Characteristics and effects of flow reversals at Madison Blue and Peacock Springs (Florida) during river flooding

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1 Characteristics and effects of flow reversals at Madison Blue and Peacock Springs (Florida) during river flooding Jonathan B. Martin Jason Gulley Elizabeth J. Screaton Geological Sciences - UF Photo thanks to Wes Skiles Funding: National Science Foundation

2 Spring Runs/Rivers Much concern about changing water quality and its effects on ecosystems Stable composition of some spring runs allow study of fluvial nutrient dynamics Nitrate and phosphate concentrations Other micro-nutrients (Fe?) But stream water composition ultimately controlled by sources of water to springs Influenced by subsurface flow paths Modifications of water chemistry in the subsurface.

3 Water in the subsurface Two primary sources to springs (in Florida at least): Conduits: rapid flow Matrix porosity: slow Darcian flow These sources will have very different chemical compositions Different flow paths & residence times Different interactions with the aquifers rocks, sediments, biota Sources of water to and exchange between conduits and matrix important for spring water composition Photo thanks to Wes Skiles

4 Baseflow karst aquifers Conduits act as drains for matrix porosity Long residence time of matrix water leads to equilibration with matrix rocks Martin and Screaton, 2001 High pco 2 ph ~ 8 High Ca concentrations Low DO concentrations Nutrients NO 3 and PO 4 concentrations depend on land use and rock type Possibly modified by subsurface reactions

5 Flood karst aquifer Martin and Screaton, 2001 Conduits flood with surface water From sinkholes From flooding rivers Flood water can flow from conduits to matrix Typical surface water characteristics: Low ph (5?) Dilute low Ca concentrations Undersaturated with respect to calcite Elevated DO and OC 1. Undersaturation leads to dissolution of aquifer material forming caves and altering spring water chemistry 2. Dissolution long believed to occur in downstream direction (e.g., Rhoades and Sinacori, 1941; J. Geology)

6 Suwannee River Watershed Geology mostly Miocene and post-miocene siliciclastic rocks -confining Some Oligocene and Eocene carbonate rocks exposed Floridan Aquifer Floridan Aquifer variably confined and unconfined Boundary is Cody Scarp Talk today focuses on two springs heavily impacted by flood waters: Madison Blue Spring Peacock Spring

7 Spring Discharge Reversals are common Occur when river elevation exceeds groundwater elevation Specific research questions: How does chemical composition of spring/river water change during and after reversals? Do reversals cause dissolution leading to formation of caves? Do caves form during upstream flow or in the classic downstream direction? 1/1/02 1/1/09 Significance: Formation mechanism controls morphology of caves Cave morphology controls groundwater flow/chemistry

8 0 Lee, FL Supersaturated Undersaturated SI Cal vs River Discharge Si cal -5 0 Q (m 3 /s) 500 Luraville, FL Supersaturated 0 Undersaturated Si cal -6 0 Q (m 3 /s) 1200 Saturation of river water with respect to calcite versus discharge Lee, FL = near Madison Blue Spring Luraville, FL = near Peacock Spring Sampled approximately monthly from 2003 to 2008 Greater scatter at Luraville Longer flow path on unconfined portion

9 Approach - Madison = CTD Loggers Nestlé water bottling facility = karst window > 8000 m of mapped passage, much more unmapped CTD loggers installed from March through July 2009 Nestlé bottling plant, commonly stops operations during flooding Tannins in wells, truck in clear water from elsewhere

10 = CTD Loggers = karst window = sample site Approach - Peacock 7,500 m of mapped passage, some unmapped Not perennial spring series of karst windows When river stage > 8 masl, overland flow to sinkholes CTD loggers installed August 2008, entrance only March through July 2009 Samples collected April through July 2009 from 2 karst windows and Suwannee River

11 Results - Peacock Spring CTD logger installed only at entrance August 2008,Tropical Storm Fay, stage increase 5.4 to 7.7 masl Stage insufficient to flood springs Drop in SpC corresponds to increase in Q river and increase in T Suggests flood waters enters system through subsurface

12 Results - Madison Blue Spring Flooding initiated Stage increase 9.3 to 19.5 masl Mixing at entrance of conduits Nearly 60 days before return to pre-flood conditions Flood waters enter conduits; Lag suggests flow rate of 0.03 m/sec Dissolution or Groundwater starts to mix with flood water Rapid return to near pre-flood conditions Conservatively estimate that 5.8 x 10 4 m 3 water entered conduit in 7.5 days

13 Results - Peacock Spring Flooding initiated Stage increase 5.7 to 14.2 masl Estimate: Flow to conduits for about 20 days Flow rate 0.14 m/sec Total infiltration around 1.2 x 10 5 m 3 Saturation index decreases to nearly -5 Assuming equilibration, about 28 m 3 of calcite dissolved Wall retreat of conduits would be about 2.6 x 10-4 m/d Average elsewhere is about 3 x 10-6 to 10-7 m/d

14 Physical Evidence for Dissolution Little River Spring: Photo courtesy of Ken Clizbe Scallop direction indicates dissolution occurs during reverse flow Flow during low discharge at equilibrium, not able to dissolve Caves thus appear to form in Florida from discharge end rather than recharge end Red coating is Fe & Mn oxides

15 Summary As river stage increases: Springs reverse flow (estavelle) More water flows into conduits than can be stored water recharge matrix Specific conductivity can be used to trace flood water Flooding can easily be traced with specific conductivity Calcite undersaturated, dissolution Conduits form from downstream end

16 Future Needs Expectations: Age of water discharging from springs should reflect introduction of new water Adds complexity to age dating spring water DO concentration of groundwater first increases, then completely reduced by organic carbon Produce CO 2, reduce ph of water As DO depleted, NO 3 should become terminal electron acceptor denitrification Metal oxides also should dissolved, more reducing, more acidic

17 Thanks Questions?

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19 Spring Runs/Rivers Classic hyporheic zone Stagnant areas in channels Pore water in sediments Also (?): Matrix porosity