GROUND WATER INTERACTION WITH SURFACE WATER BODIES AT BIG MUDDY NATIONAL FISH AND WILDLIFE REFUGE

Ground water plays an important role in the hydrologic budget and chemical composition of many surface water bodies. Many studies have investigated ground water and lake water interactions and ground water and river water interactions. At the Overton Bottoms North Unit of the Big Muddy National Fish and Wildlife Refuge, interactions among ground water, lake water, and river water are of interest as the lakes serve as habitat for migratory wildlife. The hydrogeologic setting is the Missouri River floodplain, its hydrologically connected alluvial aquifer, and two scour holes (lakes that were formed in the floodplain during the Flood of 1993).

Based on hydraulic head data, for most times of the year, the scour holes are flow-through lakes with ground water entering along the upgradient side and exiting along the downgradient side. Eventually the ground water discharges into the Missouri River. During annual flood events, the scour holes receive water from the river and from upgradient ground water. During these annual flood events, the head of the river is higher than that of the downgradient ground water. Our objective is to test the timing and linkages among the ground water, lake water, and river water using the isotopic composition of the water bodies.

Our monthly sampling efforts began in January 2004. If there were good communication among the three types of water bodies, we would expect the isotopic composition of these water bodies to be similar. To date, the average delta 18O values are -7.90 per mil for the Missouri River and -5.40 per mil for the ground water. For one scour hole, the average delta 18O value is -4.52 per mil reflecting the ground water source coupled with evaporation. For the other scour hole, the average delta 18O value is 1.31 per mil. This value indicates isolation from the river and ground water with evaporation contributing significantly to the enrichment of 18O.

Developing an understanding of the source contributions of water and accompanying nutrients in the water bodies at Overton Bottoms North is valuable to refuge managers in making habitat restoration decisions. This information will allow them to predict the hydrogeologic outcome when designing projects to create ephemeral and permanent water bodies for a host of biota.