The terrain around Keystone Lake, a 388-acre lake in northwest Hillsborough County, Florida, near the Cosme well field, is dotted with sinks that promote leakage from the surficial aquifer to the underlying Floridan aquifer. The lake, an integral part of the Brooker Creek channel, receives overland runoff from cypress swamps, pastures, citrus groves, and lakefront residential areas. The lake, a composite of many coalescing sinkholes, is generally 14 to 16 ft deep and has been dredged in places to 23 ft deep. Since 1960, rainfall in the area shows a cumulative negative departure of 86 in. from the long-term average. The mean annual lake stage in 1973 was the lowest on record. Concurrently, municipal pumpage in the Cosme well field from the Floridan aquifer peaked in 1961 and again in 1973, and in 1972 the potentiometric surface was the lowest since 1960. Comparisons of the hydrographs of lake stage and the potentiometric surface in the Floridan aquifer with rainfall graphs shows that both correspond closely to seasonal rainfall, resulting in the trend of decreasing seasonal maximum levels in the lake and the aquifer. Keystone Lake water quality is adequate for recreation and propagation of wildlife. (Woodard-USGS)
