Controls of groundwater flow in a peat dominated wetland/stream complex, Allequash Wetland, northern Wisconsin.

摘要

Field investigations and numerical simulations were used to identify processes controlling groundwater flow in Allequash Wetland, a peat dominated wetland/stream complex in northern Wisconsin. Field measurements collected from May 2005 through December 2007 include water levels at 47 piezometers, groundwater temperatures, three geophysical surveys, stream gaging, and water chemistry samples along Allequash Creek. Field results were used to develop a conceptual model of groundwater flow within the wetland. Ground penetrating radar (GPR) was used to determine the extent of peat, which forms a bowl-shaped depression. Results from the GPR survey together with the presence of gyttja (lucustrine sediment) in peat cores support previous speculation that the wetland was once a lake. Large spring complexes, ponds and zones of high (focused) groundwater discharge were observed in those areas where a break in slope in the interface between peat and the underlying sand/gravel intersects regional groundwater flow. Results from a two-dimensional groundwater flow model show the importance of low conductivity gyttja and the position of the interface between the peat and underlying sand/gravel on the formation of a spring fed pond. Discrete zones of focused groundwater discharge to Allequash Creek were identified from changes in streambed temperature using a distributed temperature sensor. Seepage meter measurements confirmed that temperature anomalies in Allequash Creek correspond to focused groundwater discharge zones, likely caused by preferential flow in soil pipes within the peat. Focused zones of groundwater discharge contribute over 75% of the discharge to Allequash Creek. This conceptual model that focused zones of groundwater discharge contribute most of the discharge to the wetland stream may be appropriate for other peat fens. Results from a three-dimensional groundwater flow model support the conceptual model consisting of increasing hydraulic conductivity with depth. Particle tracking shows water discharging to the stream is significantly controlled by low conductivity sediments at the base of the peat. Stream reaches receiving groundwater with the shortest residence times correspond with shallow peat zones identified in the GPR survey. The study also showed that hydrostratigraphy and geometry of the wetland deposits are significant in controlling groundwater flow paths and discharge zones at the wetland surface.