The Mississippi River Valley alluvial (MRVA) aquifer is an important surficial aquifer in the Mississippi Alluvial Plain (MAP) area. The aquifer is generally considered to be an unconfined aquifer (fig. 1; Clark and others, 2011), and withdrawals are primarily used for irrigation (Maupin and Barber, 2005). These groundwater withdrawals have resulted in substantial areas of water-level decline in parts of the aquifer. Concerns about water-level declines and the sustainability of the MRVA aquifer have prompted the U.S. Geological Survey (USGS), as part of the USGS Water Availability and Use Science Program and with assistance from other Federal, State, and local agencies, to undertake a regional water-availability study to assess the characteristics of the MRVA aquifer, including the potentiometric-surface altitude of the MRVA aquifer for spring 2018, and to provide information to water managers to inform their decisions about resource allocations and aquifer sustainability. The purpose of this report was to present a potentiometric-surface map for the MRVA aquifer using manually measured groundwater-altitude data and daily mean or maximum groundwater-altitude data from wells measured generally in spring 2018, which is after water levels have substantially recovered from pumping in the previous irrigation season and before pumping begins for the next irrigation season, and using the altitude of the top of the water surface in rivers in the area, hereinafter referred to as “surface-water altitude,” generally on April 10, 2018, from streamgages in the area. The term “potentiometric surface” is used in this report because it is applicable for maps of the groundwater-altitude surface in unconfined, semiconfined, and confined aquifers (Lohman, 1972). In this report, the maps of the MRVA aquifer’s groundwater surface are termed potentiometric-surface maps as opposed to water-table maps because, although the MRVA aquifer generally exhibits characteristics of unconfined conditions, where surface-water features may or may not be hydraulically connected, it also exhibits characteristics of confined or semiconfined conditions in some areas at least during part of the year. The location of these areas, where the aquifer is confined or semiconfined, is not well understood or defined (Arthur, 1994; Kleiss and others, 2000).
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