This map shows the distribution of major bedrock aquifer units within 300 feet of ground surface in Illinois. ��Major aquifers are capable of yielding 70 gallons of water per minute. ��Potable water contains less than 2,500 milligrams per liter of total dissolved solids. ��Bedrock aquifers within 300 feet of ground surface cover most of northern Illinois and are commonly overlain only by thin layers of less permeable silts and clays. ��Many are directly overlain by shallow or major sand and gravel aquifers allowing direct hydrologic communication with shallower aquifer systems. �� This feature class is intended to identify areas of Illinois where major bedrock aquifers exist. �� The scale of these data is 1:500,000.