3-DIMENSIONAL GROUNDWATER-FLOW NEAR NARROW SURFACE-WATER BODIES

A series of natural gradient tracer tests was conducted to delineate g roundwater flow patterns near a drainage ditch in glacial outwash of c entral Wisconsin. Water level and precipitation data were collected to document factors that could contribute to temporal variations in the flow field. Year-to-year variations in recharge generated shifts in fl ow paths between replicate tracer experiments. The field data provided the basis for numerical modeling designed to elucidate the sensitivit y of flow paths to variations in recharge. A three-dimensional aspect of flow paths identified in this study is a ''wrap-around'' pattern, i n which groundwater passes under a surface water body prior to turning in the downstream direction and continuing to migrate as subsurface f low. This feature was suggested by the path of a tracer injected deep in the aquifer and was simulated at all recharge rates employed in the modeling. The three-dimensional nature of the flow field and temporal variability of flow paths can have important implications for design of monitoring networks near narrow surface water bodies.