, observations of hydraulic head), suggests that waterexchanged between conduits and the matrix of the Floridan aquifer is controlled byreversals in head gradients between the matrix and conduits as a result of high allogenicinput into the conduit system. The previous work has not quantified contributions fromdiffuse recharge from the surface, the magnitude of exchange between the conduits andmatrix, the residence time of water that exchanges, or chemical modifications of theaquifer through dissolution reactions. Consequently, this proposed project will determinecoupling between aquifer components and water sources by integrating new chemicalmeasurements with previously used physical observational techniques.The field area will be the Santa Fe Sink/Rise system in north-central Florida.Observations to be made will include major element and isotope compositions of waterfrom nested new and existing wells and lysimeters (soil water samplers) for ground watercharacterization, from the River Sink, Rise, and karst windows for characterization ofconduit water, and from precipitation for characterization of the input sources. Thechemical measurements will provide flow velocities to be compared with head gradientsbetween the conduits and the matrix and saturation states with respect to carbonate andsilicate minerals. The nested wells and data on chemical composition of precipitationwill allow characterization of vertical differences in water chemistry to determinemagnitudes of diffuse recharge from the surface. These data will be compared to otherkarst aquifers with low intergranular porosity using available deterministic models suchas chemograph separation. Comparisons will also be made to results of previous and ongoingstudies of other regions of the Floridan aquifer. Because of the highertransmissivity of the highly porous Floridan aquifer, it is anticipated that the intergranularporosity component will play a large role in the overall flow field of the aquifer.Broader Impacts. This work should improve the understanding andmanagement of karst aquifers that have significant portions of their flow within matrixporosity. These aquifers commonly represent major water resources, but have previouslybeen managed using conceptual models that consider either flow primarily throughconduits or treat the system as an equivalent porous medium. In particular, these resultswill be directly applicable to management of the Floridan aquifer and results will bedisseminated to critical water managers through Martin's participation in the FloridaSprings Task Force. Human resources will be impacted through graduate andundergraduate education, both formally as a resource for currently taught courses andthrough graduate student theses. One second-year PhD student is currently working onthis topic as part of his dissertation, and two masters students and several undergraduateswill participate in the field, laboratory, and analytical work.