IMPACT OF RESERVOIR-STAGE MANIPULATION ON GROUND-WATER DISCHARGE AND CHEMICAL VARIATION IN A TRIBUTARY EMBAYMENT

Constructed surface reservoirs have become the dominant aquatic feature throughout the United States; however, a clear understanding of the geochemical and hydrologic relationships in these “unnatural” water bodies has yet to be achieved. Chemical variations in Kentucky Lake, the terminal reservoir along the Tennessee River, may be related to the biannual manipulation of reservoir stage (~1.5 m in March and September). The Ledbetter Creek embayment (area ~1.5 km²) is located on the west side of Kentucky Lake in western Kentucky. Fifteen piezometers in seven nests near the head of the embayment are being monitored to determine the influence of reservoir stage on ground-water discharge and chemical variations in the embayment. Ground water flows to the embayment year-round despite changes in reservoir stage. However, vertical hydraulic gradients in the hyporheic zone decrease during the spring, and gradient reversals have been observed at some nests following reservoir-stage increase. Since January 2004, surface- and ground-water samples have been collected from 19 monitoring points in the embayment. Samples have been analyzed for major anions, metals and metalloids, DIC, DOC, and stable isotopes of H₂O. Nitrate concentrations varied seasonally in both surface- and ground-water samples. Average surface-water NO₃^- concentrations (± standard error) decreased from January (1.21 mg/l ± 0.06) to March (0.580 mg/l ± 0.08) to April (0.551 mg/l ± 0.09). Likewise, average ground-water NO₃^- concentrations decreased from January (1.30 mg/l ± 0.5) to March (0.413 mg/l ± 0.11) to April (0.360 mg/l ± 0.1). Although the reservoir stage rose following the March sample date, changing hydraulic gradients indicate that small stage fluctuations prior to sampling could influence ground-water flow in the embayment enough to alter solute concentrations. The influence of reservoir stage on chemical variations in the embayment will continue to be examined following the stage decrease in the autumn.