Geochemical and biological consequences of groundwater augmentation in lakes of west-central Florida (USA).

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 ²²⁶ Ra 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 ²²⁶ Ra activity yet reported from a Florida lake core. Several lines of evidence suggest that groundwater augmentation is responsible for the high ²²⁶ Ra activities in recent sediments: (1) ²²⁶ Ra activity in cores increased recently, (2) the Charles, Crystal, and Saddleback cores display ²²⁶ Ra/ ²¹⁰ Pb disequilibrium at several shallow depths, suggesting ²²⁶ Ra entered the lakes in dissolved form, (3) cores show recent increases in Ca, which, like ²²⁶ Ra, is abundant in augmentation groundwater, and (4) greater Sr concentrations are associated with higher ²²⁶ Ra 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 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.