APPROACHES TO MODEL LAYERING FOR SIMULATING GROUNDWATER DISCHARGE TO BURIED VALLEY SPRINGS AND WETLANDS

Improved access to hydrostratigraphic data and computational resources have contributed to development of a number of detailed, multi-layer groundwater flow models at county or larger scales for groundwater resource evaluation and management. Some of the management concerns that prompt development of these models include impacts on springs and wetlands that are supported by groundwater discharge. Two such models are those representing Dane County, Wisconsin and the seven-county area encompassed by the SE Wisconsin Regional Planning Commission. In both of these regions, bedrock aquifer units are relatively flat lying, which makes the assignment of single or combined bedrock units to (effectively) horizontal model layers a reasonable approach. However, in both of these regions the bedrock upper surface is irregular and some bedrock units are not present over the entire model area. Unlithified deposits fill a number of deeply incised, buried bedrock valleys. The regional scale models approximate this complex geometry by assigning negligible thicknesses to layers representing the absent bedrock units and by draping one or more layers, with highly variable transmissivity, over the entire model domain to represent the unlithified deposits. Springs and wetlands in these areas are often found along the margins of the buried valleys. This presentation will compare the existing layering approach to ones that more realistically represent the bedrock-unlithified contact geometry, with particular emphasis on implications for simulating groundwater discharge locations and fluxes.