Health of the System

Confinement Condition of the Memphis Aquifer – Shelby County

Beneath much of Shelby County, the Memphis aquifer is confined, meaning that water levels in the aquifer sit above the base of the upper confining clay. Water levels in the Memphis aquifer can rise above the aquifer itself when they are under tremendous pressure, such as when they are capped by clay and hundreds of feet of sediment above that. Before local pumping from the aquifer began in 1886 (considered “predevelopment conditions”), water levels likely remained almost unchanged. As pumping has continued and withdrawals have steadily increased since 1886, these levels, known as the potentiometric surface (or potential water height), have dropped. When water levels drop enough to fall below the base of the upper confining clay, the Memphis aquifer goes unconfined, which is an undesirable situation.

Data used in this analysis comes from predevelopment levels by Waldron and Larsen (2015), Memphis aquifer water levels from 2015 by Kingsbury (2018), and geologic formation elevations derived by Van Arsdale (2015).

The percentage represents the height of 2015 Memphis aquifer water levels from the base of the upper Claiborne confining unit (UCCU) compared to the original, pre-development, (1886) water level height above the base of the UCCU, following this equation:

where, MAWL_xxxx is the Memphis aquifer water level elevation for that year (xxxx) and UCCU_base is the base elevation of the UCCU.

On the diagram, the arrow falls between 100% and 0% where 100% reflects zero water decline since pumping began in 1886, and 0% says water levels have reached the base of the confining clay and full unconfinement of the Memphis aquifer beneath Shelby County is imminent. The current 62% rating is an average among all 59 available measurements, and indicates that the Memphis aquifer remains mostly confined, meaning the water levels have dropped on average 38% below original predevelopment levels. Of the 59 water level locations surveyed in 2015 by the U.S. Geological Survey, five locations show unconfinement. Calculated values are shown in the following table (highlighted cells are unconfined).

Threats to the Groundwater- West TN

Threats to the Memphis aquifer mainly come from the surface or right below the surface from anthropogenic (human) sources. A significant release of a contaminant can migrate through the shallow subsurface to the shallow aquifer and possibly find its way through breaches in the protective clay layer, reaching our water supply source. The State of Tennessee Department of Environment and Conservation (TDEC) and the U.S. Environmental Protection Agency (EPA) identify and monitor remediation sites throughout the State. In 2018, there were 223 remediation sites in west Tennessee with 61 open cases and the remaining 157 sites were either closed, withdrawn or referred to other agencies or numbers (defined below).

The position of the arrow shows sites that remain Open (where investigation or remedial work is planned or ongoing). Other category descriptions follow:

Location of the 223 sites is shown on the map below and listed in the table provided.