Memphis Aquifer Research Articles

The role of fault pathways for modern and deep recharge to a semi-confined aquifer: revised analysis of modern water leakage pathways in the Davis wellfield, Memphis, Tennessee, USA

Hydrologic tracer, borehole log and geophysical data provide new insight into recharge sources and pathways to the semi-confined Memphis aquifer in the Davis wellfield, Memphis, Tennessee, U.S.A. New data and recent studies highlight the importance of faults as potential pathways for the migration of modern water into the Memphis aquifer, an important regional public, industrial, and agricultural water supply. Geochemical and environmental tracer data collected over the past 28 years, combined with hydrologic, stratigraphic, and structural data, were used to evaluate changes in water quality and different recharge pathways to the Memphis aquifer. Assessment of stratigraphic and structural relationships argue for the wellfield being bound by a fault along the bluff line (west) and another fault or faults along the eastern and southeastern margins. Modern water recharge (< 60 years old) occurs through the fault along the western boundary of the wellfield, whereas modern water and, seasonally, deep-seated (helium-bearing) fluids recharge along the fault on the southeastern margin. Each recharge source and pathway present different vulnerabilities, suggesting that the role of faults in wellfields within active tectonic regions needs to be assessed.

Numerical analysis of surface hydrogeological water budget to estimate unconfined aquifers recharge

Under changing climate, groundwater resources are the main drivers of socioeconomic development and ecosystem sustainability. This study assessed the contribution of two adjacent watersheds, Muse Street (MS) and West Wood (WW), with low and high urban development, to the Memphis aquifer recharge process in central Jackson, Tennessee, USA. The numerical MODFLOW model was created using data from 2017 to 2019 and calibrated using reported water budget components derived from in-situ data. The calibrated MODFLOW model was then used to investigate the impact of high and low urban developments on the recharge rate. The hydraulic parameters and recharge rates were optimized by adjusting the groundwater level based on the observed water level using PEST. The stochastic modeling was also carried out using the Latin Hypercube approach to reduce the uncertainty. The calibration results were satisfactory, with RMSE of 0.124 and 0.63 obtained in the WW and MS watersheds, respectively, indicating accurate estimation of the input parameters, precisely the hydrodynamic coefficients. The study results indicate that, per unit area, the MS watershed contributes 119% more to recharge and 186% more to riverbed leakage compared to the WW watershed. However, regarding total recharge and riverbed leakage, the WW watershed contributed more than the MS watershed. The results of this study have enhanced the knowledge of the impact of urbanization on hydrology and the recharge process in watersheds with diverse land uses.

Wellfield optimization to minimize contaminant migration from a surficial to a semi‐confined aquifer using numerical modeling

AbstractThe shallow, Memphis, and Fort Pillow aquifers are the three major water‐bearing strata beneath Memphis, Tennessee, where the Memphis aquifer serves as the primary groundwater source. The upper Claiborne confining unit (UCCU) separates shallow and Memphis aquifers across the majority of Shelby County, acting as an upper protective layer for the Memphis aquifer. However, hydrogeologic breaches within the UCCU create a hydraulic connection and provide an avenue for potential contaminant migration from the shallow to the Memphis aquifer. This research aims to minimize contaminant migration, mitigate risks, extend existing wells' life that may face water contamination, and find suitable locations for future well construction. Several strategies are developed addressing well depth, seasonal well operation, and mapping no‐drilling or red zones to provide practical solutions. A numerical groundwater modeling technique is developed for each strategy that includes stochastic simulation–optimization and customized simulation models depending on the strategy. The models result in thousands of numerical simulations for each scenario to identify recurring patterns of contaminant movement to and through the Memphis aquifer. The results indicate that optimum well positions (spatially and vertically) and modification to pumping can increase the life expectancy of wellfields, offer sustainable management of the Memphis aquifer, and reduce contaminant migration through 2050.

Possible Fault Communication between the Memphis Sand Aquifer and the Mississippi River

ABSTRACT The Eocene Memphis Sand aquifer is the major source of drinking water for municipalities in the upper Mississippi Embayment, with the city of Memphis, TN, being the largest consumer. Concerns about contamination of the Memphis aquifer from surface waters have primarily focused on local groundwater transmission through the Upper Claiborne aquitard and erosional windows through the aquitard. This study used recent fault mapping to show that faults extend upward from the Memphis aquifer to very near the surface in and adjacent to Memphis. The Meeman-Shelby and Cuba faults extend under the Mississippi River and upward to the base of the Mississippi River alluvium. Groundwater levels (potentiometric surface) in Memphis aquifer monitor wells H002 and LdF 004 and the Mississippi River water levels during the years of 2007 through 2011 show strong correlation (0.744829 and 0.779691, respectively). We believe this correlation may be due to direct connection through fault zones.

Characterization of Hydraulic Properties of the Memphis Aquifer by Conducting Pumping Tests in Active Well Fields in Shelby County, Tennessee

AbstractLimited availability of field measurements for aquifer parameters commonly leads to nonuniqueness of numerical model solutions. Six pumping tests were conducted in five municipal well fields within Shelby County following the procedure described in the ASTM D4050‐14 and using an additional qualitative matrix framework to achieve greater reliability. Drawdown data of the pumping tests were analyzed using AQTESOLV, which allowed for partial penetration of well screens and interference from neighboring production wells. The values of transmissivity have a combined range of 600–3,100 m2/day, which is 2–4 times less than published measures that used less robust data analysis and questionable or poorly documented methods. The range of storativity was 0.0005–0.002, and again, the resulting values have greater reliability than prior investigations. Robust quality assessment in the present methodology through assignment of a scoring decision matrix provides greater trust in the measurements. With a score at or above 10 considered optimal, the methodology and test environment resulted in an average score of 8.7, a vast improvement from prior investigations that together averaged 4.1. The calculated parameter values are an improvement on historical values, constraining the two critical groundwater hydraulic terms that should reduce modeling nonuniqueness of numerical modeling solutions that should lead to improved evaluation of local groundwater resources and environmental impacts.

Development of a Numerical Multi-Layered Groundwater Model to Simulate Inter-Aquifer Water Exchange in Shelby County, Tennessee

Inter-aquifer water exchange between the shallow and Memphis aquifers in Shelby County, Tennessee may pose a contamination threat due to the downward migration of younger, poor quality groundwater into deeper, more pristine aquifer. Discontinuities (breaches) in the upper Claiborne confining unit (UCCU) allow for leakage into the Memphis aquifer, a sand-dominated aquifer that provides about 95% of the groundwater used in the Memphis area. This study created a multi-layered 3D groundwater model for Shelby County using the United States Geological Survey’s MODFLOW-NWT program to evaluate water exchange for a simulation period from January 2005 to December 2016. Results indicate an overall leakage through the UCCU of 61 m3/min into the Memphis aquifer in Shelby County, accounting for 10% of its water budget inflow, with localized areas experiencing as much as 20% water exchange. As young water tends to stay in the upper part of the Memphis aquifer, water budget assessment for the upper 60 m of the Memphis aquifer revealed leakage representing 29% of the zone inflow, and as much as 53% in certain areas. More localized studies must be conducted to understand the location, characteristics, and orientation of the confining unit breaches, as well as the inter-aquifer water exchange.

Recharge pathways and rates for a sand aquifer beneath a loess-mantled landscape in western Tennessee, U.S.A.

Study region: Western Tennessee, U.S.A. Study focus: Recharge to aquifers that underlie loess and other fine-grained surficial deposits in western Tennessee as well as other areas is commonly impeded resulting in sensitivity in regard to sustainability of groundwater resources. This study investigates the role of preferential pathways of recharge to the regional Memphis aquifer in an area mantled by loess and fine-grained paleosols. New hydrological insights: Data gathered through a vadose-zone chloride mass balance analysis and a year of monitoring physical hydrologic, water chemistry, and environmental and applied tracer data within an upland watershed in western Tennessee indicate that recharge to the Memphis aquifer is dominated by lateral recharge of infiltrated stream water sources rather than vertical infiltration through loess-covered uplands. The results from this research challenge prevailing models of recharge to the Memphis aquifer and aquifers mantled by loess or other fine-grained soils in other regions that envision vertical recharge through fine-grained soils to be the dominant recharge mechanism.

Application of Numerical Tools to Investigate a Leaky Aquitard beneath Urban Well Fields

Memphis aquifer is the primary drinking water source in Shelby County (Tennessee, USA), and it supplies industrial, commercial, and residential water. Memphis aquifer is separated from the Shallow aquifer by a clayey layer known as the Upper Claiborne confining unit (UCCU). All of the production wells in the Memphis area are screened in the Memphis aquifer, or even deeper in the Fort Pillow aquifer. Traditionally, it was assumed that the UCCU could fully protect the Memphis aquifer from the contaminated Shallow aquifer groundwater. However, recent studies show that at some locations, the UCCU is thin or absent, which possibly leads to the contribution of Shallow aquifer to the Memphis aquifer. Accurately locating the breaches demands expensive and difficult geological or geochemical investigations, especially within an urban area. Hence, a pre-field investigation to identify the locations where the presence of breaches is likely can significantly reduce the cost of field investigations and improve their results. In this study, to identify the locations where the presence of breaches in the UCCU is likely, we develop a reliable MODFLOW-based numerical model, and use three different analyses: (1) pilot-point calibration (PPC), (2) velocity and flow budget (VFB), and (3) particle tracking (PT), to post-process the developed groundwater model results. These pre-field numerical investigations provide relevant and defensible explanations for groundwater flow anomalies in an aquifer system for informed decision-making and future field investigations. In this study, we identify five specific zones within the broad study area which are reasonable candidates for the future field investigations. Finally, we test the results of each analysis against other evidence for breaches, to demonstrate that the results of the numerical analyses are reliable and supported by previous studies.

Fraction of young water as an indicator of aquifer vulnerability along two regional flow paths in the Mississippi embayment aquifer system, southeastern USA

Wells along two regional flow paths were sampled to characterize changes in water quality and the vulnerability to contamination of the Memphis aquifer across a range of hydrologic and land-use conditions in the southeastern United States. The flow paths begin in the aquifer outcrop area and end at public supply wells in the confined parts of the aquifer at Memphis, Tennessee. Age-date tracer (e.g. SF6, 3H, 14C) data indicate that a component of young water is present in the aquifer at most locations along both flow paths, which is consistent with previous studies at Memphis that documented leakage of shallow water into the Memphis aquifer locally where the overlying confining unit is thin or absent. Mixtures of young and old water were most prevalent where long-term pumping for public supply has lowered groundwater levels and induced downward movement of young water. The occurrence of nitrate, chloride and synthetic organic compounds was correlated to the fraction of young water along the flow paths. Oxic conditions persisted for 10 km or more down dip of the confining unit, and the presence of young water in confined parts of the aquifer suggest that contaminants such as nitrate-N have the potential for transport. Long-term monitoring data for one of the flow-path wells screened in the confined part of the aquifer suggest that the vulnerability of the aquifer as indicated by the fraction of young water is increasing over time.

Application of Environmental Tracers in the Memphis Aquifer and Implication for Sustainability of Groundwater Resources in the Memphis Metropolitan Area, Tennessee

AbstractFourteen years of results from environmental tracer sampling for 3H, He isotopes, noble gases, and SF6 in production wells screened in the Memphis aquifer in Memphis, Tennessee, are presented and used to determine the age and mass proportion of modern water (&lt;60 years old) pumped from this regionally important public supply aquifer. The results indicate persistent presence of modern water in water pumped from one or more production wells in most of the well fields sampled. The percentage of modern water is quantified by three methods: tritium‐loading history, inverse geochemical modeling, and lumped parameter modeling. The mixing percentages and ages determined from each technique are generally in agreement, but also emphasize the unique information provided by each tracer and associated modeling approaches. The implications of the tracer data for sustainability and vulnerability are also considered, especially in regard to potential water quality threats to the water supply in the Memphis, Tennessee, metropolitan area.

Pre‐Development Groundwater Conditions Surrounding Memphis, Tennessee: Controversy and Unexpected Outcomes

AbstractReliance on groundwater resources by differing governing bodies can create transboundary disputes raising questions of ownership and apportionment as the resource becomes strained through overuse or threatened by contamination. Transboundary disputes exist at varying scales, from conflicts between countries to smaller disputes between intrastate jurisdictions. In 2005 within the United States, the State of Mississippi filed a lawsuit against its political neighbor and their utility, the City of Memphis and Memphis Light, Gas, and Water, for groundwater deemed owned by the State of Mississippi to be wrongfully diverted across the state line and into Tennessee by the defendants. The basis of the lawsuit was potentiometric maps of groundwater levels for the Memphis aquifer that showed under suggested pre‐development conditions no flow occurring across the Mississippi‐Tennessee state line, but subsequent historic potentiometric maps show a cone of depression under the City of Memphis with a clear northwesterly gradient from Mississippi into Tennessee. The suggested pre‐development conditions were derived from limited groundwater level observations between 41 and 74 years post‐development. A new pre‐development map is constructed using historic records that range 0‐17 years post‐development that shows the natural flow is northwesterly from Mississippi into Tennessee and transboundary groundwater quantities have actually decreased since pre‐development conditions.

Stream Loss Contributions to a Municipal Water Supply Aquifer in Memphis, Tennessee

Leakage of water into the semi-confined Memphis aquifer near the Sheahan well field in Memphis, Tennessee, has been suggested by many studies extending back to the 1960s. A leakage pathway from Nonconnah Creek to the Sheahan well field was investigated over a 1-year period using multiple approaches: (1) stream discharge data, (2) hydraulic head data, (3) geochemical and environmental tracer (3H/3He and chlorofluorocarbon) studies, and (4) geochemical modeling. The stream loss data strongly support the conclusion that losses from the creek contribute at least 2,600 m3/d to the shallow aquifer, and hydraulic head data consistently indicate the majority of this infiltrated surface water flows within the shallow aquifer toward the Sheahan well field. Tritium-3He data from shallow aquifer monitoring wells within the Sheahan well field are composed of modern water with ages that generally increase with distance from the creek toward the well field. The leakage pathway from the creek to the well field is interpreted to involve seepage of creek waters into the shallow aquifer, mixing with other water sources and chemical reactions as the infiltrated waters flow in the shallow aquifer along a paleovalley toward the Sheahan well field, as well as leakage from the shallow to the Memphis aquifer through a hydrologic window in the upper Claiborne confining unit. The results of this work emphasize the utility of multiple investigative approaches in studies of groundwater flow to well fields, but they also illustrate the complexity of groundwater flow and groundwater vulnerability in a stressed water-supply aquifer system.

Changes in Shallow Groundwater Quality Beneath Recently Urbanized Areas in the Memphis, Tennessee Area1

Barlow, Jeannie R.B., James A. Kingsbury, and Richard H. Coupe, 2012. Changes in Shallow Groundwater Quality Beneath Recently Urbanized Areas in the Memphis, Tennessee Area. Journal of the American Water Resources Association (JAWRA) 48(2): 336‐354. DOI: 10.1111/j.1752‐1688.2011.00616.xAbstract: Memphis, the largest city in the state of Tennessee, and its surrounding suburbs depend on a confined aquifer, the Memphis aquifer, for drinking water. Concern over the potential for downward movement of water from an overlying shallow aquifer to the underlying Memphis aquifer provided impetus for monitoring groundwater quality within the shallow aquifer. The occurrence of volatile organic compounds (VOCs), nitrate, and pesticides in samples from the shallow well network indicate a widespread affect on water quality from the overlying urban land use. Total pesticide concentration was generally higher in more recently recharged groundwater indicating that as the proportion of recent water increases, the occurrence of pesticides related to the current urban land use also increases. Groundwater samples with nitrate concentrations greater than 1.5 mg/l and detectable concentrations of the pesticides atrazine and simazine also had higher concentrations of chloroform, a VOC primarily associated with urban land use, than in other samples. The age of the water from these wells indicates that these concentrations are most likely not representative of past agricultural use, but of more recent urban use of these chemicals. Given that the median age of water represented by the shallow well network was 21 years, a lag time likely exists between changes in land use and the occurrence of constituents related to urbanization in shallow groundwater.

Resolving the source and mixing proportions of modern leakage to the Memphis aquifer in a municipal well field using geochemical and 3H/3He data, Memphis, Tennessee, USA

The Memphis aquifer in southwestern Tennessee is a confined to semi-confined unconsolidated sand aquifer and is the primary municipal water source for the Memphis metropolitan area. Leakage of modern water from shallow unconfined aquifers through the upper Claiborne confining unit locally degrades water quality in the Memphis aquifer and makes the aquifer more vulnerable to contamination. Major solute chemistry, tritium, and 3H/3He data were used to investigate the source and mixing proportions of modern water in the Memphis aquifer in the Davis well field, Memphis, Tennessee. Water quality in several production wells in the Davis well field has declined over the past 30 years, mainly through increased total dissolved solids, iron, alkalinity, and hardness. Trends in chemical data, tritium, and other hydrogeologic data support a source for the leakage from the Mississippi River Valley Alluvial aquifer. Mixing proportions of alluvial water in production well water obtained by inverse chemical modeling with PHREEQCi range from 7 to 45%. For two of the production wells, MLGW 414 and 432, 3H/3He data yield mixing ratios similar to those obtained from PHREEQCi in three of four cases; the dissimilar ratio is likely due to a poor solution from the PHREEQCi inverse modeling. Modeling of the age distribution obtained from MLGW 414 and 432 using an exponential-piston flow model (EPM) with an inverse solution computer code yielded mixed results. The EPM solution obtained for MLGW 414 converged with difficulty only for a 5-year transit time in the shallow aquifer and is consistent with a source from the Mississippi River Valley Alluvial aquifer; however, the modeled age of the water is greater than that observed. In comparison, the EPM solution for MLGW 432 converged for the 5- and 7-year transit periods in the shallow aquifer and yielded model ages consistent with observed 3H/3He ages; however, the extent of the maximum radii for infiltration source is not consistent with a Mississippi River Valley Alluvial aquifer source. Other potential sources for leakage to MLGW 432 include water from the Fluvial-terrace aquifer migrating along a fault east of the well field or infiltration of water from a lake south of the well field.

Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part II: data analysis, interpretation and traveltime tomography

ABSTRACTWe collected P‐wave seismic reflection data to image suspected breaches in the confining unit (aquitard) above the Memphis aquifer in Memphis, Tennessee, USA, where previous studies of water quality have suggested potential contamination. A 1‐km‐long reflection line was acquired across depressions of the water table that have been interpreted to reflect a breach in the aquitard. Although raw data are dominated by surface waves, after frequency filtering, shot static correction and filtering, consistent reflections can be observed. Integration of constant velocity analysis (CVS) and super gather semblance analysis was used to determine the RMS velocity field. The stack section shows a set of shallow reflectors interpreted to correspond to the top and bottom of the confining unit. The aquitard thins by about half from the north end to the south end along the reflection line. The continuity of the top and bottom of the aquitard was disrupted by a few faults interpreted along the line. None of these faults juxtapose the surficial aquifer against the Memphis aquifer but it is possible that the faults themselves may act as a hydraulic connection between the surficial aquifer and the Memphis aquifer and thus serve as pathways for a potential leakage. The refraction tomography technique was applied to the first arrival data and it revealed three depressions that are interpreted as paleochannels on the upper part of the confining unit, which is consistent with the background geology of the area. These inferred paleochannels may suggest that ancient river channel erosion may contribute to the process responsible for the formation of aquitard breaches in this area.

Mapping an aquitard breach using shear-wave seismic reflection

In multi-layered hydrostratigraphic systems, aquitard breaches caused by faulting or paleo-erosion can allow substantial quantities of water of differing quality to be exchanged between aquifers. Seismic reflection technology was used to map the extent and orientation of an aquitard breach connecting a shallow alluvial aquifer to the deeper semi-confined Memphis aquifer in southwestern Tennessee, USA. Geophysical well logs indicate the presence of the aquitard at borehole locations that define the beginning and end points on two seismic survey lines, which intersect at a borehole where the aquitard is absent. A SE–NW-oriented paleochannel, 350 m wide and approximately 35–40 m deep, is interpreted from the seismic reflection surveys. The paleochannel cuts through the aquitard and into the upper part of the Memphis aquifer, thus creating a hydraulic connection between the shallow unconfined and deeper, semi-confined aquifers. The results indicate the potential of the shear-wave seismic reflection methods to resolve shallow breaches through fine-grained aquitards given availability of sufficient well control.

Case Study of the Sheahan Wellfield Using H3∕He3 Field Data to Determine Localized Leakage Areas

Source water protection zones are not easily determined for semiconfined aquifers with highly localized areas of leakage from overlying shallow aquifers. This research is a companion case study to a theoretical study detailing the inverse modeling of aquifer mixing zones using age-distribution models. Where the first study detailed the capabilities of inverse age-distribution modeling given various sets of environmental tracer data and prior information, this study demonstrates the successful application of the technique to a real-world problem with a robust conceptual model verified in the current literature. The case study presented here considers a hydrogeologic setting in the northern Mississippi Embayment where highly localized leakage features exist between a shallow alluvial aquifer and the Memphis aquifer. Geochemical analyses, environmental, and radiochemical tracers have been used to develop and verify a conceptual model of the flow system at the Sheahan Wellfield in Memphis, Tenn. This study us...

Virus Transport during Infiltration of a Wetting Front into Initially Unsaturated Sand Columns

We investigated the effect of different flow conditions on the transport of bacteriophage phiX174 in Memphis aquifer sand. Virus transport associated with a wetting front moving into an initially unsaturated horizontal sand column was experimentally compared with that observed under steady-state saturated vertical flow. Results obtained by sectioning the sand columns showthattotal (retained and free) resident virus concentrations decreased approximately exponentially with the travel distance. The rate of decline was similar under both transient unsaturated flow and steady-state saturated flow conditions. Total resident virus concentrations near the inlet were an order of magnitude greater than the virus concentration of the influent solution in both experiments, indicating continuous virus sorption during flow through this zone. Virus retardation was quantified using the ratio of the centroids of the relative saturation and virus concentration versus relative distance functions. The mean retardation factors were 6.43 (coefficient of variation, CV = 14.4%) and 8.22 (CV = 8.22%) for the transient unsaturated and steady-state saturated flow experiments, respectively. Attest indicated no significant difference between these values at P < 0.05. Air-water and air-water-solid interfaces are thought to enhance virus inactivation and sorption to solid particles. The similar retardation factors obtained may be attributable to the reduced presence of these interfaces in the two flow systems investigated as compared to steady-state unsaturated flow experiments in which these interfaces occur throughout the entire column.

Resolving aquifer behavior near a focused recharge feature based upon synoptic wellfield hydrogeochemical tracer results

Aquitard windows are known to occur in unconsolidated aquifer systems and represent areas of focused recharge to otherwise confined underlying aquifers. The study herein focused on a known window site affecting the Memphis aquifer located in the Sheahan wellfield in Shelby County, Tennessee. Uranium- and thorium-series radioisotopes were evaluated from production wells sampled in the wellfield and compared to other geochemical characteristics of the system. From the research, a conceptual model was realized whereby higher uranium concentrations from near-surface waters flow through a redox barrier in the aquitard window and become depleted in uranium. Further, 234U/ 238U activity ratio increased downgradient of the recharge source by alpha-recoil mobilization of 234U via 234Th, and possibly leaching of uranium bound by iron hydroxides via dissolution and subsequent precipitation by reducing waters flowing to downgradient wells. The behavior of the hydrogeochemical tracers could be best described in the conceptual model by normalizing uranium to Fe 2+ throughout the system. The use of the [U]–[Fe 2+] pair provides a useful tool for understanding of the mixing behavior near focused recharge sites with redox barriers and may be useful for developing aquifer behavior models.

Efficacy of Genetic Algorithm to Investigate Small Scale Aquitard Leakage

The movement of water from a shallow aquifer to a semiconfined aquifer through a localized area of leakage in an aquitard, designated as an aquitard window, is an area of concern in certain gulf coastal plain aquifer settings. Locating these windows, or areas of leakage, has been historically arduous. The purpose of this study was to evaluate the use of a genetic algorithm (GA) technique as an inverse technique for locating highly probable areas of leakage based upon aquifer head data near the suspected area of leakage. The GA technique has previously been tested on synthetic data alone, and this research offers insight into the capabilities of the technique with actual field data from a site with a known aquitard window. Research sites with historical and ongoing data collection efforts are rare and offer the best opportunity for robust evaluation of new methodologies. Based upon the GA technique, the steady-state accretion flux to the Memphis aquifer at the Shelby Farms study site was determined to be 12,346 m3/day with a standard deviation of 821 m3/day and compared well with the range from 510 to 9,911 m3/day previously determined for the site.