INTERACTIONS BETWEEN GROUND-WATER AND SURFACE-WATER IN THE SUWANNEE RIVER BASIN, FLORIDA

Ground water and surface water constitute a single dynamic system in m ost parts of the Suwannee River basin due to the presence of karst fea tures that facilitate the interaction between the surface and subsurfa ce. Low radon-222 concentrations (below background levels) and enriche d amounts of oxygen-18 and deuterium in ground water indicate mixing w ith surface water in parts of the basin. Comparison of surface water a nd regional ground water flow patterns indicate that boundaries for gr ound water basins typically do not coincide with surface water drainag e subbasins. There are several areas in the basin where around water f low that originates outside of the Suwannee River basin crosses surfac e water basin boundaries during both low-flow and high-flow conditions . In a study area adjacent to the Suwannee River that consists predomi nantly of agricultural land use, 18 wells tapping the Upper Floridan a quifer and 7 springs were sampled three times during 1990 through 1994 for major dissolved inorganic constituents, trace elements, and nutri ents. During a period of above normal rainfall that resulted in high r iver stage and high ground water levels in 1991, the combination of in creased amounts of dissolved organic carbon and decreased levels of di ssolved oxygen in ground water created conditions favorable for the na tural reduction of nitrate by denitrification reactions in the aquifer . As a result, less nitrate was discharged by ground water to the Suwa nnee River.