|PublicationsScientific Investigation Reports|
SIR 2005-5178—ONLINE ONLY
Prepared in cooperation with:
St. Johns River Water Management District
Florida Fish and Wildlife Conservation Commission
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Purpose and Scope
Environmental Setting and Hydrogeologic Framework
Hilochee Wildlife Management Area
Hydrogeology of Karstic Wetlands
Hydraulic Properties of the Surficial Aquifer System
Hydrologic Conditions, 2001-2003
Evapotranspiration and Recharge
Wetland Lateral Ground-Water Flow Conditions
Nearshore Water-Table Features
Anthropogenic Influences on Water Levels
Hydrologic Response of Wetlands
Wetland Storage and Estimated Ground-Water Exchange
Potential Influence of Nearshore Water-Table Features on Wetlands
Two internally drained karstic wetlands in central FloridaBoggy Marsh at the Hilochee Wildlife Management Area and a large unnamed wetland at the Lyonia Preservewere studied during 2001-03 to gain a better understanding of the net-recharge function that these wetlands provide, the significance of exchanges with ground water with regard to wetland water budgets, and the variability in wetland hydrologic response to a range of climate conditions. These natural, relatively remote and unaltered wetlands were selected to provide a baseline of natural wetland hydrologic variability to which anthropogenic influences on wetland hydrology could be compared. Large departures from normal rainfall during the study were fortuitous, and allowed monitoring of hydrologic processes over a wide range of climate conditions. Wetland responses varied greatly as a result of climate conditions that ranged from moderate drought to extremely moist. Anthropogenic activities influenced water levels at both study sites; however, because these activities were brief relative to the duration of the study, sufficient data were collected during unimpacted periods to allow for the following conclusions to be made.
Water budgets developed for Boggy Marsh and the Lyonia large wetland showed strong similarity between the flux terms of rainfall, evaporation, net change in storage, and the net ground-water exchange residual. Runoff was assumed to be negligible. Of the total annual flux at Boggy Marsh, rainfall accounted for 45 percent; evaporation accounted for 25 percent; net change in storage accounted for 25 percent; and the net residual accounted for 5 percent. At the Lyonia large wetland, rainfall accounted for 44 percent; evaporation accounted for 29 percent; net change in storage accounted for 21 percent; and the net residual accounted for 6 percent of the total annual flux.
Wetland storage and ground-water exchange were important when compared to the total water budget at both wetlands. Even though rainfall was far above average during the study, wetland evaporation volumetrically exceeded rainfall. Ground-water inflow was effective in partially offsetting the negative residual between rainfall and evaporation, thus adding to wetland storage. Ground-water inflow was most common at both wetlands when rainfall continued for days or weeks, or during a week with more than about 2.5 inches of rainfall. Large decreases in wetland storage were associated with large negative fluxes of evaporation and ground-water exchange. The response of wetland water levels to rainfall showed a strong and similar relation at both study sites; however, the greater variability in the relation of wetland water-level change to rainfall at higher rainfall rates indicated that hydrologic processes other than rainfall became more important in the response of the wetland.
Changes in wetland water levels seemed to be related more to vertical gradients than to lateral gradients. The largest wetland water-level rises were associated mostly with lower vertical gradients, when vertical head differences were below the 18-month average; however, at the Lyonia large wetland, extremely large lateral gradients toward the wetland during late June 2002 may have contributed to substantial gains in wetland water. During the remainder of the study, wetland water-level rises were associated mostly with decreasing vertical gradients and highly variable lateral gradients. Conversely, wetland water-level decreases were associated mostly with increasing vertical gradients and lateral gradients away from the wetland, particularly during the dry season.
The potential for lateral ground-water exchange with the wetlands varied substantially more than that for vertical exchange. Potential for vertical losses of wetland water to ground water was highest during a dry period from December 2001 to June 2002, during the wet season of 2002, and for several months into the following dry season. Lateral head-gradients toward the wetland were relatively short-lived at the large wetland, generally lasting a week; at Boggy Marsh, these conditions generally lasted for at least 2 weeks. Lateral gains in wetland water from ground water were more common during the wet season, whereas lateral losses to ground water were more common during the dry season.
The relative importance of nearshore water-table features (mounds and troughs) in influencing ground-water exchange with the wetland was determined by computing the daily net volume of water contained in all features simultaneously present at each wetland. Nearshore water-table features were larger at Boggy Marsh than at the large wetland. The daily net volume contained in water-table features accounted for 120 percent of the daily net change in storage at Boggy Marsh, whereas this volume accounted for only 5 percent of the daily net change in storage at the large wetland. The largest net-volume proportions at Boggy Marsh were associated with small daily fluxes of storage change and ground-water exchange.
Natural variations in water-level responses at the two sites were a result of geologic heterogeneity of the surficial sediments, as indicated by test drilling and ground-penetrating radar and by the varying vertical gradients in the surficial aquifer system with respect to the underlying Upper Floridan aquifer and varying lateral water-table gradients surrounding the wetlands. Surficial, sandy sediments of Holocene to Pliocene age are 60 to greater than 100 feet thick at Hilochee, and 70 to greater than 95 feet thick at Lyonia. Sandy sediments contain interbedded clay layers that in places can create separate water-bearing units in the surficial sediments. Clayey sediments generally are more prevalent at Hilochee than at Lyonia. Hydraulic conductivity of the surficial sediments, based on slug tests, is similar at both sites, ranging from 0.2 to 20 feet per day (ft/d) at Hilochee and from 0.5 to 10 ft/d at Lyonia. Median hydraulic conductivity (4 to 5 ft/d) was nearly identical for both sites.
The Lyonia large wetland and Boggy Marsh exhibited substantially different lateral ground-water flow characteristics, both annually and seasonally. The large wetland lost water to ground water for 62 percent of the year and for nearly the entire dry season; whereas Boggy Marsh lost water to ground water for only about 1 percent of the year and only during the wet season. Ground water flowed laterally to the large wetland for 24 percent annually and only during the wet season; ground water flowed laterally to Boggy Marsh for 13 percent of the wet season and 7 percent of the dry season. Boggy Marsh was in a flow-through condition, indicating that the wetland was gaining and losing water through ground-water exchange along its shoreline, for 91 percent annually and changed only slightly from the dry to wet season. In contrast, the large wetland was in a flow-through condition for 14 percent annually, mostly during the wet season.
U.S. Department of the Interior
U.S. Geological Survey
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Altamonte Springs, FL 32714
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