Wetlands DOI 10.1007/s13157-013-0451-8
HYDROLOGIC RESTORATION
The Influence of Hydrologic Restoration on Groundwater-Surface Water Interactions in a Karst Wetland, the Everglades (FL, USA) Pamela L. Sullivan & René M. Price & Jessica L. Schedlbauer & Amartya Saha & Evelyn E. Gaiser
Received: 28 December 2012 / Accepted: 20 June 2013 # Society of Wetland Scientists 2013
Abstract Efforts to rehydrate and restore surface water flow in karst wetlands can have unintended consequences, as these highly conductive and heterogeneous aquifers create a close connection between groundwater and surface water. Recently, hydrologic restoration efforts in the karstic Taylor Slough portion of the Everglades has changed from point source delivery of canal water (direct restoration), to the use of a series of surface water recharge retention basins (diffuse restoration). To determine the influence of restoration on groundwatersurface water interactions in the Taylor Slough headwaters, a water budget was constructed for 1997–2011 using 70 hydro-
Electronic supplementary material The online version of this article (doi:10.1007/s13157-013-0451-8) contains supplementary material, which is available to authorized users. P. L. Sullivan (*) Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA e-mail:
[email protected] P. L. Sullivan : R. M. Price : A. Saha : E. E. Gaiser Southeast Environmental Research Center, Florida International University, Miami, FL 33199, USA P. L. Sullivan : A. Saha : E. E. Gaiser Department of Biology, Florida International University, Miami, FL 33199, USA R. M. Price Department of Earth and the Environment, Florida International University, Miami, FL 33199, USA J. L. Schedlbauer Department of Biology, West Chester University, West Chester, PA 19383, USA
meteorological stations. With diffuse restoration, groundwater seepage from the Everglades toward the urban boundary increased, while the downstream delivery of surface water to the main portion of the slough declined. The combined influence of diffuse restoration and climate led to increased intra-annual variability in the volume of groundwater and surface water in storage but supported a more seasonally hydrated wetland compared to the earlier direct tactics. The data further indicated that hydrologic engineering in karst wetland landscapes enhances groundwater-surface water interactions, even those designed for restoration purposes. Keywords Ecohydrology . Water budget . Climate variability . Priestley-Taylor method . Evapotranspiration
Introduction Near-surface karst regions occupy approximately 20 % of icefree terrestrial land on Earth and supply roughly 20–25 % of the world’s population with groundwater (Ford and Williams 2007). Ecologically, karst regions are important as they also support many wetlands around the world, especially in Europe and North America (Gondwe 2010). Over the last 200 years, mounting human pressure, such as agricultural and suburban expansion, has led to a substantial loss in wetland cover globally (≈50 %; Zedler and Kercher 2005), while many remaining wetlands have undergone some degree of degradation. In karst wetlands, the influence of these human modifications can be amplified (Ford and Williams 2007), as small water level changes in highly transmissive aquifers can yield significant shifts in groundwater flow (Bonacci et al. 2009).
Wetlands
As wetlands are estimated to provide approximately 40 % of the world’s ecosystem services (Zedler and Kercher 2005), the challenge facing scientists and managers now is how to balance the increasing human demand for water (Oki and Kanae 2006) while maintaining and restoring wetlands, especially in karst regions. As the site of one of the most extensive karst wetland restoration efforts to date, the Everglades, USA, provides a unique environment to study the influence of human modification and demand on wetland ecohydrology. Spanning almost the entire southern portion of the Florida peninsula (Fig. 1), the Everglades is primarily underlain by the Biscayne Aquifer (≈ 66 % of the Everglades), which supplies drinking water to a population of over 5 million. The construction of canals, dikes and levees starting around the turn of the 20th century led to the compartmentalization of the majority of the wetland and the reduction in surface water flow. Along the northern and eastern boundaries of the Everglades water levels have been significantly lowered to provide flood protected agricultural and suburban lands, while water levels in interior portions have been significantly increased to conserve water (Light and Dineen 1994). These changes in the hydraulic gradient across the Everglades have altered both regional and local groundwater flow patterns and enhanced groundwater-surface water interactions (particularly near canals; Harvey et al. 2004; Harvey and McCormick 2009). In parts of the Everglades, ecosystem level changes have been concurrent with increased nutrient and mineral loading associated with canal water inputs, but may also be attributed to the quantifiable change in the discharge of high nutrient groundwater to the oligotrophic surface water. The headwaters of Taylor Slough is just one area along the Everglades eastern boundary where the appearance of cattail (Typha domingensis; Surratt et al. 2012) and the reduction in periphyton biomass (Gaiser et al. 2013) have occurred.
Fig. 1 The headwaters of Taylor Slough (outlined in black) are located along the eastern boundary of Everglades National Park (ENP) in an area known as the Rocky Glades
These changes suggest nutrient enrichment is taking place as a result of point source canal water restoration techniques (referred to as direct hydrologic restoration efforts) used to rehydrate and restore flow to the wetland. In an effort to reduce the ecological ramifications associated with direct hydrologic restoration efforts, a more diffuse water delivery method was enacted for the first time along the Taylor Slough headwaters Everglades-Urban boundary at the end of 1999 (Fig. 2). The goal of the diffuse hydrologic restoration efforts was to use a series of water and nutrient retention basins (S332BN, B, C and D) in conjunction with associated pump stations to create a hydrologic divide between the Everglades National Park (ENP) and the adjacent canal system (SFNRC 2005; Fig. 3), thereby rehydrating the headwaters and supporting downstream freshwater delivery to Taylor Slough while reducing nutrient loading. Since operation, the four retention basins have typically acted as closed surface water systems (Hydrogeologic, Inc. 2010); however their influence on the discharge of groundwater to the surface water or the recharge of surface water to the groundwater (R; groundwater-surface water interactions) and the overall ecohydrology of the headwaters have yet to be determined.
Fig. 2 Hydro-meteorological stations (various symbols) used to determine the influence of direct and diffuse hydrologic restoration efforts on the Taylor Slough headwaters water budget (1997–2011). The L-31 N and C-111 canals form the Everglades-Urban boundary
Wetlands Rocky Glades
= water level
Retention Basin
Pump
Urban
R (?) SWrecharge Biscayne Aquifer
GWdischarge
SWrecharge
GWflow (?)
Miami Limestone
GWSeep
Canal
Fort Thompson Formation
Fig. 3 A schematic of the diffuse restoration techniques employed between 2000-present along the eastern boundary of Everglades National Park (ENP). Water budget calculations in the paper were used to determine if diffuse restoration efforts resulted in the westward groundwater flow (GWflow) and significantly influenced groundwater-surface water interaction (R=SWrecharge−GWdischarge) in the Rocky Glades or the Taylor Slough headwaters. Arrows indicate potential flow directions while inverted triangles indicate potential water levels
The goal of this paper was to determine the influence of diffuse hydrologic restoration on R in the Taylor Slough headwaters. To achieve this goal we used a water budget approach to estimate annual and monthly R from 1997 to 2011, assessed the spatial influence of diffuse hydrologic restoration efforts on the Taylor Slough headwaters, and then reviewed current literature to gain a better understanding of the ecohydrologic influence of this restoration technique on the area. Finally, future success of diffuse restoration tactics in the Taylor Slough headwaters was evaluated in terms of the hydrologic implications of predicted climate change.
Water Management and Study Area Taylor Slough is the second largest waterway within Everglades National Park (ENP; Fig. 1) and plays an important role in delivering fresh water to downstream estuaries. The construction of canals along its eastern boundary, starting in the 1950’s with the L-31 N canal, has resulted in a substantial reduction in fresh water inputs from Taylor Slough to Florida Bay, shunting water toward the Atlantic. Numerous ecohydrologic changes have been observed over this period such as hypersaline conditions in Florida Bay (Van Lent et al. 1993), saltwater encroachment on the freshwater marsh (Ross et al. 2000) and a decline in vertebrate populations (Jorenz 2013). At the headwaters of Taylor Slough, wetland drainage by the eastern canals (L-31 W, L-31 N and C-111; Fig. 2) has led to an increase in fire frequency, desiccation of peat in some areas (McVoy et al. 2011), introduction of non-native fish and vegetation (Kline et al. 2013; Rehage et al. 2013) and loss of habitat for the endangered Cape Sable Seaside Sparrow and the American Alligator (Davis et al. 2005). In 1980 direct hydrologic restoration efforts to reestablish the historic fresh water inputs from the Taylor Slough headwaters southward
were enacted. Up through 1999, water management plans authorized the inputs of canal water from L-31 N into L31 W through the S-174 spillway where it eventually discharged through the S332 pump into the wetland. The current diffuse hydrologic restoration efforts were then put into operation along the Everglades-Urban boundary between 2000 and 2002 (Fig. 2). Like most of the Everglades, this area is underlain by the Biscayne Aquifer, an extremely transmissive (≈100,000 m2 d−1) aquifer that consists of four limestone facies; bryozoan, oolitic, coralline and coquina (Genereux and Guardiario 1998; Genereux and Slater 1999). The Biscayne Aquifer is ≈13.6 m thick in the study area with the upper third consisting of the Miami Limestone Formation and remaining aquifer represented by the Fort Thompson Formation (Genereux and Guardiario 1998). The canals (L-31 N, C-111 and L-31 W) running along the eastern boundary of ENP cut through approximately 5.7 m of the upper portion of the aquifer, completely bisecting the Miami Limestone and upper portions of the Fort Thompson Formation (Genereux and Guardiario 1998). The headwaters of Taylor Slough occupy most of the Rocky Glades (Fig. 1), an oolitic limestone outcrop that helps to separate the southwestward surface water flow of Shark River Slough from Taylor Slough (Armentano et al. 2006; Price and Swart 2006). Exposed pinnacle rock and solution holes generate the small topographic differences (60 cm) found in the area (Genereux and Guardiario 1998; Davis et al. 2005). The shallow soils (≈15 cm thick; Armentano et al. 2006; Schedlbauer et al. 2010) are mainly marls. These soils are generated by highly calcareous periphyton mats (a collection of algae, bacteria and fungi) characteristic of Rocky Glades (Gottlieb et al. 2006) along with limestone bedrock weathering. While small tree islands are scattered across the Rocky Glades, they only make up 0
d = Water depth (m)
Schedlbauer et al. 2011) derived from continuous eddy covariance and micrometeorological measurements at the TSPH1 tower from 2008 to 2009 (Fig. 2) were used to estimate daily α and G in the Taylor Slough headwaters from 1997 to 2011. Monthly ET (m3) values used in the water budget were calculated as the sum of all daily ET (m3) across the Taylor Slough headwaters for a given month. Using similar methods, Price et al. (2007) determined the average error for evaporation estimates in the area was 9 %, which was also applied to this study.
eastern and southern boundaries to avoid underestimating GWseep-south. Daily Q was summed for all pixels along the eastern and southern boundaries to determine the total daily GWseep from the headwaters. Monthly GWseep (m3) was then attained by summing daily GWseep, and it had an estimated error of 8–10 %. Error was associated with the gradient calculation and did not include the variability associated with the potential range in hydrologic conductivity values.
Groundwater Seepage (GWseep)
From 1997 to 1999 the S174 spillway and the S332 pump regulated direct surface water inputs to the Taylor Slough headwaters. Between 2000 and 2002, the S332B, BN, C and D pump stations provided additional surface water input, but by 2003 surface water inputs into the Taylor Slough headwaters were predominately relegated to the S332B, BN, C and D pump stations as the S174 and S332 stations were phased out. As the retention basins were essentially isolated from the Taylor Slough headwaters, the amount of surface water that could contribute to the water budget through inflitration was estimated by taking the difference between the amounts of water pumped into the retention basins and that lost through ET. Outflow from the Rocky Glades occurred at the S175 culvert (until 2000), Taylor Slough Bridge (TSB) and 23 culverts underlying the Ingraham Highway (Fig. 2). Mean daily flow values (m3 s−1) for all structures were obtained from the SFWMD. Within ENP, flow monitored at the TSB and varying
Daily GWseep along the eastern (GWseep-East) and southern boundaries (GWseep-South) of the Taylor Slough headwaters were estimated using Darcy’s law. Along the eastern boundary, hydraulic gradient was solved as the difference between the groundwater and canal surface water levels in the adjacent pixels divided by 400 m (distance between the centers of the pixels). Pixels adjacent to the canal were selected in an effort to also capture local changes in groundwater seepage. Since no canal exists at the southern extent of the headwaters, hydraulic gradient was calculated as the difference in groundwater levels north and south of the Ingraham Highway, and then divided by 400 m. The cross-sectional area over which GWseep occurred at the pixel level was constrained by the average canal depth (5.7 m) and the width of a pixel (400 m). The same depth was used along the
Surface Water Inflows (SWin) and Outflows (SWout)
Wetlands Table 2 Annual water budget parameters for the Taylor Slough headwaters from 1997 to 2011 normalized to the area of the watershed and reported as depth (cm). Precipitation (P), and surface water (SWin) were inputs to the water budget while surface water (SWout), evapotranspiration (ET), and
groundwater seepage (GWseep) were outputs. Change in storage (ΔS) and groundwater-surface water interactions (R) fluctuated between positive and negative depending on the year
Management
Years
P
SWin
SWout
ET
GWseep-East
GWseep-South
ΔS
R
Err
S332 in operation
1997 1998 1999 2000
175 133 162 143
47 57 82 106
−62 −49 −96 −61
−97 −106 −98 −116
−9 −11 −11 −11
−9 −8 −10 −11
17 −7 12 −15
−58 −5 −12 63
5 3 4 7
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 Stdev
158 147 138 122 149 125 143 129 143 135 131 14
128 122 215 126 234 50 70 175 246 257 99 73
−49 −38 −58 −22 −61 −16 −9 −41 −46 −32 −18 23
−108 −120 −117 −107 −136 −105 −124 −124 −112 −114 −92 13
−9 −13 −16 −13 −14 −12 −11 −13 −16 −15 −8 2
−10 −12 −11 −8 −11 −10 −9 −11 −12 −11 −6 2
8 −8 9 −2 7 −14 −9 24 2 −20 17 12
19 39 67 34 57 35 25 43 58 76 −17 31
9 8 12 8 13 4 6 7 16 19 7 5
Installation of pumps & basins
S322B, C & D pumps and retention in operation
culverts was correlated to surface water stage and used to calculate mean daily surface water flow (m3 s−1) out of Taylor Slough with a rating curve developed by ENP (DataforEVER;
[email protected]). Mean daily inflow and outflow (m3 s−1) was multiplied by the number of seconds in a day (83,400 s) to determine the total daily flows (m3 d−1), and summed to determine the monthly surface water inflows and outflow for the Taylor Slough headwaters. Error associated with flow was estimated at 7.5 % by the SFWMD (Imru and Want 2005). Change in Storage (ΔS) Monthly ΔS (m3), or volume of water in storage, was calculated as the difference between the surface water level surfaces on the first and last day of each month; positive values represented the loss in storage over the month. If d was less than or equal to 0 m, the monthly water level difference was multiplied by the porosity. Ground penetrating radar and borehole images in the Rocky Glades indicated the total porosity, within the first 2-m of the aquifer, ranged between 0.40 and 0.50 (Cunningham 2004). The lower porosity (0.40) boundary was used to represent the effective porosity in calculating ΔS.
and negative values indicated groundwater recharged by the surface water. Data Analysis Water budget parameters were averaged according to the three distinct water management periods: I Direct Hydrologic Restoration Efforts (S332 operation, 1997–1999), II Transition (2000–2002), and III Diffuse Hydrologic Restoration Efforts (operation of retention basins, 2003–2011). To determine the influence of direct vs. diffuse hydrologic restoration on the groundwater-surface water interactions and the Rocky Glades water budget, yearly and monthly averaged inputs and outputs were compared to pump discharge using regression analysis. Linear regressions were also developed to evaluate significant trends over time, while one-way analyses of variance (ANOVAs), along with post-hoc Tukey tests (α=0.05), were used to determine significant differences among the three time periods.
Results Climate
Groundwater-Surface Water Interactions (R) R across the system was estimated as the residual of the water budget variables and solved on the monthly time step. Positive R values indicated groundwater discharged to the surface water
Annual precipitation (P) inputs ranged from 122 to 175 cm between 1997 and 2011 (Table 2; Fig. 4). For 60 % of the study time, P inputs were above the 60-year mean of 138 cm (Royal Palm Station; EMS 1). Below-average annual P inputs occurred
Wetlands
Fig. 4 Annual amount (cm) of precipitation (black) compared to the 60-year mean precipitation (Royal Palm Station, dashed line) and annual amount of evapotranspiration (gray)
between 2004 and 2011 when the retention basins were fully operational. Regression analysis of annual inputs revealed a relatively steady decline in P over the study period (R2 =0.33, p=0.02). Annual ET losses represented 55–96 % of the annual P inputs and ranged between 92 and 136 cm y−1 (Table 2; Fig. 4), with the largest annual losses between 2002 and 2008. Daily ET averaged 0.31 cm with values ranging between 0.10 and 0.60 cm d−1 over 96 % of the study period. A one-way ANOVA indicated that annual ET losses were substantially greater between 2000–2002 and 2003–2011 as compared to 1997–1999 (p=0.005; Fig. 4), most of which was accounted for by a significant increase in wet season ET during this time (p
