In any given area, the groundwater within an aquifer, or the groundwater produced by a well, has some vulnerability to contamination from society's activities. Groundwater vulnerability is a function of the geologic setting of an area, as this largely controls the amount of time that has passed since the water fell as rain, infiltrated through the soil, reached the water table, and began flowing to its present location. This amount of time is called the residence time of the groundwater. Most sources of contamination are located on or near the land surface, and are of quite recent origin from a geological perspective. In Iowa, groundwater that is older than 200 years interacted with the land surface before factories, gas stations, landfills, or other potential sources of contamination existed. Groundwater that is older than 35 years interacted with the land surface before the extensive agricultural use of commercial fertilizers and pesticides. In addition to controlling residence times of groundwater, the geologic setting of an area affects the rate at which any contamination will degrade or break down. Understanding how different geologic settings affect residence times and contaminant degradation have been an important part of the Geological Survey Bureau's investigations of the relationships between land use and groundwater quality (see section on Water Quality and Agriculture). The map Groundwater Vulnerability Regions of Iowa summarizes groundwater vulnerability on a regional basis for both aquifers and typical wells in the differing geological settings of the state. For further information contact Bob Libra (Robert.Libra@dnr.iowa.gov) at 319-335-1575.