338-346 Gw M-j 04

Much of the existing conceptualization of fluid flow in karst aquifers is based on work in extensively cemented and recrystallized Paleozoic and Mesozoic carbonates. Within these aquifers, flow is often primarily through conduits (White 1969, 1977; Smart and Hobbs 1986; Ford and Williams 1989), although diffuse flow may occur through secondary porosity, such as fractures and joints (Shuster and White 1971). In contrast, other karst aquifer systems occur in younger rocks that have significantly greater primary porosity and permeability than recrystallized carbonates (Budd and Vacher 2002; Martin et al. 2002; Vacher and Mylroie 2002). Palmer (2002) suggests that hydraulic conductivity outside of conduits averages about four orders of magnitude greater in young aquifers such as the Floridan (Paleocene to Miocene) and the Edwards (Cretaceous) than in Paleozoic aquifers. This high permeability is believed to allow a significant quantity of flow through the matrix of these young carbonate aquifers (Miller 1986; Smart and Hobbs 1986). Storage and specific yield values also are greater in younger aquifers (Palmer 2002). Thus, the matrix porosity of such aquifers contains the bulk of the useable water supplies and commonly represents primary regional water resources. Along with the high matrix conductivity, the unconfined Floridan Aquifer has extensive conduit development typical of the branchwork and spongework forms of Palmer’s (1991) classification scheme. Many of the conduits result from the focused input of surface streams where they reach the unconfined portion of the Floridan Aquifer (Upchurch and Lawrence 1984) because surface water is typically undersaturated with respect to carbonate minerals, leading to dissolution. During high flow events, monitoring Abstract Exchange of water between conduits and matrix is an important control on regional chemical compositions, karstification, and quality of ground water resources in karst aquifers. A sinking stream (Santa Fe River Sink) and its resurgence (River Rise) in the unconfined portion of the Floridan Aquifer provide the opportunity to monitor conduit inflow and outflow. The use of temperature as a tracer allows determination of residence times and velocities through the conduit system. Based on temperature records from two high water events, flow is reasonably represented as pipe flow with a cross-sectional area of 380 m2, although this model may be complicated by losses of water from the conduit system at higher discharge rates. Over the course of the study year, the River Rise discharged a total of 1.9 × 107 m3 more water than entered the River Sink, reflecting net contribution of ground water from the matrix into the conduit system. However, as River Sink discharge rates peaked following three rainfall events during the study period, the conduit system lost water, presumably into the matrix. Surface water in high flow events is typically undersaturated with respect to calcite and thus may lead to dissolution, depending on its residence time in the matrix. A calculation of local denudation is larger than other regional estimates, perhaps reflecting return of water to conduits before calcite equilibrium is reached. The exchange of matrix and conduit water is an important variable in karst hydrology that should be considered in management of these water resources.