Groundwater Flow and Storage Depletion within the Mississippi Embayment Regional Aquifer Study (MERAS)

The U.S. Geological Survey (USGS) Groundwater Resources Program supports projects to assess groundwater availability in multiple areas across the United States. One project is the Mississippi Embayment Regional Aquifer Study (MERAS). Billions of gallons per day of groundwater are used within the MERAS area for primarily agricultural, municipal, and industrial supply. Declining water levels within the MERAS area necessitate tools that can be used to better manage the groundwater resource. The primary tool used by the USGS to evaluate groundwater availability in the Mississippi embayment is the MERAS groundwater flow model. The construction of the MERAS model, using the USGS MODFLOW-2005 modeling software, included over 2,600 geophysical logs for hydrogeologic framework development; 137 years of groundwater withdrawal information; approximately 70,000 groundwater withdrawal locations; 39 rivers comprising about 6,900 river miles; and precipitation, land use, surficial geology, and aquifer properties covering approximately 78,000 square miles.
In the Grand Prairie area of central Arkansas, water levels within the surficial aquifer have declined as much as 100 feet since the early 1920’s because of intense groundwater pumping for agricultural purposes. Similar water level declines occur along the west side of Crowleys Ridge in Arkansas, and in an area in Mississippi. Reductions in available groundwater in the surficial aquifer caused by pumping in excess of recharge for agricultural needs have resulted in pumping from deeper aquifers to offset demand in some areas. Pumping from deeper aquifers has changed the direction of groundwater flow in the system within and between these aquifers. Simulated groundwater flow between some aquifers has tripled in some areas. Simulated groundwater flow has reversed in other areas from downward flow in predevelopment periods to upward flow in recent decades. Initial simulations indicate cumulative losses from storage of 9,000 to 17,000 acre-feet per square mile in areas of Arkansas and Mississippi.