A major objective of the Gulf Coast Regional Aquifer-System Analysis is to use digital models of regional groundwater flow systems to develop better understanding and to improve management of the resource. Modeling is used to synthesize information about the aquifer systems and to test hypotheses about the relative importance of the components of the systems. The 290,000-sq mile study area in the Gulf of Mexico Coastal Plain includes the Mississippi embayment, Gulf Coastal Plain of Texas, and the Continental Shelf that are underlain by deposits of Tertiary and younger age, which contain fresh and saline water. A 10-layer, finite-difference, variable density model, with blocks 10 miles on a side, was used to simulate groundwater flow before development and in 1980, assuming steady- state conditions. Preliminary results indicate that the major factors controlling predevelopment regional flow are the topography, land-surface outcrop pattern, and geometry of aquifers and confining units. Geologic structure and the distribution of precipitation were less significant factors. The density of saline water in the deeper parts of the aquifer system probably has a substantial effect on regional groundwater flow that extends into the freshwater part of the system. Variable water density may be a significant driving force that transports salt great distances in many directions, including updip. The distribution and rates of regional recharge and discharge have been substantially changed by development. Groundwater pumpage in 1980 was about five times the value of predevelopment regional recharge. About 80% of the pumpage was supplied from increased regional recharge. Also resistance to vertical flow caused by many fine-grained beds within the permeable zones can be as important as resistance caused by regional confining units.
