CHANGES IN GROUNDWATER FLOW PATHS WITHIN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER: 1870–2007

The Mississippi River alluvial plain in northwestern Mississippi (referred to as the Delta), once a floodplain to the Mississippi River covered with hardwoods and marshland, is now a highly productive agricultural region of large economic importance to Mississippi. Water for irrigation is supplied primarily by the Mississippi River Valley alluvial aquifer, and although the alluvial aquifer has a large reserve, there is evidence that the current rate of water use from the alluvial aquifer is not sustainable. Using an existing regional groundwater flow model, changes in the inflows and outflows to the aquifer were assessed over a time period from 1870 to 2007. Prior to extensive use of groundwater for irrigation, the regional groundwater flow path generally followed the topography of the alluvial plain, discharging to the streams and rivers within the Delta. Presently (2011), the regional groundwater flow path is intercepted by a large cone of depression in the central Delta with a maximum drawdown occurring in Sunflower County, formed as a result of groundwater pumping for irrigation. Water-level declines have resulted in decreased groundwater discharge to streams and increased surface-water leakage to the aquifer to the extent that many streams in the Delta are presently net losing streams throughout the year. Additionally, the Mississippi River, which in the past was a net importer of water from the alluvial aquifer, is now a net exporter of water to the alluvial aquifer. These changes in flow to and from the aquifer have increased the vulnerability of the alluvial aquifer to agricultural chemicals in surface runoff and from the Mississippi River and have diminished many ecosystem services provided by groundwater discharge to streams such as maintaining baseflow in streams, regulating stream temperature regimes for aquatic biota, and buffering the transport of contaminants through the streambed interface.