This report is a companion volume to the groundwater monitoring report for the Hanford Site, which is produced annually. It contains background information that does not change significantly from year to year. Unconsolidated sediment of the Hanford and Ringold formations comprises the uppermost aquifer beneath the Hanford Site. These formations contain highly permeable layers of sand and gravel interbedded with less permeable layers of silt and clay. Groundwater flows primarily from west to east, but influences of liquid waste disposal disrupt this pattern locally. Facilities that produced significant amounts of liquid waste, or waste sites that require groundwater monitoring, are located mainly in the 100, 200, and 300 areas. A few additional sites are located in the 400,600, and Richland North areas. Most of these waste sites are inactive. Selection of groundwater monitoring wells, constituents, and sampling frequencies are based on knowledge of waste disposal practices, regulatory requirements, proximity to disposal areas, contaminant mobility, and site hydrogeology.