Exchange of water between conduits and matrix in the Floridan aquifer
Introduction
Carbonate karst aquifers have long been conceptualized as containing three types of porosity: intergranular porosity within the matrix rocks, small aperture fracture porosity and large cavernous conduit porosity (e.g., Smart and Hobbs, 1986, White, 1999). These types of porosity and their relative proportions within a karst aquifer can cause permeability to span many orders of magnitude resulting in both laminar and turbulent flow and widely ranging flow rates (e.g., Hickey, 1984, Wilson and Skiles, 1988, Halihan et al., 1999). Differences in flow rates as well as storativity of conduit and intergranular porosity mean that most flow is concentrated in conduits while most water is stored in the intergranular porosity Atkinson, 1977a, Atkinson, 1977b.
Conduit porosity can be connected to surface water through cavernous opening such as sinkholes and sinking streams. These openings allow rapid and extensive mixing of surface and ground water, cause natural changes in the chemical composition of ground water and increase its vulnerability to contamination (e.g., Field, 1988, Field, 1993, White et al., 1995, Memon and Prohic, 1998). If surface water flows rapidly through the conduit system, however, it will have little impact on water quality within the intergranular porosity, which commonly is the primary water supply in karst areas because of its high specific storage. In contrast, the exchange of water between conduit and intergranular porosity could affect the quality of water in intergranular porosity and disturb chemical equilibrium established between the water and surrounding rock. The extent of this exchange is thus critical to management of water supplies, as well as the understanding of karst hydrodynamics and of fluid–solid reactions within the aquifer.
Many well-studied examples of karst aquifers occur in dense and recrystallized rocks, which restrict much ground water flow to conduits. The dominance of conduit flow in these aquifers has lead to a research focus on changes in discharge at springs (e.g., their “flashiness”) and how these changes reflect flow through the subsurface (e.g., Pitty, 1968, Shuster and White, 1971, Ternan, 1972, Smart and Ford, 1986, Hess and White, 1988, Felton and Currens, 1994, Padilla et al., 1994, Ryan and Meiman, 1996, Halihan et al., 1998, White, 1999). For example, mathematical relationships between rainfall and spring discharge can be used in certain cases to predict recharge areas and variations in flow with recharge (e.g., Dreiss, 1983, Dreiss, 1989a, Dreiss, 1989b, Wicks and Hoke, 1999). One important control on the flashiness is the volume of recharge in a particular area provided by sinking streams Newson, 1971, Atkinson, 1977b. Information on flow-through intergranular, fracture and conduit porosity can also be obtained through sampling and observations of behavior of water levels in wells Hickey, 1984, Shevenell, 1996. In reality, a comprehensive approach using multiple techniques is required to understand and characterize flow in all parts of the porosity systems of karst aquifers Smart and Hobbs, 1986, Worthington, 1999.
Both conduit and matrix flow could contribute significantly to flow in carbonate aquifers that are composed of unaltered carbonate rocks with high primary porosity or where carbonate rocks have extensively developed secondary matrix porosity and/or fracture systems (e.g., Atkinson, 1977b, Beck, 1986, Wilson and Skiles, 1988, Recker et al., 1988, Smart and Hobbs, 1986, Ford and Williams, 1989, Sanford and Konikow, 1989, Mylroie and Carew, 1995, Shevenell, 1996). In such aquifers, it will be important to include the matrix in models of both regional and local ground water flow. In order to construct such models, the relative contributions of matrix and conduit flow to the total ground water system must be known, as well as the potential for, the controls on, and the extent of exchange of water between the conduit and intergranular porosity of the matrix rocks. Although some estimates have been made for the flow of water from matrix rocks to the conduits (e.g., Newson, 1971, Atkinson, 1977b), few attempts have been made to observe or quantify the flow of water from conduits to the matrix (e.g., Wilson and Skiles, 1988).
This paper uses chemical composition of water in a sinking river, its resurgence and two nearby water supply wells to make observations about exchange of water between conduits and matrix porosity. The study area is along a 5-km section of the Santa Fe River in north-central Florida Skirvin, 1962, Hisert, 1994, Martin and Dean, 1999. The time that water from the sinking stream remains underground can be measured at high resolution using temperature data for water flowing into the Sink (Martin and Dean, 1999). This information on residence time allows the collection of water samples from a single pulse of river water that passes through the conduits that source the resurgence. Changes in the chemical composition of the water suggest that a large fraction of water discharging from the Rise is derived from the matrix during low flow. In contrast, during flooding, water appears to flow from the conduits and through matrix porosity for at least several kilometers down the regional ground water gradient following floods.
Section snippets
General geologic and hydrogeologic background
The post-Cretaceous lithostratigraphy of the Florida platform can be crudely divided into two major units: pre-Miocene carbonate-dominated rocks, which are further subdivided into several distinct formations, and Miocene and younger siliciclastic-dominated rocks, which in part comprise the Hawthorn Group (Fig. 1)Scott, 1988, Scott, 1992, Groszos et al., 1992. These stratigraphic units largely control the hydrogeology of the region with the Floridan aquifer formed mostly of pre-Miocene carbonate
Field sampling
Water samples were collected from three locations along the river: at the River Sink, at Sweetwater Lake and at the River Rise (Fig. 3). The River Sink samples were collected ∼40 m upstream from the Sink. The Rise samples were collected ∼100 m downstream from the two primary resurgence points (Hisert, 1994) in order to allow for complete mixing of water from those springs. In addition, water was collected from two local water supply wells, one located ∼500 m upstream from the Sink and the other
Precipitation and river stage
Precipitation, river stage and discharge records for the year of the study are shown in Fig. 4. Precipitation is measured at the entrance of O'Leno State Park, approximately 3 km west of the River Sink. The stage of the River is measured approximately 500 m upstream from the River Sink. Precipitation data and stage are collected daily by park staff and reported to the Suwannee River Water Management District (SRWMD). Discharge measurements are collected by the US Geological Survey (USGS) ∼5 km
Discussion
Regional scale mapping shows that the potentiometric surface of the upper Floridan aquifer slopes to the southwest through north-central Florida (Meadows, 1991). Consequently, the regional flow of ground water should be from the northeast to the southwest through the study area (Fig. 3). No detailed potentiometric surface maps have been constructed in the area; however, such maps would provide important information about the local direction of flow of the ground water. Nonetheless, if ground
Conclusions
The temporal and spatial variations of the chemical composition of water in the Santa Fe Sink/Rise system appears to reflect the exchange of water between matrix and conduits in the Floridan aquifer. Available discharge measurements show that more water discharges from the River Rise than enters the River Sink at low to intermediate discharge rates, indicating an additional source of water, probably from the eastern conduits. The chemical composition of water at the River Rise also suggests
Acknowledgements
We thank Mr. Dale Kendrick and his staff at O'Leno State Park for allowing us frequent access to the park at odd hours. This work would not have been possible without the help and support of the park staff. Mike Poucher kindly provided preliminary versions of the cave maps. We also thank the two anonymous reviewers for Chemical Geology who made numerous and very helpful comments. The work has been supported by NSF Grant EAR-9725295.
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2021, Journal of HydrologyCitation Excerpt :The storage zones refer to eddies or vortices in a conduit and matrix surrounding the conduit that are stagnant relative to the faster fluids near the center of the conduit (Runkel, 1998); solutes reside in the storage zones temporarily before returning into the main conduit. For the matrix with significant permeability, the water and solute are exchanged between the conduits and matrix (Martin and Dean, 2001; Raeisi et al., 2007; Ronayne, 2013) due to the hydraulic pressure difference between them (Li et al., 2008; Frank et al., 2019) and the possible matrix diffusion, leading to the tailing of BTC if the solutes forced into the matrix subsequently back again. However, this paper mainly focuses on solute transport in the turbulent conduit flow of highly karstified aquifers with limited conduit-matrix exchange (Peterson and Wicks, 2005; Field and Pinsky, 2000).
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