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Lesson 00 · Scale
Before we zoom in to Shelby County, here's the scale of the groundwater system Memphis is part of.
The Mississippi Embayment is a trough-shaped depression that opens southward toward the Gulf of Mexico. Over the past 70 million years, as it subsided and was periodically flooded by the ocean, it collected stacked layers of sand (aquifers) and clay (confining units).
Today those stacked layers form one of the largest, most productive groundwater systems in North America — supplying municipal water for Memphis, agricultural irrigation for the Mississippi Delta, and industrial supply across eight states.
Memphis is the largest US metropolitan area that relies 100% on groundwater. It sits at a critical point in the embayment: the Memphis Sand is very close to the surface here (relatively speaking), pumping concentrations are heavy, and the protective confining layer is perforated by breaches.
Whatever happens in Memphis — contamination, over-pumping, or successful protection — becomes a model for every other community drinking from the embayment.
The Mississippi Embayment Regional Aquifer Study (MERAS) is the USGS-maintained MODFLOW model of this whole system. It covers ~70,000 square miles, integrates 100,000+ wells and 2,600+ geophysical logs, and models 10 hydrogeologic units across the eight states.
The CAESER Shelby County model you explored on the "See the Scale" page is a local refinement: higher resolution, more wells per square mile, and incorporating the University of Memphis's breach research. Both models complement each other.
Authoritative references: USGS SIR 2009-5172 · CAESER MLGW Study