J Paleolimnol (2006) 36:371–383 DOI 10.1007/s10933-006-9008-7
ORIGINAL PAPER
Geochemical and biological consequences of groundwater augmentation in lakes of west-central Florida (USA) Mark Brenner Æ Thomas J. Whitmore Æ Melanie A. Riedinger-Whitmore Æ Brandy DeArmond Æ Douglas A. Leeper Æ William F. Kenney Æ Jason H. Curtis Æ Byron Shumate Received: 21 May 2005 / Accepted: 6 March 2006 / Published online: 14 September 2006 Springer Science+Business Media B.V. 2006
Abstract We studied sediment cores from four Florida (USA) lakes that have received groundwater hydrologic supplements (augmentation) for >30 years to maintain lake stage. Top samples (0–4 cm) from sediment cores taken in Lakes Charles, Saddleback, Little Hobbs, and Crystal had 226Ra activities of 44.9, 17.5, 7.6, and 8.5 dpm g)1, respectively, about an order of magnitude greater than values in deeper, older deposits. The surface sample from Lake Charles yielded the highest 226Ra activity yet reported
M. Brenner (&) Æ B. DeArmond Æ W. F. Kenney Æ J. H. Curtis Æ B. Shumate Department of Geological Sciences and Land Use and Environmental Change Institute, University of Florida, Gainesville, FL 32611, USA e-mail:
[email protected] T. J. Whitmore Æ M. A. Riedinger-Whitmore Environmental Science, Policy, and Geography, University of South Florida, St. Petersburg, FL 33701, USA D. A. Leeper Southwest Florida Water Management District, 2379 Broad Street, Brooksville, FL 34604, USA B. DeArmond ARCADIS, 3903 Northdale Blvd Suite 120W, Tampa, FL 33624, USA B. Shumate 8739 Susquehanna St., Lorton, VA 22079, USA
from a Florida lake core. Several lines of evidence suggest that groundwater augmentation is responsible for the high 226Ra activities in recent sediments: (1) 226Ra activity in cores increased recently, (2) the Charles, Crystal, and Saddleback cores display 226Ra/210Pb disequilibrium at several shallow depths, suggesting 226Ra entered the lakes in dissolved form, (3) cores show recent increases in Ca, which, like 226Ra, is abundant in augmentation groundwater, and (4) greater Sr concentrations are associated with higher 226Ra activities in recent Charles and Saddleback sediments. Sr concentrations in Eocene limestones of the deep Floridan Aquifer are high relative to Sr concentrations in surficial quartz sands around the lakes. Historical water quality inferences for the lakes were based on diatom assemblages in sediments. Recent alkalization in Lakes Charles, Saddleback, Little Hobbs, and Crystal was inferred from weighted-averaging calibration (WACALIB). The lakes also show recent trophic state increases based on WACALIB-derived estimates for limnetic total P. Although residential and agricultural sources might contribute to increased P loading, P in augmentation waters probably has had significant influence on eutrophication. Dystrophic diatoms were abundant in the early history of Lakes Saddleback, Little Hobbs, and Crystal, which suggests that these lakes contained more tannic waters during the past than at present, perhaps as a consequence of
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greater inflows from surrounding wetlands. Ionic content of lake waters increased, as indicated by diatom autecological analysis. Recent geochemical and biological changes detected in cores from these lakes probably are a result of deliberate groundwater augmentation, although inputs of groundwater pumped for agricultural and residential development in the watersheds also might have contributed to limnological changes. Keywords Diatoms Æ Florida Æ Groundwater Æ Radium-226 Æ Sediment Æ Trophic state Æ Water quality
Introduction Large amounts of groundwater have been withdrawn from the Floridan Aquifer at wellfields in Hillsborough County, Florida, USA (Fig. 1). Removal of deep groundwater increased downward seepage of both shallow groundwater and surface water, causing local lake-level declines. Lake stage decreases were exacerbated by droughts, stormwater diversion, road construction, and residential development (Stewart and Hughes 1974). Some Hillsborough County lake levels have been maintained by
Florida 0 50 100
km N
Study Lakes
Hillsborough County Fig. 1 Map of Florida (USA) and inset map of Hillsborough County showing the approximate location of groundwater augmented study Lakes Charles, Saddleback, Little Hobbs, and Crystal
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pumping deep groundwater from lakeside wells directly into the water bodies. Several lakes began to receive groundwater augmentation in the 1960s. In the 1970s, a few studies evaluated the effects of augmentation on lake hydrologic budgets and water chemistry (Stewart and Hughes 1974; Martin et al. 1976a; Dooris and Martin 1979). Other investigations explored the biological consequences of augmentation (Dooris et al. 1982; Martin et al. 1976b). Groundwater inputs altered the chemistry of lake waters (Martin et al. 1976a; Dooris and Martin 1979). Prior to augmentation, most area lakes were ‘‘soft’’ and dominated by ) sodium (Na+), sulfate (SO2) 4 ), and chloride (Cl ). Augmentation with deep groundwater converted water bodies into calcium (Ca2+) bicarbonate (HCO)3 ) systems. Augmented lakes have water column ion concentrations with relative proportions similar to groundwater, and display high hardness, bicarbonate concentration, and pH (Martin et al. 1976a). Phytoplankton diversity is greater in augmented lakes and positively correlated with water-column inorganic carbon (IC) concentration (Dooris et al. 1982). Martin et al. (1976b) suggested that groundwater augmentation might promote growth of exotic Hydrilla verticillata. Previous paleolimnological study of groundwater-augmented Round Lake (A=5 ha, zmean=2.4 m) near Tampa, Florida indicated that augmentation, which began in 1966, increased 226 Ra input to the water body (Brenner et al. 2000). The lake receives about half its annual hydrologic budget from pumped groundwater. Surface sediments (0–4 cm) from two Round Lake cores had 226Ra activities of 26.9–1.0 and 26.8–0.3 dpm g)1 dry (Brenner et al. 2000), the highest 226Ra activities that had been measured in Florida lake sediments (Brenner et al. 1994, 1997). These high activities were attributed to inputs of groundwater that is rich in dissolved 226 Ra, which adsorbs to near-surface sediments (Brenner et al. 2004). Augmentation waters also showed higher concentrations of total P and specific conductivity than in Round Lake, and diatom analyses from a core suggested that groundwater augmentation contributed to higher dissolved ion concentrations, slightly increased
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pH, and higher trophic state conditions (Brenner and Whitmore 1999). Radium-226 activity in topmost Round Lake sediment samples exceeded total 210Pb activity, indicating disequilibrium between 226Ra and supported 210Pb. This phenomenon had been reported for only one other Florida waterbody, Lake Rowell (Brenner et al. 1994). Isotopic disequilibrium in Round Lake was linked to augmentation groundwater that passed through 238 U-rich carbonate–fluorapatite deposits in the underlying bedrock (Kaufmann and Bliss 1977; Upchurch and Randazzo 1997). The upper Floridan Aquifer can have 226Ra activities that exceed the drinking water standard of 11 dpm l)1 (Kaufmann and Bliss 1977). Coastal surface waters off west central Florida that receive groundwater inputs display high 226Ra activities (Fanning et al. 1982; Miller et al. 1990). High 210 Po activities have been measured in Florida groundwater (Harada et al. 1989) and some of the highest values have been measured in Hillsborough County (Upchurch and Randazzo 1997). The Round Lake study also showed that 226Ra had accumulated in the lake’s flora and fauna (Brenner et al. 2000). Divalent 226Ra apparently substitutes for calcium in plant tissues and the shells, bones, and flesh of animals. Accumulation of 226Ra in sediments and food webs of groundwater-augmented Florida lakes may prove to be common. Likewise, groundwater pumping for other purposes such as agricultural irrigation, residential, and industrial uses, may also contribute 226Ra to Florida’s aquatic ecosystems (Brenner et al. 2004). Several factors determine dissolved 226Ra concentrations in Florida lake waters and the amount of 226Ra adsorbed to recent sediments: (1) 226Ra activity in pumped groundwater, which is influenced by local geology, (2) rate of groundwater pumping, (3) proportional contribution of groundwater to the lake’s annual hydrologic budget, (4) lake water residence time, and (5) mixing of augmentation water throughout the lake. Sediment composition, particle size, deposition rate, and the through-flow of 226Ra-rich water may also influence the stratigraphic distribution of 226 Ra activity in sediments. For instance, DeArmond et al. (in press) found a positive correlation between 226Ra activity and organic matter content
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in a suite of surface deposits from groundwateraugmented Lake Charles. Previous studies showed that groundwater in Hillsborough County can contain high levels of dissolved 226Ra. Groundwater augmentation of local lakes was also shown to have ecological impacts, including the accumulation of 226Ra in sediments and aquatic food webs, and the alteration of diatom communities. Preliminary findings also suggested potential human health risks. Analysis of the data from the Round Lake study indicated that regular consumption of unionid mussels from that basin would likely raise cancer mortality and morbidity risk above the acceptable range (Hazardous Substance and Waste Management Research, Inc. 2000). Cancer risk might also increase if augmented lakes were to desiccate and sediments became exposed, thereby increasing the probability of ingestion, inhalation, and external exposure to alpha emitters (Hazardous Substance and Waste Management Research, Inc. 2004). Given the potential environmental effects of groundwater augmentation, we decided to analyze sediment cores from four groundwateraugmented lakes in Hillsborough County, Florida, USA (Charles, Saddleback, Little Hobbs [Lutz], Crystal) (Fig. 1). Our objective was to assess the geochemical and biological effects of long-term augmentation. Lakes Charles and Saddleback began receiving groundwater supplements in summer 1968. Little Hobbs was first augmented in the early 1970s, and augmentation at Crystal began in fall 1973. Recent analyses by the Florida Department of Health showed that 226 Ra enters all four basins with groundwater input. Triplicate samples run on augmentation water at Lakes Charles, Saddleback, Little Hobbs, and Crystal yielded mean 226Ra concentrations of 3.11, 3.26, 0.82, and 1.41 dpm l)1, respectively. We evaluated stratigraphic changes of organic matter, phosphorus, radionuclides (210Pb, 226Ra, 137 Cs), cations (Ca, Mg, Sr), and diatom assemblages in the sediment cores from the lakes.
Methods Sediment cores were collected in 2003 from Lakes Charles, Saddleback, Little Hobbs, and Crystal
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using a sediment–water interface piston corer (Fisher et al. 1992). Cores were taken in areas with substantial sediment accumulation and minimal sand content. They were sectioned in the field at 4-cm intervals in a vertical position. Sediments were extruded into a tray and samples were placed in weighed, labeled containers. Wet subsamples (~1 cm3) for diatom analysis were removed from each stratigraphic section and stored in labeled scintillation vials. In the laboratory, remaining wet sediment was weighed, dried, re-weighed and ground for radionuclide, nutrient, and cation analyses. Total 210Pb, 226Ra, and 137Cs activities were measured in contiguous samples from the mud/ water interface to depths where total 210Pb and 226 Ra activities were low and similar to one another. Tared SarstedtTM tubes were filled with dry, ground sediment to a height of ~30 mm. Sample mass was determined and tubes were sealed with epoxy glue and set aside for three weeks to allow 214Bi and 214Pb to equilibrate with in situ 226Ra. Radioisotope activities were measured by gamma counting, using ORTECTM Intrinsic Germanium Detectors connected to a 4096 channel, multi-channel analyzer (Schelske et al. 1994). Total 210Pb activity was obtained from the photopeak at 46.5 keV. Radium-226 activity was estimated by averaging activities of 214 Pb (295.1 keV), 214Pb (351.9 keV), and 214Bi (609.3 keV) (Moore 1984). The 137Cs activity was determined from the 662 keV photopeak (Krishnaswami and Lal 1978). Activities are expressed as decays per minute per gram dry sediment (dpm g)1). Organic matter in dry sediments was measured by weight loss on ignition at 550 C for 1 h (Ha˚kanson and Jansson 1983). Total phosphorus (P) was measured using a Technicon Autoanalyzer II with a single-channel colorimeter, following sediment digestion with H2SO4 and K2S2O8 (Schelske et al. 1986). Calcium (Ca), magnesium (Mg), and strontium (Sr) concentrations in sediments were measured following digestion of dry, weighed sediment samples in 1 N HCl (Andersen 1976). Concentrations in solution were read on a Perkin Elmer 3100 Atomic Absorption Spectrophotometer. Data are expressed as amount per g dry sediment.
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Diatom samples were digested using hydrogen peroxide and potassium dichromate (Van der Werff 1955), then settled, aspirated carefully using a micropipette with an inverted tip, and rinsed until the dichromate color disappeared. Supernatants were dried on coverslips, and samples were mounted in Naphrax mounting medium on glass slides. At least 500 valves were counted in samples with adequate diatom abundance and preservation. Several samples demonstrated substantial breakage and scarcity of valves. In such cases, counting was limited to several hundred individuals, which revealed the dominant taxa despite information loss. Diatoms were identified to the lowest possible taxonomic level using phasecontrast microscopy at 1,500· magnification. Past limnetic total P concentrations and limnetic pH values were inferred for sediment core samples using diatom data and weighted-averaging tolerance calibration methods (WACALIB: Line et al. 1994). The WACALIB model used to infer past limnetic total P has been applied in previous Florida paleolimnological studies (e.g., Riedinger-Whitmore et al. 2005). Past limnetic pH values were inferred using a calibration set of 74 Florida lakes (r2 adj.=0.83, s.e. pred. = 0.519). Ionic concentration trends over time were examined using summary ecological data for diatom salinity preferences in sediment core samples. Past specific conductivity was not inferred by WACALIB because exploratory analyses suggested that specific conductivity inferences might be statistically influenced by variation in pH. Summary ecological data were obtained by merging the data set containing diatom counts with an autecological database compiled from multiple sources (e.g., Hustedt 1930; Lowe 1974; Patrick and Reimer 1966–1975; Whitmore 1989; Van Dam et al. 1994; Krammer and Lange-Bertalot 1986–1991). Ecological preferences along the salinity gradient were described using the terminology of Gasse et al. (1987) as modified by Reed (1998). Halobion classifications from diatom floras and from ecological compilations (e.g., Lowe 1974; Van Dam et al. 1994) were converted to saline ecological classifications using the approach outlined in Whitmore et al. (1996). Percentages of diatoms in salinity ecological
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categories were summed for each sample. Percentages of taxa that spanned more than one ecological category were divided equally among those ecological categories.
Results Geochemistry Cores from the groundwater-augmented lakes all display upcore increases in 226Ra activity (Fig. 2). Surface sediments (0–4 cm) possess about an order of magnitude more 226Ra activity than do deeper sediments. Topmost samples range in 226 Ra activity from 7.6 dpm g)1 in Little Hobbs Lake to 44.9 dpm g)1 in Lake Charles. Surface sediments exceed 5 dpm g)1, a value suggested by Brenner et al. (2004) as indicative of anthropogenic 226Ra enrichment. Stratigraphic distribution of 226Ra in the cores indicates that enrichment occurred recently. Several depths in the Lake Charles core possess 226Ra activities >30 dpm g)1 and are the highest values yet reported for a Florida lake sediment core. Isotopic disequilibrium (226Ra/210Pb) is apparent at depths in the Charles, Saddleback and Crystal cores where 226Ra activity exceeds total 210Pb activity. This suggests the lakes received dissolved 226Ra that separated from its parent isotopes (e.g., 238U) and did not yet equilibrate with its daughters (e.g., 210Pb). Disequilibrium is most obvious in shallow deposits of the Charles and Saddleback cores where 226Ra> total 210Pb. In these recent sediments, 226Ra activity exceeds the combined activities of supported and unsupported (excess) 210 Pb, indicating the presence of significant 226Ra without its daughters. Radium-226 stratigraphy in each core was compared with the stratigraphic distribution of other geochemical variables (organic matter [OM], calcium [Ca], magnesium [Mg], and total phosphorus [P]) to explore the mode of radium delivery to the lakes. Previous studies of Florida lake cores showed a strong correlation between sediment total P and 226Ra, suggesting that both were delivered by a common mechanism. It was
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proposed that colluvial or aeolian processes had delivered phosphate-rich, Ra-containing particles to the lakes (Brenner et al. 1994, 1997). Study of groundwater-augmented Round Lake, however, showed a strong correlation between 226Ra activity and both inorganic (carbonate) carbon and Ca in sediment cores. This suggested that bicarbonate-rich groundwater was the source of 226 Ra entering Round Lake (Brenner et al. 2000). High levels of 226Ra in Round Lake biota also pointed to a dissolved source of the radionuclide. Sediment cores from Lakes Charles, Saddleback, Little Hobbs, and Crystal show a strong stratigraphic correlation between 226Ra and Ca concentrations (Table 1; Figs. 2, 3). Among lakes, the correlation is weakest in Lake Charles (r=0.82), but is nonetheless strong (p < 0.001) (Table 1). The Lake Charles core does not display a significant correlation between 226Ra and total P over its entire length, but 226Ra and total P are highly correlated in the top 52 cm (r=0.68, p < 0.01), where 226Ra activity increases abruptly (Fig. 2). Radium-226 activity and OM content are positively correlated throughout the Charles core (Table 1) and this correlation is stronger (r=0.89, p < 0.001) when only the topmost sediments (0–52 cm) are considered. In the Crystal Lake core, the only variable significantly correlated with 226Ra activity was Ca (r=0.94, p < 0.001) (Table 1). In the Little Hobbs core, Ca, Mg, P, and OM were positively correlated with 226Ra at p < 0.01 or p < 0.001 (Table 1). The Saddleback core displays a very strong positive correlation between Ca and 226Ra (r=0.99, p < 0.001), as well as between Mg and 226Ra (r=0.95, p < 0.001) (Table 1). The correlation between 226Ra and P is weaker, but significant (r=0.69, p < 0.05). Radium-226 activity in the Saddleback core shows a strong negative correlation with organic matter. Strontium was measurable in the Charles and Saddleback sediments. In the Saddleback deposits, Sr concentrations were highest in near-surface sediments and highly correlated with 226Ra (r=0.98, p < 0.001). In the Lake Charles core, highest Sr concentrations were associated with recent sediments, with the exception of a single peak at 104–108 cm. Excluding this sample, Sr concentrations and
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0
Lake Charles Core
Saddleback Lake Core
Activity (dpm/g)
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Crystal Lake Core
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Pb-210
30 35
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40 Cs-137
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Cs-137 45 50
Fig. 2 Radionuclide (210Pb, 226Ra, 137Cs) activities (dpm g)1) in sediment cores from groundwater augmented study Lakes Charles, Saddleback, Little Hobbs, and Crystal 226
Ra activities are strongly correlated (r=0.68, p < 0.001). Attempts to date the cores using gamma counting proved problematic. None of the cores showed a distinct 137Cs peak (Fig. 2). The 1963, bomb fallout peak is commonly blurred in Florida lake sediments that lack 2:1 layer clays and bind
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cesium poorly (Brenner et al. 2004). Soluble 137Cs is prone to transport within sediments and probably moves downward through the sediment lens in these hydrologically ‘‘leaky’’ lakes. High and variable 226Ra activities in the cores from Lakes Charles, Saddleback, Little Hobbs, and Crystal create difficulties for 210Pb dating. Disequilibrium
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Table 1 Stratigraphic correlation between 226Ra activity and concentrations of calcium (Ca), magnesium (Mg), total phosphorus (P), and organic matter (OM) in sediment cores from Lakes Charles, Saddleback, Little Hobbs, and Crystal
Charles (226Ra) Saddleback (226Ra) Little Hobbs (226Ra) Crystal (226Ra)
Ca
Mg
P
OM (%LOI)
0.82*** 0.99*** 0.94*** 0.94***
0.03ns 0.95*** 0.68*** 0.44ns
0.03ns 0.69* 0.89*** 0.03ns
0.61*** )0.96*** 0.56** )0.08ns
Values are correlation coefficients (r) *p < 0.05, **p < 0.01, ***p < 0.001, ns = not significant (p>0.05). n values for Charles = 29 (Ca, Mg) and 26 (P, OM), Crystal = 12, Little Hobbs = 22, Saddleback = 10
between 226Ra and supported 210Pb makes it impossible to estimate unsupported 210Pb activity accurately and confounds dating (Brenner et al. 2004). The large amount of dissolved 226Ra introduced into the lakes will equilibrate with (supported) 210Pb only after ~110 years, i.e. about five half-lives (22.3 yr) of 210Pb. Deeper sediments in the cores, where 226Ra and 210Pb activities are low and similar, are probably >100 years old, as they apparently possess no unsupported 210 Pb. In all four cores, it appears that more than a century of accumulated sediment was retrieved (Fig. 2). Diatoms Diatom assemblages in all four lakes display relative increases in eutrophic and alkaline indicators and corresponding decreases in oligotrophic and acidic taxa proceeding upward in the cores. Dystrophic diatoms, characteristic of tannic lake waters, are abundant at the base of the cores from Lakes Saddleback, Little Hobbs, and Crystal. Observed diatom taxa that indicate dystrophic or oligotrophic to dystrophic waters include Eunotia carolina Patr., E. vanheurckii Patr., Frustulia rhomboides (Erh.) DeT., F. rhomboides var. capitata (Mayer) Patr., and Neidium ladogense var. densestriatum (Østr.) Foged. Dystrophic diatom percentages decline toward the tops of the sediment cores as diatoms that indicate clearer waters and higher productivity become abundant. In the 0–4 cm interval of the Lake Saddleback core, 63.7% of the assemblage is represented by Staurosira construens var. pumila (Grun.) W. & R., a hypereutrophic and circumneutral indicator in Florida (Whitmore 1989). Staurosira construens (Ehr.), an alkaliphilous and generally eutrophic
indicator in Florida, is abundant between the 28 and 4 cm levels. Frustulia rhomboides and Pinnularia legumen (Ehr.) Ehr. dominate in the 28–32-cm interval, indicating more oligotrophic and acidic conditions. In the Lake Charles core, Staurosira construens is 50% of the assemblage in the 0–4-cm interval, but this species is less abundant in deeper sediments that are dominated instead by Cyclotella stelligera (Cl. & Gr.) V.H. and Nitzschia denticula Grun., which indicate less productive conditions. In the Crystal Lake core, top samples are dominated by Cyclotella stelligera, which occurs in circumneutral, oligotrophic to eutrophic waters in Florida (Whitmore 1989), whereas assemblages near the base of the core are characterized by Frustulia rhomboides, Eunotia pectinalis var. minor (Kutz.) Rab., and Eunotia carolina. Proceeding downwards in the Little Hobbs Lake core, Staurosira contruens var. venter (Grun.) W. & R. and Cyclotella stelligera are replaced by oligotrophic to dystrophic indicators that include Frustrulia rhomboides and F. rhomboides var. capitata. All four lakes show evidence of recent alkalization based on WACALIB-derived pH inferences (Table 2). Modern pH is underestimated slightly in Lakes Charles, Saddleback, and Little Hobbs (Table 3), though distinct trends towards alkalization nevertheless are apparent. WACALIB-derived limnetic total P inferences also suggest that trophic state increased in all four lakes during recent decades (Table 2). Recent WACALIB inferences for limnetic total P closely approximate modern measured values in Lakes Charles, Saddleback, and Crystal (Table 3). Diatom preservation was poor in samples from the lower portion of the Lake Saddleback core
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Fig. 3 Calcium concentration (mg g)1) in sediment cores from groundwater augmented study Lakes Charles, Saddleback, Little Hobbs, and Crystal
and in several samples from the Crystal Lake core, so quantitative estimates of past water quality for these samples are tentative. Salinity preferences of diatoms in the sediment cores are shown in Fig. 4. With respect to dissolved solute concentrations, freshwater diatoms indicate low salinity (0–0:5 ppt), oligosaline individuals demonstrate higher salinity tolerance
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(0.5–6 ppt), and mesosaline individuals tolerate moderate salinity (6–23 ppt). Proceeding upward in the sediment cores from all four lakes, diatom salinity preferences show a progressive decline in freshwater taxa and an increase in oligosaline individuals, indicating a shift to higher dissolved ion concentrations in the lake waters. In Lake Charles, in contrast, mesosaline diatoms show
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Table 2 Limnetic total P and pH inferences for sediment core samples based on sedimented diatom assemblages
Table 3 Modern reported values for limnetic total P and pH in the study lakes
Lake
Lake
Total P (lg l)1)
pH
37L9 25L8 12L9 18L7 17L9 15.5L3 18L9 32.3L3
8.4L9
Charles
Saddleback
Little Hobbs
Crystal
Depth in sediment (cm) 0–4 4–8 8–12 12–16 16–20 20–24 24–28 28–32 32–36 36–40 60–64 80–84c 0–4 4–8 8–12 12–16a 16–20b 20–24b 24–28b 28–32a,c 0–4 4–8 8–12 12–16 20–24 28–32 44–48 56–60 0–4c 4–8 8–12 12–16c 16–20b 20–24b 24–28 28–32c
Inferred )1
Total P (lg l )
pH
Charles
37.11 38.20 41.46 41.42 35.26 37.82 27.02 17.30 14.32 18.97 16.98 11.01 12.25 33.88 47.03 13.11 5.45 5.43 5.58 4.90 29.96 28.90 29.84 28.47 22.73 19.86 10.66 3.65 20.94 19.86 19.58 11.25 3.27 3.79 2.72 2.91
7.86 7.87 7.83 7.84 7.85 7.83 7.76 7.05 6.92 7.44 7.21 5.84 7.31 7.88 7.96 6.55 6.39 5.73 5.89 5.23 7.78 7.74 7.77 7.71 7.51 7.03 6.02 4.97 7.19 7.41 7.30 5.82 5.06 5.22 4.93 4.98
Saddleback
a
Excessive breakage compromised statistical inferences
b
Excessive breakage might have compromised statistical inferences c
Breakage and scarcity limited counts to < 500 valves
highest percentages between the 12 and 40 cm levels, which suggests a period of highest salinity at intermediate depths in the sediment core.
Discussion Sediment cores from Lakes Charles, Saddleback, Little Hobbs, and Crystal all show recent
Little Hobbs Crystal L9
8.1L9 8.1L9 7.0L9
Florida Lakewatch (1999)
Hillsborough County Water Resources Atlas presentation of L7Florida Lakewatch data for 1997; L8Florida Lakewatch data for 1998; L3Florida Lakewatch data for 2003
enrichment with 226Ra. Radium-226 activities in surface sediments are about an order of magnitude greater than ‘‘baseline’’ activities in the deepest levels of the cores. Topmost (0–4 cm) samples in all four cores had 226Ra activities >5 dpm g)1, and Lakes Saddleback and Charles displayed values of 17.5 and 44.9 dpm g)1, respectively. The Lake Charles surface sample yielded the highest activity yet measured in a Florida lake core. These 226Ra values are much greater than those measured in lakes and wetlands that receive most of their hydrologic input from surface waters (Brenner et al. 1999, 2001, 2004). High activities were measured previously in surface deposits of groundwater-augmented Round Lake (26.9 and 26.8 dpm g)1) (Brenner et al. 2000), and in topmost sediments from Lake Rowell (22.6 dpm g)1) (Brenner et al. 1994). Lake Rowell is not deliberately augmented, but we now suspect that it, too, may receive substantial groundwater inputs from water pumped for mining and domestic use. Pumped Floridan Aquifer water is probably the source of much 226Ra activity in sediment cores from the study lakes. Several lines of evidence implicate augmentation water. First, the 210 Pb and 226Ra stratigraphies show that the increase in 226Ra activity occurred recently, probably within the last few decades. Second, deep groundwaters in the region contain appreciable 226 Ra. Augmentation water entering the study lakes ranges from 0.82 dpm l)1 (Little Hobbs) to 3.26 dpm l)1 (Saddleback). Third, there is
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and Sacks 2002), hence their need for groundwater supplements. This downward transport of 226 Ra may also explain why the initial rise of 226 Ra activity in cores may appear in sediments deposited prior to the onset of groundwater augmentation. Diatoms indicate that recent alkalization occurred in all four of the study lakes. Lake waters apparently changed from pH values in the range of 5–5.8 to pH values in the range of 7.2–8. Ionic content also increased in all four study lakes, as indicated by diatom salinity autecological data. Both of these changes are consistent with what might be expected from the addition of groundwaters that are high in base cation content. Trophic state appears to have increased in all four lakes, as demonstrated by WACALIB-derived estimates for limnetic total P. In a similar study of nearby Round Lake (Brenner and Whitmore 1999), augmentation groundwater had higher total P content (60 lg l)1) and specific conductance (495 lS cm)1) than lake waters (8.2 lg l)1 limnetic total P and 253 lS cm)1), which led to an increase in trophic state along with alkalization. Although it is difficult to assess the relative contribution of groundwater augmentation, agriculture, and residential development to trophic-state changes in Lakes Charles, Saddleback, Little Hobbs, and Crystal, it is probable that augmentation with nutrient-rich groundwater was a significant factor in eutrophication. Dystrophic diatoms were abundant at the base of the cores from Lakes Saddleback, Little Hobbs, and Crystal. These three lakes appear to have contained waters that were relatively high in humic content prior to the period of augmentation. Modern humic content of waters in these lakes is relatively low. Crystal Lake currently has a color value of 47 Pt–Co units, Saddleback has a color value of 12 Pt–Co units, and Little Hobbs has a color value of only 4 Pt–Co units. The observed declines in dystrophic diatoms are probably, in part, a direct result of augmentation with clear groundwater. Additionally, alteration of wetlands that surrounded the lakes also might have reduced humic inputs to the waterbodies over time. As agricultural and residential development occurred in the watersheds of these lakes,
Ra/210Pb disequilibrium at depths in the Charles, Crystal, and Saddleback cores. This suggests that 226Ra enters the lakes in dissolved form, separated from its precursors and daughters. Fourth, all four cores show a strong stratigraphic correlation between 226Ra and Ca, and both elements are recent additions to the sediment. Groundwater is rich in calcium bicarbonate and was added to the lakes in substantial quantity only with the onset of augmentation. Fifth, greater Sr in the recent deposits of Charles and Saddleback lakes also indicates groundwater input. Strontium concentrations in limestone are high compared to concentrations in the surficial quartz sands that surround the lakes. Although augmentation water is the probable source of 226 Ra that reaches the lake sediments, recent increases in 226Ra have also been detected in cores from Florida lakes that do not receive deliberate augmentation (Brenner et al. 2004). Groundwater input may also be the source for 226 Ra in these lakes, but may be pumped to the surface for other reasons, including crop irrigation, residential, mining, and industrial uses, after which it runs off into local water bodies. In most Florida lake sediment cores that display an upcore increase in 226Ra activity, the change is rather continuous over the uppermost deposits, reaching highest activities in surface or near-surface layers. There are several possible explanations for this smooth trend. First, since the onset of groundwater input, there may have been a steady increase in the amount of groundwater reaching the lakes. This may be true for Florida lakes with watersheds that have been subject to increasing development over the last century, but it is an unlikely explanation for this trend in groundwater-augmented systems. The latter group of lakes has received groundwater supplements at varying rates over the years, depending on rainfall amount and the need for augmentation. The pattern of 226Ra distribution in the sediments of groundwater-augmented lakes may reflect, in part, downward transport and adsorption of dissolved 226Ra on the sediment lens, with most 226Ra adhering to near-surface deposits. Florida lakes can lose a substantial amount of water to downward leakage (Deevey 1988) and augmented lakes are especially ‘‘leaky’’ (Metz
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Fig. 4 Percentages of diatoms in salinity preference categories vs. depth in sediment cores from study Lakes Charles, Saddleback, Little Hobbs, and Crystal. The
bottom two samples in Lakes Charles and in Little Hobbs are not contiguous (see Table 2), so shading at the base of these figures reflects broader-interval sampling
nearby wetlands may have been filled in, and their hydrologic influence on the lakes was diminished. Sediment cores from Lakes Charles, Saddleback, Little Hobbs, and Crystal display geochemical changes in their uppermost deposits. Cores from all four lakes show upcore increases in 226 Ra activity, Ca, and Mg content. The Charles
and Saddleback cores display a general increase in Sr concentration. Diatom evidence from the cores suggests that the lakes have experienced increases in limnetic nutrient concentrations, pH, and ionic content. The recent geochemical and biological changes in the sediment records are consistent with shifts that might be expected as a
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consequence of deliberate groundwater augmentation. Inadvertent addition of deep groundwater associated with past agriculture and residential development in the watersheds also might have contributed to changes that are evident in the sediment record. Acknowledgements Funding was provided by the Coastal Rivers, Hillsborough River, and Northwest Hillsborough Basin Boards of the Southwest Florida Water Management District. Interpretations and conclusions are not necessarily those of the supporting agency. Adam Munson and Jaime Escobar assisted with core collection. Cory Catts, Luciana Spatazza, and Nia Thommi helped with laboratory analyses. Atte Korhola and an anonymous reviewer provided helpful comments on the manuscript. This is a publication of the University of Florida Land Use and Environmental Change Institute.
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