MLGW Aquifer Study - CAESER - University of Memphis

MLGW Aquifer 1 Study
2018-2023

Shelby County, Tennessee: Aquifer Study

Project

Memphis Light Gas and Water (MLGW) Aquifer Study

In the winter of 2018, the Memphis City Council approved the establishment of a special fund, paid for by the citizens of Memphis and unincorporated Shelby County, to conduct an important scientific investigation on Memphis’ vital drinking water resource, the Memphis aquifer. This research focuses on the presence of breaches in the confining (protective) clay layer, using several techniques seeking to determine their existence, size, orientation, and ability to support inter-aquifer water exchange (shallow to Memphis). Therefore, this study covered 22 projects that used a suite of scientific methods and tools to help target potential breach locations and understand more about their properties and what that means to the bigger picture. These methods included chemical analysis and age-dating of groundwater, numerical (groundwater) modeling, subsurface mapping using historical drilling records, geophysical techniques, physical measurements of water movement and levels, and evaluating the patterns and interactions between streams and the shallow aquifer. Additionally, the impact of breaches on the Memphis aquifer was tested using groundwater models and conducting laboratory experiments. This study vastly increased our knowledge of these breaches, expanding the number of suspected breaches from 6 to 35 and having verified through drilling six known breaches (one being a historically identified known breach). Breaches contribute modern water, typically of poorer quality, to the Memphis aquifer. The largest contribution of suspected breaches came from AEM conducted in northern Shelby County north of the Loosahatchie River. As the geology in north Shelby County is not different from the rest of Shelby County, the 23 suspected breaches identified through the AEM may reflect what could be present in the remaining two-thirds of the county, meaning there may be many more breaches. With the Memphis area being heavily urbanized and having occurrences of localized contamination, the continued need to better understand threats to the Memphis aquifer, our primary drinking water source, has been highlighted by this study. Read our Executive Summary of this study below:5

Research Projects

Projects are listed as defined in the contract. For example, Year 1 Deliverable 1 is Project 1-1, Year 2 Deliverable 1 is Project 2-1, so on and so forth. Click on the project name to read the project’s executive summary.

Project 1-1: Determine impact of known breaches in the Sheahan well field, determine presence of new breaches in the well field, and assess impact of Former Custom Cleaners site. 

Project 1-2: Determine possible new breach locations proximal to the Wolf River by conducting riverbed seepage measurements, performing detailed discharge measurements, and developing well transects to monitor groundwater/surface water exchange. 

Project 1-3: Perform aquifer characterization across Shelby County to constrain numerical model parameter estimation. 

Project 1-4: Map potential aquitard breaches in Ensley Bottoms near the Davis well field, TVA, and proximal to the Allen well field using geophysical techniques. 

Project 1-5: Groundwater pumping optimization to minimize contaminant movement from the water table aquifer to Memphis Aquifer using stochastic modeling. 

Project 2-1: Use geophysical well records to investigate hypothetical paleo-drainage network atop the upper Claiborne confining unit. This will potentially help identify new breach locations and better inform numerical groundwater flow simulations.  

Project 2-2: Subsurface mapping of geologic units to identify the presence of aquitard breaches and characterize the hydraulic properties of identified breaches using geophysical techniques in conjunction with other traditional methods. 

Project 2-3: Conduct multi-scale investigation of surface water-groundwater interactions along the Loosahatchie River and Nonconnah Creek using a variety of methodologies to identify breaches. Incorporate these findings plus those of the Wolf River (Project 1-2) into Shelby County numerical groundwater model. 

Project 2-4: Development of hypothetical groundwater models focusing on groundwater sustainability and modeling the fate and transport of various contaminants while conducting bench scale testing of retardation reactions. 

Contaminant Transport Study.

Laboratory Work Study.

Project 2-5: Build upon Davis well field age-dating results to further refine and quantify source waters to the Memphis aquifer at the Davis well field through sampling water chemistry, groundwater age-dating, characterization of the hydrogeologic properties of a known breach impacting Davis, and development of a conceptual model of groundwater flow for later incorporation into a numerical model. 

Project 2-6: Determine in-situ riverbed properties (hydraulic conductivity and thickness) for the Loosahatchie River, Wolf River and Nonconnah Creek to further constrain the Shelby County numerical groundwater model and for site-scale hydrogeologic analyses. 

Project 2-7: Investigate Shaw well field to determine source waters and potential for modern water migration into the Memphis aquifer, development of unconfined conditions and vulnerability to nearby contaminated sites. 

Project 2-8: Determine recharge mechanisms and rates to the shallow aquifer within Shelby County that contributes to its replenishment and source of additional inflow to the Memphis aquifer through aquitard breaches. 

Project 2-9: Determine numerical modeling best practice for simulating groundwater conditions in the shallow aquifer that better represent groundwater levels and flow direction, vertical leakage through aquitard breaches, and avoiding inherent cell flooding (too much recharge) and drying (thin saturation depths). 

Project 2-10: Numerical modeling to correlate age-dating and geochemical observations to known/potential breaches that will include possible paleo-drainage atop the upper Claiborne confining unit. 
 

Project 2-11: Formulate and test methodologies to reduce or eliminate preferential inter-aquifer water exchange. 

Project 3-1: Investigate a suspected aquitard breach near McCord well field by using electric resistivity and possibly other geophysical techniques within available open space. Project to include drilling as well for stratigraphic control. 

Project 3-2:  Investigate Mallory well field to determine source waters and potential for modern water migration into the Memphis aquifer field through sampling water chemistry, groundwater age-dating, mapping of the subsurface stratigraphy, and development of a conceptual model of groundwater flow for later incorporation into a numerical model. 

Project 3-3:  Incorporate more complete age-dating of Sheahan production wells into numerical model to resolve the probable location of breach(es) in the southern portion of the well field with attempted validation through geophysical techniques or drilling of an observation well.  

Project 3-4: Conduct a county-wide water level survey of the shallow aquifer.  

Project 4-1: Develop lithologic database of well logs for Shelby County for 3D representation and use for stratigraphic mapping and other upscaling tasks. 

Project 4-2: Fly AEM (Airborne Electromagnetism) over section of north Shelby County that includes a suspected breach location. 