Environ Earth Sci (2012) 65:1759–1780 DOI 10.1007/s12665-011-1157-y

ORIGINAL ARTICLE

Processes affecting geochemistry and contaminant movement in the middle Claiborne aquifer of the Mississippi embayment aquifer system Brian G. Katz • James A. Kingsbury • Heather L. Welch • Roland W. Tollett

Received: 17 May 2011 / Accepted: 17 June 2011 / Published online: 17 August 2011 Ó Springer-Verlag (outside the USA) 2011

Abstract Groundwater chemistry and tracer-based age data were used to assess contaminant movement and geochemical processes in the middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system. Water samples were collected from 30 drinking-water wells (mostly domestic and public supply) and analyzed for nutrients, major ions, pesticides, volatile organic compounds (VOCs), and transient age tracers (chlorofluorocarbons, tritium and helium-3, and sulfur hexafluoride). Redox conditions are highly variable throughout the MCA. However, mostly oxic groundwater with low dissolved solids is more vulnerable to nitrate contamination in the outcrop areas east of the Mississippi River in Mississippi and west Tennessee than in mostly anoxic groundwater in downgradient areas in western parts of the study area. Groundwater in the outcrop area was relatively young (apparent age of less than 40 years) with significantly (p \ 0.05) higher dissolved oxygen and nitrate–N concentrations and higher detections of pesticides and VOCs compared to water samples from wells in downgradient

B. G. Katz (&) U.S. Geological Survey (USGS), 2639 N. Monroe St., Ste. A200, Tallahassee, FL 32303, USA e-mail: [email protected] J. A. Kingsbury USGS, 640 Grassmere Park, Ste. 100, Nashville, TN 37211, USA H. L. Welch USGS, 308 South Airport Rd, Jackson, MS 39208, USA R. W. Tollett USGS, 3095 West California Ave., Ruston, LA 71270, USA

areas. Oxygen reduction and denitrification rates were low compared to other aquifers in the United States (zero order rate constants for oxygen reduction and denitrification were 4.7 and 5–10 lmol/L/year, respectively). Elevated concentrations of nitrate–N, and detections of pesticides and VOCs in some deep public supply wells ([50 m depth) indicated contaminant movement from shallow parts of the aquifer into deeper oxic zones. Given the persistence of nitrate in young oxic groundwater that was recharged several decades ago, and the lack of a confining unit, the downward movement of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing older oxic water. Keywords Middle Claiborne aquifer  Mississippi embayment aquifer system  Nitrate  Pesticides  Memphis aquifer

Introduction Groundwater is one of the most important resources in the United States and is the source of drinking water for about 38% of the population, or about 114 million residents in 2005 (Kenny et al. 2009). Because groundwater is used as a source for public-water supplies and because of the potential for groundwater to affect surface-water quality and ecological and recreational resources, degradation of groundwater quality as a result of anthropogenic activities is a continuing concern. Recent studies have shown that contaminants associated with agricultural and urban land use have degraded groundwater quality in many aquifers across the US (e.g. Gilliom et al. 2006; Zogorski et al. 2006; Dubrovsky et al. 2010). In a national study of major aquifers in the US, water from domestic wells had higher

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Environ Earth Sci (2012) 65:1759–1780

nitrate concentrations in areas of agricultural land use than in areas of other land uses (DeSimone 2009). Nitrate concentrations were highest in areas with well-drained soils and nitrate concentrations in shallow groundwater were most affected by redox conditions and nonpoint source nitrogen inputs (Burow et al. 2010). The Mississippi embayment aquifer system (MEAS) is the primary source of drinking water in areas of the southcentral United States underlain by the MEAS (Fig. 1). In 2000, the MEAS provided about 1.82 million m3/day (480 million gallons per day) of water for public supply, irrigation, and self-supplied industrial uses in northwest Mississippi and west Tennessee (Maupin and Barber 2005). The middle Claiborne aquifer (MCA) (Table 1) is the most productive of the aquifers in this aquifer system in west Tennessee and northwest Mississippi and is the primary source for water supply in this area. The MCA is protected from contamination by the middle Claiborne confining unit (MCCU) that overlies the aquifer in most areas down dip of the MCA outcrop area. In parts of northwest Mississippi Fig. 1 Map showing the location of the study area for the middle Claiborne aquifer of the Mississippi embayment aquifer system, sampled wells, and land use

and west Tennessee, however, this confining unit locally may be absent or may consist of fine sand, conditions that create the potential for downward movement of contaminants from shallow units that overlie the MCCU into the underlying MCA. Furthermore, groundwater withdrawals throughout the MCA have lowered water levels, enhancing the movement of shallow groundwater into the deep aquifers of the MEAS (Graham and Parks 1986; Welch et al. 2009). The potential for downward leakage into the MCA in the Memphis, Tennessee, area has been known for some time (Graham and Parks 1986; Parks 1990; Parks et al. 1995). Larsen et al. (2003) found that modern water from shallow units and a nearby stream in an area in Memphis was leaking through the MCCU into the MCA. Previous studies of various aquifers in the MEAS have found that the occurrence of elevated concentrations of nitrate, pesticides, and volatile organic compounds (VOCs) was related to redox conditions, degree of aquifer confinement, and the intensity of agricultural activities (Larsen et al. 2003; Welch et al. 2009).

92°0'0"W

91°0'0"W

90°0'0"W

89°0'0"W

88°0'0"W

37°0'0"N 30

Study area

Mississippi embayment aquifer system

19

29

18 28 20

EXPLANATION 36°0'0"N

Study area boundary

siss

Land use Category

21 17

07

Mis

Lakes and rivers Urban

22

TENNESSEE

25

Memphis

^ 06

Barren

35°0'0"N

26

Riv

Cities

ippi

^

er

Well location and number

27

Forest

08 23 16 24 15

Shrubland

05

Grasslands

09

Agriculture

04

Wetlands

03

ARKANSAS 34°0'0"N

14

!

10

MISSISSIPPI 02 11 01

12

0 0

13

33°0'0"N

123

15 20

30 40

60 Miles 80 Kilometers

Environ Earth Sci (2012) 65:1759–1780

1761

Table 1 Stratigraphic column of the Mississippi embayment aquifer system (modified from Renken 1998; Lloyd and Lyke 1995)

The geochemical processes affecting the regional groundwater quality in shallow and deep parts of the MCA are not well understood, with the exception of a few localscale studies (e.g. Graham and Parks 1986; Parks 1990; Parks et al. 1995; Larsen et al. 2003). Results of these and other localized studies indicate that the fate of nitrate and pesticides in the MCA is controlled by the oxidation– reduction conditions in the subsurface, extent and permeability of confining units, and mixing of young groundwater from shallow parts of the aquifer with older waters from deeper parts of the MCA and other units of the MEAS. In 2007, as part of the US Geological Survey’s National Water Quality Assessment Program (NAWQA), 30 wells used primarily for public and domestic supply in northwest Mississippi and west Tennessee (Fig. 1) were sampled to characterize the water quality of the MCA and to assess the susceptibility of the aquifer to contamination from activities at land surface. The specific objectives of this paper are to (1) to characterize the occurrence of nitrate, pesticides, and VOCs with respect to hydrologic and geochemical data to gain a better understanding of their fate and transport in the MCA, (2) link groundwater age information with geochemical data to estimate denitrification rates in the aquifer, and (3) determine the dominant geochemical processes occurring along flow paths that affect groundwater quality. The new information from this study will benefit water-resource managers in assessing the vulnerability of the MCA to contamination, evaluating natural remediation

processes for nitrate in groundwater, and reducing potential impacts to groundwater quality and human health from increased groundwater withdrawals and changing land-use activities.

Description of study area The study area covers approximately 34,550 km2 in northwest Mississippi and west Tennessee (Fig. 1). In 2010, the total population of counties within or partially within the study area boundary was about 2,041,370 (US Census Bureau 2010). The population of Shelby County, Tennessee, which includes the City of Memphis, was about 928,000 people in 2010 (US Census Bureau, http:// 2010.census.gov/news/releases/operations/cb11-cn93.html; accessed 4/3/11). The climate in northwest Mississippi and west Tennessee is humid and subtropical. The mean annual rainfall for the study area ranges from about 132 to 153 cm. Mean annual rainfall in the Memphis area is 139 cm. The mean annual temperature for the study area ranges from about 14.0 to 17.2°C. Mean annual temperature for the Memphis area is 16.8°C (http://cdo.ncdc.noaa. gov/climatenormals/) (accessed 15 November 2010). Hydrogeology Wells sampled for this study are completed in the Tertiaryage MCA, which is referred to locally as the Memphis

123

1762

aquifer in Tennessee and the Sparta aquifer in Mississippi. The MCA is one of several regional aquifers that make up the MEAS (Table 1) and has been described in detail (Hosman and Weiss 1991; Lloyd and Lyke 1995; and Renken 1998). The predominantly unconsolidated to semiconsolidated deposits that makeup the MCA and the other regional aquifers of the MEAS (Table 1) were deposited in the Mississippi embayment, a trough-like structure with its axis roughly coincident with the Mississippi River. The MCA is as much as 270 m thick in the Memphis area and thins up dip to the east and north. The aquifer crops out in a north–south trending belt, the eastern limits of which form the eastern extent of the study area. In the west part of the study area, the MCCU separates the MCA from the overlying Holocene-age Mississippi River Valley alluvial aquifer and shallow aquifers in the overlying Pliocene(?)-Pleistocene-age terrace deposits (Table 1). Groundwater generally moves from areas of recharge within the outcrop area along the east part of the study area, downdip, or westward, to areas of discharge near the Mississippi River (Fig. 2). The MCA is mostly unconfined in the outcrop area, and becomes confined by the MCCU downgradient towards the Mississippi River. Groundwater discharge is to pumping wells, incised rivers and streams, and as upward leakage to shallower aquifers (Lloyd and Lyke 1995). Withdrawals for public-supply, industrial, and irrigation uses have altered groundwater-flow directions locally throughout the study area. The primary example of these effects is in the Memphis area, including the northwestern-most counties of Mississippi, where groundwater flow directions in the MCA are toward the center of a large cone of depression that has formed from extensive withdrawals (Fig. 2). In addition, withdrawals in other parts of the study area have increased recharge to the MCA by inducing downward leakage through the alluvium, Pliocene(?)-Pleistocene-age terrace deposits, and the upper Claiborne aquifer in the west part of the study area, as well as from incised streams in outcrop areas to the east (Graham and Parks 1986; Parks 1990; Larsen et al. 2003). Aquifer mineralogy Poorly crystalline kaolinite is the predominant clay mineral in the sediments of the Claiborne Group (Table 1), with smaller amounts of smectite, illite, and illite–smectite mixed-layer clay present (Hosterman 1984). Recent petrologic analyses of sediment from the MCA (Memphis aquifer) indicate that the clay mineralogy changes from kaolinite north of the state boundary between Mississippi and Tennessee to mixed kaolinite/smectite near the boundary to smectite-dominated composition south of the state boundary (Lumsden et al. 2009). Lignite is found as

123

Environ Earth Sci (2012) 65:1759–1780

both disseminated grains and in lenticular layers throughout the Claiborne Group. Based on a limited number of mineralogical analyses of samples from the MCCU and upper part of the MCA in the Memphis area, Spann (1998) found that the clay fraction was dominated by kaolinite, with minor amounts of illite and smectite (mean cation exchange capacity, 13.4 meq/100 g). The reactive mineral phases in the MCCU and MCA used in a geochemical model in the Memphis area included clay minerals, pyrite, and Fe hydroxides (Larsen et al. 2003). Water use Groundwater represents the primary source of drinking water for most of the population in the study area. About 1.8 million m3/day (480 Mgal/day) of water was withdrawn from the Tertiary aquifers for public supply, irrigation, and self-supplied industrial uses in 2000 in Mississippi and Tennessee (Maupin and Barber 2005). Some of the largest withdrawals from the Tertiary aquifers are from the MCA and lower Claiborne-upper Wilcox aquifer (LCUWA). Specifically, large withdrawals for public supply occur in west Tennessee, particularly the Memphis area, and parts of northwest Mississippi. During 2000, 0.7 million m3/day (188 Mgal/day) of groundwater was pumped from the MCA in west Tennessee for public supply (Webbers 2003). In Mississippi, the MCA is available for water supply in more than 40% of the state, which is a larger percentage of area than any other aquifer in the state (Newcome 1976). Withdrawals of groundwater from the MCA have lowered water levels particularly near pumping centers (Criner and Parks 1976; Kingsbury 1996; Renken 1998; Schrader 2007). Prior to these withdrawals, groundwater levels in parts of the aquifer were higher than in shallow overlying units, such as the Mississippi River Valley alluvial aquifer (MRVAA), and groundwater moved upward into the MRVAA. In the Memphis area, large groundwater withdrawals from the MCA (Memphis aquifer) have lowered groundwater levels as much as 30 m and induced downward leakage from the MRVAA and the Pliocene(?)/ Pleistocene-age terrace deposits to the Memphis aquifer (Renken 1998). Land use and potential sources of groundwater contamination The primary land uses in the study area (Fig. 1) are agriculture (49%); forested and shrublands (28%); lakes, rivers, and wetlands (15%); and urban (8%). Much of the agricultural land use lies in the west part of the study area along the Mississippi River, and accounts for about 62% of the land use in that area. The principal crops grown in the

Environ Earth Sci (2012) 65:1759–1780

1763

Fig. 2 Map showing the location of sampled wells, the potentiometric surface contours, and wells along selected groundwater flow paths in the middle Claiborne aquifer of the Mississippi embayment aquifer system

study area are corn, soybeans, and cotton. Most of the forest and shrubland is found throughout the outcrop area of the MCA in the east part of the study area, and accounts for about 49% of the total land use in this area. The most

widespread urban land use is in the west-central part of the study area in and around Memphis. Urban and agricultural land uses are potential sources of a wide variety of contaminants to groundwater. Volatile

123

1764

organic compounds, and in particular chlorinated solvents, are among the most common contaminants associated with urban land use (Zogorski et al. 2006). Generally, nitrate and pesticides are the contaminants of concern in areas of row crop agriculture. Fertilizer nitrogen usage in the study area [based on countywide fertilizer sales information (Ruddy et al. 2006)] increased steadily from the mid-1940s to the mid-1970s where it fluctuated somewhat but remained fairly constant until 2001 (last available records). Most of the agricultural land use in the study area occurs in the MRVA, which overlies and is hydraulically connected to the MCA. Sixty percent of the area overlying the MRVA is agricultural land used mostly to cultivate soybeans, cotton, corn, and rice (Coupe 2000).

Methods Groundwater network The sampling network included 30 wells used primarily for public and domestic supply in northwest Mississippi and west Tennessee (Table 2). Well site selection criteria (Lapham et al. 1995) were used to create a sampling network designed to characterize water quality in the MCA in northwest Mississippi and west Tennessee (Fig. 2). The main criterion used for site selection was that the wells were located in unconfined and confined areas of the MCA (depths less than 150 m). A geographical information system (GIS) based computer program (Scott 1990) used an equal-area method to select sampling locations for wells screened in the MCA at various locations throughout the study area. Most of the selected wells were used for drinking-water supply [14 public supply (PS) wells and 10 domestic (D) supply wells]; however, there were three agricultural-type wells [two irrigation (I) and one stock (S)], one commercial (C) use well, and one unused well (U) sampled for the study (Table 2). Sampling and analyses Water samples were collected once from each well during April to June 2007, although five additional samples for age dating were collected during May 2009. Specific conductance (SC), pH, temperature, and dissolved oxygen (O2) were measured in the field using a multi-parameter probe with a flow-through cell. Alkalinity was determined in the field by analyzing acidimetric-titration data using the inflection point titration method. Turbidity, in nephelometric turbidity units (NTU), was measured using a portable meter (O’Dell 1993). Sample collection began after purging several (at least three) casing volumes and stabilizing field measurements (Koterba 1998). Major ions were

123

Environ Earth Sci (2012) 65:1759–1780

measured using various methods (Fishman and Friedman 1989). Pesticides were extracted from samples through a carbon (C)-18 solid-phase extraction column and were analyzed using gas chromatography/mass spectrometer (GC/MS) and high-performance liquid chromatography/ mass spectrometer (Zaugg et al. 1995; Furlong et al. 2001). VOCs were analyzed using a purge and trap capillarycolumn GC/MS (Connor et al. 1998). Age dating groundwater Water samples were collected from 17 of the 30 selected wells in 2007 and analyzed for the transient tracers chlorofluorocarbons [CFCl3 (CFC-11), CF2Cl2 (CFC-12), and C2F3Cl3 (CFC-113)], tritium (3H), and its decay product helium-3(3He) to estimate apparent ages (the time elapsed since recharge to the water table) of groundwater samples in various zones in the MCA. Samples from five additional wells were collected and analyzed for sulfur hexafluoride (SF6) in 2009. Anthropogenic activities, such as industrial processes and atmospheric testing of thermonuclear devices, have released CFCs, SF6, and 3H into the atmosphere in low but measurable concentrations (Fig. 3). Precipitation that incorporates CFCs, and SF6, and 3H from the atmosphere infiltrates into the ground and carries a particular chemical or isotopic signature related to atmospheric concentrations of these compounds at the time of recharge to the aquifer. These dating methods assume that gas exchange between the unsaturated zone and air is rapid, but that shallow groundwater remains closed to gas exchange after recharge (Schlosser et al. 1989; Plummer and Busenberg 2000; Busenberg and Plummer 2000). The continued decrease and low concentrations of 3H in rainfall in the southern US have resulted in limited use of the 3 H method for age dating groundwaters recharged during the past 2–3 decades. However, by measuring tritiogenic helium-3 (3Hetrit), the stable daughter product of 3H decay that has accumulated in groundwater systems, the dating range and precision can be enhanced (Plummer et al. 1993). When combined, measurements of 3H and 3Hetrit, define a relatively stable tracer of the initial 3H input to groundwater, and their ratio can be used to calculate the 3H/3Hetrit age from a single water sample (Schlosser et al. 1988, 1989; Solomon and Sudicky 1991). The 3H/3Hetrit ratio yields the following equation for the piston-flow assumption (tracer concentrations are not altered by transport processes; i.e. no mixing or dispersion and all groundwater flow lines have similar velocity) in which the apparent age (T, years) can be expressed as (Torgersen et al. 1979) h  i T ¼ 1=kT ln 1 þ3 Hetrit =3 H ; ð1Þ where kT is the radioactive decay constant for 3H, 3H is the concentration in tritium units (TU), and 3Hetrit is the

04/25/07

06/26/07

05/30/07

06/19/07

04/16/07

04/18/07

04/05/07

05/15/07

06/28/07

06/11/07

05/07/07

05/12/09

05/23/07

05/22/07

04/19/07

05/16/07

06/12/07

06/12/07

SPRT2

SPRT3

SPRT4

SPRT5

SPRT6

SPRT7

SPRT8

SPRT9

SPRT10

SPRT11

SPRT12

SPRT13

SPRT14

SPRT15

SPRT16

SPRT17

SPRT18

SPRT19

04/04/07

05/09/07

SPRT1

SPRT20

Sample date

Well_#

PS

PS

PS

PS

PS

D

D

S

D

D

D

D

PS

I

PS

U

D

I

I

D

Well type

101

170

130

71.3

53

15.2

36.6

58

46.3

176

22.6

48.8

95.7

165

136

67.1

63.4

160

99.1

71.6

Well depth (m)

16.8

16.7

19.7

17.0

16.5

17.1

17.5

19.1

19.9

20.6

17.8

17.8

16.4

16.9

17.0

18.2

18.0

19.7

21.7

19.1

Temp

116

45

288

57

44

43

85

78

259

268

88

151

71

282

97

156

106

0.2

3.9

0.1

0.2

6.2

2.5

5.6

5.6

1.5

0.1

6.5

5.6

4.2

0.6

4.2

0.2

0.3

5.9

5.7

6.3

5.4

4.8

5.4

4.7

5.5

7.5

8.1

5.6

5.4

5.7

6.3

5.4

6.3

5.7

7.5

890 \0.1

7.7

pH

7.5

3.4

DO

7.7

413

226

SC

0

23

184

22

12

16

13

30

144

152

24

30

33

172

43

86

54

375

253

144

HCO3

0.01

0.03 \0.06

\0.02 \0.06

0.07

E0.21 E0.004 \0.4

E0.004

E0.32

0.07 \0.06 \0.02 \0.02

E0.26

E0.005

\0.02 \0.06

E0.24

0.01

0.87

E0.22

\0.02

E0.005

E0.23

na

0.11

E0.005

0.01

E0.28

0.47

E0.33

E0.31

E0.01

1.5

\0.02

0.08

0.41 \0.06 0.14

0.31

0.83 \0.06

\0.02

0.03

1.1

\0.02

0.01

4.0

\0.02

E0.31

E0.22

E0.35

\0.4

1.01

0.92

E0.23

DOC

E0.005 \0.4

0.08

E0.003

0.03 \0.06 \0.02

0.01

0.47

\0.02

0.01

E0.02 \0.06

0.61

0.39 \0.06 0.06

0.12

0.38 \0.06

1.09 \0.06

PO4–P

NO3– N

NH4– N

10

3.0

28

3.3

2.3

2.5

3.0

4.1

29

1.2

4.2

6.6

4.7

30

5.6

13

7.3

24

0.49

8.5

Ca

4.9

1.1

12

1.2

0.76

0.9

1.7

1.6

8.9

0.60

1.7

2.3

1.9

14

2.3

6.3

3.1

8.5

0.24

2.6

Mg

6.3

4.1

4.4

5.1

4.6

3.7

8.4

8.4

7.0

61

11

18

6.2

8.8

11

8.1

7.7

167

99

38

Na

1.5

0.43

7.9

0.34

0.71

0.71

0.64

0.91

7.5

2.0

0.57

0.62

0.56

1.5

0.73

1.0

1.5

3.2

1.3

4.6

K

0.86

SO4

2.0

1.4

1.2

1.7

3.0

2.1

7.1

3.9

1.2

1.9

10

19

3.5

2.5

5.6

3.7

3.9

\0.1

\0.1

0.17

\0.1

E0.09

E0.06

0.20

0.31

E0.08

F

3.0

1.0

6.4

3.5

E0.09

\0.1

0.12

\0.1

0.29 \0.1

15

11

12

12

17

14

0.58 \0.1

19

33

17

18

\0.1

E0.08

0.19

31

18

13

14

17

18

30

14

30

31

SiO2

\0.1 13

3.7

7.3

6.0

0.28 \0.1

1.1

2.1

3.3

2.2

2.1

2.2

\0.18

5.8 \0.18

2.1

111

Cl

Table 2 Well depth, concentrations of selected chemical constituents in water from sampled wells, redox categories, and geographical location grouping

610

E6

12,300

260

\6

\6

15

\6

120

31

\6

16

\6

1,270

\6

1,070

2,640

600

290

37

Fe

171

0.68

196

9.1

0.39

0.33

32

E0.2

23

5.1

0.74

0.68

0.64

88

\0.2

13

18

29

11

8.4

Mn

83

33

156

41

35

27

64

67

156

166

78

97

49

156

65

104

98

528

308

149

DS

Environ Earth Sci (2012) 65:1759–1780 1765

123

123

06/19/07

04/16/07

04/18/07

04/05/07

05/15/07

06/28/07

06/11/07

05/07/07

05/12/09

05/23/07

05/22/07

04/19/07

05/16/07

SPRT-6

SPRT-7

SPRT-8

SPRT-9

SPRT-10

SPRT-11

SPRT-12

SPRT-13

SPRT-14

SPRT-15

SPRT-16

SPRT-17

PS

SPRT-5

05/08/07

SPRT30

PS

05/30/07

04/24/07

SPRT29

D

SPRT-4

04/17/07

SPRT28

D

06/26/07

05/08/07

SPRT27

PS

SPRT-3

04/04/07

SPRT26

PS

04/25/07

06/18/07

SPRT25

PS

05/09/07

05/31/07

SPRT24

PS

SPRT-2

04/23/07

SPRT23

C

SPRT-1

06/27/07

SPRT22

PS

Sample date

04/18/07

SPRT21

Well type

Well_#

Sample date

Well_#

Table 2 continued

Central

East

Central

East

East

Central

West

Central

Central

Central

West

West

West

West

West

West

West

Location group

163

171

35.1

42.7

93

57.9

79.3

91.4

41.8

88.4

Well depth (m)

56

35

151

44

44

123

47

28

84

78

SC

4.1

5.4

3.6

5.4

7.3

9.3

7.6

7.1

1.0

5.4

DO

Fe(III)/SO4

O2

O2

O2

O2

O2-Fe(III)/SO4

Suboxic

O2

O2

O2

O2-Fe(III)/SO4

O2

Fe(III)/SO4

Fe(III)/SO4

CH4

O2-CH4

O2

Redox process

16.9

16.2

16.3

16.2

15.8

16.5

16.2

16.9

17.7

16.6

Temp

24

18

50

20

11

11

7

14

29

32

HCO3

1.2 0.31 1.5 3.5 0.89 E0.05

\0.02 \0.02 \0.02 \0.02 \0.02 \0.02

E0.02

\0.02

Anoxic

Oxic

Oxic

Oxic

Oxic

Mixed (oxic-anoxic)

Suboxic

Oxic

Oxic

Oxic

Mixed (oxic-anoxic)

Oxic

Anoxic

Anoxic

Anoxic

Mixed (oxic-anoxic)

Oxic

E0.23

E0.28

0.01 2.0 3.3

E0.004 \0.4

6.2

2.3

2.5

4.3

2.2

1.5

3.9

3.9

Ca

E0.2

0.78

E0.004 \0.4

E0.006 \0.4

0.01

E0.004 \0.4

E0.004 \0.4

E0.005

1,890

4,280

114

2,650

4,280

341

95

2,270

Pump rate

E0.05

DOC

E0.005 \0.4

PO4–P

E0.05 \0.01

1.5

0.67

\0.02

2.36

NO3– N

NH4– N

Redox category

5.5

5.2

5.5

5.3

5.2

5.5

5.4

4.9

5.7

5.3

pH

1.2

0.73

4.4

0.88

0.98

1.6

0.87

0.49

1.4

1.4

Mg

4.8

3.4

5.1

4.6

3.5

16

3.9

2.7

13

9.3

Na

0.84

0.37

4.8

0.41

0.81

0.77

0.85

0.67

0.50

0.38

K

3.9

1.2

9.0

5.1

2.0

1.0

7.2

1.7

2.8

18

Cl

\0.1

\0.1

F

\0.1

3.2

0.9

\0.1

\0.1

14

9.9

16

\0.1 4.4

12

15

19

14

14

18

13

SiO2

0.86 \0.1

0.18 \0.1

3.5

0.33 \0.1

0.21 \0.1

2.2

2.8

SO4

110

3

\6

\6

\6

25

E5.4

\6

6.5

E4.9

Fe

8.9

0.60

495

0.42

1.0

1.8

0.92

0.32

0.28

0.30

Mn

39

35

84

34

38

87

41

348

60

64

DS

1766 Environ Earth Sci (2012) 65:1759–1780

05/08/07 SPRT-30

CFC-12

TRITIUM, Rainfall, St. Louis, MO

CFC-11

CFC-113 SF6 X10

1940

1950

1960

1970

1980

1990

2000

TRITIUM CONCENTRATION, TU

CFC OR SF6 CONCENTRATION, PPTV

Concentrations are in milligrams per liter unless otherwise noted;temp temperature in degrees celsius; SC specific conductance in uS/cm; DO dissolved oxygen; pH standard units; DS dissolved solids; Fe and Mn, in micrograms per liter; na not analyzed; E estimated, value between method detection limit and laboratory reporting level; well types: PSW public supply; D domestic; I irrigation; S stock; Ind industrial; C commercial; U unused; pump rate, liters per minute

3,970

4,920 Oxic

Mixed (oxic-anoxic) O2-Fe(III)/SO4

Mixed (oxic-anoxic)

04/24/07 SPRT-29

Central

O2

O2-Mn(IV) 04/17/07 SPRT-28

East

05/08/07 SPRT-27

East

Oxic O2

04/04/07 SPRT-26

Central

757

3,790 Oxic O2

06/18/07 SPRT-25

East

2,270

Oxic O2

Oxic 05/31/07

East

1,890 Oxic

O2

Oxic

SPRT-24

East

O2

O2

04/23/07 SPRT-23

Central 06/27/07 SPRT-22

Central

3,790

545 Anoxic

Oxic O2

Fe(III)/SO4

04/18/07 SPRT-21

Central

04/04/07 SPRT-20

West

3,790

4,060 Anoxic

Oxic O2

Fe(III)/SO4 West 06/12/07

06/12/07

SPRT-18

Central

1767

SPRT-19

Sample date Well_#

Table 2 continued

Location group

Redox process

Redox category

Pump rate

Environ Earth Sci (2012) 65:1759–1780

2010

Fig. 3 Atmospheric input curves for chlorofluorcarbons (CFC-11, CFC-12, CFC-113), sulfur hexafluoride (SF6), and tritium (3H) in rainfall from St. Louis, Missouri

tritiogenic 3He content (in TU). One TU is equal to one 3H atom in 1018 hydrogen atoms or 3.2 picocuries/L. A helium (He)-isotope mass balance is used to calculate the amount of tritiogenic and non-tritiogenic 3He in the sample. Nontritiogenic 3He, which generally is negligible in a shallow aquifer with local recharge, is corrected by using measured concentrations of helium-4 (4He) and neon (Ne) in the water sample and assuming solubility equilibrium with air at the water temperature measured during sampling (Schlosser et al. 1988, 1989). Concentrations of 3H and 3 Hetrit in groundwater are assumed not to be affected by contamination, sorption, and microbial degradation processes (Plummer et al. 1993). The distribution of 3H and 3 Hetrit, however, can be affected by hydrodynamic dispersion, mixing of different age waters, and diffusive fractionation (Solomon and Sudicky 1991; Reilly et al. 1994; LaBolle et al. 2006). Information about groundwater transit time or apparent age (includes unsaturated zone travel time) in the study area can be obtained by comparing measured 3H concentrations in groundwater with the long-term 3H input function of rainfall measured at the International Atomic Energy Agency (IAEA) precipitation monitoring station in St. Louis, Missouri (Michel 1989) (Fig. 3), which is located approximately 460 km north of the study area. Atmospheric weapons testing beginning in the early 1950s increased 3H concentrations in rainfall at St. Louis to a maximum of several hundred TU during the mid-1960s, followed by a nearly logarithmic decrease in concentrations to present values of \2 TU (Fig. 3). Water samples for the determination of 3H/3Hetrit, 4He, and Ne were collected in pinched-off copper tubes (10 mm diameter, 80 cm length, approximately 40 mL volume) while applying back pressure to prevent formation of gas bubbles. Samples were analyzed at the Noble Gas Laboratory of Lamont-Doherty Earth Observatory using

123

1768

quantitative gas extraction followed by mass-spectrometric techniques (Schlosser et al. 1989; Ludin et al. 1998). Tritium samples were analyzed using the direct liquid-scintillation counting method (Thatcher et al. 1977) at the USGS Tritium Laboratory in Menlo Park, California. Analytical uncertainty (1r) for 3H using the low-level counting procedure is approximately ±0.15–0.30 TU. The CFC and SF6 age-dating techniques rely on the stability of these halogenated hydrocarbon and sulfur compounds in the hydrosphere, which has led to their effective use as tracers to date groundwater recharged during the past 50 years (Plummer and Busenberg 2000; Busenberg and Plummer 2000). These techniques presume that CFC and SF6 concentrations in the aquifer have not been altered by biological, geochemical, or hydrologic processes. Apparent ages for CFCs and SF6 are estimated based on the equilibrium partitioning between recharging groundwater and the partial pressures of trichlorofluoromethane (CCl3F, CFC-11), dichlorodifluoromethane (CCl2F2, CFC-12), trichlorotrifluoroethane (C2Cl3F3, CFC113), and SF6 in the troposphere or soil atmosphere (Fig. 3). CFC and SF6 concentrations in groundwater are functions of the atmospheric partial pressures and the temperature at the base of the unsaturated zone during recharge. The recharge temperature and the quantity of dissolved excess air (Heaton and Vogel 1981) are determined from analyses of nitrogen (N2) and argon (Ar) gases analyzed using gas chromatography on the headspace of water samples collected in the field (Busenberg et al. 1993). Samples for CFCs and SF6 were collected using procedures outlined by the USGS CFC Laboratory in Reston, Virginia (http:// water.usgs.gov/lab/chlorofluorocarbons/sampling/bottles/; accessed September 22, 2009). Procedures for CFCs and SF6 analyses are described by Busenberg and Plummer (1992, 2000). Ancillary datasets Ancillary data for buffer areas around the sampled wells include land use and soil properties. Land use was estimated using a 30-m resolution digital data set based on 2001 National Land Cover Data (Homer et al. 2004) with selected classifications from the USGS aerial photographbased Land Use Land Cover Data of the mid-1970s to mid1980s. A 500-m radius circular buffer was delineated around each site, and the fraction of the total buffer represented by each land-use category was calculated. Areaweighted and depth-weighted average values for soil organic matter, average clay content, and soil hydrologic group data were compiled for buffer areas from Soil Survey Geographic (SSURGO) Database data (Natural Resources Conservation Service 2007).

123

Environ Earth Sci (2012) 65:1759–1780

Geochemical modeling along groundwater flow paths Geochemical mass-balance modeling techniques (NETPATH; Plummer et al. 1994) were used to quantify mass transfer associated with sources and sinks of major dissolved constituents along groundwater flow paths. The mass-transfer models were constrained by the concentration of selected dissolved constituents in initial and final waters along the flow path. In addition to these chemical constraints, an electron balance is included to account for conservation of electrons under redox conditions. The models contain solid phases based on the aquifer mineralogy. Plausible reaction models for this study are valid within the constraints of available thermodynamic data.

Results and discussion Groundwater chemistry and redox conditions Overall, water from sampled wells generally has low dissolved solids concentrations (median, 72 mg/L) (Table 2). Water types generally are mixed with no dominant cations or anions. Differences in groundwater chemistry throughout the study area can be attributed in part to the location of wells relative to the outcrop area for the MCA. Wells were grouped into three locations within the study area: east (in and near outcrop areas), central, and west (farthest downgradient from outcrop areas) (Table 2). As presented, the order of this grouping corresponds to the general direction of groundwater flow from the outcrop areas westward toward the Mississippi River. Water from seven of eight wells located in the east part of the study area (Table 2; location grouping) is predominantly oxic (dissolved O2 [ 0.5 mg/L) and has significantly higher dissolved O2 and nitrate–N concentrations (p \ 0.05) than water from wells located in the west part of the study area. Redox processes were classified based on threshold concentrations of dissolved O2, nitrate, manganese, iron, and sulfate (McMahon and Chapelle 2008; Chapelle et al. 2009) and were used to group the groundwater samples into four categories: oxic, suboxic, mixed source, and anoxic. Groundwater in the central part of the study area is mostly oxic, although water from two wells is a mixed source and water from a third well is anoxic (Table 2). Dissolved O2 and nitrate–N concentrations in water from wells in the central part of the study area are not significantly different from those in the east and west parts of the study area. Groundwater from the west part of the study area is mostly anoxic, suboxic, or mixed (8 of 10 wells) (Table 2). Differences in groundwater chemistry also are related to the depth of the sampled aquifer zone. Well depths in the

Environ Earth Sci (2012) 65:1759–1780

east part of the study area are shallower than those in the west part of the study area (p \ 0.05), but well depths in the central part of the study area are not significantly different from those in the east or west parts. Groundwater age Apparent groundwater ages (assuming piston flow) ranged from about 25 to more than 60 years based on measured CFC-12, CFC-113, 3H and 3He concentrations, and atmospheric inputs of these tracers over the past 50–60 years (Table 3). Overall, CFC apparent ages generally are concordant with apparent ages obtained from 3H/3He data (Fig. 4). Given the limitations associated with each dating method, such as degradation of CFC-11 and possible gas fractionation or gas loss during sampling for noble gases, the groundwater age uncertainty is approximately ±5 years but likely is higher for old waters with low tracer concentrations. Many of these water samples (9 out of 16 wells) where 3H analyses were conducted contain very low concentrations of 3H (\0.1 TU), which likely indicates that groundwater (from these wells) was recharged prior to the atmospheric testing of thermonuclear weapons in the 1950s. Groundwater from wells in the MCA outcrop area in the east part of the study area has significantly (p \ 0.05) younger apparent ages than those in the downgradient west part of the study area. There also is a significant correlation between apparent ages and well depth (Table 4). CFC concentrations were not used to estimate apparent ages where geochemical conditions indicated likely degradation of CFC compounds. Degradation of CFC-11 likely has occurred in anoxic and mixed waters, as CFC-11 apparent ages are older than those for CFC-12 and CFC113 in water samples from five of seven wells in these redox categories (Table 3). In contrast, for oxic groundwater samples, only three of 10 wells have CFC-11 apparent ages that are older than CFC-12 or CFC-113 apparent ages. Based on a comparison of apparent ages derived from SF6 and CFCs (wells SPRT-8, SPRT-20, and SPRT-29), SF6 apparent ages are younger than CFC-12 and CFC-113 apparent ages. The older CFC ages in samples from these wells may indicate CFC degradation or possibly mixtures of waters of different ages, but there is considerable uncertainty in estimating apparent ages given the low concentrations of CFCs and SF6. Mixing of waters from various vertical zones in the MCA does not appear to be occurring as age-tracer concentration data are more consistent with output for a piston-flow model than for output curves for other lumped-parameter models (such as exponential mixing, binary mixing, or dispersion). CFC-12 and CFC-113 apparent ages are younger than the 3H/3He apparent age for anoxic water samples from well SPRT-18, which would indicate that CFC degradation is less likely.

1769

The apparent-age and well-depth data tend to cluster into three distinct groups (Fig. 5). Group 1 contains shallow wells (\80 m) and apparent ages \40 years (Table 3). This group has 3H concentrations [1 TU (wells SPRT-9, SPRT-10, SPRT-14, and SPRT-24) and the highest CFC concentrations compared to the other two groups. Group 2 also contains shallow wells (depths \80 m) but older apparent ages ([45 years). Three water samples in group 2 had 3H concentrations that were between 0.1 and 1.0 TU [wells SPRT-22 (0.84 TU), SPRT-19 (0.18 TU), and SPRT-17 (0.49 TU)]. Water from wells SPRT-1, -5, -15, and -27 had 3H concentrations \0.1 TU. SPRT-22 has a 3 H/3He apparent age of about 30 years; whereas the CFC apparent ages are considerably older (46–54 years) and may indicate some degradation of CFCs. Group 3 contains deep wells ([120 m) and old waters ([40 years) (wells SPRT-3, SPRT-11, SPRT-18, and SPRT-19). For all three dating techniques, the apparent age increases with increasing well depth and the slopes (negative) of the trend lines (linear regression) fall into three groups that range from 1.1 (for 3H/3He apparent age data) to 1.65 (for CFC-113 apparent age data) (Fig. 5). This finding indicates that for every meter increase in well depth, the age increases by 1.1–1.65 years. The r2 values generally were low for best-fit linear regression lines (Fig. 5), and the highest r2 value (0.26) was for the 3H/3He apparent age data and well depth (slope 1.1). Reduction in dissolved oxygen and nitrate concentrations with groundwater age and depth Dissolved O2 concentrations were inversely correlated (p \ 0.05) with the apparent age of groundwater (based on CFC-12 and CFC-113 data). Dissolved O2 generally decreased with increasing depth in the MCA, although the decrease was not statistically significant. Oxic conditions are highly variable throughout the MCA as indicated by elevated dissolved O2 concentrations ([125 lmol/L or [4 mg/L) in some deep wells ([80 m). However, dissolved O2 was inversely correlated (p \ 0.05) with the apparent age of groundwater (based on CFC-12 and CFC113 data). The rate of oxygen reduction in the MCA is estimated by assuming that oxygen loss follows a zeroorder rate expression: C ¼ C ð0Þ  k0 t;

ð2Þ

where C is the concentration of dissolved O2 at the time of interest, C(0) is the initial dissolved O2 concentration, k0 is the zero-order reaction constant, and t is time. Initial dissolved O2 concentration was estimated to be 297 lmol/L, which is the O2 concentration at saturation in water at 17°C (the median measured groundwater temperature) and 750 mm Hg. The zero-order rate constant (4.7 lmol/L/year)

123

123

05/16/07

06/12/07

06/12/07

SPRT-17

SPRT-18

SPRT-19

05/30/07

05/08/07

SPRT-24

SPRT-27

5.5

140.1

12.8

4.3

1.7

6.9

1.1

5.0

358.6

30.4

25.1

27.3

17.1

11.6

268.3

0.0

7.3

0.0

2.1

0.0

1.2

0.0

24.2

50.9

29.6

46.5

51.7

54.4

50.1

56.1

33.4

55.1

51.9

59.4

25.7

48.2

48.9

48.9

52.6

47.0

30.1

59.9

50.8

C contaminated, pptv parts per trillion by volume, mg/L milligrams per liter

SPRT-29

06/27/07

SPRT-22

SPRT-20

05/22/07

SPRT-15

105.6

0.0

62.9 48.8

54.4

54.4

34.9

54.5

43.9

54.4

47.9

54.4

26.9

54.4

37.2

25.7

27.4

24.4

15.8

05/19/09

05/09/09

05/12/09

05/23/07

4.5

0.0 10.3

28.5

SPRT-14

1.8

19.1

24.9

30.0

05/23/07

0.0

11.0

31.7

SPRT-13

27.1

05/07/07

33.5

28.6

54.5

36.4

06/11/07

16.7

30.6

58.5

59.7

23.6

SPRT-12

247.7

38.6

63.0

62.9

54.5

47.1

SF6 sample date

SPRT-11

102.6

18.9

0.0

7.7

56.2

53.4

CFC113/CFC-12 apparent age (years)

06/28/07

244.5

5.9

5.0

63.0

62.9

CFC-113 apparent age (years)

SPRT-10

44.0

0.0

0.0

0.0

1.7

CFC-12 apparent age (years)

05/08/09

06/19/07

SPRT-5

10.9

15.5

CFC-11 apparent age (years)

05/06/09

05/30/07

SPRT-4

0.0

0.0

CFC113 (pptv)

05/15/07

06/26/07

SPRT-3

CFC12 (pptv)

SPRT-9

05/09/07

SPRT-1

CFC11 (pptv)

SPRT-8

CFC sample date

Well-#

0.342

0.203

69.3

33.6

0.459

SF6, pptv

35.9

40.9

C

C

33.8

SF6 apparent age (years)

5/8/2007

5/31/2007

6/27/2007

41.9

6/12/2007

5/12/2007

5/16/2007

5/22/2007

5/23/2007

5/7/2007

6/11/2007

6/28/2007

5/5/2007

9/19/2007

5/30/2007

6/26/2007

5/9/2007

3H sample date

-0.04

4.9

0.84

0.18

-0.02

0.49

0.02

4.02

0.01

-0.05

3.34

1.1

-0.09

0.01

-0.01

0.07

3H (TU)

2.85

15.43

4.21

-0.83

-0.65

-0.54

0.33

8.15

5.11

8.76

5.89

-1.7

5.67

0

0

3He (TU)

25.3

31.8

63.8

52.4

19.7

22.9

33

54

65

3H, apparent age (years)

18.5

18.9

20.4

19.5

17.7

N2 (mg/L)

0.652

0.660

0.682

0.652

0.626

Ar (mg/L)

6.52

0.271

6.02

6.74

3.75

O2 (mg/L)

62.0

83.9

98.4

95.2

70.2

CO2 (mg/L)

0

6E-04

0

0

0

CH4 (mg/L)

Table 3 Summary of calculated atmospheric partial pressures for CFC-11, CFC-12, CFC-113, and SF6; tritium, helium-3, and dissolved gas concentrations; and apparent groundwater ages and recharge dates using the piston flow model

1770 Environ Earth Sci (2012) 65:1759–1780

Environ Earth Sci (2012) 65:1759–1780

1771

CFC-11 CFC-12 CFC-113

70

1:1 line

60 50 40 30 20

3

3

H/ He Apparent Age, years

80

10 0 0

10

20

30

40

50

60

70

80

CFC Apparent Age, years

Fig. 4 3H/3He apparent ages versus CFC apparent ages for water from wells sampled for age tracers

in the MCA was determined by fitting a linear regression line to a plot of C versus t (r2 = 0.30; p = 0.024). The reduction in dissolved O2 concentrations with groundwater age was also evaluated using a first-order rate expression: C ¼ C ð0Þexpðk1 tÞ

ð3Þ

where C, C(0), and t are as described above and k1 is the first-order rate constant. The first-order rate constant (0.08 year-1) was determined by fitting a linear regression line to a plot of lnC versus apparent groundwater age (Fig. 6) (r2 = 0.25; p = 0.04) after rearranging Eq. (3). The zero- and first-order rate expressions fit the data similarly; however, first-order rate expressions tend to provide a better fit to data with low O2 concentrations (Tesoriero and Puckett, 2011). Both the zero- and firstorder rate constants indicate that there is a small amount of O2 consumed as groundwater age increases. The zero- and first-order rate constants for the MCA are slightly lower than median zero- and first-order rate constants (7.6 lmol/ L/year and 0.11 year-1, respectively) obtained for shallow aquifers in 12 study areas around the United States (Tesoriero and Puckett 2011). The low rate of O2 consumption indicates that reactivity of O2 with electron donors (such as dissolved organic carbon and sulfate) in the MCA is generally low, although the reaction rate likely could be higher in localized zones (e.g. with higher amounts of organic matter). Nitrate–N concentrations in groundwater also are variable in the MCA, ranging from \0.06 to 4 mg/L (Table 2). Nitrate–N concentrations are significantly higher in the east part of the study area (oxic groundwater) than in the west and central parts (mostly anoxic groundwater), and are inversely correlated (p \ 0.05) with well depth, alkalinity, and pH. Nitrate–N concentrations were \0.06 mg/L in all Fe- and SO4-reducing waters and in water from well SPRT3, which was methanogenic (Table 2). In all well waters with mixed redox conditions (oxic-anoxic) (Table 2),

nitrate–N concentrations also were \0.06 mg/L, with the exception of nitrate–N (1.5 mg/L) in water from well SPRT-28 where the concentration was 1.5 mg/L (a mixture of O2- and Mn-reducing conditions). This well is the only one located in the east part of the study area with water categorized as having mixed redox conditions. Overall, there was a positive correlation between nitrate–N and dissolved O2 concentrations (p \ 0.05) (Table 4). There were no correlations between nitrate–N concentrations and percentage of various land use types (agriculture, urban, forest, wetlands, and rivers and lakes) in circular buffer areas around wells (1,000-m radius). Nitrate–N concentrations in the MCA, however, were correlated with the percentage of sand and inversely correlated to the percentage of silt in soils within a 500-m radius area around each well (Table 4). This is consistent with findings at other NAWQA sites where higher nitrate concentrations were found in shallow groundwater beneath well-drained soils (Nolan and Hitt 2006; Burow et al. 2010; Puckett et al. 2011). The percentage of sand in soils was significantly higher in the east part of the study area relative to the central and west parts of the study area; however, the percentage of silt and clay was not significantly different among geographical groupings. Nitrate–N concentrations were inversely correlated with the apparent age of groundwater, based on measurements of CFC-11, CFC-12, and 3H/3He (Table 4). For CFC-113 apparent ages, nitrate was inversely correlated but at a higher alpha level (p = 0.079). There are two possible scenarios that would account for this relation: (1) lower nitrogen input during past recharge of older waters and (2) denitrification. Fertilizers likely are the dominant source of nitrogen to the MEAS (Welch et al. 2009). Nitrate–N concentrations in water from the MCA plotted by apparent recharge dates for age-dated samples generally follow nitrogen inputs to groundwater inferred from county fertilizer sales data for the study area (Fig. 7). Median fertilizer sales data for counties in the study area show an increasing trend over time from the mid-1940s to 2001 (Fig. 7). If it is assumed that fertilizer usage (based on sales) is a reasonable indication of the amount of nitrogen applied to the land surface, then one could infer that nitrate–N concentrations may continue to increase over time in parts of the MCA that are oxic, as fertilizer usage has increased after 1990 (close to the water sample (SPRT-12) with the youngest apparent recharge date). The apparent decreases in nitrate–N concentrations with age of water from sampled wells likely are related to denitrification, as nitrate–N and dissolved O2 concentrations are correlated. The rate of nitrate–N loss due to denitrification in the MCA has been estimated using zeroand first-order rate expressions similar to the reduction of

123

123

Porosity

KSAT

OM_pct

Ag_pct

H3_ageyr

cfc113age

cfc12_age

cfc11_age

Cl

DOC

NO3-N

DO

wdepth_m

-0.571

0.002

-0.065

0.742

0.610

28

0.140

28

28

28

-0.101

0.379

0.495

-0.286

-0.173

30

30

0.135

-0.252

0.179

-0.297

9

9

0.111

0.009

0.205

17

-0.803

17

0.467

-0.700

0.002

17

17

0.289

0.080

0.323

0.260

-0.437

17

17

0.255

0.002

0.068

0.020

-0.439

28

0.178

0.262

28

0.084

-0.332

30

0.771

-0.055

9

0.026

-0.729

17

0.079

-0.437

17

0.001

-0.741

17

0.004

-0.655

30

30

-0.706

30

0.453

0.636 0.000

0.301

29

0.106

29

29

0.548

-0.116

-0.361

0.948

0.312

30

0.050

-0.013

0.195

0.002

30

0.008

30

1

30

30

0.538

-0.474

30

0.538 0.002

1

30

0.008

30

-0.474

-0.284

NO3-N

0.129

DO

0.129

-0.284

30

1

wdepth_m

0.112

0.333

0.194

27

0.087

0.335

27

0.188

0.261

29

0.100

0.312

9

0.515

0.251

16

0.679

0.540

-0.121

28

0.040

0.390

28

0.438

-0.153

30

0.140

0.276

9

0.067

-0.633

17

0.626

-0.127

17

0.456

0.156 16

-0.194

17

0.567

-0.150

30

1

29

0.249

0.221

30

0.000

0.636

30

0.106

0.301

30

0.050

-0.361

Cl

0.372

16

0.262

0.298

29

0.249

0.221

29

1

29

0.548

-0.116

29

0.948

-0.013

29

0.312

0.195

DOC

0.003

0.691

16

0.888

-0.038

16

0.362

0.244

17

0.232

0.306

9

0.004

0.850

17

0.007

0.632

17

0.001

0.730

17

1

17

0.567

-0.150

16

0.262

0.298

17

0.004

-0.655

17

0.002

-0.706

17

0.068

0.453

cfc11_age

0.001

0.756

16

0.311

-0.271

16

0.176

0.356

17

0.189

0.335

9

0.005

0.833

17

0.056

0.472

17

1

17

0.001

0.730

17

0.456

-0.194

16

0.156

0.372

17

0.001

-0.741

17

0.080

-0.437

17

0.323

0.255

cfc12_age

0.213

0.329

16

0.200

-0.338

16

0.405

0.224

17

0.171

0.348

9

0.125

0.550

17

1

17

0.056

0.472

17

0.007

0.632

17

0.626

-0.127

16

0.679

0.112

17

0.079

-0.437

17

0.002

-0.700

17

0.260

0.289

cf113_age

0.058

0.690

8

0.779

-0.119

8

0.693

0.167

9

0.265

0.417

9

1

9

0.125

0.550

9

0.005

0.833

9

0.004

0.850

9

0.067

-0.633

9

0.515

0.251

9

0.026

-0.729

9

0.009

-0.803

9

0.205

0.467

H3_ageyr

0.474

0.141

28

0.658

0.088

28

0.523

0.126

30

1

9

0.265

0.417

17

0.171

0.348

17

0.189

0.335

17

0.232

0.306

30

0.140

0.276

29

0.100

0.312

30

0.771

-0.055

30

0.179

-0.252

30

0.111

-0.297

Ag_pct

0.035

0.400

28

0.589

-0.107

28

1

28

0.523

0.126

8

0.693

0.167

16

0.405

0.224

16

0.176

0.356

16

0.362

0.244

28

0.438

-0.153

27

0.188

0.261

28

0.084

-0.332

28

0.379

-0.173

28

0.495

0.135

OM_pct

0.117

0.303

28

1

28

0.589

-0.107

28

0.658

0.088

8

0.779

-0.119

16

0.200

-0.338

16

0.311

-0.271

16

0.888

-0.038

28

0.040

0.390

27

0.087

0.335

28

0.178

0.262

28

0.610

-0.101

28

0.140

-0.286

KSAT

1

28

0.117

0.303

28

0.035

0.400

28

0.474

0.141

8

0.058

0.690

16

0.213

0.329

16

0.001

0.756

16

0.003

0.691

28

0.540

-0.121

27

0.333

0.194

28

0.020

-0.439

28

0.002

-0.571

28

0.742

-0.065

Porosity

0.938

0.015

28

0.002

0.569

28

0.696

0.077

28

0.064

0.355

8

0.352

-0.381

16

0.274

-0.291

16

0.200

-0.338

16

0.923

-0.026

28

0.002

0.559

27

0.512

0.132

28

0.025

0.423

28

0.471

0.142

28

0.084

-0.333

Sand_pct

0.435

0.154

28

0.008

-0.490

28

0.618

0.099

28

0.185

-0.258

8

0.071

0.667

16

0.704

0.103

16

0.192

0.344

16

0.820

-0.062

28

0.006

-0.505

27

0.337

-0.192

28

0.029

-0.414

28

0.415

-0.160

28

0.359

0.180

Silt_pct

Table 4 Spearman correlation coefficients (r), p values for statistical significance that r exceeds 0 (p \ 0.05 are considered a statistically significant correlation), and number of observations

1772 Environ Earth Sci (2012) 65:1759–1780

28

0.142

-0.333

0.084

28

28

28

0.029

-0.160

0.415

0.180

0.359

-0.414

0.025 28

0.471

28

28

0.423

28

28

27

0.337

-0.192

27

0.512

0.132

27

DOC

28

0.006

-0.505

28

0.002

0.559

28

Cl

16

0.820

-0.062

16

0.923

-0.026

16

cfc11_age

16

0.192

0.344

16

0.200

-0.338

16

cfc12_age

16

0.704

0.103

16

0.274

-0.291

16

cf113_age

8

0.071

0.667

8

0.352

-0.381

8

H3_ageyr

28

0.185

-0.258

28

0.064

0.355

28

Ag_pct

28

0.618

0.099

28

0.696

0.077

28

OM_pct

28

0.008

-0.490

28

0.002

0.569

28

KSAT

28

0.435

0.154

28

0.938

0.015

28

Porosity

28

<.0001

-0.886

28

1

28

Sand_pct

28

1

28

<.0001

-0.886

28

Silt_pct

ln [O2 ], mol/L

Italic highlight denotes significant positive correlation; bold highlight denotes significant inverse correlation

C-C(0), mol O2 /L

Well Depth, meters, below land surface

DO dissolved oxygen, NO3–N nitrate as N, DOC dissolved organic carbon, Cl chloride, cfc11_age apparent age of groundwater based on CFC-11, cfc12_age apparent age of groundwater based on CFC-12, cfc113_age apparent age of groundwater based on CFC-113, H3_ageyr apparent groundwater age based on tritium and helium-3, Ag_pct percentage of agricultural land use, OM_pct percentage of organic matter in soils, KSAT saturated hydraulic conductivity, porosity porosity of soils in 500-m circular areas around wells, sand_pct percentage of sand in soils, silt_pct percentage of silt in soils

Silt_pct

Sand_pct

NO3-N

DO

wdepth_m

Table 4 continued

Environ Earth Sci (2012) 65:1759–1780 1773

0

20

-350

10

40

60

80

120 3

140

160

180 0

-50

0

0

H/ He

3

CFC-113

SF6

10

-200

-250

10

10

20

20

20

10

14

Group 1 9

30

10 15

14 14 9

100

24 24

22 9 4

8

24

10 SPRT- Well #

CFC-12 29

30

30

40

40

3.0

y = -0.078x + 7.3 r² = 0.27

2.0

k 1= 0.08/yr

40

15 15

12 22 12

Linear regression equations: 2 CFC-12: y=1.3x + 16.2 (r =0.08) 2 CFC-113: y=1.65x + 2.1 (r =0.12) 3 3 2 H/ He: y=1.14x + 12 (r =0.26)

18

19 19

22 27 13

11 3

50

Group 2 27

5 5 5 1 17 17 1

13

20

18

Group 3

18

11 3

200

Apparent age, years

50

50

60

60

60

70

Fig. 5 Apparent groundwater ages for various tracers as a function of well depth

0

ZERO ORDER

-100

-150

y =-4.69x + 18.8 r² = 0.40 k0= 4.7 mol/L/yr

-300

CFC-12 APPARENT AGE, YRS 70

6.0

5.0

FIRST ORDER

4.0

1.0

0.0

CFC-12 APPARENT AGE, YRS 70

Fig. 6 Concentrations of O2 in relation to apparent groundwater age. Regression statistics are for the fit of zero-order reaction rate expression and for the fit of first-order reaction rate expression (see text for rate expressions and more explanation)

oxygen with groundwater age, as described above using expressions (2) and (3), respectively. In order to more realistically estimate denitrification rates, only groundwater samples with dissolved O2 concentrations \60 lmol/L (1.9 mg/L) were included. Tesoriero and Puckett (2011)

123

Environ Earth Sci (2012) 65:1759–1780

Median County Fertilizer Nitrogen

4.5

10.0

4.0 3.5 3.0

1.0

2.5 2.0 Millions Kg N/yr

1.5

2

kg N/km X1000

0.1

Apparent Recharge Date

1.0

CFC-12 CFC-113

0.5 0.0 1930

3

1940

1950

1960

1970

1980

3

H/ He

1990

2000

0.0 2010

Nitrate-N, mg/L, By Apparent Recharge Date

1774

Fig. 7 Median fertilizer nitrogen inputs to the study area [curves for total amount (kg) per year and amount per unit area (kg/km2)] inferred from county fertilizer sales data versus nitrate–N concentrations by apparent recharge dates

found that denitrification occurred in nearly all samples with dissolved O2 concentrations \60 lmol/L based on their study of denitrification in 12 shallow aquifers around the US zero-order rate constants for denitrification in the MCA ranged from 5.0 to 9.7 lmol/L/year based on a C(0) of 5.0 mg/L (357 lmol/L) and CFC-12 apparent groundwater ages. The zero-order rate constant (k0) is dependent on the initial nitrate–N concentration [C(0)], which was estimated by adding 1.0 mg/L (71 lmol/L) to the highest nitrate–N concentration (4 mg/L; 286 lmol/L) measured in groundwater from the shallow well SPRT-9 (to account for possible loss of nitrate due to denitrification in the soil zone). If C(0) is doubled to 10 mg N/L (714 lmol N/L; an unlikely high initial concentration, but useful for sensitivity analysis), the range of k0 values would increase to 11.5–19.5 lmol/L/year. First-order rate constants (k1) for denitrification in the MCA ranged from 0.08 to 0.16/year for C(0) = 357 lmol/L. The first-order rate constant is considerably less sensitive to changes in C(0), as k1 only increases slightly (0.09–0.17/year) when C(0) is doubled from 357 to 714 lmol/L. Both the zero- and first-order denitrification rate constants are low as compared to denitrification rates in other areas. For suboxic shallow aquifer systems around the United States, zero-order rate constants ranged from 15 to 70 lmol/L/year and first-order rate constants ranged from 0.06 to 0.29/year (Tesoriero and Puckett 2011). This indicates that there is a small amount of nitrate–N consumed in the MCA as water moves downgradient and from recharge to discharge area in the aquifer, although nitrate–N concentrations also could be affected by oxic conditions and the amounts of organic carbon, both of which are highly variable throughout the aquifer (Table 2). Zero-order rate constants in the MCA also are lower than the zero-order rate constant of 0.49 mg/L/year (35 lmol/L/year)

123

predicted by Welch et al. (2011) in groundwater from a nearby study area in the MRVAA in northern Mississippi. Given that organic carbon concentrations are considerably higher in the MRVAA than in the MCA, one would expect that denitrification rates would be lower in the MCA than in the MRVAA. Another possible indicator of denitrification in groundwater is excess N2. This is calculated by subtracting the estimated concentration of atmospheric nitrogen derived from both air–water equilibration at the mean recharge temperature (inferred from N2 and Ar gas data) and excess air, from that of the total amount of N2 measured in the groundwater samples. Dissolved gases (N2 and Ar) were only measured in a subset of five wells in 2009 (SPRT-8,-9, -13,-20,-29) (Table 3). Water from these wells is oxic with the exception of SPRT-20, which is anoxic and has a trace amount of methane (Table 3). No excess N2 was calculated for any of these groundwater samples, and N2 and Ar concentrations generally are consistent with atmospheric equilibration during groundwater recharge with minor amounts of excess air added during recharge. Oxic conditions are defined as groundwater having dissolved O2 concentrations [0.5 mg/L; however, denitrification is frequently observed in samples with higher concentrations of O2 due to mixing of oxic and anoxic waters (Green et al. 2008, 2010; Tesoriero and Puckett 2011). Geochemical reactions along groundwater flow paths Geochemical mass transfer reactions were modeled along three flow paths using NETPATH (Plummer et al. 1994). Information needed for mass-balance modeling in NETPATH includes initial and final water compositions (Table 2), and reactive mineral phases along the flow path. Initial and final wells along each flow path are listed in Table 5. Reactive minerals in the aquifer material include dolomite, clay minerals (kaolinite, illite, smectite, vermiculite), pyrite, and Fe oxyhydroxides (Hosterman 1984; Spann 1997; Larsen et al. 2003). NETPATH models for wells along flow paths contained the following potential reactants: organic matter (represented by CH2O), goethite (used to represent Fe oxyhydroxides and Fe hydroxides), pyrite, Na-montmorillonite, kaolinite, CO2, O2, and Ca– Mg–Na exchange on clay minerals. The models were constrained using C, S, Na, Fe, Mg, Ca, and redox state. Redox values were calculated from measured dissolved O2 concentrations. NETPATH models indicated that there were relatively low amounts of mass transfer (B1.1 mmol/kg) from aqueous and solid phases between sites along the groundwater flow paths (represented by the initial and final wells) (Table 5). Dominant geochemical processes along the groundwater flow paths included oxidation of organic

Environ Earth Sci (2012) 65:1759–1780

1775

Table 5 Results of NETPATH mass-balance modeling for wells along various flow path SPRT-# Initial well ? SPRT-# final well

Mass transfer associated with various phases (mmol/kg H2O) CH2O

Dolomite

Goethite

CO2

Namontmorillonite

Ca/Na exchange

Mg/Na exchange

O2

Pyrite

Kaolinite

Flow path 1 25 ? 22

0.28

22 ? 17 17 ? 7

0.002

0.006 0.66

0.016 -0.021

-0.6

0.0086

0.0062

-0.11 1.14

0.019

-1.1 -0.27

0.017

0.0091 0.12

-0.0066 0.0028

0.0068 -0.0015

0.11

-0.37

0.019

0.015

0.22

-0.00063

Flow path 2 24 ? 23

0.00064

23 ? 16

0.013

0.021

-0.00039

-0.097

0.19

0.01

0.00039

16 ? 8

0.041

0.06

-0.0074

0.19

0.14

0.013

0.0092

0.15

-0.0085

0.15

0.68

Flow path 3 15 ? 6

matter, dissolution of dolomite, varying amounts of exchange between Ca and Mg for Na, and minor amounts of Fe dissolution and precipitation. Some of the models indicated small amounts of goethite precipitation, which is common in soils and near-surface sediments. Most models involved dissolution of CO2, which is consistent with dissolution of soil-derived CO2 and oxidation of organic matter. Small amounts of CO2 (0.1 mmol/kg) degassed in three models along sites in flow paths 1 and 2 (Table 5); however, given the small amounts of carbon mass transfer these values are within uncertainty limits. The variability in groundwater chemistry is evident when comparing the end points of flow path 1 (well SPRT-7) and 2 (well SPRT-8). Water from the relatively deep well SPRT-7 (165 m) is mixed (oxic-anoxic), has a higher pH, alkalinity (accounted for by higher dissolution of dolomite), and dissolved solids concentration (Tables 2, 5) than water from shallower well SPRT-8 (96 m), which is oxic and has measurable nitrate (0.08 mg/L). The mass transfer results (Table 5) are consistent with chemical measurements and available mineralogical data. The results may vary under different assumptions about the mineral phases present and physical processes, such as mixing, affecting the mass-balance models. However, the dominant geochemical processes occurring along groundwater flow paths identified in this study are consistent with those found in studies of other parts of the MEAS (Parks et al. 1995; Larsen et al. 2003). Changes in nitrate–N concentrations along the three groundwater flow paths are related to changes in redox conditions, groundwater age, and depth zone sampled in the MCA. Unfortunately, groundwater age data are available for only a limited number of wells along the flow paths. For sites with groundwater age data, along flow path 1, nitrate–N concentrations decrease downgradient from 3.5 mg/L (well SPRT-25) to \0.06 mg/L (well SPRT-17).

0.069

0.086

0.085

0.0085

Water from well SPRT-17 is deeper, likely older, and more strongly reducing (Fe(III) and SO4 reduction) than from well SPRT-25 (Tables 2, 3). Along flow path 2, nitrate–N concentrations decrease downgradient from 1.5 mg/L (well SPRT-24) to 0.08 mg/L (well SPRT-8). Although water from well SPRT-8 remains oxic (dissolved O2 decreased along flow path from 7.6 to 4.2 mg/L), the apparent groundwater age increases slightly from well SPRT-24 to the deeper well SPRT-8 (Table 3). Along flow path 3, dissolved O2 concentrations increase from 2.5 mg/L in well SPRT-15 to 4.2 mg/L in well SPRT-6, and nitrate–N concentrations also slightly increase (0.11–0.47 mg/L) (Table 2). Groundwater along this short flow path is oxic even though the downgradient well (SPRT-6) is much deeper (136 m) than well SPRT-15 (15 m). Deep wells in the MCA are vulnerable to contamination, which is most likely related to the presence and thickness of confining units and variable redox conditions. Relation of pesticides, VOCs, and nitrate occurrence to well depth and groundwater age Pesticide detections in the sampled wells generally appear to be related to well locations in the study area, depth, and age of groundwater. Water samples from five of the 30 sampled wells (SPRT-9, -24, -25, -26, and -28) had two or more pesticides detected in low concentrations (Table 6). Four of the five wells that had pesticide detections (wells SPRT-24, -25, -26, -28) are located in the east part of the study area near the outcrop area for the MCA. Water samples also were collected for groundwater age dating from two of the five wells (SPRT-9 and SPRT-24). Well SPRT-9, a domestic well (49 m deep), and well SPRT-24, a public-supply well (79 m deep), both produced water that was young (\40 years) (Table 3). The other three wells that had pesticide detections but were not sampled for

123

123

DEA deethylatrazine, E estimated concentration less than laboratory method reporting level but greater than detection limit, PS public supply well, D domestic well, C commercial well

0.17

\0.02 \0.06 \0.02 \0.04 \0.04 E0.0054 D 28

E0.0069

E0.0072

E0.0074

\0.009

0.0111

\0.1

\0.04

\0.02 \0.06

\0.06 E0.03

\0.02 \0.04

\0.04 0.93 E0.06

\0.04 \0.04 \0.1

\0.1

0.0143

E0.007 E0.0042

\0.01 \0.009

E0.0031 E0.0055

E0.0185 0.0108

0.014 0.0176

E0.0029 PS

PS

25

26

\0.02

\0.02 \0.06

\0.06 \0.02

\0.02 \0.04

\0.04 \0.04

\0.04 0.17

E0.04 \0.1

0.49 E0.005

\0.007 \0.01

\0.01

\0.009

\0.009 E0.0063

\0.01

\0.01

\0.006 PS

PS

23

24

\0.006

\0.014

\0.02

\0.02 \0.06

\0.06 \0.02

\0.02 \0.04

\0.04 \0.04

\0.04 0.1

0.14 \0.1

\0.1 \0.007

\0.007 \0.01

\0.01 \0.009

\0.009

\0.01

\0.014

\0.01

C 22

\0.006

PS 21

\0.006

\0.014

\0.02

\0.02 0.07

\0.06 \0.02

\0.02 \0.04

\0.04 \0.04

\0.04 \0.04

E0.04 \0.1

\0.1 \0.007

\0.007 \0.01

\0.01 \0.009

\0.009 \0.014

\0.014

\0.01

\0.01

\0.006

\0.006

D

PS

15

17

\0.02

\0.02 \0.06 \0.02 E0.09 \0.04 \0.04 \0.1 \0.007 \0.01 \0.009 \0.01 D 14

\0.006

\0.014

\0.02

\0.06

\0.02

\0.06 \0.02

\0.02 \0.04

\0.04 \0.04

\0.04 E0.06

0.1 \0.1

\0.1 \0.007

E0.007 E0.0092

\0.01 \0.009

\0.009 E0.007

\0.014

\0.01

\0.01

\0.006

\0.006

D

D

9

10

\0.06

\0.06 \0.02

\0.02 \0.04

\0.04 \0.04

\0.04 E0.07

E0.05 \0.1

\0.1 \0.007

\0.007 \0.01

\0.01 \0.009

\0.009 \0.014 \0.01

\0.01

\0.006 PS

PS

6

8

\0.006

\0.014

Carbon disulfide Trichloroethene (TCE) 1,1,1-Trichloroethane Tetrachloroethene (PCE) Chloroform 1,2-Dichloroethane Atrazine Metolachlor Dieldrin DEA Prometon Simazine Well type SPRT-#

Table 6 Concentrations (in micrograms per liter) of pesticides and volatile organic compounds detected in domestic and public supply wells

\0.02

Environ Earth Sci (2012) 65:1759–1780 cis-1,2-Dichloroethene (DCE)

1776

age-dating (although likely have a component of young water) include a domestic well (SPRT-28; depth 35 m) and two public supply wells (SPRT-25, depth 58 m; and SPRT26, depth 93 m). Well SPRT-26 has a 30-m screened interval and depth to the top of the screen is about 60 m. The pesticide metolachlor, which was detected in water from wells SPRT-9, -26, and -28 (Table 6), was registered for usage in 1977 (http://www.epa.gov/oppsrrd1/REDs/ factsheets/0063fact.pdf). The registration date would indicate that the water from these wells was recharged sometime after 1977. Well SPRT-9, which had an apparent recharge date of 1980 (based on the ratio of CFC-113 to CFC-12) is the only one of the three wells where metolachlor was detected that had groundwater-age data that are consistent with the registered usage date of this herbicide. Another possibility is that a minor amount of mixing could produce a 3H/3He recharge date of around 1976 and still include some water recharged after 1977. Of interest, the sample results show that the highest number of pesticides (6) were found in the public supply well SPRT-26, which has only 3.4% agricultural land use but 81% urban land use in the circular area (500-m radius) around the well. Three of the pesticides found in SPRT-26, simazine, atrazine, and metolachlor, are herbicides that also have non-agricultural use. These herbicides also were detected in water samples from Pliocene(?)/Pleistoceneage terrace deposits and shallow Tertiary-age wells in a previous study, with the largest number of different pesticides detected in wells in the Memphis area (Welch et al. 2009). The higher number of pesticide detections in a developed area could originate from their use for weed control along rights-of-way in urban areas in the study area. Well SPRT-26 is a relatively high capacity supply well, and the land use inventory for the 500-m buffer area may not reflect land uses in the contributing area of this well, as has been shown for public-supply wells in other areas (Landon et al. 2008). Agricultural land use in the 500-m buffer areas for the other five wells ranged from 31-75%. Pesticides were more commonly detected in groundwater where soils had high infiltration rates and low organic matter content in a previous MEAS study (Welch et al. 2009). No relation was observed between pesticide detections and soil properties in 500-m buffer areas around wells in the present MCA study area. Water samples from all five of the wells that had two or more pesticides detected also had nitrate concentrations [0.80 mg/L and dissolved O2 concentrations [5.5 mg/L (Tables 2, 6). Four other wells had nitrate–N concentrations [0.8 mg/L (SPRT-10, -14, -16, and -22), but no pesticides were detected in water samples from these wells. In addition to the six pesticides (Table 6) detected in water samples from well SPRT-26, four VOCs [chloroform, tetrachloroethylene (PCE), trichloroethylene (TCE),

Environ Earth Sci (2012) 65:1759–1780

and cis-1,2-dichloroethylene (DCE)] also were detected in low concentrations (E0.06, 0.93, E0.03, and 0.17 ug/L, respectively; where E denotes estimated concentrations, which are quantifiable values above the method detection limit but below the laboratory reporting level). Well SPRT26 is located in the east part of the study area in the outcrop area of the MCA where the aquifer is unconfined and this well is relatively shallow with a screened interval from 61 to 91 m. The relatively high pumping rate for this publicsupply well (about 3,800 L/min) likely induces the movement of groundwater from shallow zones into the screen. A similar pumping rate for a public supply well in the northern High Plains aquifer (an aquifer with similar lithology to the MCA) drew contaminants from shallow unconfined parts into the deeper confined parts of the aquifer (Landon et al. 2008). Two other wells, SPRT-14 (domestic well) and SPRT-24 (public supply well) also are located in the east part of the study area where the MCA crops out (Fig. 1) and had water samples with at least one VOC detection (chloroform). Chloroform also was detected in SPRT-10, a shallow well (23 m depth) in the southcentral part of the study area, and carbon disulfide in well SPRT-17, a public supply well with a screened interval from 56 to 70 m below land surface (located in the central part of the study area). Chloroform was the most commonly detected VOC (in water samples from 10 of 30 wells) with concentrations ranging from 0.04 to 0.17 lg/L (Table 6). In a previous study of the MEAS, chloroform, dichlorodifluoromethane, tetrahydrofuran, and methyl-tertbutyl-ether (MTBE), were the most frequently detected VOCs in groundwater (Welch et al. 2009), which is consistent with the many varied uses and localized sources of these compounds (such as chlorinated solvents, fuel additives, and their degradation products). Implications for further groundwater quality impacts The presence of elevated nitrate–N concentrations, pesticides, and VOCs in groundwater samples from the MCA indicates that contaminants have moved into the aquifer at locations in the aquifer outcrop area in the east part of the study area and at a few other locations where the aquifer is confined to semi-confined. Several of the wells where contaminants were detected in groundwater samples are used for public-water supply. Fertilizer nitrogen usage (county sales data) remains near peak levels and it is possible that nitrate concentrations and detections could increase in areas where groundwater is oxic, particularly in and near the outcrop area for the MCA. Results from this study show that in these areas, groundwater in the MCA is relatively young (\30 years) and is vulnerable to contamination, particularly in areas with a high percentage of agricultural or urban land use. Given the persistence of nitrate in young oxic

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groundwater that was recharged several decades ago and low denitrification rates in many parts of the MCA, the downward movement of young contaminated water may result in higher nitrate concentrations over time in deeper parts of the aquifer containing old oxic water where the confining unit is thin and or leaky. In a previous study of the MEAS, Welch et al. (2009) documented the widespread occurrence of nitrate, pesticides, and VOCs in samples from wells in the Pliocene(?)/Pleistocene-age terrace deposits in the Memphis area. Consistent with other studies of the potential for contamination of the Memphis aquifer in the Memphis area (Graham and Parks 1986; Parks 1990), they postulated that these contaminants have the potential to move into the upper part of the Memphis aquifer at locations where the confining unit is thin or absent. Welch et al. (2009) also found that samples from each of eight drinking water wells completed in the upper part of the MCA in the Memphis area contained at least one pesticide and samples from two wells contained VOCs. In the Memphis area, large withdrawals of groundwater for public supply have lowered water levels in the MCA and increased the potential for movement of groundwater and contaminants to the aquifer from shallower zones. Also, low rates of oxygen reduction in the MCA compared to other aquifers in the US also enhances the persistence of nitrate and likely contributes to the low overall denitrification rates in the aquifer. The presence of elevated nitrate levels in parts of the MCA also has health and ecological implications. For example, exposure to nitrate concentrations in groundwater much lower than US Environmental Protection Agency and World Health Organization standards (10 mg/L as N) has been linked to several cancers and reproductive problems (Ward et al. 2005; Dubrovsky et al. 2010). The persistence of elevated nitrate concentrations in oxic zones of the MCA that were recharged several decades ago also has implications for surface-water quality; as in oxic parts of the aquifer, shallow groundwater with elevated nitrate concentrations may eventually discharge to surface water and impact stream water quality.

Summary and conclusions Groundwater quality in the middle Claiborne aquifer (MCA) of the Mississippi embayment aquifer system (MEAS) is most vulnerable to contamination in the outcrop areas in a study area east of the Mississippi River in northwest Mississippi and west Tennessee. Based on concentrations of transient tracers (chlorofluorocarbons, tritium and its decay product helium-3, and sulfur hexafluoride), water samples from wells in the outcrop area are relatively young (apparent age \40 years) with higher dissolved oxygen and nitrate–N concentrations and higher

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detections of pesticides and VOCs compared to water samples from wells in downgradient areas in the central and west parts of the study area. Given the persistence of nitrate in oxic young groundwater that was recharged several decades ago, nitrate concentrations may increase in deeper parts of the aquifer containing old oxic water as a result of the downward movement of young contaminated water. Also contributing to the persistence of nitrate in the MCA are the low oxygen reduction and denitrification rates compared with other aquifers in the US. Estimated zeroorder rate constants for oxygen reduction and denitrification are 4.7 and 5–10 lmol/L/year, respectively. Dominant processes that affect water quality along three studied groundwater flow paths include oxidation of organic matter to produce CO2, dissolution of dolomite, exchange between Ca and Mg for Na on clay minerals, and minor amounts of iron dissolution and precipitation. The amount of mass transfer associated with these processes is small (generally B1.1 mmol/kg) in the MCA, as evidenced by generally low dissolved solids concentrations (median of 72 mg/L) in water from wells sampled for the study. Nitrate, pesticides, and VOCs were detected in some public supply wells ([50 m deep) in the outcrop area. Higher pumping rates and larger zones of contribution for public supply wells compared with domestic wells may induce the movement of these contaminants from shallow depths into deeper zones, especially in areas where the confining unit is thin or leaky. For future work, to better understand sources of nitrate contamination and extent of denitrification in groundwater from various zones in the MCA, it would be useful to collect nitrogen isotope samples along with dissolved gases. In addition, stable isotopes of water would be useful for estimating recharge from return flow of groundwater used for irrigation and urban supply. Initial estimates from this study suggest a comparatively slow rate of denitrification and a better understanding of the fate of nitrate and other nonpoint source contaminants is needed to insure the MCA has the capacity to meet future demand. Acknowledgments This research study was funded by the US Geological Survey’s National Water Quality Assessment Program (NAWQA). Special thanks are extended to the homeowners and utilities for allowing us to collect water samples from their wells. The authors gratefully acknowledge R.C. Seanor for GIS support; and J.K. Carmichael, C.T. Green, S.C. Cooper, and several anonymous reviewers for their comments and suggestions that significantly improved earlier versions of this paper.

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