Estimation of downward leakage from Florida lakes1

The karst lake district of peninsular Florida shares hydrologic properties with certain glaciated districts, such as outwash plains, but differs from most well-watered regions in losing much more water to subsurface effluents than to surficial runoff. In nutrient-loading models that begin as hydrologic throughflow models, groundwater inflow and outflow are commonly ignored or estimated from errors of closure of water budgets. Such models are inapplicable to seepage lakes, unless deep-seepage outflow can be measured. Estimates of downward leakage from Florida lakes with and without outlets suggest that conventional nutrient-loading models are inapplicable to any lake not in a watertight rock basin. Downward leakage is measurable during months when lake level falls by an amount exceeding the net precipitation deficiency. To measure it, USGS stage data for 20 lakes from north, central, and south sectors of the peninsula were compared with three 32-yr sets of mean monthly meteorologic measurements. Monthly pan-evaporation data were converted to lake evaporation by factors estimated at Lake Okeechobee. Leakage estimates (geometric means) range from 28.4 to 50.9 cm yr-I in 18 of 20 lakes; two exceptionally astatic lakes gave 90.1 and 141.2 cm yr-I. Presence of outlets in 15 of 20 lakes makes no detectable difference in leakage. Adding evaporative losses (and ignoring surface outflow if any), residence times for the 20 lakes averaged 2.67k 1.33 yr. Florida lake levels fluctuate in sympathy with each other and respond with little or no lag to monthly net precipitation. Longer term swings are also evident and are more highly correlated with artesian pressure in the deep limestone aquifer than with meteorologic variables. Few if any lakes receive direct injections from this aquifer, and artesian influence is exerted on lakes and local water tables through many meters of surficial (Mio-Pliocene) deposits. In limnological usage, drainage lakes are 40). If closed lakes are thus restricted to rock also open lakes, whereas closed lakes lose basins in semi-arid regions, "all seepage water only by evaporation (Wetzel 1975, p. lakes are in this sense almost certainly open" (Hutchinson 1957, p. 23 1); in other words, seepage lakes are th;subset of drainage lakes that lack surface outlets. That seepage lakes Supported by the National Science Foundation (DAR 79-24812, DEB 82-1 1380) and by a grant from have effluents groundwater has been surthe Whitehall Foundation. mised by many observers, beginning at least * Died 29 November 1988. with Thoreau. Considering drainage lakes I 1309 Leakage from Florida lakes like Thoreau's Sandy (Flint's) Pond, however, later limnologists have been misled by the fact that "most small lakes not in rock basins are separated from the ground water by a clay seal, formed as an early lake sediment" (Hutchinson 1957, p. 248). Linsley Pond is such a drainage lake. Although its clay seal was found to admit some inflowing 14C-deficient groundwater (Deevey and Stuiver 1964), the crude water budget gave no hint of outflow via a second effluent. Mirror Lake, downstream from the Hubbard Brook Experimental Forest, is also insulated from that system's "watertight bedrock" (Likens et al. 1977) by a clay lens overlying sandy till and outwash. Collectively known as drift, these glacial and early postglacial deposits provide the only subsurface aquifers of the region. Their output, the groundwater that enters streams and lakes and nourishes forest trees, is deep seepage. Although Mirror Lake itself is a source of this output, the magnitude of downward seepage through its floor has not yet been measured directly (Winter 1985). When estimated by the lake's water budget, measured inputs (rainfall plus deep and surface inflow) exceed surface outflow by 3.6 x 1O5 m3 yr,or 43% of the total (Likens 1985: table 1V.E-4). Proofthat this huge loss is in fact deep seepage was obtained during 1970-1 972, when the lake level (measured daily) was below the dam during 11 of 28 months. Leakage through the drift dam was measured at V-notch weirs downstream and appears as outflow in the budget. Accurately measured and corrected nonevaporative losses from the lake were then 3.1 x 1O5 m3 yr-'; the remaining discrepancy of budget closure, averaging 49.7 +22.3 x 1 03 m3 during 10 yr or +5.9% yr-', was also assigned to deep seepage. In this paper I follow the practice of Florida hydrologists in calling deep-seepage outflowdownward leakage (Hammett 198 1). In Mirror Lake it is the fraction that did not seep outward through the dam. It amounted to 42% of the mean depth or 240 cm of water per year. Implications of this extraordinarily large figure are discussed below. In modem limnology, water budgets are necessary first steps toward nutrient or solute budgets; hydrologists' estimates ofwater throughflow are commonly accepted by professional courtesy. If this confidence is misplaced (Winter 198 1 ; LaBaugh and Winter 1984), most limnologists would probably prefer not to hear about it. Among the hundreds of small lakes for which nutrient-loading models have been constructed (Chapra and Reckhow 1983), few if any have water budgets as closely monitored as that of Mirror Lake. Nearly all these lakes lie in well-watered regions, where rainfall exceeds evaporation by 70-85% (Deevey and Brooks 1963; Kohler et al. 1959). It is therefore not surprising that rapid hydrologic throughflow is a key component of the nutrient-loading paradigm as used in eutrophication studies. If deep seepage and groundwater flow contribute some inconvenient variance to these models, it is easy to overlook the variance in larger discrepancies in water or nutrient budgets. Unfortunately for modelers, the nutrientloading paradigm is inapplicable to seepage lakes. It is the thesis of this paper that it is also highly questionable for drainage lakes not in rock basins. In this study of the rather special hydrology of the karst lake district of Florida, I report some statistical measurements of deep seepage. The study area is mapped in Fig. 1. Its northern third is shown as seen by satellite in Fig. 2. This northern sector receives 1,359.44207.8 ( 4 1 SE) mm (53.52 k 8.1 8 inches) of annual rainfall; with lake evaporation averaging 1,288.0 +73.7 mm (50.7 1 k2.90 inches), net rainfall (the difference, with a large SE) is 7 1.44220.5 mm (2.8 1 f8.68 inches). In the years of record (water-years 1954-1 986), net rainfall in the central third of the lake district was 49.4k228.6 mm (1.95k9.00 inches); in the slightly drier southern third the net was negative, -38.4k260.9 mm (1.5 1 +10.27 inches). As would be expected from the variance ofthese figures most of the lakes are strongly astatic. An extreme example (Pebble Lake) has fallen 93 cm (3.05 ft) in a given month and risen by 78.6 cm (2.58 ft) in another; over 40 yr its mean depth has varied between 72 and 529 cm, i.e. by a factor of 7.3. If a 2-m excursion of lake level is a Walden unit, 11 of 20 lakes in our data set (Table