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Abstract In 1997 natural-gamma logs were made in 31 monitoring wells ranging from 30 to 109 feet in depth to determine the stratigraphy of the shallow hydrogeologic units in the Memphis, Tennessee, area. The hydrogeologic units studied in this investigation include the alluvium and loess of Holocene and Pleistocene age, the fluvial deposits of Pleistocene and Pliocene(?) age, and the Jackson Formation, Cockfield Formation, Cook Mountain Formation, and Memphis Sand of Eocene age. The gamma logs for these wells were interpreted by considering many sources of information, including published and unpublished geologic maps, logs based on auger cuttings and driller’s information, potentiometric-surface maps, split-spoon samples, and the character of the gamma-log curves. Introduction In 1991, the U.S. Geological Survey (USGS) began full-scale implementation the National Water-Quality Assessment (NAWQA) program. The long-term goals of the NAWQA Program are to describe the status of and trends in the quality of a large part of the Nation’s water resources and to identify the major natural and human factors that affect the quality of these resources. To achieve these goals, 60 hydrologic systems that include parts of most major river basins and aquifer systems were selected for study. The Mississippi Embayment study unit (MISE)(fig. 1) is one such hydrologic system (Mallory, 1994) . |
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One of the issues under investigation within the MISE study area is the effect of urban development on the quality of shallow ground water in the Memphis, Tennessee, area. A shallow, unconfined to semi-confined aquifer consisting of alluvial and fluvial deposits underlies most of the Memphis area. This shallow aquifer generally is separated from the deeper Memphis aquifer by a confining unit. The Memphis aquifer is the primary source of water supply for the city of Memphis. A network of 31 monitoring wells was installed within the uppermost aquifer of the Memphis area (fig. 2) so that samples could be collected and researchers could assess water quality. As part of this effort, natural-gamma logs were made in 31 monitoring wells ranging from 30 to 109 feet in depth to determine the stratigraphy of the hydrogeologic units. |
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Purpose and Scope The purpose of this report is to describe the stratigraphy of the hydrogeologic units at each of the 31 monitoring wells. The report presents the natural-gamma-log data that were measured for the Mississippi Embayment study unit monitoring wells in the Memphis, Tennessee, area and interpretations of these logs. Acknowledgments The author would like to thank William S. Parks (USGS) for significant technical contributions to this report; Gerard J. Gonthier (USGS) for providing sample data, field logs, and review; Larry B. Thomas and William K. Kelly (USGS) for their efforts in installing the monitoring wells; and Michael J. Mallory, William T. Oakley, and Bobby R. Richards (USGS) for their efforts in acquiring the gamma logs. Hydrogeologic Setting The hydrogeologic units of interest to this investigation include the alluvium and loess of Holocene and Pleistocene age, the fluvial deposits of Pleistocene and Pliocene(?) age, and the Jackson Formation, Cockfield Formation, Cook Mountain Formation, and Memphis Sand of Eocene age (fig. 3). The lower part of the alluvium, the fluvial deposits, and the Memphis Sand are aquifers; the upper part of the alluvium, the loess, and the Cockfield and Cook Mountain Formations generally are confining units. The Jackson, Cockfield, and Cook Mountain Formations are referred to in this report as the "Jackson-upper Claiborne confining unit" after Parks (1990). |
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The alluvium consists of gravel, sand, silt, and clay that were deposited by streams. In the western part of the Memphis area, the alluvium comprises the Mississippi River alluvial aquifer, a regionally continuous and heavily pumped source of fresh water. Alluvium of interest to this investigation, however, consists of local deposits that underlie the alluvial plain of Nonconnah Creek. Loess is predominantly composed of silt that has primarily been deposited by the wind. In Tennessee and southward into Mississippi, these deposits generally are thickest in the hills immediately adjacent to the eastern edge of the Mississippi River alluvial plain and thin eastward (Parks, 1993). Loess deposits encountered in this investigation generally ranged from 5 to 40 feet in thickness. In the Memphis area, loess deposits often contain geosols, which are relatively thin layers characterized by relatively high gamma-ray emissions (Parks, 1993). The fluvial deposits consist primarily of sand and gravel transported and deposited by streams. These deposits occur beneath the uplands and valley slopes in the Memphis area, and are highly variable in thickness due to erosional surfaces at the top and base of the deposits (Graham and Parks, 1986). The fluvial deposits also have been referred to as "terrace deposits," the "water-table aquifer," or the "fluvial deposits aquifer" in the Memphis area. The fluvial deposits aquifer provides water to domestic and farm wells in rural areas (Parks, 1990). The Jackson-upper Claiborne confining unit includes equivalents of the Jackson, Cockfield, and Cook Mountain Formations (fig. 3), which consist primarily of clay, silt, sand, and lignite. This confining unit generally separates the alluvium and fluvial deposits from the Memphis Sand in the Memphis area; however, the confining unit is highly variable in thickness and locally may be absent. The Jackson-upper Claiborne confining unit thins to the east until it is absent. The depositional extent of the confining unit occurs near the Shelby County boundary in the eastern part of the Memphis area (Parks, 1990). The Memphis Sand consists primarily of a body of sand ranging from 650 to 900 feet in thickness with minor lenses of clay and silt (fig. 3). The Memphis Sand is stratigraphically equivalent to the Tallahatta Formation, Winona Sand, Zilpha Clay, and Sparta Sand of Mississippi, and the Carrizo Sand, Cane River Formation, and Sparta Sand of southern Arkansas (Hosman, 1988). The Memphis Sand is an aquifer that is regional in extent, generally under confined conditions, and the primary source of fresh water for the Memphis area and western Tennessee (Parks and Carmichael, 1990). Methodology Gamma radiation occurs naturally in geologic materials due to gamma-emitting radioisotopes of potassium-40 and nuclides of the radium and thorium decay series. Measurements of natural-gamma radiation were made from January 27 to January 29, 1997, in 31 monitoring wells that had 2-inch polyvinyl chloride (pvc) casings and ranged from 30 to 109 feet in depth in the Memphis, Tennessee, area. A logger with a sodium iodide detector was used to measure natural-gamma radiation in counts per second. Because the rate at which the detection probe is lowered in the well affects the resolution of the acquired data, the probe was lowered at a constant rate of 15 feet per minute in all of the wells. The equipment was checked prior to field operations by logging a well with existing geologists’ and natural-gamma logs and then comparing the results. Gravel, sand, silt, and clay are the primary sediment types in the shallow subsurface of the Memphis, Tennessee, area. Therefore, the following general assumptions were applied in making interpretations: (1) clay and silt have the highest levels of gamma radiation; (2) sand and gravel have the lowest levels. Although these general assumptions may not be true elsewhere, they generally are confirmed for the Memphis area by analyses of split-spoon samples and geologists’ logs. Gamma logs made during this investigation were interpreted by considering many sources of information. Published and unpublished maps were used in determining the base of the alluvial and fluvial deposits (Parks, 1973, 1975, 1977, 1979, 1987, 1990) and the top of the Memphis Sand (Kingsbury and Parks, 1993). Logs based on auger cuttings and driller’s information were used to aid in the interpretations of the natural-gamma and lithologic logs; however, auger cuttings are returned from depth to the surface and are subject to mixing with cuttings from overlying deposits. Therefore this information was used with caution. All of the monitoring wells were screened at the bottom 10-foot interval. Potentiometric-surface maps of the Memphis aquifer (Kingsbury, 1996) and of the fluvial deposits aquifer (Parks, 1990) were helpful in determining the hydrogeologic unit in which the wells were screened by comparing the published water levels with water levels measured at each well at the time the gamma logs were made. It should be noted that the water-level measurements made during this investigation were taken prior to well development; however, the water levels from the monitoring wells were generally in agreement with published water levels from the corresponding aquifer. In addition, water levels measured just prior to and after well development were within 0.5 foot of agreement for each of the wells–the exception being the water level at UR26 which dropped more than 50 feet after the well was developed (G.J. Gonthier, USGS, written commun., 1997). Two split-spoon samples, each measuring 2 feet in length, were generally taken at various depths during construction of each of the monitoring wells. The analyses of these samples provided a field calibration for interpreting the gamma logs for most of the wells. Finally, the character of the curves produced by plotting counts per second of gamma radiation versus depth was used to determine the top and bottom altitudes of the hydrogeologic units. After analyzing all of the gamma logs, certain patterns emerged. An idealized log was created (fig. 4) to illustrate these general patterns. |
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However, the thickness and presence of a particular hydrogeologic unit varies from well to well, as can the level of gamma radiation for a given hydrogeologic unit. Generally, the different hydrogeologic units encountered in this investigation produced different levels of gamma emissions. Therefore, transitions between hydrogeologic units generally are reflected on the gamma logs as either a gradual or sharp change in the slope of the curve. The depth to a change in hydrogeologic units generally was chosen at the midpoint of this slope of the curve on the gamma log produced by either the increasing or decreasing gamma emissions (see fig. 4 for examples). This placement of the depth to the change in hydrogeologic units is consistent with previous interpretations of gamma logs in the Memphis area (W.S. Parks, USGS, oral commun., 1997). Discussion Of Results The results of natural-gamma-radiation measurements are plots of the number of counts per second of gamma radiation detected versus depth. Levels of naturally occurring gamma radiation vary not only with the type of geologic material encountered (such as clay or sand), but may also vary areally for the same type of geologic material. Therefore, the results of natural-gamma-radiation measurements are subject to interpretation. Several different factors were considered in interpreting each gamma log. However, the general criteria, based on the characteristics of the gamma curves, for determining the top and bottom elevations of the hydrogeologic units, as well as some general observations for the gamma logs (figs. 5-35, access them through fig. 2) are as follows:
The characteristics of the gamma-log curves and the observations discussed above may be used as a guide to understand the interpretations presented in this report. However, due to the somewhat subjective nature of gamma-log interpretations, a different investigator may have a different interpretation of the tops and bottoms of the hydrogeologic units investigated in this study. Summary In 1991, the U.S. Geological Survey began the NAWQA program to describe the status of and trends in the quality of a large part of the Nation’s water resources and to identify the major natural and human factors that affect the quality of these resources. One of the issues under investigation within the Mississippi Embayment study unit is the effect of urban development on the quality of shallow ground water in the Memphis, Tennessee, area. A network of 31 monitoring wells was installed within the uppermost aquifer of the Memphis area so that samples could be collected and researchers could assess water quality. As part of this effort, natural-gamma logs were made in 31 monitoring wells ranging from 30 to 109 feet in depth to determine the stratigraphy of the hydrogeologic units. The hydrogeologic units of interest to this investigation include alluvium, loess, fluvial deposits, the Jackson Formation and upper part of the Claiborne Group, and the Memphis Sand. The gamma logs that were made during this investigation were interpreted by considering many sources of information, including published and unpublished geologic maps, logs based on auger cuttings and driller’s information, potentiometric-surface maps, split-spoon samples, and the character of the gamma-log curves. The loess deposits generally have a natural-gamma radiation level of about 50 to 80 counts per second. A spike in the gamma logs occurs occasionally near the base of the loess deposits indicating a high level of gamma radiation for only a few feet of the layer’s thickness. The spike may indicate a geosol, or ancient soil layer within the loess. The fluvial deposits generally are directly beneath the loess deposits. The top of the fluvial deposits generally is characterized by a gradual or sharp decrease in gamma emissions. Within the fluvial deposits there is infrequently a spike in the gamma logs indicating a high level of gamma radiation for only a few feet of the layer’s thickness; these spikes are interpreted to be clay lenses. Logs based on auger cuttings and driller’s information indicate a general coarsening of the fluvial deposits with depth. The gamma logs may reflect the coarsening of deposits with depth as gamma radiation decreases due to decreasing amounts of clay and silt which have relatively high levels of gamma radiation. The Jackson-upper Claiborne confining unit generally is directly beneath the fluvial deposits. The top of the Jackson-upper Claiborne confining unit generally is characterized by a gradual or sharp increase in gamma emissions. It is often very difficult to distinguish between the bottom of the Jackson-upper Claiborne confining unit and the top of the Memphis Sand. The upper part of the Memphis Sand is lithologically similar to the Jackson-upper Claiborne confining unit. In addition, the Jackson-upper Claiborne confining unit may also contain significant lenses of sand. Within the Memphis Sand there is occasionally a spike in the gamma logs indicating a high level of gamma radiation for only a few feet of the layer’s thickness, similar to the spike infrequently observed in the fluvial deposits. These spikes are interpreted to be clay lenses. Alluvial deposits were interpreted on several logs based on the physiography of the site. Gamma logs of these deposits appear quite similar to gamma logs of the loess deposits because much of the alluvium consists of reworked or colluviated loess. References Graham, D.D., and Parks, W.S., 1986, Potential for leakage
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