From raised bogs (Large et al., 2007) and vernal pools (Schlising and Saunders, 1982; Rains et al., 2006) to fens (Wilcox et al., 1986) and Carolina bays (Lide et al., 1995), studies of GIWs have long characterized hydrologic control along a continuum of local forces, from precipitation to groundwater of a surficial aquifer. Few studies, however, document hydrologic control by groundwater of a regional aquifer. This study provides physical and chemical evidence that GIWs and waters in the sandhill of west-central Florida are hydraulically connected to the U Fldn—a large, regional water supply aquifer. This connection distinguishes the sandhill wetlands and waters of west-central Florida from most other GIWs and waters and places them at the far end of the hydrologic continuum (Fig. 10). These findings are important both to the field of wetland ecohydrology and to the proper identification, management and protection of these unique and vulnerable natural resources.
Figure 10 Sandhill wetlands as regional groundwater endmembers along a geographically isolated wetland hydrologic continuum
Characteristic Hydrology & Evidence of U-Fldn Connectivity
This study examines water levels and water geochemistry of sandhill wetlands and waters in west-central Florida. It documents their characteristic (and certain uncharacteristic) hydrologic attributes and compares them to those of the U Fldn as evidence (where applicable) of a hydraulic connection with the regional aquifer. The key physical attributes of sandhill wetland and water hydrology, as defined here, are widely ranging water levels (2–5 meters or more) that are synchronous with those of the U Fldn (Figs. 3a-b), deviating in consistent and predictable patterns (Figs. 4a-b), which result in very high correlations (84 ≤ R2 ≤ 99%) (Table 3). Within these attributes, sandhill wetlands and waters may vary markedly in their hydrologic expression—from small, shallow elliptical wetlands that remain dry for years (e.g., Croom Road Marsh, Figs. 1 and 5a) to deep circular ponds and large amorphous lakes that maintain permanent inundation (e.g., Capuchin Pond and Hunter’s Lake, Figs. 1 and 5b, respectively). Each of these features, though markedly different in expression, are exemplars of the sandhill type.
Findings here expand on a study by Henderson (1986), which shows similar behavior and correlations (80 ≤ R2 ≤ 87%) between a lake in this study (Hunter’s Lake) and nearby U Fldn monitor wells and between that lake and a wetland in this study (Weeki Wachee Prairie). Similarly widely ranging water levels and “sympathetic fluctuations” between potential sandhill waters elsewhere in the state were reported by others (Deevey, 1988), but these features are not hydraulically connected to the U Fldn, nor does it control their hydrology (Sacks et al., 1998; and Swancar et al, 2003).
Geochemical attributes are more variable than the physical attributes. Most features express a predominantly calcium-bicarbonate water type (or sodium-calcium/calcium-sodium bicarbonate) similar to the limestone water samples (Fig. 7, Tables 4a-b), but vary in their specific conductance, Ca2+ and/or pH. In many shallow features (e.g., wetlands), specific conductance, Ca2+ and/or pH are low and reflective of rainwater chemistry. In many deep features (e.g., lakes and ponds), these attributes are elevated, reflective of mineralized water in contact with limestone (Figs. 6a-b). In the remaining features (wetlands, ponds, and lakes included), the attributes are intermediate, suggesting a mix of the rainwater and limestone water types.
Some of the features with intermediate or elevated Ca2+ are proximal to sources of leachate (e.g., septic fields and fertilized lawns or agricultural areas irrigated with groundwater). It is plausible their higher values are artificially inflated by the leachate. Two sites provide evidence of a naturally high source of Ca2+. At one wetland (String of Pearls Marsh, SOP) located in the middle of a state forest outside the influence of cultural leachate, Ca2+ in the shallow groundwater is elevated (22 mg/L) in the wet season and intermediate (3.9 mg/L) in the dry season (Fig. 6b). With no artificial source of Ca2+, one may conclude its higher Ca2+ is due to mixing with water residing in limestone. At Hunter’s Lake (HL), which is located in a residential area with irrigated lawns and septic fields, Ca2+ values in the surface water are elevated (13–17 mg/L, dry and wet seasons) (Fig. 6b). Historical data show these values were comparably elevated in the 1980s (14–26 mg/L) following near build-out of the area, but were lower (6–7 mg/L) in the mid-1960s before the onset of development (Henderson, 1986). The intermediate values of the pre-development period may suggest the lake water is naturally high in Ca2+ but is made higher by the leachate. Historical data are not available for the other features with intermediate or elevated Ca2+, but it is possible they too have naturally higher Ca2+ due to their connection to water residing in limestone, and that water also may be enhanced by cultural leachate from adjacent residential areas.
The low specific conductance, Ca2+, and/or pH at the other wetlands and ponds (BP, CaPo, CrRi, CrRo, ESP, R4, R8) do not negate their connectivity to the U Fldn. These features simply do not intersect that part of the U Fldn that contains water residing in (or formerly in contact with) limestone. Water in the U Fldn is chemically stratified into an upper rainwater lens residing in the surficial sands, a lower body of Ca2+-rich water residing in the limestone, and a transitional, or mixing, zone in between (Fig. 11). The position of the mixing zone, which is controlled by the expansion and shrinking of the rainwater lens, and the depth of the features determine what type of water chemistry the features will have. Water in features that are not deep enough to intercept the mixing zone will maintain the chemistry of the rainwater lens. Water in features that are deep enough to intercept the mixing zone (or the limestone water itself) will reflect a mixed rainwater-limestone (or limestone water) chemistry.
Figure 11 Conceptual model of generalized sandhill wetland & water geochemistry
Uncharacteristic Hydrology & Unexpected Findings
Four features in this study are considered exceptional for their uncharacteristic or unexpected water level behavior. At Sand Point Pond, the elevation offset (relative to the ROMP 107 U Fldn monitor well) is markedly high (12.5 m) and the correlation markedly low (R2 = 0.43) (Figs. 3b and 5f). At Perry Oldenburg Marsh, the elevation offset (relative to the WR-6b Shallow U Fldn monitor well) is much less (0.12 m), but this value is artificially low considering the 3 m drop in head that occurs along the regional hydraulic gradient between the well and wetland (see contour elevations, Fig. 1). More importantly, the offset is highly variable over the POR, and the correlation is very low (R2 = 0.48) (Fig. 3b). Water levels at both wetlands appear to be perched, with no connectivity to the U Fldn. The presence of remnant clay along the Brooksville Ridge (where both wetlands are located), creates the opportunity for perching. For their lack of connectivity to or hydrologic control by the U Fldn, neither wetland would be considered of the sandhill type.
At Banshee Pond, wetland water levels were unexpectedly dichotomous relative to those of the U Fldn—synchronous during periods of high water, but not low water (Fig. 5e). Physical evidence of historical excavation is present on site and is believed to have deepened the wetland bottom into the limestone residuum, which is typically found at depth. This would have not only increased the hydroperiod of the wetland (from intermittent to semi-permanent), but also altered the way surface water drains from it. The now near-surface residuum, which has a clayey texture, is believed to perch surface water at a certain threshold elevation (at or around 13.5 m), disconnecting it from the U Fldn water table as it drains. As the water table rises above the threshold, it converges with the perched surface water and they reconnect. This is evident in the hydrograph where both the elevation offset and correlation coefficient shift markedly between the low and high water periods. This wetland would still be considered a sandhill wetland, but with a modified hydrology due to excavation. Sandhill wetlands/waters of Florida have historically been excavated to capture stormwater and to increase hydroperiods for aesthetic or agricultural purposes, increasing the possibility of scenarios like this at other locations of similar geology.
At Weeki Wachee Prairie, the elevation offset relative to the U Fldn monitor well (WWP) was unexpectedly high (0.5 m) (Fig. 5d). The offset is fairly consistent across the POR, and the water levels are highly correlated (R2 = 0.88), as is typical for sandhill wetlands; but considering the wetland and well share the same position along the regional hydraulic gradient, a lower offset was anticipated. Such is the case at Hunter’s Lake whose U Fldn monitor well (HUNT) also sits at the shoreline, and where the elevation offset is comparatively small (0.2 m). The study by Henderson (1986), which describes the close relationship between Hunter’s Lake and the U Fldn, may help explain this. Henderson describes both features as flow-through systems, where groundwater enters from one side and leaves from another. At Hunter’s Lake, the U Fldn well is located on the up-gradient shoreline where U Fldn water table contours are gradual and translate to a smaller offset; on the down-gradient shoreline, the contours are steeper and translate to a larger offset. At Weeki Wachee Prairie, the U Fldn well is (presumably) located on the down-gradient shoreline, where contours may be similarly steep. Detailed contours are not available to confirm this, but similar to Hunter’s Lake, Weeki Wachee Prairie is a large feature situated at the base of a parabolic dune train (Upchurch et al, 2018) and may be subject to similar local gradients. Only at Hunter’s Lake and Weeki Wachee Prairie are the U Fldn wells located at the shorelines. Monitor wells evaluated for all other features are located 1–10 km away because adjacent wells do not exist. Water levels at these more distant wells may better reflect the regional groundwater flow system because they are not subject to the local complexities and vertical gradients that are created by the surface water features themselves.
Of added intrigue, although neither characteristic nor uncharacteristic of sandhill wetlands, is a rarely noted phenomenon called the Lisse Effect, which appears to occur at the three smaller wetlands where organic material accrues above the sandy bottom (e.g., Ref 4, Ref 8, String of Pearls Marsh) (Nowicki et al, unpublished field data). The Lisse Effect occurs in response to intense rainfall, which inundates the wetland so rapidly it traps air beneath the soil wetting front. The trapped air builds up pressure, which artificially raises the head in the shallow monitor well. When the pressure is released, the water level in the monitor well equilibrates with the water table, reflecting the actual recharge that occurred from the rainfall event (Heliotis et al., 1987; Weeks, 2002). On a hydrograph, this would produce a zig-zag response in shallow groundwater levels as is shown in Fig. 12. This phenomenon is not well documented and may help explain some of the residual variation between water levels at these wetlands and the U Fldn.
Figure 12 Apparent Lisse effect in shallow groundwater levels at wetland Ref 4. The zig-zag pattern, represented by a spike and dip in the shallow groundwater levels may represent the rarely noted Lisse Effect. This occurs when intense rainfall seals the surface and builds up pressure in the monitor well. The pressure creates an artificial rise in head in the well, which equilibrates shortly after to reflect actual recharge (Heliotis et al., 1987; Weeks, 2002)
Differences in residual variation among features also may be related to factors inherent to them (e.g., size, shape, and depth) and their situation (e.g., antecedent conditions, landscape setting, adjacent land use/land cover, and rainfall intensity/duration). The effects of these factors may be greatest at: 1) the larger lakes and wetlands (e.g., Tooke Lake, Willow Sink) whose greater surface areas may contribute greater losses to evaporation and whose longer shorelines may contribute more opportunities for surface water-groundwater exchange (Wetzel, 2001; Lee et al., 1997); and 2) the seasonally inundated wetlands (e.g., Ref 4, Ref 8, String of Pearls Marsh), where water more frequently shifts between surface water and shallow groundwater phases and where accumulation of organic material may slow or speed losses to leakage or evapotranspiration, respectively. The relative simplicity of the ponds (i.e., smaller size, simpler shape, and single surface water phase) may explain why their residual variation is least among feature types. Similarly, the smaller size, simpler shape, and largely groundwater phase at Croom Road Marsh may explain its near perfect water level correlation with that of the WR-6b Shallow U Fldn monitor well. For this wetland, its close proximity to the monitor well (< 1 km) is likely a proxy for hydrogeologic similarity, which is believed to be among the most important factors contributing to the strong correlation between their water levels.