This report describes the geohydrologic framework of the coastal lowlands aquifer system, one of the three regional aquifer systems delineated in the Gulf Coast RASA (Gulf Coast Regional Aquifer - System Analysis) study area. The coastal lowlands aquifer system is composed predominantly of Oligocene and younger sediments, whereas the Mississippi embayment aquifer system and the Texas coastal uplands aquifer system are composed predominantly of Eocene sediments. Each of the three aquifer systems is composed of thousands of feet of deposits, which contain numerous aquifers, permeable zones, and confining units. Some of these geohydrologic units are regionally extensive, whereas others are of local importance.