Water-Resources Investigations Report 92-4092 

GEOCHEMISTRY OF AND RADIOACTIVITY 

IN GROUND WATER OF THE HIGHLAND 

RIM AND CENTRAL BASIN AQUIFER 

SYSTEMS, HICKMAN AND MAURY 

COUNTIES, TENNESSEE 

URANIUM-23.5 

c 

THORIUM-232 

Prepared by the 

U.S. GEOLOGICAL SURVEY 

in cooperation with the 

TENNESSEE DEPARTMENT OF ENVIRONMENT AND CONSERVATION 
GEOCHEMISTRY OF AND RADIOACTIVITY 

IN GROUND WATER OF THE HIGHLAND 

RIM AND CENTRAL BASIN AQUIFER 

SYSTEMS, HICKMAN AND MAURY 

COUNTIES, TENNESSEE 

By Gregg E. Hileman and Roger W. Lee 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 92-4092 

Prepared in cooperation with the 

TENNESSEE DEPARTMENT OF ENVIRONMENT AND CONSERVATION 

Nashville, Tennessee 

1993 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

For additional information write to: Copies of this report can be purchased from: 

District Chief 

U.S. Geological Survey 

810 Broadway, Suite 500 

Nashville, Tennessee 37203 

U.S. Geological Survey 

Books and Open-File Reports Section 

Federal Center 

Box 25425 

Denver, Colorado 80225 
CONTENTS 

Abstract 1 

Introduction 1 

Purpose and scope 2 

Description of the study area 2 

Hydrogeologic setting 2 

Geochemistry of ground water 7 

Major constituents and physical properties 7 

Geochemical processes 12 

Radioactivity in ground water 15 

Radiochemical constituents 16 

Relations between constituents 20 

Summary and conclusions 22 

Selected references 24 

ILLUSTRATIONS 

Figure 1. Map showing physiographic provinces of Tennessee, study area, 

and idealized section through part of the Highland Rim 

and Central Basin provinces 3 

2. Map showing location of wells and springs sampled for study 4 

3. Boxplots showing major ion constituents in ground water of 

the Highland Rim and Central Basin aquifer systems, 

Hickman and Maury Counties, Tennessee 10 

4. Trilinear diagram illustrating principal ion composition of 

ground water at 13 sites tapping the Highland Rim aquifer 

system in Hickman County, Tennessee 11 

5. Trilinear diagram illustrating principal ion composition of 

ground water at 27 sites tapping the Central Basin aquifer 

system in Hickman and Maury Counties, Tennessee 12 

6-9. Graphs Showing: 

6. Calcite and dolomite saturation indexes of ground water from the 

Highland Rim and Central Basin aquifer systems, Hickman and 

Maury Counties, Tennessee, calculated using the WATEQF program 13 

7. Calcite and gypsum saturation indexes of ground water from the 

Highland Rim and Central Basin aquifer systems, Hickman and 

Maury Counties, Tennessee, calculated using the WATEQF program 14 

8. Relation between dissolved calcium plus dissolved magnesium and total 

inorganic carbon in ground water of the Highland Rim and Central 

Basin aquifer systems, Hickman and Maury Counties, Tennessee 15 

9. Relation between dissolved calcium and dissolved magnesium in ground 

water of the Highland Rim aquifer system, Hickman County, Tennessee 16 

10. Schematic diagram showing uranium-238 and thorium-232 radioactive decay series 

11. Boxplots showing selected radioactive constituents in ground water of the 

Highland Rim and Central Basin aquifer systems, Hickman and Maury 

Counties, Tennessee 19 

12. Map showing distribution of radon-222 in ground water of the study area 21 

17 

Contents iii 
13-16. Graphs showing: 

13. Relation between dissolved calcium and dissolved uranium in 

ground-water samples from the Central Basin aquifer system, 

Hickman and Maury Counties, Tennessee 22 

14. Relation between pH and total radon-222 in ground-water samples 

from the Highland Rim aquifer system, Hickman County, Tennessee 

15. Relation between dissolved phosphorus and dissolved uranium in 

ground-water samples from the Highland Rim aquifer system, 

Hickman County, Tennessee 23 

16. Relation between dissolved phosphorus and dissolved uranium in 

ground-water samples from the Central Basin aquifer system, 

Hickman and Maury Counties, Tennessee 23 

TABLES 

Table 1. Bedrock units underlying the study area 5 

2. Summary statistics of ground-water quality, exclusive of radionuclides, 

Highland Rim and Central Basin aquifer systems, Hickman and 

Maury Counties, Tennessee 8 

3. Summary statistics of selected radionuclides in ground water of the 

Highland Rim and Central Basin aquifer systems, Hickman and 

Maury Counties, Tennessee 18 

22 

iv Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED WATER-QUALITY UNITS 

MultiDlv Bv To obtain 

foot (rt) 

mile (mi) 

0.3048 meter 

1.609 kilometer 

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929-a geodetic datum derived from 

a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. 

Water-aualitv units 

mg/L 

pci5 

@cm 

Other Abbreviations 

R* 

milligrams per liter 

picocuries per liter 

microsiemens per centimeter at 25 degrees Celsius 

coefficient of correlation squared 

Conversion factors, vertical datum, and V 

abbreviated water-quality units 
GEOCHEMISTRY OF AND RADIOACTIVITY 

IN GROUND WATER OF THE HIGHLAND 

RIM AND CENTRAL BASIN AQUIFER 

SYSTEMS, HICKMAN AND MAURY 

COUNTIES, TENNESSEE 

By Gregg E. Hileman and Roger W. Lee 

ABSTRACT 

A reconnaissance of the geochemistry of and 

radioactivity in ground water from the Highland Rim 

and Central Basin aquifer systems in Hickman and 

Maury Counties, Tennessee, was conducted in 1989. 

Water in both aquifer systems typically is of the 

calcium or calcium magnesium bicarbonate type, but 

concentrations of calcium, magnesium, sodium, potassium, chloride, and sulfate are greater in water of the 

Central Basin system; differences in the concentrations 

are statistically significant. Dissolution of calcite, 

magnesium-calcite, dolomite, and gypsum are the 

primary geochemical processes controlling groundwater chemistry in both aquifer systems. Saturationstate calculations using the computer code WATEQF 

indicated that ground water from the Central Basin 

system is more saturated with respect to calcite, 

dolomite, and gypsum than water from the Highland 

Rim system. Geochemical environments within each 

aquifer system are somewhat different with respect to 

dissolution of magnesium-bearing minerals. Water 

samples from the Highland Rim system had a fairly 

constant calcium to magnesium molar ratio, implying 

congruent dissolution of magnesium-bearing minerals, 

whereas water samples from the Central Basin system 

had highly variable ratios, implying either incongruent 

dissolution or heterogeneity in soluble constituents of 

the aquifer matrix. 

Concentrations of radionuclides in water were 

low and not greatly different between aquifer systems. 

Median gross alpha activities were 0.54 picocuries per 

liter in water from each system; median gross beta 

activities were 1.1 and 2.3 picocuries per liter in water 

from the Highland Rim and Central Basin systems, 

respectively. Radon-222 concentrations were 559 and 

422 picocuries per liter, respectively. Concentrations 

of gross alpha and radium in all samples were substantially less than Tennessee’s maximum permissible levels 

for community water-supply systems. The data indicated no relations between concentrations of dissolved 

radionuclides (uranium, radium-226, radium-228, 

radon-222, gross alpha, and gross beta) and any key 

indicators of water chemistry, except in water from the 

Highland Rim system, in which radon-222 was 

moderately related to pH and weakly related to 

dissolved magnesium. The only relation among 

radiochemical constituents indicated by the data was 

between radium-226 and gross alpha activity; this 

relation was indicated for water from both aquifer 

systems. 

INTRODUCTION 

Much of the ground water used for domestic 

and municipal supply in Middle Tennessee is 

withdrawn from two regional aquifer systems: the 

Highland Rim aquifer system and the Central Basin 

aquifer system. Both systems are comprised of 

sedimentary rock that includes phosphatic limestone and shale. Low-level radioactivity commonly is associated with phosphatic limestones and 

black shales, and results from the incorporation of 

uranium and other radionuclides into the rock 

matrix. Because the radioactive constituents are 

soluble, ground water percolating through these 

rocks has acquired small concentrations of 

radionuclides. 

Despite awareness that ground water from 

these aquifer systems contains low concentrations 

of radionuclides and that the long-term ingestion of 

water containing radioactive matter presents 

potential health risks, little study has been made of 

the identity and concentrations of radionuclides in 

ground water or of the geochemical environments 

in which they are either gained or removed from 

solution. To provide water managers with data 

necessary to ensure the suitability of the water 

Introduction 1 
source for public and domestic supply, and to 

enhance knowledge of the geochemistry of these 

systems, the U.S. Geological Survey (USGS), in 

cooperation with the Tennessee Department of 

Environment and Conservation (formerly called the 

Tennessee Department of Health and Environment), conducted a reconnaissance of the quality of 

ground water from the two regional aquifer 

systems during spring and summer 1989. The 

study was limited to Hickman and Maury Counties 

(fig. I), which were selected as representative of 

areas where each aquifer system occurs within a 

few hundred feet of land surface. 

Objectives of the study were to identify and 

measure concentrations of selected radionuclides in 

ground water from each of the two aquifer 

systems, to characterize the geochemical environment of ground water flowing through the two 

aquifer systems, and to determine if a relation 

exists between water geochemistry and radionuclide concentrations. The first objective was met 

with the publication of data collected during the 

investigation (Hileman, 1990). 

Purpose and Scope 

The purpose of this report is to present 

interpretive results of the study described above. 

The report describes the hydrogeology of the area, 

summarizes results of the chemical analyses, 

discusses major geochemical processes, and 

examines possible relations between radionuclides 

and some of the constituents for which analyses 

were made. 

To characterize water quality in the 

Highland Rim and Central Basin aquifer systems, 

17 wells and 3 springs from Hickman County and 

20 wells from Maury County were sampled and 

the water analyzed for selected water-quality 

constituents and characteristics. Chemical and 

radiochemical analyses of ground water from the 

two aquifer systems are summarized and compared. Geochemical environments are characterized by calculating saturation indexes of key 

minerals, and major geochemical processes are 

discussed. Relations between radiochemical constituents and key indicators of water chemistry and 

relations among radiochemical constituents in 

ground water are investigated. 

Description of the Study Area 

Hickman and Maury Counties are located 

about 25 to 50 miles southwest of the Nashville 

city limit in Middle Tennessee. Most of Hickman 

County and the northwestern and southwestern 

comers of Maury County are included in the Highland Rim physiographic province, whereas much of 

Maury County and part of the Duck River valley 

in Hickman County are included in the Central 

Basin province (figs. 1 and 2). The Highland Rim 

province within the study area is a rolling upland 

that stands about 300 feet above the Central Basin. 

Most of the isolated hills in western Maury County 

are erosional remnants of the Highland Rim. The 

Central Basin in this area is characterized by gently 

rolling hills drained by the Duck River. The 

boundary between the two provinces is marked by 

an escarpment of minor relief. Elevations range 

from about 400 feet above sea level along the 

Duck River in western Hickman County to 1,100 

feet above sea level along the drainage divide 

between the Duck and Elk Rivers in southern 

Maury County. 

Land use is primarily agricultural except 

where communities have become established. The 

population of Hickman County in 1990 was 16,754 

and 54,812 in Maury County (U.S. Department of 

Commerce, 1991). Mean annual rainfall of this 

area is about 47 inches (National Oceanic and 

Atmospheric Administration, 1987). 

HYDROGEOLOGIC SETTING 

The Highland Rim aquifer system and the 

Central Basin aquifer system are the primary 

sources of ground water in Hickman and Maury 

Counties. These aquifer systems have been described by Brahana and Bradley (1986a, 1986b) 

and are named for the physiographic provinces 

where they are located. Formations comprising 

these aquifer systems range in age from Ordovician 

to Mississippian (table 1). 

Ground water in both aquifer systems generally is near land surface (within 300 feet). Both 

systems are recharged by precipitation. Ground 

water moves through soil and regolith, to gravelrich zones within the regolith, then into solution 

openings in the underlying bedrock. Ground water 

2 Geochemistry of and r8diOSctiVity in ground wstsr of the Highland Rim 

snd Central B88in aquifer 8y8tam8. Hickmsn snd Maury GOUntieS, Tennessee 
Study area 

h 

HIGHLAND 

Fad’ 

Modified from NM Fenneman 

(1938) and R.A. Miller (1974) 

j-p CENTRAL BASIN - 

V” 100 KILOMETERS M’LES 

t-HIGHLAND_I RIM 

Not fo scale 

EXPLANATION 

a - MISSISSIPPIAN SYSTEM 

b - DEVONIAN SYSTEM 

HIGHLAND RIM 

AQUIFER SYSTEM 

c - SILURIAN SYSTEM 

d - ORDOVICIAN SYSTEM 

CENTRAL BASIN 

AQUIFER SYSTEM 

----- 

CONTACT BETWEEN SYSTEMS 

A-A’ TRACE ON MAP AND LINE OF SECTION 

Figure 1. Generalized physiographic provinces of Tennessee, study area, and idealized section through part of the 

Highland Rim and Central Basin provinces. 

Hydrogeologic setting 3 
8740’ 8730’ 87’20’ W-10 

I I I I 

HICKMAN COUNTY 

1:2.5do.odo 

cl 

cl 

n 

l 

._.................... 

. . . . . . . . . . . . . . . . . 

MAURY COUNTY 

. . Eli+- . :::::.:.:. ‘. :: YJ@:; z . t-r -1.. 

Base modified from U.S. Geological Survey. 

DIgItal line graph; hydrography, 1980. 

1 :I 00.000: COlmtY outline. 1980. 

35’30’ - 

EXPLANATION 

OUTCROP AREA OF THE HIGHLAND RIM 

AQUIFER SYSTEM--Corresponds to 

Highland Rim physiographic 

province within the study area 

OUTCROP AREA OF THE CENTRAL BASIN 

AQUIFER SYSTEM--Corresponds to 

Central Basin physiographic 

province within the study area 

WELL OR SPRING OPEN TO THE HIGHLAND 

RIM AQUIFER SYSTEM 

WELL OPEN TO THE CENTRAL BASIN 

AQUIFER SYSTEM 

Figure 2. Location of wells and springs sampled for study. 

4 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems. Hickman and Maury Counties, Tennessee 
Table 1. Bedrock units underlying the study area 

[Modified from Brahana and Bradley, 1988a, 1988b] 

Group Formation Thickness 

(feet) Lithology 

I St. Louis Limestone 1 80-175 1 L lmestone, generally massive where unweathered. 

I 100 I Limestone, massive; shale; sandstone. 

I Fort Payne Formation 

I I 

loo-350 Limestone, with dolomite, siltstone, and chert stringers. 

Evaporites present locally. 

Shale, mudstone, and siltstone; glauconite, with abundant phosphate- nodules. 

I Chattanooga Shale 

I 

5-20 

I 

Shale, black, fissile, uraniferous. 

Brownsport Formation 

I 

O-80 

I 

Limestone and shale in alternating thin beds. 

Leg0 Limestone 045 Limestone. 

Waldron Shale o-5 

E 

Shale and shaly limestone. 

s Laurel Limestone O-30 Limestone. 

I Osgood Limestone I O-15 1 Shale and limestone, argillaceous. 

Leipers Formation 

Catheys Formation 

o-75 

125-400 

Limestone, cherty and locally glauconitic. 

Shale and argillaceous limestone. 

Limestone and shale. 

Limestone with interbedded shale; phosphatic. 

Limestone with shale, locally phosphatic. 

Limestone, sandy; has phosphatic facies. 

Limestone, shaly, sandy, phosphatic. 

Limestone; contains four thin beds of bentonite. 

Limestone. 

$ 

.$ Ridley Limestone 100 Limestone, dolomite. 

8 

E Pierce Limestone 25 Limestone, shaly. 

CA 

Murfreesboro Limestone 425 Limestone, massive, cherty. 

Pond Spring Formation O-150 Limestone, silty dolomite, dolomitic limestone. 

Hydrogeologic setting 5 
from the aquifer systems discharges through seeps 

and springs and as base flow to area streams. 

The Highland Rim aquifer system within the 

study area includes the Mississippian St. Louis 

Limestone, Warsaw Limestone, Fort Payne Formation, and Maury Shale (Brahana and Bradley, 

1986a). For the purposes of this report, the 

Chattanooga Shale, a regional confining unit, also 

is assigned to the Highland Rim system. The St. 

Louis Limestone generally is represented only by 

residual clay with blocks of siliceous limestone. 

The Warsaw Limestone is typically a fine- to 

coarse-grained fossil fragmental limestone. The 

Fort Payne Formation consists of argillaceous and 

siliceous limestone and calcareous siltstone, and 

includes much chert, both nodular and bedded. In 

some areas, gypsum and other evaporites are 

present. The Maury Shale, a thin, glauconitic 

formation containing phosphatic nodules, lies 

between the Fort Payne Formation and the Chattanooga Shale. Total thickness of formations of the 

Highland Rim aquifer system in the study area 

ranges from about 300 to 600 feet. 

The Warsaw Limestone and Fort Payne Formation weather deeply, producing a clay-rich, and 

in the case of the Fort Payne Formation, a chertrich regolith as much as 100 feet thick. The regolith contains a reservoir of water that supplies 

ground water to openings along bedding planes and 

solution-enlarged fractures in the bedrock. These 

openings form by the dissolution of soluble minerals, mostly carbonates, reacting with slightly acidic 

recharge water. The openings represent secondary 

porosity and permeability in. an otherwise nonporous rock mass and are highly irregular in their 

distribution. 

The Chattanooga Shale is a black shale rich 

in organic matter that separates the Highland Rim 

aquifer system from the Central Basin aquifer 

system and plays a significant role in the hydrology 

of the area. The Chattanooga Shale, although not 

thick in the study area, is considered to be an 

effective barrier to the vertical flow of ground 

water between the Highland Rim aquifer system 

and the underlying Central Basin aquifer system. 

In addition, the shale might affect water chemistry. 

Iron sulfide, radionuclides, and other constituents 

in the shale might dissolve in water that comes in 

contact with the rock unit, either along its surface 

or in fractures or partings within the rock. 

Sources of uranium in rocks of the Highland 

Rim aquifer system include phosphatic nodules of 

the Maury Shale, and probably the entire section of 

the Chattanooga Shale. The Gassaway (or upper) 

Member of the Chattanooga Shale contains about 

0.006 percent uranium (Swanson, 1961). This 

uranium probably does not occur as a distinct 

uranium mineral, but rather is associated with organic matter - pyrite complexes (Bates and others, 

1954). The Hardin Sandstone (or lowest) Member 

of the Chattanooga Shale is a nodular phosphatic 

zone, and exposures along one of the tributaries to 

the Duck River in Hickman County were formerly 

mined commercially for “blue rock” phosphate 

(Theis, 1936, p. 110). In phosphatic rock, uranium 

commonly substitutes for calcium in fluorapatite 

[Ca,(PO,)‘F] (Altschuler and others, 1958). 

The Central Basin aquifer system includes 

the Silurian- and Ordovician-age formations from 

the base of the Chattanooga Shale to the top of the 

lower Ordovician Knox Group. The Central Basin 

aquifer system includes numerous interbedded 

limestone, shale, and siltstone units and various 

admixtures of these lithotypes. Limestones in this 

stratigraphic interval are much less siliceous than 

those of the Mississippian system, but commonly 

are argillaceous and locally, glauconitic. 

Limestones of some of the Ordovician formations 

are massive; some are dolomitic. Although 

thickness of the formations in the Central Basin 

aquifer system in the study area is over 1,000 feet, 

most of the water-bearing units are within a few 

hundred feet of land surface. 

The hydraulic properties of the Central 

Basin system generally resemble those of the 

Highland Rim system. Recharge flows through 

openings along bedding planes and solutionenlarged fractures that are highly irregular in 

distribution. The rock mass itself has little or no 

primary permeability. Regolith in the Central 

Basin is thin, generally 20 feet or less, thus storage 

capacity is limited. 

Uranium and other radionuclides in rocks of 

the Central Basin aquifer system most commonly 

are formed in phosphate- and shale-rich zones. 

The Leipers Formation, Catheys Formation, Bigby 

Limestone, and Hermitage Formation (all of 

Ordovician age) contain phosphatic intervals or 

facies. At the time of Theis’ (1936) study, the 

weathered Bigby Limestone was the most 

6 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
important source of phosphate in Tennessee. 

Phosphate nodules from Ordovician rocks in 

Middle Tennessee are typically 0.003 percent 

uranium (Altschuler and others, 1958), whereas 

nonphosphatic rocks such as chert and “pure” 

limestone commonly are less than 0.0005 percent 

(Swanson, 1961). These phosphatic limestones and 

numerous shales in the aquifer system could be 

sources of dissolved uranium and other radionuclides in the ground-water supply. 

GEOCHEMISTRY OF GROUND WATER 

The Highland Rim aquifer system and the 

Central Basin aquifer system are both primarily 

limestone aquifers. However, considerable 

difference exists in the mineralogy and geochemistry of these two aquifer systems and 

between the individual formations within each 

system. To provide an understanding of the geochemistry and geochemical processes operating 

within the two aquifer systems, inorganic chemical 

characteristics of water from each system were 

analyzed, saturation indexes of certain ions were 

determined, and key geochemical processes were 

evaluated. 

Major Constituents and Physical Properties 

Seventeen wells and 3 springs in Hickman 

County and 20 wells in Maury County were 

sampled for this investigation (fig. 2). All of the 

sites are used for domestic supply. The geologic 

unit supplying water to each well was determined 

by examination of drillers’ logs and information 

provided by well owners. Of the Hickman County 

samples, 13 were interpreted to be from sources 

tapping the Highland Rim aquifer system, and 7 

from sources tapping the Central Basin aquifer 

system. All Maury County wells tapped the 

Central Basin aquifer system. Thus, the total 

number of sites in the Highland Rim and Central 

Basin aquifer systems were 13 and 27, 

respectively. 

Samples were collected from existing plumbing lines and treated in accordance with current 

USGS guidelines. Alkalinity, pH, temperature, 

and specific conductance were measured in the 

field at each site. Concentrations of principal 

cations and anions, trace metals, and total organic 

carbon were determined in the laboratory. A 

description of sample collection and treatment is 

included in the data report of this study (Hileman, 

1990). 

Summary statistics (table 2) describe the 

quality of water from each aquifer system. Boxplots presented in figure 3 for several constituents 

in water provide a visual comparison of the quality 

of water from each system. 

Most of the water-quality measures indicate 

that moderate differences exist between the aquifer 

systems. Median values for the specific conductance of water from the Highland Rim and Central 

Basin aquifer systems were 150 and 500 pS/cm, 

respectively; for dissolved solids, 101 and 

271 mg/L; for alkalinity, 73 and 206 mg/L; and 

for pH, 7.2 and 7.2 standard units. Each measure 

showed greater variation for the Central Basin 

system, which might partly result from the greater 

number of samples representing that system and 

the greater number of formations sampled. 

Concentrations of many ions typically are 

greater in water of the Central Basin system than 

in the Highland Rim system, as indicated by larger 

mean and median values for calcium, magnesium, 

sodium, potassium, chloride, sulfate, fluoride, 

phosphorous, organic carbon, boron, and most of 

the trace metals (table 2). Average silica and 

barium concentrations in water from the two 

systems are about equal. In contrast, the mean and 

median concentrates of dissolved oxygen in water 

from the Highland Rim system are two to three 

times greater than corresponding values for the 

Central Basin system. 

All 13 of the samples collected from the 

Highland Rim aquifer system were of the calcium 

bicarbonate type. The overall similarity in major 

ion composition of water from this system is 

shown by the clustering of points on the three parts 

of a trilinear diagram (fig. 4). The composition of 

water in samples from the Central Basin system 

showed more variability than did samples from the 

Highland Rim system. Of the 27 samples collected, 18 were of the calcium bicarbonate type; 3 

were of the calcium magnesium bicarbonate type; 

3 were of the calcium sulfate type; 1 was a calcium 

magnesium sulfate type; and 2, of mixed types. 

Variability in the major ion composition of water 

from the Central Basin system is shown by the 

Geochemistry of ground water 7 
h 

. 

N 

ei 

5 

. 

0 

G 

Kl 

In 

ui 

-0 

. 

ul 

0 

. 

N 

0 

cd 

z 

.-N-O.- 

. . . . . 

z 

c - c v 

5 .C 

3 

8 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
Geochemistry of ground water 9 
r 

CALCIUM MAGNESIUM SODIUM ALKALINITY SULFATE CHLORIDE 

(as Cocos) 

EXPLANATION 

H- VALUE OUTSIDE 

QOTH PERCENTILE 

- QOTH PERCENTILE 

- 75TH PERCENTILE 

BOX AND WHISKER PLOT OF 

DATA FROM HIGHLAND RIM 

AQUIFER SYSTEM--Number 

I of analys es: 13 

I: - MEDIAN 

- 25TH PERCENTILE 

- 1OTH PERCENTILE 

BOX AND WHISKER PLOT OF 

DATA FROM CENTRAL BASIN 

AQUIFER SYSTEM--Number 

of analyses: 27 

x-VALUE OUTSIDE 

1 OTH PERCENTILE 

Figure 3. Major ion constituents in ground water of the Highland Rim and Central Basin aquifer systems, Hickman 

and Maury Counties, Tennessee. Minimum reporting level for analytical methods is 1 milligram per liter. 

10 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems. Hickman and Maury Counties, Tennessee 
CALCIUM CHLORIDE, FLOURIDE, NITRITE + NITRATE 

PERCENTAGE REACTING VALUES 

Figure 4. Trilinear diagram illustrating principal ion composition of ground water at 13 sites tapping the Highland 

Rim aquifer system in Hickman County, Tennessee. Percentages are computed from concentrations in 

milliequivalents per liter. 

scattering of points on the three parts of a trilinear 

diagram (fig. 5). 

Differences and similarities between the 

ground-water chemistries of the Highland Rim and 

Central Basin aquifer systems are supported by 

statistical comparison of the data sets. The 

Wilcoxon-Mann-Whitney rank sum test (Iman and 

Conover, 1983) was used to test for statistically 

significant differences between the two aquifer 

systems for the constituents and physical 

characteristics listed in table 2. 

Results of the statistical testing indicated 

differences between the two aquifer systems at the 

9%percent confidence level for temperature, 

specific conductance, dissolved solids, alkalinity, 

dissolved oxygen, organic carbon, calcium, 

magnesium, sodium, potassium, chloride, sulfate, 

iron, strontium, and fluoride. These results 

support differences observed in data presented in 

figure 3 and table 2. The statistical test failed to 

indicate any significant differences (at the 

95percent confidence level) between the two 

aquifer systems for pH, dissolved silica, 

ammonium, nitrite plus nitrate, phosphorus, 

manganese, and barium. 

Geochemistry of ground water 11 
CALCIUM CHLORIDE, FLOURIDE. NITRITE + NITRATE 

PERCENTAGE REACTING VALUES 

Figure 5. Trilinear diagram illustrating principal ion composition of ground water at 27 sites tapping the Central 

Basin aquifer system in Hickman and Mauty Counties, Tennessee. Percentages are computed from concentrations 

in milliequivalents per liter. 

Geochemical Processes 

Based on lithologic descriptions and 

predominant ions in ground-water samples, the 

primary geochemical changes within both aquifer 

systems probably result from the dissolution or 

precipitation of calcite, dolomite, magnesium 

calcite, and gypsum. Chemical equations for these 

reactions are: 

CaCO, + H&O, = Ca++ + 2HCO; 

(calcite) 

(1) 

CaMg(CO& + 2HzCOs = Ca++ + Mg++ + 4HCO; (2) 

(dolomite) 

Ca,,&fg,-,,CO, + H&O, = 0.9Ca++ + O.lMg++ + 2HCO; 

(magnesium calcite,) (3) 

C&30, * 2Hz0 = Ca++ + SO,- + 2H20. (4) 

kwsum) 

Saturation indexes are indicators of whether 

a mineral is likely to precipitate or dissolve in a 

fluid such as ground water. The saturation index 

for a solid species is based on the chemical 

characteristics of the fluid, and is determined by 

comparing the actual ion activity product of the 

dissolved constituents of the solid species with its 

solubility product. A value less than zero indicates 

that the ground water is undersaturated with 

respect to the mineral and net dissolution of the 

mineral from solid phase should occur. If the 

value equals zero, the ground water is at 

equilibrium with respect to the mineral. When the 

value is greater than zero, the ground water is 

oversaturated with respect to the mineral, and net 

precipitation should occur (Matthess, 1982). 

Saturation indexes of calcite, dolomite, and 

gypsum, the three principal soluble minerals in the 

aquifers, were determined using the computer code 

12 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
WATEQF (Plummer and others, 1976). Most 

samples representing the Highland Rim aquifer 

system ranged from significantly undersaturated to 

nearly saturated with respect to calcite; one sample 

was slightly oversaturated (fig. 6). The water was 

significantly undersaturated to nearly saturated with 

respect to dolomite (fig. 6), and significantly 

undersaturated with respect to gypsum (fig. 7). 

Almost every sample from the Central Basin 

system was nearly saturated to slightly oversaturated with respect to calcite (fig. 6). Most 

samples were undersaturated with respect to 

dolomite, although three samples were slightly 

oversaturated (fig. 6). All samples were undersaturated with respect to gypsum (fig. 7). The 

plots of saturation indexes indicate that, overall, 

water in the Highland Rim aquifer system is less 

saturated with respect to these three minerals than 

water in the Central Basin aquifer system. 

The two aquifer systems are similar in terms 

of primary geochemical reactions and the 

interactions of ground water with respect to calcite, 

dolomite, magnesium calcite, and gypsum. An 

evaluation of specific geochemical processes 

further supports some similarities observed and 

indicates that some differences exist between the 

two aquifer systems. 

Similarities, in terms of carbonate 

dissolution, are indicated by the relation between 

dissolved calcium plus dissolved magnesium and 

total inorganic carbon. Least-square regression 

lines through the data points (fig. 8) show that 

approximately two moles of inorganic carbon are 

in water for every mole of dissolved calcium plus 

dissolved, magnesium. Regression lines through 

data sets for each aquifer system have similar 

slopes (0.54 and 0.38 for the Highland Rim aquifer 

system and Central Basin aquifer system, 

respectively), indicating similar molar ratios in 

ground water from both aquifer systems. Total 

inorganic carbon (COJ in each ground-water 

sample was calculated from field alkalinity using 

part of the WATEQF code @.unmer and others, 

1976). The two to one molar ratio between 

inorganic carbon and calcium plus magnesium is 

characteristic of the dissolution of calcite, 

dolomite, and magnesium calcite (equations 1, 2, 

and 3). Analyses for five sites were not included 

1 I 1 I I 

1 I 

t: 0 Oversaturated l - 

Undeisaturated J l 

is 

l 

- _ 

l SAMPLE FROM CENTRAL ; 1 

-1 

E t 

BASIN AQUIFER SYSTEM 

E 

d -2 n SAMPLE FROMHIGHLAND l m . . 

RIM AQUIFER SYSTEM . l 

s 

I l 

Ln -3 

-6 L I I I I I 

-2.5 -2.0 -1.5 -1.0 -0.5 0 

CALCITE SATURATION INDEX 

0.5 

Figure 6. Calcite and dolomite saturation indexes of ground water from the Highland Rim and Central Basin aquifer 

systems, Hickman and Maury Counties, Tennessee, calculated using the WATEQF program. 

Geochemistry of ground water 13 
1 

1: 

0 

n 

Z 

- -1 

2 i= -2 d 

2 

;f -3 

zi -4 

ii! 

G -5 

-6 

Ovrrsaturated 

Undersaturated 

l SAMPLE FROM CENTRAL . . 

BASIN AQUIFER SYSTEM . - 

. 

n SAMPLE FROM HIGHLAND . . 

. 

RIM AQUIFER SYSTEM .‘. *.a 

. 

l * l m l n . 

n + n 

t 

n n 

l n 

m 8 

t 

I I I I I I I 

,2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 

CALCITE SATURATION INDEX 

Figure 7. Calcite and gypsum saturation indexes of ground water from the Highland Rim and Central Basin aquifer 

systems, Hickman and Maury Counties, Tennessee, calculated using the WATEQF program. 

in the regression. These samples contained higher 

concentrations of dissolved calcium and dissolved 

sulfate, and might indicate areas where gypsum 

dissolution (equation 4) affects water composition. 

Comparison of dissolved calcium to dissolved magnesium (fig. 9) indicates slightly different geochemical processes with respect to the 

dissolution of magnesium-bearing carbonate 

minerals present in each aquifer system. The 

regression through the data, calculated with values 

from the Highland Rim aquifer system only, has a 

correlation coefficient squared (R2 value) of 0.87. 

This strong correlation, combined with thermodynamic calculations showing undersaturation of 

carbonates, suggests that dissolution of 

calcium- and magnesium-bearing carbonates from 

the Highland Rim aquifer system is congruent. In 

this process, calcite dissolves without simultaneous 

precipitation of calcite. Furthermore, the aquifer 

material that reacts with the water in the Highland 

Rim aquifer system has a fairly constant calcium to 

magnesium molar ratio of about 5.3 to 1. 

Data for ground-water samples from the 

Central Basin aquifer system are more variable and 

do not plot along this same regression line. 

Possible explanations for the scatter of data from 

the Central Basin aquifer system are incongruent 

dissolution of calcium- and magnesium-bearing 

minerals or dissolution of aquifer material with 

variable calcium to magnesium ratios. Incongruent 

dissolution involves dissolution of a magnesiumrich solid phase such as dolomite (equation 2) or 

magnesium calcite (equation 3) followed by 

precipitation of a calcium-rich phase such as calcite 

(equation 1). Evidence for the likely precipitation 

of calcite is the nearly saturated to oversaturated 

state of calcite in most samples from the Central 

Basin aquifer system, as previously noted. Alternatively, variable calcium to magnesium ratios in 

the aquifer materials of the Central Basin aquifer 

system associated with changes in lithology could 

have a similar effect. For example, pure dolomite 

dissolving in water (equation 2) will result in a 

lower calcium to magnesium ratio than will pure 

calcite dissolving in water (equation 1). 

Sulfate is the predominant anion in water 

from five sites; several processes can account for 

high sulfate concentrations. Dissolution of 

gypsum, a calcium sulfate evaporite mineral 

(equation 4), will increase the sulfate concentration 

14 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
I I 

A 

A A 

Highland Rim aquifer system 

Least square regression line 

A keTat!Z Ge63EF21 ~~ aa. 

squared (FL) is 0.9s / , 

is In 

E 

Correlation coefficient 

squared (R2) is 0.72 

0 

0 2 4 6 8 10 

TOTAL INORGANIC CARBON (C02), 

IN MILLIMOLES PER LITER 

EXPLANATION 

l SAMPLE FROM CENTRAL A SAMPLE DATA NOT 

BASIN AQUIFER SYSTEM USED IN LINE FIT 

m SAMPLE FROM HIGHLAND 

RIM AQUIFER SYSTEM 

Figure 8. Relation between dissolved calcium plus dissolved magnesium and total inorganic carbon in ground 

water of the Highland Rim and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee. 

in water. Pyrite oxidation also will contribute 

sulfate to solution, although in smaller amounts 

because oxygen availability often is limited in 

ground water. Alternatively, water containing high 

concentrations of dissolved sulfate might move 

upward from a deeper aquifer. Most of the high 

sulfate concentrations reported in this study 

probably are the result of gypsum dissolution 

because dissolved oxygen concentrations are low in 

most samples and because gypsum observed in 

rock outcrops confirms that the mineral is present. 

RADIOACTIVITY IN GROUND WATER 

The most common radionuclides in ground 

water are from the uranium-238 and the thorium232 decay series (fig. 10). Uranium-238 is a 

naturally occurring, long-lived isotope (half life, 

4.49x109 years) present in small concentrations in 

many rocks world-wide. This radionuclide slowly 

decays into a sequence of daughter products, 

ending in a stable isotope of lead. Among the 

daughters of uranium-238 are uranium-234 (half 

life, 2.48~105 years), radium-226 (half life, 

1,622 years), and radon-222, a gas (half life, 

3.825, days). Thorium-232 also is a naturally 

occurring, long-lived isotope (half life, 1.41~10’~ 

years) present in many rocks. Among its 

daughters is radium-228 (half life, 5.75 years). 

Other radionuclides in the decay series of these 

two elements and all isotopes of the uranium-235 

decay series are highly immobile or have short half 

lives, and thus are not present in significant 

amounts in ground water (Zapecza and Szabo, 

1988). 

Radioactivity in ground water 15 
Least square regression line 

i2el~+l~;P:“,,kki?g1 

squared (Rz) is 0.87 

0 0.1 0.2 0.3 

DISSOLVED MAGNESIUM, IN MILLIMOLES PER LITER 

Figure 9. Relation between dissolved calcium and dissolved magnesium in ground water of the Highland Rim 

aquifer system, Hickman County, Tennessee. 

Radiochemical Constituents 

Samples of water for radiochemical analyses 

were collected concurrently from the same wells 

and springs as the samples for analyses of major 

constituents. Radon-222 concentrations in ground 

water were measured at each site using a Lucas 

cell technique. Gross alpha and gross beta 

activity, dissolved uranium, radium-226, and 

radium-228 concentrations were determined in the 

laboratory. Summary statistics of the analyses are 

given in table 3. 

Differences in activity levels of constituents 

between aquifer systems generally were small 

(fig. 11). The median concentration of gross alpha 

activity in samples from each aquifer system was 

0.54 pCi/L. The median concentration of gross 

beta activity in samples from the Highland Rim 

system was 1.1 pCi/L; and in samples from the 

Central Basin system, 2.3 pCi/L. Median 

concentrations of dissolved uranium (all isotopes) 

in water from the Highland Rim and Central Basin 

aquifer systems were 0.09 and 0.12 pCi/L, 

respectively; of radium-226,0.09 and 0.07 pCi/L; 

and of radium-228, 0.04 pCi/L and 0.04 pCi/L. 

The largest difference was between median 

concentrations of radon-222. Median values were 

559 pCi/L for the Highland Rim system and 

422 pCi/L for the Central Basin system. The 

range in concentrations of each constituent was 

greater for samples from the Central Basin system, 

which might reflect the greater number of samples 

representing that system and the greater number of 

formations sampled. A Wilcoxon-Mann-Whitney 

rank sum test (Iman and Conover, 1983) was used 

to test for statistical differences between concentrations of radioactive constituents in water from 

each aquifer system. The test failed to reveal 

statistical differences at the 95percent level of 

significance for any of the radioactive constituents. 

None of the samples exceeded the radioactivity standards adopted by the Tennessee 

Department of Health and Environment (1991) for 

naturally occurring radioactivity in water used for 

community water supplies (a maximum of 

15 pCi/L for gross alpha activity and 5 pCi/L for 

radium-226 plus radium-228 concentrations). Of 

the 40 samples collected, the largest gross alpha 

activity measured was 6.4 pCi/L, and the largest 

radium-226 plus radium-228 concentration was 

2.7 pCi/L. Both maxima are identified with the 

same well, located in the Central Basin. 

16 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems. Hickman and Msury Countiee. Tennessee 
URANIUM-238 THORlUM-232 

Pa-234 

1.18 m 

radionuclides 

U - URANIUM 

Pa - PROTACTINIUM 

Th - THORIUM 

Ra - RADIUM 

EXPLANATION 

Other 

radionuclides 

with short 

half-lives 

Rn - RADON 4 ALPHA DECAY 

Pb-LJUD f BETA DECAY 

AC - ACTINIUM 

Figure 10. Uranium-238 and thorium-232 radioactive decay series. Stippling pattern indicates those radionuclides 

most frequently detected in ground water. Times shown are half-lives: y, years; d, days; h, hours; m, minutes. 

(Modified from O.S. Zapecza and Z. Szabo, 1988.) 

Radioactivity in ground water 17 
t 

v, 

0 

T 

0 

V 

* 

6 

V 

2 

\9 

V 

0 

\9 

V 

2 

18 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
w 

x 

x 

0 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

0.001 ’ I 

GROSS ALPHA GROSS BETA URANIUM RADIUM-226 RADIUM-228 RADON/l ,000 

EXPLANATION 

x- VALUE OUTSIDE 

90TH PERCENTILE 

- 90TH PERCENTILE 

BOX AND WHISKER PLOT OF 

DATA FROM HIGHLAND RIM 

AQUIFER SYSTEM--Number 

- 75TH PERCENTILE 

of analyses: 13 

r 

- MEDIAN 

- 25TH PERCENTILE 

- 1OTH PERCENTILE 

BOX AND WHISKER PLOT OF 

DATA FROM CENTRAL BASIN 

AQUIFER SYSTEM--Number 

of analyses: 27 

x- VALUE OUTSIDE ......... MINIMUM REPORTING LEVEL WITH 

1 OTH PERCENTILE ANALYTICAL METHOD USED 

Figure 11. Selected radioactive constituents in ground water of the Highland Rim and Central Basin aquifer 

systems, Hickman and Maury Counties, Tennessee. Radon activities shown are 1 /I,000 of actual measurements. 

Radioactivity in ground water 19 
Concentrations in samples from most wells were 

substantially less than these maxima. Gross alpha 

activity was less than 2 pCi/L in about 90 percent 

of the samples from each aquifer system. The 

combined radium concentrations were less than 0.9 

pCi/L in all samples from the Highland Rim 

aquifer system, and less than 0.9 pCi/L in all 

except two samples from the Central Basin aquifer 

system. 

Neither the Tennessee Department of Health 

and Environment nor the U.S. Environmental 

Protection Agency has established a standard for 

radon in public drinking-water supplies. However, 

health physicists have proposed a 10,000 pCi/L 

limit for radon-222 (Cross and others, 1985). 

Concentrations of radon-222 in all samples 

collected for this study were substantially less than 

the proposed standard. Radon-222 in samples 

from the Highland Rim aquifer system ranged from 

225 to 1,000 pCi/L, and in samples from the 

Central Basin aquifer system, 63 to 1,490 pCi/L. 

The geographic distribution of radon-222 in 

samples from the Highland Rim aquifer system 

suggests a trend. Six sites with concentrations less 

than the median value (559 pCi/L) are in northern 

Hickman County and seven sites with concentrations equal to or greater than the median value are 

in the southwestern two-thirds of the county 

(fig: 12). The data show no consistent pattern for 

the Central Basin aquifer system; however, 

maximum and minimum values are associated with 

wells less than one-half mile apart, and other large 

differences were observed in values between wells 

that were not far apart. 

Relations Between Constituents 

Correlation coefficients between radioactive 

constituents and key indicators of water quality 

were calculated to evaluate possible relations. The 

radioactive constituents were uranium, radium-226, 

radium-228, radon-222, gross alpha, and gross 

beta. Each of these constituents was paired 

individually, by aquifer system, with calcium, 

magnesium, sodium, barium, phosphorus, 

alkalinity, sulfate, dissolved oxygen, and pH. 

Although barium is usually considered a trace 

constituent, it was included because its chemical 

behavior is similar to that of radium. In addition, 

correlation coefficients were calculated by pairing 

each of the radionuclides, gross alpha, and gross 

beta with the other radionuclides of that aquifer 

system. For those data sets resulting in a 

correlation coefficient greater than 0.2, data were 

plotted to visually examine the correlation and 

evaluate the potential relation between the 

radionuclide and the constituent or property. 

No relation was found in most of the data 

sets analyzed for this reconnaissance. Correlation 

coefficients generally were less than 0.2. A 

common pattern seen in plotted data (correlation 

coefficient greater than 0.2) was for most of the 

points to cluster in one part of the graph and one 

or two points to fall in outlying areas. The plot of 

uranium and calcium in water from the Central 

Basin aquifer system is typical (fig. 13), indicating 

that no relation exists between concentrations of 

radionuclides (dissolved uranium, radium-226, 

radium-228, and radon-222) in water and any of 

the key indicators of water chemistry in either 

aquifer system. One exception is radon-222 in 

water of the Highland Rim aquifer system, which 

is moderately related to pH (R2 of 0.71) and 

weakly related to dissolved magnesium (R2 of 

0.51). 

The relation of radon-222 concentration to 

pH might be indicative of hydrogeochemical environments in the lower part of the Highland Rim 

aquifer system. A graph of the relation between 

radon-222 concentrations and pH shows that they 

are inversely related (fig. 14). The six samples 

having radon-222 concentrations greater than 

600 pCi/L are from water associated with the 

Chattanooga Shale; two of the samples are from 

contact springs at the top of this formation. The 

comparatively low pH of these samples indicates 

that water in this part of the aquifer has not 

substantially interacted with carbonate rock and, 

that in the area represented by these samples, the 

lower part of the aquifer probably contains little 

carbonate material. A possible explanation is that 

the carbonate bedrock in this area has been reduced 

to regolith to at least the bottom of the Fort Payne 

Formation, readily permitting radon-222 from the 

decay of uranium in the Chattanooga Shale to enter 

ground water in the regolith immediately above the 

shale. In areas characterized by lower radon levels 

and higher pH values, relatively unweathered bedrock overlies the Chattanooga Shale and prevents 

20 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
3800 

3550' 

3540' 

w-40' 8730' 8720' 8710' 

I I I I 

HICKMAN COUNTY 

I 

.,.............. QP.~.~.~.~.~.~.~.~.~.~.~.~.~. 

I I 

Easr modlflod from U.S. Geological Survey. 

Dl ltal Ilno graph: hydrography, 1980. 

1: rl 00.000: counfy outllne, 1980, 

1:2,500.000 

3530'- 

MAURY COUNTY 

EXPLANATION 

cl OUTCROP AREA OF THE HIGHLAND RIM 

. . 

. AQUIFER SYSTEM 

q OUTCROP AREA OF THE CENTRAL BASIN 

AQUIFER SYSTEM 

559 

n WELL OPEN TO THE HIGHLAND RIM 

AQUIFER SYSTEM--Number indicates 

concentration of radon-222 in ground 

water, in picocuries per liter 

l 157 WELL OPEN TO THE CENTRAL BASIN 

AQUIFER SYSTEM--Number indicates 

concentration of radon-222 in ground 

water, in picocuries per liter 

Figure 12. Distribution of radon-222 in ground water of the study area. 

Radioactivity in ground water 21 
. 

0 0.4 0.8 1.2 1.6 

DISOLVED URANIUM CONCENTRATION. 

IN PICOCIJRIES PER LITER 

Figure 13. Relation between dissolved calcium and 

dissolved uranium in ground-water samples from the 

Central Basin aquifer system, Hickman and Maury 

Counties, Tennessee. 

the entry of other than minor amounts of gas into 

the aquifer above the shale. The presence of 

carbonate bedrock in the aquifer is expressed 

geochemically by the comparatively higher pH 

values of ground water that has come in contact 

with the rock. 

The association of uranium with phosphatic 

rock was previously noted. However, the data did 

not indicate any relation between uranium and 

phosphorus concentrations in water from either 

aquifer system (figs. 15 and 16). 

None of the isotopes in water from either 

aquifer system were related to other isotopes or to 

gross alpha or gross beta activities, with the 

exception of radium-226. Radium-226 concentrations were weakly related to gross alpha activity 

in water from the Highland Rim aquifer system (R’ 

of 0.48) and moderately related to gross alpha 

activity in water from the Central Basin aquifer 

system (IX2 of 0.88). The latter value might be 

misleading, however, owing to outlying points in 

the data set which tend to make the correlation 

seem stronger than the distribution of points 

otherwise implies. 

SUMMARY AND CONCLUSIONS 

A reconnaissance of the geochemistry of and 

radioactivity in ground water from the Highland 

7.8 

f 7.6 

= 7.4 

& 

d 

7.2 

z 7.0 

2 vl 6.8 

d 6.6 

6.4 _. 

200 400 600 800 1,000 1.200 

TOTAL RADON-222 CONCENTRATION, 

IN PICOCURIES PER LITER 

Figure 14. Relation between pH and total radon-222 in 

ground-water samples from the Highland Rim aquifer 

system, Hickman County, Tennessee. 

Rim and Central Basin aquifer systems in Hickman 

Maury Counties, Tennessee, was conducted in 

1989 by the U.S. Geological Survey in cooperation 

with the Tennessee Department of Health and 

Environment. For the study, chemical and 

radiochemical analyses were made of ground-water 

samples from 13 sites tapping the Highland Rim 

aquifer system and 27 sites tapping the Central 

Basin aquifer system. The results of the analyses 

were summarized and the geochemical 

environments characterized by calculating saturation indexes of key minerals and discussing major 

geochemical processes indicated by the relations of 

constituents in ground-water samples. Relations 

between radiochemical constituents and key 

indicators of water chemistry and relations among 

radiochemical constituents also were determined. 

Most of the measures of water chemistry 

indicate moderate differences in geochemistry 

between the two aquifer systems. Median values 

for the specific conductance of ground-water 

samples from the Highland Rim and Central Basin 

aquifer systems were 150 and 500 j&/cm, respectively; for dissolved solids, 101 and 271 mg/L; and 

for pH, 7.2 and 7.2 standard units. Concentrations 

of many ion ground-water samples typically were 

lower in the Highland Rim than in the Central 

Basin aquifer system. Comparisons of water 

quality between the two aquifer systems indicated 

22 Geochemistry of and radioactivity In ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee 
0.04 -. 

0.03 - .’ 

0.02 * 

0.01 ” l . - 

o-“-““““““’ 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 

DISSOLVED URANIUM CONCENTRATION. 

IN PICOCURIES PER LITER 

. 

’ l * 

‘. l 

. 

d.4 . , , , , . , , , , , , , 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 

DISSOLVED URANIUM CONCENTRATION, 

IN PICOCURIES PER LITER 

Figure 15. Relation between dissolved phosphorus and Figure 16. Relation between dissolved phosphorus and 

dissolved uranium in ground-water samples from the dissolved uranium in ground-water samples from the 

Highland Rim aquifer system, Hickman County, Central Basin aquifer system, Hickman and Maury 

Tennessee. Counties, Tennessee. 

statistically significant differences among the major 

dissolved ions concentrations of calcium, magnesium, sodium, potassium, chloride, and sulfate. 

Despite differences between aquifer systems in 

major ion concentrations, the types of water in 

both systems are similar. Calcium and bicarbonate 

were the predominant ions in all 13 samples from 

the Highland Rim aquifer system. Calcium or 

calcium and magnesium were the predominant 

cations in 25 of the 27 Central Basin aquifer 

system samples and bicarbonate was the major 

anion in 21 of 27 samples. 

Dissolution of calcite, magnesium calcite, 

dolomite, and gypsum are the primary geochemical 

processes controlling ground-water chemistry in 

both aquifer systems. For the Highland Rim 

aquifer system, saturation state calculations using 

chemical analyses of ground-water samples and the 

computer code WATEQF indicate that, with 

respect to calcite saturation, most of the groundwater samples ranged from significantly undersaturated to nearly saturated, except one sample 

that was slightly oversaturated; with respect to 

dolomite saturation, samples were significantly 

undersaturated to nearly saturated; and with respect 

to gypsum, all samples were significantly undersaturated. For the Central Basin aquifer system, 

saturation state calculations indicate that with 

respect to calcite, almost every ground-water 

sample was nearly saturated to slightly oversaturated; with respect to dolomite, all samples 

were undersaturated to nearly saturated except 

three samples that were oversaturated; and with 

respect to gypsum, all samples were undersaturated. Overall, ground-water samples from the 

Central Basin aquifer system were more saturated 

with respect to calcite, dolomite, and gypsum than 

samples from the Highland Rim aquifer system. 

The relation of dissolved calcium to 

dissolved magnesium and the saturation state 

calculations indicate that slightly different geochemical environments exist in each aquifer system 

with respect to the dissolution of magnesiumbearing minerals. Ground-water samples from the 

Highland Rim aquifer system had a fairly 

consistent calcium to magnesium molar ratio of 

about 5.3 to 1, which suggests dissolution of 

aquifer material with a consistent calcium to 

magnesium ratio. The highly variable calcium to 

magnesium molar ratios in ground-water samples 

from the Central Basin aquifer system might 

indicate incongruent dissolution of magnesiumbearing minerals or heterogeneity in magnesium 

content of the aquifer. 

Concentrations of radiochemical constituents 

were low and generally similar in ground-water 

samples from both aquifer systems. For the 

Highland Rim aquifer system, the median activity 

Summery end conclusions 23 
in ground-water samples for gross alpha was 

0.54 pCi/L, and for gross beta, 1.1 pCi/L. The 

median concentration of dissolved uranium (all 

isotopes) was 0.09 pCi/L; radium-226,0.09 pCi/L; 

radium-228, 0.04 pCi/L; and radon-222, 

559 pCi/L. For the Central Basin aquifer system, 

the median activity of gross alpha was 0.54 pCi/L, 

and of gross beta, 2.3 pCi/L. The median concentration of dissolved uranium was 0.12 pCi/L;, 

radium-226,0.07 pCi/L; radium-228,0.04 pCi/L; 

and radon-222, 422 pCi/L. Radium-226 plus 

radium-228 concentrations were less than 

0.9 pCi/L in all ground-water samples from the 

Highland Rim aquifer system and less than 

0.9 pCi/L in all except two samples from the 

Central Basin aquifer system. Concentrations of 

radium-226 plus radium-228 in all samples from 

both aquifer systems were less than Tennessee’s 

maximum permissible contaminant level of 5 pCi/L 

for radium isotopes in community water-supply 

systems. 

Data collected for this reconnaissance 

generally did not indicate any relations involving 

SELECTED REFERENCES 

Altschuler, Z.S., Clarke, R.S., Jr., and Young, E.J., 

1958, Geochemistry of uranium in apatite and 

phosphor&: U.S. Geological Survey Professional Paper 314-D, p. 45-90. 

Bates, T.F., Strahl, E.O., Short, N.M., and others, 

1954, Mineralogy and petrography of the 

Chattanooga Shale [abs.]: Geological Society of 

America Bulletin, v. 65, p. 1230. 

Bradley, M.W., 1984, Ground water in the Dickson 

area of the western Highland Rim of Tennessee: 

U.S. Geological Survey Water-Resources 

Investigations Report 824088, 42 p. 

Brahana, J.V., and Bradley, M.W., 1986a, Preliminary 

delineation and description of the regional 

aquifers of Tennessee--The Highland Rim 

aquifer system: U.S. Geological Survey WaterResources lnvestiga-tions Report 82-4054,38 p. 

----- 1986b, Preliminary delineation and description of 

the regional aquifers of Tennessee--The Central 

Basin aquifer system: U.S. Geological Survey 

Water-Resources Investigations Report 82-4002, 

35 p. 

radiochemical constituents in ground-water 

samples. For either aquifer system, no relation 

exists between concentrations of the radionuclides 

(uranium, radium-226, radium-228, and radon-222) 

dissolved in water and any of the key indicators of 

water chemistry, with the exception of water from 

the Highland Rim aquifer system, for which a 

moderate relation (R2 of 0.71) was indicated 

between radon-222 and pH, and a weak relation 

(R2 of 0.51) was indicated between radon-222 and 

dissolved magnesium. The relation of radon-222 

activity to pH might be indicative of hydrogeochemical environments of the Highland Rim aquifer system where carbonates are absent and 

radon-222 can easily enter ground water. None of 

the radionuclide concentrations in water from 

either aquifer system correlated to other radionuclides or to gross alpha or gross beta activities, 

with the exception of radium-226. Radium-226 

correlated to gross alpha activity in water from 

both the Highland Rim and Central Basin aquifer 

systems (R2 of 0.48 and 0.88, respectively). 

Brutsaert, W.F., Norton, S.A., Hess, C.T., and 

Williams, J.S., 1981, Geologic and hydrologic 

factors controlling radon-222 in ground water in 

Maine: Ground Water, v. 19, no. 4,407-417 p. 

Bureau of Radiological Health and Training Institute, 

Environmental Control Administration, 

compilers, 1970, Radiological Health Handbook 

(rev. ed.): Rockville, Maryland, U.S. 

Department of Health, Education and Welfare, 

Public Health Service, 458 p. 

Coker, Gene, and Olive, Robert, 1989, Radionuclide 

concentrations from waters of selected aquifers 

in Georgia: U.S. Environmental Protection 

Agency, Ground-Water Protection Branch, 

Region IV, 21 p. 

Cross, F.T., Harley, N.H., and Hofmzum, W., 1985, 

Health effects and risks from Rn-222 in drinking 

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26 Geochemistry of and radioactivity in ground water of the Highland Rim 

and Central Basin aquifer systems, Hickman and Maury Counties, Tennessee l USGPO 1993-750-232/80003