DOI: 10.5066/p9fzv4ed
title: Groundwater withdrawal zones for drinking water from the Mississippi River Valley alluvial aquifer and Mississippi embayment aquifers

description: Of the approximately 6.6 million people living in the Mississippi embayment (MISE) region in the central United States, approximately 65 percent rely on groundwater for their drinking water (Dieter, Linsey, and others, 2017). Regional assessments of water quality in principal aquifer systems provide context for the long-term availability of these water resources for drinking-water supplies. To assess the current (2018) status of water quality in MISE in relation to drinking water supplies, groundwater withdrawal zones used for domestic and public supply were modeled using available groundwater well and hydrogeologic framework data. Three dimensional surfaces were modeled to map the depth zones at which groundwater is withdrawn for drinking water. These surfaces will be used to model groundwater quality as part of the U.S. Geological Survey National Water Quality Assessment project's intensive principal aquifer analysis. The MISE region includes two principal aquifer systems: the surficial aquifer system, which is dominated by the Quaternary Mississippi River Valley Alluvial aquifer (MRVA), and the Mississippi embayment aquifer system, which includes deeper Tertiary aquifers and confining units. Based on the distribution of groundwater use for drinking water, the modeling effort is focused on MRVA and two hydrogeologic units from the deeper system, including the middle Claiborne aquifer (MCAQ) and lower Claiborne aquifer (LCAQ). The MRVA is a surficial, unconfined to semi-confined, highly productive aquifer used mostly for irrigation, with a lesser amount of groundwater use for public supply and domestic self-supply (Clark and others, 2011; Maupin and Barber, 2005). The median thickness of the MRVA is about 130 feet (ft) but it can be as much as 290 ft thick (Hart and others, 2008). The MCAQ is confined where overlain by the Middle Claiborne confining unit and is used dominantly for public supply. Domestic self-supply occurs along outcrop areas where the unit is shallower or crops out. The unit consists mostly of the Sparta Sand, but north of approximately the 35th parallel (near the border between Tennessee and Mississippi), the underlying lower Claiborne confining unit (LCCU) undergoes a facies change and the Memphis Sand is included in the MCAQ (Hosman and Weiss, 1991). The MCAQ has a median thickness of about 805 ft, but it can be as much as 1,890 ft thick (Hart and others, 2008). Although not as regionally important as MRVA or MCAQ, domestic and public supply wells withdraw groundwater from LCAQ, especially on the margins of the Mississippi embayment where LCAQ is relatively shallow or crops out. The aquifer does not extend north of approximately the 35th parallel because of a facies change in the LCCU. The aquifer is relatively thin, ranging from 50 to 195 ft thick with a median thickness of 125 ft (Hart and others, 2008). Continuous surfaces representing groundwater withdrawal zones used for drinking water were created for MRVA (combined domestic and public supply), MCAQ-domestic, MCAQ-public supply, LCAQ-domestic, and LCAQ-public supply, where the surfaces represent the altitude (in feet above North American Vertical Datum of 1988) of the bottom and top of the screened interval. Surfaces were created by kriging well points using Empirical Bayesian Kriging in ArcMap version 10.4 (ESRI, 2016). Well construction information for public supply (P) and domestic (D) wells and aquifer surfaces from the Mississippi Embayment hydrogeologic framework (Hart and others, 2008) were used to populate as much information as available about well use, well depth, screened interval, and aquifer as to improve the modeled surfaces. To assess error on the modeled surfaces, well datasets were separated into training (90 percent) and testing (10 percent) datasets for kriging and root mean square error was calculated. The number of wells used for kriging varied for each surface (WellsSummary.csv). A shapefile representing the density of wells within each raster cell was also produced to aid users in understanding general well locations and errors (ps_wellerrors.shp). For a full explanation of methods, see Processing Steps. Clark, B.R., Hart, R.M., and Gurdak, J.J., 2011, Groundwater availability of the Mississippi embayment: U.S. Geological Survey Professional Paper 1785, 62 p., accessed July 29, 2015, at http://pubs.er.usgs.gov/publication/pp1785. Maupin, M.A., and Barber, N.L., 2005, Estimated withdrawals from principal aquifers in the United States, 2000: Circular 1279, accessed December 8, 2016, at http://pubs.er.usgs.gov/publication/cir1279. Hart, R.M., Clark, B.R., and Bolyard, S.E., 2008, Digital Surfaces and Thicknesses of Selected Hydrogeologic Units within the Mississippi Embayment Regional Aquifer Study (MERAS): U.S Geological Survey Scientific Investigations Report 2008-5098, accessed August 28, 2015, at http://pubs.er.usgs.gov/publication/sir20085098.