Environmental tracer evidence for connection between shallow and bedrock aquifers and high intrinsic susceptibility to contamination of the conterminous U.S. Glacial Aquifer

•Groundwater age distributions were estimated for the glacial alluvial aquifer.•Equal area sampling provides representative mean age estimates for large aquifer.•Tracers show likely connection between bedrock and shallow alluvial aquifer.•New method was developed to assess likelihood of groundwater contamination.•Susceptibility Index is better predictor of contaminant presence than mean age.