Transient groundwater interactions and lake stage were simulated for Lake Barco, an acidic seepage lake in the mantled karst of north central Florida. Karst subsidence features affected groundwater flow patterns in the basin and groundwater fluxes to and from the lake. Subsidence features peripheral to the lake intercepted potential groundwater inflow and increased leakage from the shallow perimeter of the lake bed. Simulated groundwater fluxes were checked against net groundwater flow derived from a detailed lake hydrologic budget with short-term lake evaporation computed by the energy budget method. Discrepancies between modeled and budget-derived net groundwater flows indicated that the model underestimated groundwater inflow, possibly contributed to by transient water table mounding near the lake. Recharge from rainfall reduced lake leakage by 10 to 15 times more than it increased groundwater inflow. As a result of the karst setting, the contributing groundwater basin to the lake was 2.4 ha for simulated average rainfall conditions, compared to the topographically derived drainage basin area of 81 ha. Short groundwater inflow path lines and rapid travel times limit the contribution of acid-neutralizing solutes from the basin, making Lake Barco susceptible to increased acidification by acid rain. This paper is not subject to U.S. copyright. Published in 1996 by the American Geophysical Union. Paper number 96WR00162. water flow systems can be extremely complex and varied. Few studies, however, have attempted to directly quantify the effect of karst features on groundwater exchange with lakes. The chemistry of seepage lakes in Florida is equally varied [CanfieM, 1981; Brenner et al., 1990] and in part reflects differences in the solute contributions received from groundwater [Lee and Sacks, 1991; Stauffer, 1991]. According to the 1984 Eastern Lake Survey performed by the U.S. Environmental Protection Agency, Florida has the greatest number of acidic seepage lakes (pH < 5.0) in the country [Eilers et al., 1988]. Quantifying groundwater inflow to this special class of Florida lakes is critical to their management, as groundwater contributes the only external source of acid-neutralizing capacity (ANC) [Baker et al., 1988; Pollman et al., 1991]. Pollman et al. [1991] performed a preliminary analysis of the controls on ANC in two acidic seepage lakes in Florida, Lake Barco and Lake Five-O, using the geochemical tracer approach of Stauffer [1991]. Groundwater inflow was estimated to contribute little (9-14%) of the total inflow (rainfall plus groundwater inflow) to Lake Barco, but approximately 50% of the total ANC budget of the lake. This paper describes physically based models of the groundwater fluxes and flow patterns in the Lake Barco basin. The objectives are (1) to define the relation between hydrogeologic setting and groundwater interactions with a lake in karst terrain; (2) to quantify transient groundwater fluxes from May 1989 to December 1990 and compare these fluxes to a detailed hydrologic budget of Lake ...

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