U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Lone Star Groundwater Conservation District

Estimated Rates of Groundwater Recharge to the

Chicot, Evangeline, and Jasper Aquifers

By Using Environmental Tracers in Montgomery

and Adjacent Counties, Texas, 2008 and 2011

3

H

F

Cl Cl Cl

C

Scientific Investigations Report 2013–5024

(Revised May 31, 2013) 
Front cover:

Left, CFC-11 (trichlorofluoromethane) molecule.

Middle, U.S. Geological Survey scientist collects a sample for CFC analysis (in bucket).

Right, Hydrogen isotope, tritium (3H).

Background, Sulfur hexafluoride molecule and an outline of Montgomery county, Texas.
Estimated Rates of Groundwater Recharge to 

the Chicot, Evangeline, and Jasper Aquifers By 

Using Environmental Tracers in Montgomery 

and Adjacent Counties, Texas, 2008 and 2011

By Timothy D. Oden and Margot Truini

Prepared in cooperation with the Lone Star Groundwater Conservation District

Scientific Investigations Report 2013–5024

(Revised May 31, 2013)

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Oden, T.D. and Truini, Margot, 2013 (revised May 31, 2013), Estimated rates of groundwater recharge to the Chicot, 

Evangeline and Jasper aquifers by using environmental tracers in Montgomery and adjacent counties, Texas, 

2008 and 2011: U.S. Geological Survey Scientific Investigations Report 2013–5024, 50 p., http://pubs.usgs.gov/

sir/2013/5024.
iii

Acknowledgments

The authors thank the San Jacinto River Authority as well as all the domestic and communitysupply well owners and operators who allowed access to their wells for sample collection. 

The authors also thank L. Niel Plummer and Ed Busenberg of the U.S. Geological Survey (USGS) 

Reston Chlorofluorocarbon Laboratory, and Mark Kasmarek, Jason Ramage, and Dexter Brown of 

the USGS Texas Water Science Center for their assistance in the data collection, interpretation 

of hydrogeology, and the interpretation of the isotope data.
iv
v

Contents

Abstract ..........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Previous Investigations........................................................................................................................2

Hydrogeologic Setting...................................................................................................................................6

Methods...........................................................................................................................................................8

Sample Design.......................................................................................................................................8

March–September 2008 Sample Collection.....................................................................................8

April–May 2011 Sample Collection ..................................................................................................10

Sample Analysis..................................................................................................................................10

Quality Control.....................................................................................................................................11

Geophysical Data Collection.............................................................................................................12

Environmental Tracers ................................................................................................................................12

Chlorofluorocarbons...........................................................................................................................12

Sulfur Hexafluoride.............................................................................................................................14

Tritium ..................................................................................................................................................14

Helium-3 and Tritium ..........................................................................................................................16

Helium-4 ...............................................................................................................................................16

Carbon-14.............................................................................................................................................16

Major Ion Chemistry as an Indicator of Groundwater Evolution and Travel Time............................17

Dissolved Gases...........................................................................................................................................17

Groundwater Age.........................................................................................................................................19

Modern Environmental Tracers........................................................................................................19

Paleowaters.........................................................................................................................................19

Environmental Tracer Data and Apparent Groundwater Age..............................................................26

Modern Environmental Tracers........................................................................................................26

Modeling of Carbon-14 with NETPATH............................................................................................28

Estimated Rates of Groundwater Recharge............................................................................................29

Aquifer Properties for Estimation of Groundwater Recharge Rates..........................................32

Porosity........................................................................................................................................32

Aquifer Thickness......................................................................................................................32

Distance Measurement ............................................................................................................32

Recharge Estimates for the Unconfined Chicot, Evangeline, and Jasper Aquifers ................33

Recharge Estimates for the Confined Zones of the Evangeline and Jasper Aquifers ............33

Uncertainty Analysis of the Recharge Rates .................................................................................33

Limitations of Estimating Recharge Rates......................................................................................42

Summary .......................................................................................................................................................44

References....................................................................................................................................................45
vi

Figures

1. Map showing locations of wells sampled during March–September 2008

and April–May 2011 in Montgomery County and adjacent counties, Texas.......................3

2. Graph showing total annual total precipitation during 1947–2008

in Houston, Texas..........................................................................................................................4

3. Map showing soil types based on drainage class in Montgomery County,

Texas, and surrounding counties...............................................................................................5

4. Hydrogeologic section showing the Gulf Coast aquifer system

in part of Montgomery County and adjacent counties, Texas ..............................................7

5. Local geologic section showing borehole geophysical data

in Montgomery County, Texas ..................................................................................................13

6. Graph showing atmospheric concentrations of chlorofluorocarbons,

sulfur hexafluoride, and tritium in precipitation data in Waco, Texas,

decayed to 2008 ..........................................................................................................................15

7. Map showing water chemistry in the Chicot, Evangeline, and Jasper aquifers

in April–May 2011, Montgomery, Walker, and Waller Counties, Texas .............................18

8. Diagram showing conceptual approaches to radiocarbon dating

of dissolved inorganic carbon in groundwater....................................................................20

9. Map showing adjusted carbon-14 ages from wells developed into the Chicot,

Evangeline, and Jasper aquifers in Montgomery, Waller, and

Walker Counties, Texas .............................................................................................................30

10. Diagram showing conceptual model of idealized flow through an

unconsolidated aquifer..............................................................................................................31

11. Map showing recharge estimates at wells completed in the Chicot aquifer

in Montgomery County, Texas, during March–September 2008

and April–May 2011....................................................................................................................35

12. Map showing recharge estimates at wells completed in the Evangeline

aquifer in Montgomery County, Texas, during March– September 2008

and April–May 2011....................................................................................................................37

13. Recharge estimates at wells completed in the Jasper aquifer in Montgomery,

Walker, and Waller County, Texas, during March–September 2008 and

April–May 2011............................................................................................................................39

14. Scatter plot showing uncertainty analysis of recharge rates for

unconfined wells screened in the Chicot, Evangeline, and Jasper aquifers ...................40

15. Scatter plot showing uncertainty analysis of recharge rates for

confined wells screened in the Evangeline and Jasper aquifers ......................................41

Appendixes (available at http://pubs.usgs.gov/sir/2013/5024/) 

1. Environmental and quality control samples for 24 wells sampled in Montgomery 

County, Texas, 2011.

2. Well construction information for four additional groundwater wells sampled in the 

Jasper aquifer in Montgomery County, Texas, 2011.

3. Borehole geophysical data from wells in Montgomery and Walker Counties, Texas, 

April 2010.

4. Dissolved-gas concentrations measured in groundwater samples collected from the 

Evangeline and Jasper aquifers in Montgomery County, Texas, 2011.

5. Helium-4 (4He) measured in groundwater samples collected from the Evangeline and 

Jasper aquifers in Montgomery County, Texas, 2011.
vii

Tables

1. Wells sampled in Montgomery, Walker, and Waller Counties

Texas March–September 2008 and April–May 2011 ..............................................................9

2. Geophysical logging methods used at wells where borehole

geophysical data were collected in April 2010......................................................................12

3. Summary of recharge temperatures and field water temperatures

for wells sampled in and around Montgomery County, Texas

during March–September 2008 and April–May 2011 ...........................................................19

4. Summary of adjusted carbon-14 ages and mineral mass transfer

in the Chicot, Evangeline, and Jasper Aquifers,

Montgomery County, Texas.......................................................................................................22

5. Sensitivity analysis for modeled carbon-14 adjusted ages for selected wells

within the Chicot, Evangeline, and Jasper Aquifers,

Montgomery County, Texas.......................................................................................................24

6. Apparent groundwater age derived by using samples collected

during March–September 2008 from wells completed in the

Chicot, Evangeline or Jasper aquifer, and the environmental tracer

used for apparent age determination, Montgomery County, Texas...................................26

7. Comparison of apparent ages determined from environmental tracers

for groundwater samples collected during March–September 2008

from wells completed in the Chicot aquifer in Montgomery County, Texas .....................27

8. Apparent groundwater ages and recharge estimates derived by

using samples collected during March–September 2008 and

April–May 2011 from wells completed in the Chicot aquifer

in Montgomery County, Texas ..................................................................................................34

9. Apparent groundwater ages and recharge estimates derived by

using samples collected during March–September 2008 and

April–May 2011 from wells completed in the Evangeline aquifer

in Montgomery County, Texas ..................................................................................................36

10. Apparent groundwater ages and recharge estimates derived by

using samples collected during March–September 2008 and

April–May 2011 from wells completed in the Jasper aquifer in

Montgomery, Walker, and Waller County, Texas...................................................................38

11. Recharge methods and estimates of annual recharge rates

previously determined for four wells completed in the Chicot aquifer

in Montgomery County, Texas ..................................................................................................43

12. Comparision of recharge rates determined in Montgomery County

during during March–September 2008 by using environmental

age tracers with recharge rates from previous studies that used various

methods in the Gulf Coast aquifer system in Montgomery County

and adjacent counties in Texas................................................................................................43
viii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

cubic inch (in3) 16.39 cubic centimeter (cm3)

Flow rate

inch per hour (in/h) 0 .0254 meter per hour (m/h)

inch per year (in/yr) 2.54 centimeter per year (cm/yr)

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

Radioactivity

picocurie per liter (pCi/L) 0.313 Tritium units (TU) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88) and the National Geodetic Vertical Datum of 1929 (NGVD 29). Horizontal coordinate 

information is referenced to the North American Datum of 1927 (NAD 27) and the North 

American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the 

vertical datum.

 Acronyms and Chemical Abbreviations and Units

CFC Chlorofluorocarbon

DG Dissolved gases

LSGCD Lone Star Groundwater Conservation District

NACP North Atlantic Coastal Plain

USGS U.S. Geological Survey

3

H tritium

3

He/3H helium-3/tritium

4

He helium-4

SF6 sulfur hexafluoride

14C carbon-14, radio-isotope of carbon

cm3/g STP H20 cubic centimeter per gram at standard temperature and pressure water

pCi/L picocurie per liter

TU tritium units
Estimated Rates of Groundwater Recharge to the Chicot, 

Evangeline, and Jasper Aquifers By Using Environmental 

Tracers in Montgomery and Adjacent Counties, Texas, 

2008 and 2011

By Timothy D. Oden and Margot Truini

Abstract 

Montgomery County is in the northern part of the 

Houston, Texas, metropolitan area, the fourth most populous 

metropolitan area in the United States. As populations have 

increased since the 1980s, groundwater has become an 

important resource for public-water supply and industry in the 

rapidly growing area of Montgomery County. Groundwater 

availability from the Gulf Coast aquifer system is a primary 

concern for water managers and community planners in 

Montgomery County and requires a better understanding of 

the rate of recharge to the system. The Gulf Coast aquifer 

system in Montgomery County consists of the Chicot, 

Evangeline, and Jasper aquifers, the Burkeville confining 

unit, and underlying Catahoula confining system. The 

individual sand and clay sequences of the aquifers composing 

the Gulf Coast aquifer system are not laterally or vertically 

continuous on a regional scale; however, on a local scale, 

individual sand and clay lenses can extend over several 

miles. The U.S. Geological Survey, in cooperation with the 

Lone Star Groundwater Conservation District, collected 

groundwater-quality samples from selected wells within or 

near Montgomery County in 2008 and analyzed these samples 

for concentrations of chlorofluorocarbons (CFCs), sulfur 

hexafluoride (SF6), tritium (3H), helium-3/tritium (3He/3H), 

helium-4 (4He), and dissolved gases (DG) that include argon, 

carbon dioxide, methane, nitrogen and oxygen. Groundwater 

ages, or apparent age, representing residence times since 

time of recharge, were determined by using the assumption 

of a piston-flow transport model. Most of the environmental 

tracer data indicated the groundwater was recharged prior 

to the 1950s, limiting the usefulness of CFCs, SF6, and 3H 

concentrations as tracers. In many cases, no tracer was usable 

at a well for the purpose of estimating an apparent age. Wells 

not usable for estimating an apparent age were resampled 

in 2011 and analyzed for concentrations of major ions and 

carbon-14 (14C). At six of these wells, additional 4He and DG 

samples were collected and analyzed.

Recharge rates estimated from environmental tracer 

data are dependent upon several hydrogeologic variables and 

have inherent uncertainties. By using the recharge estimates 

derived from samples collected from 14 wells completed in 

the Chicot aquifer for which apparent groundwater ages could 

be determined, recharge to the Chicot aquifer ranged from 0.2 

to 7.2 inches (in.) per year (yr). Based on data from one well, 

estimated recharge to the unconfined zone of the Evangeline 

aquifer (outcrop) was 0.1 in./yr. Based on data collected from 

eight wells, estimated rates of recharge to the confined zone of 

the Evangeline aquifer ranged from less than 0.1 to 2.8 in./yr. 

Based on data from one well, estimated recharge to the 

unconfined zone of the Jasper aquifer (outcrop) was 0.5 in./yr. 

Based on data collected from nine wells, estimated rates of 

recharge to the confined zone of the Jasper aquifer ranged 

from less than 0.1 to 0.1 in./yr. The complexity of the 

hydrogeology in the area, uncertainty in the conceptual model, 

and numerical assumptions required in the determination 

of the recharge rates all pose limitations and need to be 

considered when evaluating these data on a countywide or 

regional scale. The estimated recharge rates calculated for 

this study are specific to each well location and should not 

be extrapolated or inferred as a countywide average. Local 

variations in the hydrogeology and surficial conditions can 

affect the recharge rate at a local scale. 

Introduction

Montgomery County is in the northern part of the 

Houston, Texas, metropolitan area, the fourth most populous 

metropolitan area in the United States (U.S. Census Bureau, 

2011a). As populations have increased since the 1980s, 

groundwater has become an important resource for publicwater supply and industry. Groundwater is withdrawn from the 

Gulf Coast aquifer system, which comprises, in descending 

order, the Chicot aquifer, Evangeline aquifer, Burkeville 
2 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

confining unit, Jasper aquifer, and the Catahoula confining 

system. A good understanding of the rate of recharge is 

important to water managers to help them ensure that the 

amount of groundwater withdrawn from the Gulf Coast 

aquifer system in Montgomery County is sustainable. 

The Lone Star Groundwater Conservation District 

(LSGCD) was created in 2001 by the 77th Texas Legislature 

and was charged with managing and protecting the 

groundwater resources of Montgomery County. The U.S. 

Geological Survey (USGS) has worked cooperatively with the 

LSGCD since 2001 to monitor and appraise the Gulf Coast 

aquifer system. Water-level measurements are recorded and 

periodic water-level change maps are produced to provide 

short- and long-term analyses of the effects of groundwater 

withdrawal on the regional aquifer flow system. In 2003, 

the LSGCD set a maximum amount of sustainable yield 

for the Gulf Coast aquifer system in Montgomery County 

equal to previously determined estimates of recharge (Lone 

Star Groundwater Conservation District, 2008). In 2007, the 

LSGCD began planning for an alternative source of water, 

so further detailed information on recharge was needed in 

Montgomery County, especially for the different aquifers 

within the Gulf Coast aquifer system. Accordingly, the 

USGS, in cooperation with the LSGCD, estimated rates of 

groundwater recharge in 2008 and 2011 to the Gulf Coast 

aquifer system, namely the Chicot, Evangeline and Jasper 

aquifers, at selected wells by using a combination of different 

environmental tracers in Montgomery County. 

Purpose and Scope

The purpose of this report is to document groundwater 

recharge rates estimated by using environmental tracers at 

selected wells screened in the Gulf Coast aquifer system in 

Montgomery County. This report provides an overview of the 

hydrogeology, estimates of recharge rates at selected wells, 

and a discussion of the appropriate use and limitations of the 

recharge rates documented in this report. Recharge rates are 

provided on a well-by-well basis because the data did not 

support a determination of the average rate of recharge for 

Montgomery County. 

Description of Study Area

Montgomery County is rapidly growing and includes 

Magnolia, Conroe, and The Woodlands (fig. 1). From April 1, 

2000 to July 1, 2009, the population of Montgomery County 

increased from 293,767 to 447,718 (U.S. Census Bureau, 

2011b), making it one of the fastest growing counties in the 

Houston area. Montgomery County is forecasted to remain 

one of the fastest growing counties in the Houston area, 

more than doubling its 2000 population to about 750,000 by 

2035 (Houston–Galveston Area Council, 2006). Land use 

in Montgomery County was historically mostly forest and 

rangeland. 

The topography of Montgomery County ranges from 

generally flat toward the coast and near larger streams to 

rolling hills in the northern part of the county. Land-surface 

altitude ranges from about 79 feet (ft) above North American 

Vertical Datum of 1988 (NAVD 88) in the southeastern corner 

of the county to about 330 ft in the northwestern corner (U.S. 

Department of Agriculture, 1972). Montgomery County 

is entirely within the San Jacinto River drainage basin, 

which trends from the northwest to the southeast. Streams 

in Montgomery County include Lake Creek, West Fork San 

Jacinto River, Peach Creek, and Caney Creek (fig. 1). Lake 

Conroe impounds the West Fork of the San Jacinto River 

upstream from Conroe. Montgomery County has a humid 

subtropical climate characterized by hot, humid summers 

and cool winters. The average annual precipitation in the 

Houston area was 51.67 inches (in.) during 1947–2008 

(National Climatic Data Center, 2010). Substantial droughts 

(operationally defined for the purposes of this report as years 

with less than 30 in. of precipitation) occurred in 1954, 1956, 

and 1988 (fig. 2). 

Montgomery County has complex soil types with 

different infiltration rates, primarily related to the drainage 

network (fig. 3). Although the infiltration rates of the soils in 

Montgomery County range from high to low, the predominant 

soil associations are poorly to moderately drained soils 

consisting primarily of clay or clay-pan features in the 

underlying geology (fig. 3). Permeability of the soils is 

highly variable, as presumed from the drainage rates ranging 

from about 20 in. per hour (hr) in well drained soils to less 

than 0.06 in./hr in poorly drained soils (U.S. Department 

of Agriculture, 1972). The higher permeability soils are 

generally associated with the major river drainages of the West 

Fork San Jacinto River and Lake Creek. 

Previous Investigations

Groundwater recharge is a component of conceptual 

groundwater flow models and water-budget calculations, 

and recharge in these models is rarely measured empirically. 

Typically, recharge rates are estimated on the basis of other 

groundwater flow components such as river base flow, 

withdrawal rate, and groundwater-level fluctuations. Several 

approaches to estimating recharge have been attempted in the 

Gulf Coast aquifer system underlying the greater Houston area 

including the numerical groundwater flow models developed 

by Ryder (1988), Williamson and others (1990), Kasmarek 

and Strom (2002), and Kasmarek and Robinson (2004).

Previous estimated recharge rates, derived from 

calibrated models of the regional flow system in the 

Montgomery County area, were determined and ranged from 

0.0 to 6.0 in. per year (yr) (Ryder, 1988) and from 0.0 to 

0.7 in./yr (Williamson and others, 1990). Results from both 

studies associated smaller recharge rates with the far downdip 

areas along the Texas Gulf of Mexico coast where the aquifer 

system discharges through diffuse upward leakage into the 
Introduction 3

Figure 1. Locations of wells sampled during March–September 2008 and April–May 2011 in Montgomery County and

adjacent counties, Texas.

Bayou

2, TS-60-53-720

5, TS-60-45-414

13, TS-60-26-901

14, TS-60-26-208

15, TS-60-45-513

16, TS-60-34-301

17, YW-59-64-206

19, TS-60-63-110

20, TS-60-37-309

21, TS-60-54-702

23, TS-60-55-710

24, YU-60-28-802

27, TS-60-43-514

28, TS-60-45-716

29, TS-60-42-902

31, TS-60-36-410

34, TS-60-37-909

35, TS-60-59-102

38, TS-60-51-814

39, TS-60-52-199

40, TS-60-51-599

42, TS-60-35-703

43, TS-60-42-206

44, TS-60-36-812

6, TS-60-54-805

7, TS-60-54-806

10, TS-60-54-807

32, TS-60-35-907

4, TS-60-45-413

8, TS-60-35-504

9, TS-60-35-505

11, TS-60-35-503

18, TS-60-43-511

22, TS-60-44-212

33, TS-60-37-806

12, TS-60-44-806

25, TS-60-45-114

26, TS-60-53-210

30, TS-60-53-516

41, TS-60-55-313

36, TS-60-51-809

37, TS-60-51-811 •

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

• •

•

•

• • •

•

••

•

•

•

• •

•

1, TS-60-53-719

3, TS-60-44-805

17, YW-59-64-206

38, TS-60-51-814

35, TS-60-59-102

•

•

•

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

Sampled monitor well and 

contributing aquifer

—Map identifier and 

state well number 

(see tables 1, 8, 9, and 10)

Chicot aquifer

Evangeline aquifer

Jasper aquifer

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K 

COUNTY

HA

R R I S 

COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES C

OUNTY

SA

N JACINTO COUNTY

W

ALKER COUNTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

Aquifer data from Strom and others, 2003 a,b,c

and Kasmarek and Robinson, 2004

TEXAS

Study area

Location map
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 1947 1952 1957 1962 1967 1972 1977 1982 1987 1992 1997 

Annual Precipitation (inches) Year Average annual precipitation =51.67 inches Figure 2. Total annual precipitation during 1947–2008 in Houston, Texas (National Climatic Data Center, 2010).
Introduction 5

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1 1

1

1

1 1 1

1

11

1

1

1

1

1

1

PO L K 

COUNTY

HA

R R I S 

A US T I N COUNTY

COUNTY

WA S HI N G T ON 

COUNTY

BR A Z O S

COUNTY

MONTGOMERY COUNTY 

WALLER COUNTY

GR

MI ES COUNTY

SA

N JACINTO COUNTY

W

ALKER COUNTY

MADISON COUNTY

LIBERTY 

COUNTY

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28 29

30

31

32

33 34

35

36

37

38

39

40

41

42

43

44

35

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey, Digital data, 1:24,000

Albers Equal Area Projection, Texas Mapping System

North American Datum of 1983

EXPLANATION

Drainage class—dominant condition

Water body

Moderately well drained

Poorly drained

Somewhat excessively drained

Somewhat poorly drained

Well drained

1 Recharge sampling site and map identifier

 (see tables 1, 8, 9, and 10.)

 Soil data from U.S. Department of Agriculture

Natural Resources Conservation Service, 2006

Figure 3. Soil types based on drainage class in Montgomery County, Texas, and surrounding counties.
6 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Gulf of Mexico; larger recharge rates were associated with 

more transmissive updip outcrop areas of the aquifer system.

Unpublished analyses done in the 1990s by USGS 

hydrologists (R.K. Gabrysch and F. Liscum, U.S. Geological 

Survey, retired, written commun., 1995) estimated the 

potential recharge in the Houston area by using 30 years of 

streamflow data and water-budget information. Their analyses 

indicated that area-weighted runoff was about 7 in./yr less in 

four stream basins on outcrops of water-bearing units of the 

Gulf Coast aquifer system (Chicot and Evangeline aquifers, 

which are discussed in the “Hydrogeologic Setting” section) 

than in two stream basins on outcrops of the confined zones 

of the two aquifers. These results indicated that recharge on 

outcrops of the unconfined zones of the Chicot and Evangeline 

aquifers is substantially larger compared to recharge on 

outcrops of the confined zones.

In other studies, Noble and others (1996) sampled 

41 wells screened in the Chicot and Evangeline aquifers for 

environmental tritium concentrations in northern Harris and 

Montgomery Counties and estimated a maximum recharge rate 

of 6 in./yr using a tritium front method assuming a uniform 

porosity of 23 percent. Scanlon, Dutton, and Sophocleous 

(2002) did a literature review to estimate that the recharge 

rate to the Gulf Coast aquifer system along the Texas Gulf of 

Mexico coast ranged from 0.0004 to 2 in./yr. Nolan and others 

(2007) showed how estimated recharge rates varied depending 

on methodology and concluded that utilizing multiple 

methods is beneficial because each method has advantages 

and disadvantages. Scanlon and others (2011) conducted a 

data compilation and analysis for estimation of groundwater 

recharge to the Gulf Coast aquifer in Texas, including 

Montgomery County. Their results for the Chicot aquifer in 

Montgomery County showed a range of 0.8–4.8 in./yr by 

using well hydrograph analysis and groundwater chloride 

mass balance.

Hydrogeologic Setting

The Gulf Coast aquifer system in Montgomery County 

consists of the Chicot aquifer, Evangeline aquifer, Burkeville 

confining unit, and Jasper aquifer and the underlying 

Catahoula confining system. The Chicot aquifer is unconfined 

in Montgomery County and consists of Pleistocene and 

Holocene-age sediments (Baker, 1979). The underlying 

Evangeline aquifer consists of Miocene- and Pliocene-age 

sediments. Underlying the Evangeline aquifer is the Jasper 

aquifer, which consists of Miocene-age sediments (Baker, 

1979) and is separated from the Evangeline aquifer by the 

Burkeville confining unit. At the base of the Gulf Coast 

aquifer system, the Catahoula confining system consists 

of Oligocene-age sediments; it contains small amounts of 

freshwater in shallow restricted sands near the outcrop area 

to the north (Baker, 1979). Detailed information on the 

hydrogeology of the area can be found in Popkin (1971), 

Baker (1979 and 1986), Ryder (1988), Chowdhury and Turco 

(2006), and Kasmarek and others (2010).

The sediments of the Gulf Coast aquifer system were 

deposited by fluvial-deltaic processes and subsequently eroded 

and redeposited. The processes of deposition, erosion, and 

redeposition created a thick accretionary sediment wedge 

of interbedded sand and clay layers exceeding 7,600 ft near 

the Gulf of Mexico coast (Chowdhury and Turco, 2006). 

Many clay layers in the region are not laterally extensive 

and generally are not mappable beyond the county level 

(Ryder, 1996). The hydrologic units within the Gulf Coast 

aquifer system dip and thicken from the outcrop areas in 

the northwest—where they exist in outcrop—toward the 

southeast, becoming more deeply buried beneath overlying 

sediments (fig. 4). The Chicot aquifer outcrops throughout 

much of Montgomery County and can be differentiated from 

the geologically similar Evangeline aquifer on the basis of 

hydraulic conductivity (Carr and others, 1985, p. 10). The 

Chicot aquifer is thinner in Montgomery County than in 

counties to the south (Kasmarek and others, 2010). The Chicot 

and the Evangeline aquifers are hydrologically connected 

with no separating confining units. In central and southern 

Montgomery County, the Chicot aquifer is not productive for 

the purpose of groundwater withdrawal, and the water-bearing 

sediments of the Evangeline aquifer progressively thin from 

south to north (Kasmarek and others, 2010). The Evangeline 

aquifer outcrops in parts of northwest Montgomery County, 

whereas the Jasper aquifer outcrops northwest of Montgomery 

County. There is little information available on the thickness 

and spatial characteristics of the Catahoula confining system 

in Montgomery County. 

Water recharges the aquifers in the unconfined 

outcrop areas, moves downward and coastward through the 

interbedded sands, and discharges into streams (as base flow) 

by upward leakage to shallow aquifers or into the Gulf of 

Mexico (Ryder, 1996). Much of the water that infiltrates into 

the saturated zone flows through locally continuous shallow 

sand layers and discharges into streams; the remainder of the 

water flows to intermediate and deep zones of the aquifer 

system southeastward of the outcrop where it is captured and 

withdrawn by wells or naturally discharged by upward leakage 

in topographic lows along the coast (Kasmarek and others, 

2010). In Montgomery County, groundwater withdrawal has 

had an appreciable effect on regional and local groundwater 

flow. Based on historical groundwater-level change, major 

areas of withdrawal include Conroe, The Woodlands, 

and Magnolia (Kasmarek and others, 2010). Compared 

to predevelopment conditions, large areas of decreased 

groundwater levels in these populated areas have induced flow 

between the Chicot and Evangeline aquifers and caused flow 

directions to reverse (Kasmarek and others, 2010) that imply 

aquifer dewatering. 
Jasperoutcrop

Catahoula

 outcrop

Tertiary outcrop Quaternary outcrop

Evangeline

 outcrop

A

A'

10

1

2

3 4 5

6

7

8 9

11

WALLER COUNTY

GRIMES COUNTY

GALVESTON COUNTY

HARRIS COUNTY

MONTGOMERY COUNTY

HUMBLE No. 1 Sealy EXETER No. 1 Bradley KURDICK No. 1 Stoneham LONE STAR No. 1 Goforth SUPERIOR No. B-1 McWhorter NEWMAN & GEOTEK No. 1 ZiegenhainSPARTA No. 1 Suttles et. al. TENNESSEE GAS No. 1 Martin Unit HUGHES No. 1 Goar Est. PHILLIPS No. 5 Guaranty 

Well number on location map

A 

NORTHWEST 

COUNTY

MONTGOMERY

COUNTY GRIMES HARRIS COUNTY GALVESTON C

+400

-400

-800

-1,200

-1,600

-2,000

-4,400

-4,800

-4,000

-3,600

-3,200

-2,800

-2,400

-5,200

-7,600

-7,200

-6,800

-6,400

-6,000

-5,600

NAVD 88

FEET

NAVD 88, North American Verti

1 2 3 4 5 6 7 8 9 10 

VERTICAL SCALE GREATLY EXAGGERATED

0 4 8 12 16 MILES

0 4 8 12 16 KILOMETERS

Anahuac

Formation Formation

Frio

Upper part of 

Catahoula

Sandstone

Base of 

Catahoula 

Sandstone 

Catahoula 

confining 

system (restricted) Jasper aquifer Base of Fleming Formation

Chicot aquifer

Evangeline aquifer

Burkeville

confining

unit

Approximate land surface

Figure 4. Hydrogeologic section of the Gulf Coast aquifer system in part of Montgomery County and adjacent counties, Texas (modified from 

(Holocene- and Pleistocene-age sedime

(Pliocene- and Miocene-age sediments)

(Miocene-ag

(Miocene-ag

(Miocene-age sediments) Undifferentiated Pre-Miocene deposits

Figure 4. Hydrogeologic section of the Gulf Coast aquifer system in part of Montgomery County and adjacent counties, Texas (modified from B
8 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Methods

In total, 32 sample sites with 40 existing groundwater 

wells in or near Montgomery County were selected for 

sampling in 2008. Well nests at seven sites contained two or 

more closely spaced wells. Wells screened in the Gulf Coast 

aquifer system were selected for sampling in Montgomery, 

Waller, and Walker Counties—38 wells were in Montgomery 

County, 1 well was in Waller County, and 1 well was in 

Walker County (fig. 1 and table 1). Groundwater-quality 

samples and physicochemical properties were collected once 

from each of the 40 wells during March–September 2008; 

water-level data were collected from those wells where it 

was possible to insert either a steel tape or an electric waterlevel contact tape (e-line). Groundwater-quality samples were 

analyzed for chlorofluorocarbons (CFCs), sulfur hexafluoride 

(SF6), tritium (3H), helium-3/tritium (3He/3H), helium-4 (4He), 

and dissolved gases (DG). Water samples were collected and 

processed onsite by using methods designed to minimize 

changes to the water-sample chemistry or contamination from 

the atmosphere. Replicate samples for quality assurance and 

quality control were collected with each environmental sample 

(Oden, 2011).

In the spring of 2011, 20 of the sites sampled in 2008 

were resampled, along with 4 additional sites that were added 

to the study for a total of 24 sites sampled in 2011. The four 

wells were added to the study because previously sampled 

wells in 2008 that were to be resampled were either down for 

maintenance in 2011 (two wells) or as replacements for wells 

that were not available in 2011 (two wells). Groundwaterquality samples were collected and physicochemical properties 

and water-level data (when accessible) were measured once 

from each of the 24 wells during April–May 2011 (appendix 

1). Groundwater-quality samples were analyzed for major 

ions and carbon-14 (14C) at all 24 wells. At the four wells 

added in 2011 DG and 4He were also collected. Water samples 

were collected and processed onsite by using methods 

designed to minimize changes to the water-sample chemistry, 

contamination, and in the case of 4He and dissolved gases, 

exposure to the atmosphere. Blank and replicate samples for 

quality assurance and quality control were collected for major 

ions and replicate analysis only for 14C, DG, and 4He.

Sample Design

Existing groundwater wells in or near Montgomery 

County that met the following screening criteria were 

considered for the initial sampling in 2008:

1. The well depth was known in the targeted aquifer;

2. The length of open interval was small as possible (ideally 

less than 30 ft) to facilitate the collection of discrete 

samples of uniform age rather than samples representing 

groundwater of different ages; and

3. The well was completed in a single aquifer unit.

After the initial screening, wells were selected to 

provide an areal distribution across the study area. Wells with 

relatively short open intervals (less than 30 ft) were preferred. 

In a few instances to meet spatial distribution, wells with open 

intervals greater than 30 ft were selected. Well depths for the 

40 selected wells ranged from 52 to 1,240 ft with a median 

depth of 170 ft. Open intervals for the wells ranged from 10 

to 118 ft. Well construction information is documented in a 

previous report (Oden, 2011).

The four additional wells sampled in Montgomery 

County in 2011 met the newly developed screening criteria: 

1. The well depth was known;

2. The well was competed solely in the Jasper 

aquifer; and

3. The well had an open interval of less than 75 ft. 

The four wells had well depths ranging from 581 to

1,639 ft, and open intervals ranged from 20 to 75 ft 

(appendix 2).

March–September 2008 Sample Collection

Groundwater samples were collected once from 40 wells 

during March–September 2008 by using procedures described 

in the USGS “National Field Manual for the Collection of 

Water-Quality Data” (U.S. Geological Survey, variously 

dated) and provided by the USGS Chlorofluorocarbon 

Laboratory, Reston, Virginia (U.S. Geological Survey, 2009a). 

Groundwater-quality samples, physicochemical properties, 

and water-level data (when access allowed) were collected. 

Groundwater-quality samples were collected and analyzed 

for concentrations of CFCs, SF6, 3H, 3He/3H, 4He, and DG. 

Water levels in wells were measured manually at the time 
Methods 9

Table 1. Wells sampled in Montgomery, Walker, and Waller Counties Texas March–September 2008 and April–May 2011.

Map identifier/

sample sequence Station number Station name Aquifer County Year sampled

1 300824095274703 TS-60-53-719 Chicot Montgomery 2008

2 300824095274702 TS-60-53-720 Chicot Montgomery 2008

3 301505095343702 TS-60-44-805 Chicot Montgomery 2008, 2011

4 301948095290003 TS-60-45-413 Chicot Montgomery 2008

5 301948095290004 TS-60-45-414 Chicot Montgomery 2008

6 300833095173201 TS-60-54-805 Chicot Montgomery 2008, 2011

7 300833095173202 TS-60-54-806 Chicot Montgomery 2008

8 302636095422802 TS-60-35-504 Burkeville1 Montgomery 2008

9 302636095422803 TS-60-35-505 Burkeville1 Montgomery 2008

10 300833095173203 TS-60-54-807 Chicot Montgomery 2008

11 302636095422801 TS-60-35-503 Burkeville1 Montgomery 2008, 2011

12 301505095343703 TS-60-44-806 Chicot Montgomery 2008

13 303222095455301 TS-60-26-901 Jasper Montgomery 2008

14 303610095484501 TS-60-26-208 Jasper Montgomery 2008

15 301912095253701 TS-60-45-513 Chicot Montgomery 2008, 2011

16 302836095452701 TS-60-34-301 Evangeline Montgomery 2008

17 300542096045403 YW-59-64-206 Jasper Waller 2008, 2011

18 301904095414801 TS-60-43-511 Evangeline Montgomery 2008, 2011

19 300642095131701 TS-60-63-110 Chicot Montgomery 2008

20 302850095241801 TS-60-37-309 Chicot Montgomery 2008

21 300958095221901 TS-60-54-702 Chicot Montgomery 2008

22 302014095343201 TS-60-44-212 Evangeline Montgomery 2008

23 300849095143301 TS-60-55-710 Evangeline Montgomery 2008, 2011

24 303143095334801 YU-60-28-802 Jasper Walker 2008, 2011

25 302040095281701 TS-60-45-114 Chicot Montgomery 2008, 2011

26 301338095272301 TS-60-53-210 Evangeline Montgomery 2008

27 301917095413101 TS-60-43-514 Jasper Montgomery 2008, 2011

28 301614095284201 TS-60-45-716 Evangeline Montgomery 2008, 2011

29 301612095450901 TS-60-42-902 Evangeline Montgomery 2008

30 301228095272501 TS-60-53-516 Evangeline Montgomery 2008, 2011

31 302651095362901 TS-60-36-410 Jasper Montgomery 2008, 2011

32 302412095382101 TS-60-35-907 Jasper Montgomery 2008, 2011

33 302436095263501 TS-60-37-806 Evangeline Montgomery 2008

34 302452095242001 TS-60-37-909 Evangeline Montgomery 2008, 2011

35 300658095443101 TS-60-59-102 Evangeline Montgomery 2008

36 300853095412701 TS-60-51-809 Chicot Montgomery 2008, 2011

37 300849095412601 TS-60-51-811 Chicot Montgomery 2008, 2011

38 300849095415701 TS-60-51-814 Chicot Montgomery 2008, 2011

39 301332095361901 2TS-60-52-199 Evangeline Montgomery 2008, 2011

40 301057095421901 2TS-60-51-599 Evangeline Montgomery 2008, 2011

41 301443095091801 TS-60-55-313 Jasper Montgomery 2011

42 302240095440101 TS-60-35-703 Jasper Montgomery 2011

43 302145095473901 TS-60-42-206 Jasper Montgomery 2011

44 302459095335801 TS-60-36-812 Jasper Montgomery 2011

1

Wells listed in U.S. Geological Survey database as being in Evangeline, but determined to be completed in Burkeville confining unit.

2

Final State well number assignment pending Texas Water Development Board. 
10 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

of sampling, when possible, by using an electric tape or 

steel tape. In some wells with pumps, when water levels 

were not obtained because of either a lack of access point 

or an obstruction in the well, the most recent water-level 

measurement stored in the USGS National Water Information 

System (NWIS) Groundwater Site Inventory (GWSI) database 

(Oden, 2011) was used.

Observation wells were pumped by using an electric, 

portable, submersible, positive displacement pump (Grundfos 

Redi-flo2, Redi-flo-3 or Fultz SP400R) constructed of stainless 

steel and Teflon. When practical, the intake on the portable 

submersible pump was set at a position about 10 times the 

diameter of the well above the top of the first screen in the 

well. Additionally, a stainless steel splitter was installed on 

top of the pump, above the check valve, allowing water to 

simultaneously discharge for physicochemical properties 

monitoring and waste through Teflon or polyethylene tubing 

and to discharge for sample collection through a refrigerationgrade copper line. Water was pumped from domestic and 

municipal wells by using existing pumps, and samples were 

collected at the wellhead prior to installation of any pressure 

tanks or filtering or other treatment devices. Prior to any 

treatment, a connection was made for purging and sampling 

by using a brass connector with compression fitting to 

refrigeration-grade copper tubing.

Before sample collection, one to three casing volumes 

were purged from the well, depending on well type, either 

observation or supply. For wells that are continuously pumped 

(or pumped regularly every few hours) such as those used for 

public supply, domestic supply, or industrial purposes, purging 

less than three casing volumes is permissible (U.S. Geological 

Survey, variously dated, chapter A4). The purge procedure 

removes stagnant water in the well, reduces chemical artifacts 

of well installation or well construction materials, or mitigates 

effects of infrequent pumping. After purging was complete, 

the physicochemical properties of dissolved oxygen, pH, 

specific conductance, and water temperature were measured 

until readings were stable (Oden, 2011). Once readings 

stabilized, water samples were collected in new, precleaned 

bottles. Water samples were collected and processed onsite by 

using methods designed to minimize changes to the watersample chemistry or contamination from the atmosphere. 

So there would be no degradation of water samples and to 

maintain initial concentration of compounds between the time 

of sample collection and laboratory analyses, bottle lids were 

taped on, copper tubes were crimped shut, and samples were 

stored according to the laboratory protocols, which might 

include storing upside down, or storing either chilled or at 

room temperature.

All equipment used for the collection of the 

environmental tracers during March–September 2008 was 

cleaned with tap water and a native water rinse. The external 

parts of the submersible pump were rinsed with tap water, and 

the interior of sample and discharge lines was rinsed multiple 

times with native water prior to collection. The introduction 

of solvents to the cleaning procedures, such as methanol or 

detergents, was not done as it could have affected quality of 

the samples.

April–May 2011 Sample Collection

Groundwater-quality samples were collected once 

from 24 wells during April–May 2011 and analyzed for 

concentrations of major ions and 14C, with 6 samples analyzed 

for 4He and DG. Samples were collected and processed by 

using the same procedures as in 2008 with the exception of 

using low-density polyethylene (LDPE) tubing for sampling 

lines rather than copper. 

Samples for major ions, field alkalinity, and 14C were 

collected through a preconditioned (2 liters [L] of deionized 

water [DIW] passed through a filter), 0.45-micron pore size, 

disposable capsule filter. Samples for 14C were filtered through 

the preconditioned, 0.45-micron pore size, disposable capsule 

filter with an additional 10–12 in. long piece of CFlex tubing 

on the outflow to allow for bottom filling of a 1-L glass 

bottle, leaving about 1 cubic centimeter (cc) of headspace in 

the bottle and closing the bottle with a polyseal cone cap. To 

prevent degradation of water samples and maintain the initial 

concentration of compounds between the time of sample 

collection and laboratory analyses, bottle lids for 14C were 

taped on, and samples for cations were chemically treated with 

2 milliliters of Ultrex 7.5-7.7N nitric acid then were stored 

according to the laboratory protocols, which include storing 

chilled (14C) or at room temperature (major ions). Samples 

were shipped overnight once a week to analyzing laboratories 

after being chilled to 4 oC or less.

All equipment used for the collection of the samples was 

cleaned with 0.1-percent nonphosphate detergent rinse, tap 

water rinse, and DIW as described in the USGS National Field 

Manual (U.S. Geological Survey, variously dated, chapter 

A3). LDPE tubing used for domestic wells was precleaned in 

the laboratory, then cut to length in the field with clean tubing 

cutters, then discarded after a one-time use. The same cleaning 

procedures were used for immediate cleaning of sample 

equipment that had portable pumps. Sample equipment for use 

with the portable pumps was cleaned immediately after use at 

the site.

Sample Analysis

SF6 and CFC analyses were done at the USGS 

Chlorofluorocarbon Laboratory in Reston, Va., by using 

a purge-and-trap gas chromatography procedure with an 

electron capture detector (ECD) documented by Busenberg 

and Plummer (1992, 2000). 3He/3H analyses were done at the 

Noble Gas Laboratory of Lamont-Doherty Earth Observatory 

(LDEO) of Columbia University, Palisades, New York, by 

using 3He-ingrowth with mass spectrometry for 3H (Clarke and 

others, 1976) and mass spectrometry for 3He as described in 

Schlosser and others (1988) and Ekwurzel and others (1994). 

3

H analyses were done at the USGS Tritium Laboratory, 
Methods 11

Menlo Park, California, by using electrolytic enrichment 

(Östlund and Werner, 1962) and liquid scintillation (Thatcher 

and others, 1977). Sample analysis for 4He and dissolved gases 

was done at the USGS Dissolved Gas Laboratory, Reston, 

Va., by using gas chromatography as described in Busenberg 

and others (1993) and Busenberg and others (2001). Major 

ion samples (calcium, magnesium, potassium, sodium, 

laboratory alkalinity, chloride, fluoride, silica, sulfate, and 

iron) were analyzed at the USGS National Water Quality 

Laboratory in Lakewood, Colorado, by using methods 

described in Fishman and Friedman (1989), Fishman (1993), 

and American Public Health Association, American Water 

Works Association, and Water Environment Federation (1998). 

14C analysis was conducted by a USGS contract laboratory, 

Woods Hole Oceanographic Institution, National Ocean 

Sciences Accelerator Mass Spectrometry (NOSAMS ) Facility, 

in Woods Hole, Massachusetts, by using accelerator mass 

spectrometry (AMS) (Woods Hole Oceanographic Institution, 

2011). Field alkalinity samples were collected and measured 

by field personnel and processed by using methods described 

in Rounds (2006).

Quality Control

No blank samples were collected as part of the study for 

the sampling completed in March–September 2008 because 

water free of environmental tracers and DG is not easily 

prepared or available (L. Niel Plummer, U.S. Geological 

Survey, written commun., 2009). For quality assurance, all 

samples were collected and analyzed as replicate samples 

(Oden, 2011) in accordance with standard procedures for 

collecting the environmental tracers specific to this report. A 

summary of replicate analyses and relative percent differences 

for environmental tracers (CFCs, SF6, 3H, 3He/3H, 4He, and 

DG) is not included in this report. Although samples were 

collected as replicate samples, all values were analyzed, 

reviewed, and included in the determination of apparent 

groundwater ages. Summary statistics for environmental 

tracers (CFCs, SF6, 3H, 3He/3H, 4He, and DG) are not provided. 

The evaluation of an individual analyte in relation to its 

replicate, for example CFCs, is also dependent upon the SF6

or 3He/3H result provided, or both. Even if a result for CFCs 

appears valid, it may be contradicted by evidence in one of the 

other environmental tracers, therefore making the result not 

valid.

Quality-control (QC) samples for groundwater samples 

collected April–May 2011 were collected as described 

in “National Field Manual for the Collection of WaterQuality Data” (U.S. Geological Survey, variously dated) 

and analyzed by the same laboratories and methods as the 

environmental samples. QC samples include an equipment 

blank for major ions (one), field blanks for major ions (two), 

and sequential replicate samples (major ions [three], 14C 

[four], 4He, and DG [six]). QC samples were collected to 

evaluate any contamination, as well as bias and variability 

of the water chemistry data, that might have resulted from 

sample collection, processing, transportation, and laboratory 

analysis. QC results are listed in italics in appendix 1. An 

equipment blank was collected prior to environmental sample 

collection (USGS Houston Laboratory at Shenandoah, Tex., 

April 7, 2011) in a controlled environment to determine if the 

cleaning procedures for sample containers and the equipment 

for sample collection and sample processing were sufficient to 

produce contaminant-free samples. Manganese (measured in 

micrograms per liter (μg/L) was the only constituent detected 

in the equipment blank (0.30 μg/L) above the laboratory 

reporting level (LRL). Two field blanks were collected 

and processed at sampling sites prior to the collection of 

environmental samples. Constituent concentrations in field 

blank samples were less than the LRL except for manganese 

(appendix 1), which was detected in both field blanks 

0.27 μg/L (May 5, 2011) and 0.19 μg/L (May 12, 2011). With 

manganese being detected in the equipment and both field 

blanks at low concentrations, censoring of low concentration 

of manganese occurred. The mean of the three detected 

blank concentrations (0.253 μg/L) was multiplied by three 

(0.76 μg/L), and all environmental sample concentrations (5) 

equal to or less than that value were rejected from the dataset. 

Sequential replicate samples (hereafter referred to as 

“replicate samples” in this report) were collected during the 

study. Replicate samples are collected sequentially by filtering 

environmental samples through a preconditioned 0.45 micron 

disposable capsule filter, discarding the filter, attaching a 

new preconditioned 0.45 micron disposable capsule filter, 

and collecting another set of samples to provide a measure of 

the variability of sample processing and analysis. Replicate 

samples were compared to the associated environmental 

samples by computing the relative percent difference (RPD) 

for each constituent. RPD was computed by using the equation 

 RPD = |C1 – C2| / ((C1 + C2)/2) x 100, (1)

where 

C1 = concentration from environmental sample; and 

C2 = concentration from replicate sample. 

RPDs of 10 percent or less indicate good agreement 

between analytical results if the concentrations are sufficiently 

large compared to the LRL. The RPD exceeded 10 percent 

for one of the sample pairs of major ions and for two of the 

sample pairs for 14C. The RPD for the manganese replicate 

pair was 15.7 percent and was considered acceptable with 

concentrations in 3 μg/L range. The large RPDs for 14C are 

related to calculating percent difference on percent, and both 

replicated pairs were at or near the method detection limit, a 

concentration range in which a small change creates a large 

relative percent difference.
12 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Geophysical Data Collection

Five observation wells from which groundwater samples 

were collected for this study and three additional wells at the 

same sampling sites containing wells sampled for this study 

(wells in the same well nest as those sampled) were logged 

in April 2010 by the USGS with borehole geophysical tools 

(table 2). All geophysical tools interfaced to a Century System 

VI log acquisition system in the logging truck by way of 

0.25-inch-(6-millimeter-) diameter four conductor wireline. 

Standard borehole geophysical logs, including natural gamma 

radiation (natural gamma), fluid resistivity, conductivity, 

and temperature, and electromagnetic induction (induction/

conductivity) are provided in appendix 3. All results are 

referenced to feet below land surface (land-surface datum). 

A description of standard borehole geophysical logging 

methods and interpretation is given by Keys (1990, 1997). For 

example, gamma counts increase with increasing clay content 

and resistivity increases with increasing freshwater contents 

in sands; however, resistivity can also be an indicator of clay 

content, decreasing with increasing clay content.

The borehole geophysical logging was done to gain 

a better understanding of the physical characteristics of 

the material immediately surrounding the wells and of 

the physical condition of each well (fig. 5). The logging 

survey provided additional information on the local scale 

complexities of the subsurface that might affect the traveltime 

of groundwater and, consequently, groundwater ages and 

recharge rates.

Environmental Tracers

Samples of CFCs, SF6, 3H, and 3He/3H were used to 

estimate apparent ages of recently recharged (post-1940s) 

groundwater. Additionally, 4He samples were collected and 

analyzed with the less sensitive gas-chromatography technique 

initially as a screening tool to evaluate suitability for 3He/3H 

analysis. 14C data were used to estimate apparent ages of 

premodern groundwater recharged more than 1,000 years ago.

Chlorofluorocarbons

CFCs are stable, synthetic organic compounds that were 

developed in the early 1930s as safe alternatives to ammonia 

and sulfur dioxide in refrigeration; CFCs have been used in a 

wide range of industrial and refrigerant applications (Plummer 

and Friedman, 1999). Production of dichlorodifluoromethane 

(CFC-12) began in 1931, followed by trichlorofluoromethane 

(CFC-11) in 1936, and then by many other CFC compounds, 

most notably trichlorotrifluoroethane (CFC-13). The 

CFC-11 and CFC-12 compounds were used as coolants in 

air conditioning and refrigeration; blowing agents in foams, 

insulation, and packing materials; propellants in aerosol 

cans; and solvents (Plummer and Busenberg, 2000). The 

CFC-113 has been used primarily by the electronics industry 

in semiconductor chip manufacturing, in vapor degreasing and 

cold immersion cleaning of microelectronic components, and 

in surface cleaning. Probably better known as Freon, CFCs 

Table 2. Geophysical logging methods used at wells where borehole geophysical data were collected in April 2010.

[PVC, polyvinyl chloride; Yes, method used; No, method not used; --, Well not in table 1- see footnote]

Map 

identifier 

(fig. 1).

Station name Logging

date Aquifer Well depth (feet)

Casing

material

Geophysical logging method

Naturalgamma 

radiation 

(natural 

gamma)

Fluid resistivity, 

conductivity, and 

temperature 

Electromagnetic 

induction 

(induction/ 

conductivity)

-- 1TS-60-53-718 4/29/2010 Evangeline 246 PVC Yes Yes Yes

6 TS-60-54-805 4/29/2010 Chicot 165 Steel Yes Yes No

23 2TS-60-55-710 4/29/2010 Evangeline 600 Steel Yes Yes No

30 TS-60-53-516 4/29/2010 Evangeline 807 Steel Yes Yes No

-- 3TS-60-44-807 4/29/2010 Evangeline 845 Steel Yes Yes No

11 TS-60-35-503 4/30/2010 Evangeline 118 PVC Yes Yes Yes

-- 4TS-60-45-412 4/30/2010 Chicot 261 PVC Yes Yes Yes

24 YU-60-28-802 4/30/2010 Jasper 181 PVC Yes Yes Yes

1

Well not sampled as part of study; however, co-located with wells TS-60-53-719 and TS-60-53-720, where samples were collected.

2

Well not logged to bottom; obstruction in well reported around 285 feet below land surface.

3

Well not sampled as part of study; however, co-located with wells TS-60-44-805 and TS-60-44-806, where samples were collected.

4

Well not sampled as part of study; however, co-located with wells TS-60-45-413 and TS-60-45-414, where samples were collected.
Depth

1:1250

Lithology WLBLSD

01

Natural Gamma

Increasing counts per second

Depth

1:1250

Lithology

Natural Gamma

0 ° 90° 180° 270° 0 °

01

Depth

1:1250

Lithology WLBLSD

01

Natural Gamma

0 ° 90° 180° 270° 0 °

? 

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

TS-60-35-503 (table 2) TS-60-45-412 (table 2) TS-60-53-516 (table 2) 

50

100

50

100150200250

50

100150200250300350400450500550600650700750800

 ? 

 ? 

Figure 5. Local geologic section showing borehole geophysical data in Montgomery County, Texas.

MONTGOMERY COUNTY

Increasing counts per second NGVD 29 Increasing counts per second 

WLBLSD

TS-60-35-503

TS-60-45-412

TS-60-53-516

TS-60-53-718

EXPLANATION

Lithology contact from gamma analysis

Sand unit

Clay unit

Clay unit

Relative natural gamma

Sandy clay unit

? Unknown lithology contact

WLBLSD Water level below land surface datum

Water saturated portion of wellSand unit

Figure 5. Local geologic section showing borehole geophysical data in Montgomery County, Texas.
14 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

are non-toxic, non-flammable, and non-carcinogenic, but they 

contribute to depletion of the global ozone layer (Plummer 

and Friedman, 1999). In 1987, 37 nations signed an agreement 

to limit release of CFCs and to reduce CFC emissions by 

2000 (Plummer and Friedman, 1999). This agreement was 

strengthened in 1990 and 1992 to establish a cutoff date of 

1996 to cease production of CFCs in industrialized nations. 

The United States ceased production on January 1, 1996, 

as a regulatory requirement under the U.S. Environmental 

Protection Agency Clean Air Act (International Atomic 

Energy Agency, 2006).

CFCs can be used to estimate the apparent age of water 

that has recharged an aquifer in approximately the last 50 

to 60 years, and they also can be used to estimate the time 

of recharge (Plummer and Friedman, 1999). Groundwater 

age dating with CFCs is based on Henry’s law of solubility, 

which is the concentration of the gas dissolved in water in 

equilibrium with air proportional to the partial pressure of the 

gas in air (Plummer and Busenberg, 2000). Under favorable 

conditions, sufficient concentrations of CFC‐12, CFC‐11, 

and CFC‐113 can dissolve into water and allow dating of 

groundwater recharged since approximately 1941, 1947, and 

1955, respectively. Groundwater age dating with CFC‐12, 

CFC‐11, and CFC‐113 is possible because (1) the atmospheric 

mixing ratios of these compounds (that is, concentrations 

of the compounds in air) are known, and (or) have been 

reconstructed over the past 50 years, (2) the Henry’s law 

solubility in water is known, and (3) concentrations of these 

CFCs in air and young water are relatively high and can be 

measured (Plummer and Busenberg, 2000). The concentrations 

of the CFCs in the groundwater at the time of recharge are 

proportional to the concentrations in the atmosphere at the 

same time, which are well known, and can be readily used for 

comparison. Atmospheric concentrations of CFC‐12, CFC‐11, 

and CFC‐113 in air peaked in about 2001, 1994, and 1996, 

respectively (fig. 6). Because of nearly level atmospheric 

input functions of CFC mixing ratios (CFC‐12, CFC‐11, 

CFC‐113) in the 1990s and early 2000s, it is not possible to 

resolve modern CFC ages as precisely as can be done with 

waters recharged earlier (between 1940s and 1980s), when 

atmospheric concentrations of CFC‐12, CFC‐11 and CFC‐113 

were increasing rapidly (Plummer and others, 2006).

Sulfur Hexafluoride

 SF6 is primarily an anthropogenic gas but can occur 

naturally in some minerals or rocks of igneous or volcanic 

origin. The primary uses of SF6 are as an insulator in highvoltage switches and transformers and as a blanket gas 

in melting operations of magnesium metal production 

(Busenberg and Plummer, 2000). Production of SF6 began in 

the 1950s and shows an upward pattern (fig. 6) that is expected 

to continue (Busenberg and Plummer, 1997). 

Sulfur hexafluoride is a tracer and dating tool of 

water recharged during the last 40 years. Under favorable 

conditions, detectable concentrations of SF6 can accumulate 

in groundwater and allow for dating of waters since 

approximately 1970. Groundwater dating with SF6 is possible 

because (1) the atmospheric history is well established (Maiss 

and Brenninkmeijer, 1998); (2) sources are identified, and 

there is a large contrast between a small natural background 

and a large anthropogenic signal; (3) the Henry’s law 

solubility in water is known; and (4) concentrations in 

groundwater are sufficient to be measured (Busenberg and 

Plummer, 1997). The concentration of SF6 in groundwater at 

the time of recharge is proportional to the concentration in the 

atmosphere at the same time, which is well known, and can be 

readily used for comparison (Busenberg and Plummer, 1997).

Tritium

The radioactive isotope of hydrogen, 3H, was used 

to identify modern groundwater recharge or groundwater 

mixtures that contain some modern groundwater recharge. 

3

H substitutes for a hydrogen (1H) atom or deuterium (2H) 

atom in the water molecule and can serve as a tracer because 

water containing a 3H atom follows the same pathway through 

the environment as water that does not contain a 3H atom 

(Plummer and others, 1993); that is, transport is conservative 

with no retardation. The half-life of 3H is 12.32 years and 

commonly is measured in picocuries per liter or tritium units 

(TU) in which 1 TU = 3.19 pCi/L (Lucas and Unterweger, 

2000). Low levels of 3H are produced naturally by the 

interaction of the atmosphere with cosmic rays from solar 

wind. Atmospheric thermonuclear weapons testing from 1952 

to 1964 introduced a large amount of 3H to the atmosphere 

that was incorporated directly into water molecules of 

precipitation. Concentrations of 3H in precipitation have been 

decreasing since that time (fig. 6) because of the exchange of 

water with the oceans, short half-life of 3H, and atmospheric 

thermonuclear weapons testing was stopped. Because the 

3

H concentration in the atmosphere was high for a relatively 

short period (10–15 years) about 50 years ago (fig. 6), its 

presence in groundwater can identify water that has been 

recharged during the last 50 years or mixtures that contain a 

fraction of post-1950s water. The 3H activities in precipitation 

prior to thermonuclear weapons testing are not well known 

but probably did not exceed about 2 to 8 TU (Plummer and 

others, 1993). Because 3H has a half-life of 12.32 years, water 

derived from precipitation before thermonuclear weapons 

testing would contain a maximum 3H activity of about 0.12 to 

0.5 TU by the early 2000s. Clark and Fritz (1997) stated that 

during the mid-1990s, waters with 3H activities exceeding 

about 30 TU contained a considerable component of recharge 

from the 1960s or the 1970s, and activities exceeding 50 TU 

indicated recharge predominantly from the 1960s.
0

100200300400500600700

1940 1950 1960 1970 1980 1990 2000 

Tracerconcentrationsinpartspertrillionbyvolume

Year

3H

CFC-12

CF

CFC-113

Figure 6. Atmospheric concentrations ofchlorofluorocarbons, sulfur hexafluoride, and tritium in precipitation (tritium data f

stopped collection in 1985, 1986-2007 data decayed from 1985 data. Chlorofluorocarbon and sulfer hexafluoride data compiled b

Survey Chlorofluorocarbon Laboratory).

Dichlorodifluoromethane (CFC-12)Trichlorofluoromethane (CFC-11)Trichlorotrifluoroethane (CFC-113) Sulfur hexafluoridex 100Tritium,Waco, Texas, decayed to 2008

EXPLANATION

Figure 6. Atmospheric concentrations of chlorofluorocarbons, sulfur hexafluoride, and tritium in precipitation data in Waco, Texas, decayed to 2008 Energy Agency, 2010) (chlorofluorocarbon and sulfur hexafluoride data compiled by U.S. Geological Survey Chlorofluorocarbon Laboratory, U.S. Geo
16 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Helium-3 and Tritium 

The radioactive decay product of 3H is the noble gas 3

He. Simultaneous measurement of 3H and 3He provides an 

environmental tracer. Additionally, the 3H/3He ratio allows 

determination of the 3H/3He age of the groundwater through 

use of the equation of radioactive decay (Poreda and others, 

1988; Schlosser and others, 1988; Schlosser and others, 

1989;). For the concentration gradient of 3He to be used in 3

H/3He dating, the diffusive loss of the 3He that is derived from 

the degradation of 3H in the groundwater to the unsaturated 

zone must be minimal; therefore, 3He must be confined in the 

aquifer. The 3He confinement is the efficiency with which the 3

He produced by 3H decay is retained in the aquifer. The 3He 

produced in the aquifer by 3H decay is transported to the water 

table by diffusion against the advective downward movement 

of the water; also lateral and vertical dispersion needs to be 

small enough not to substantively alter the 3H/3He ratio (Szabo 

and others, 1996). 

The limitations of 3He/3H for age dating are many, but 

two of the important factors to consider are the presence of 

detectable 3H and minimal excess 4He. Initially, a sample 

would need to contain at least 1 TU to be datable by the 

3

He/3H method (L. Niel Plummer, U.S. Geological Survey, 

written commun., 2009), and water recharged prior to 1950 

(prebomb) would contain less than 0.5 TU (Solomon and 

Cook, 2000). Another factor to consider would be the presence 

of helium derived from the Earth’s mantle, referred to as 

mantle helium, or derived from uranium and thorium decay of 

the aquifer solids, referred to as “terrigenic helium.” Excess 

terrigenic helium indicates water that is primarily older 

or contains very little younger water except where a local 

geological source of helium exists. 

Helium-4 

Water that has been in contact with the aquifer for 

long periods of time will accumulate 4He (primarily from 

uranium and thorium decay in the aquifer), which can be 

used as a potential dating tool for groundwater recharged 

101 to 108 years before present (Solomon and others, 1996). 

Samples with excess 4He might indicate older, pre-1950s 

water— water that (in addition to other factors precluding 

3

He/3H age dating) is potentially not datable by the 3He/3H 

method because of excess 4He (Solomon and Cook, 2000). 

Dissolved helium in groundwater is derived from terrigenic 

and atmospheric sources. The 4He analysis was done 

primarily as an initial screening of samples collected for 

3

He/3H analysis; large amounts of excess 4He indicate water 

that cannot be dated by the 3He/3H method, as described in 

the previous section. Helium accumulation in groundwater as 

indicated by 4He concentration can also be another indicator 

of groundwater age (Solomon and others, 1996; Solomon, 

2000; Hunt and others, 2010).

The concept of dating groundwater with 4He is the longer 

the groundwater is in contact with the aquifer sediments, 

the greater the concentration of 4He in groundwater (above 

air-water equilibrium) (Solomon, 2000). If external sources 

of 4He are minimal (derived from the mantle or local uranium 

ore), then 4He concentrations should correspond to travel times 

of the water. The 4He is produced by decay of uranium-235/

uranium-238 and thorium-232 in aquifer sediments. The 

4

He method provides the opportunity to estimate the age of 

groundwater that recharged between 50 and 500 years—a 

range not covered by other techniques. A complicating factor 

is excess 4He derived from external sources to the aquifer, 

such as underlying rocks or the mantle (Solomon, 2000).

The use of 4He as an indicator of groundwater age can 

be difficult. A local helium accumulation rate needs to be 

determined, which is not always possible. Calculating a local 

helium accumulation rate requires data regarding the average 

concentration of uranium and thorium, on the flow path, along 

with other factors, for the aquifer (U.S. Geological Survey, 

2011a, b). The quantification of the 4He release rate can be 

difficult and is best suited for 4He samples collected at sites 

where groundwater samples datable by other methods have 

been collected. The pairing of 4He analysis with datable 

groundwater samples allows for the determination of the 4He 

release rate (Solomon, 2000). Estimates of travel time for 

water from nearby wells completed in the same aquifer can be 

evaluated on the basis of 4He concentration.

Carbon-14

14C is an important radio-isotope tool for dating deep 

groundwater. It is continuously being produced by reaction of 

cosmic ray neutrons with nitrogen-14 (14N) in the atmosphere 

and decays with a half-life of 5,730 years. 14C is quickly 

oxidized to carbon dioxide (CO2) where it is incorporated into 

the water cycle. With a relatively long radioactive half-life 

(5,730 years), 14C is suitable for dating groundwaters that are 

thousands to tens of thousands of years old. 

Dating of groundwater with 14C is accomplished by 

measuring 14C activities of dissolved inorganic and dissolved 

organic carbon in the water (DIC and DOC). Dissolved CO2, 

carbonic acid, bicarbonate, and carbonate are the four main 

species of DIC in ground water (Hem, 1985; Clark and Fritz, 

1997). Groundwater cut off from the modern atmospheric 

reservoir can be dated by using 14C if sufficient time has passed 

for measurable (detectable) decay of the initial 14C activity 

(Ao) which is dependent upon the analytical capabilities of 

the laboratory, namely the detection capability and precision 

associated with it. Measured 14C activities are expressed as 

a percent of modern carbon (pmc); the activity of modern 

carbon is defined as 95 percent of the 14C activity in 1950 of the 

National Bureau of Standards (NBS) oxalic standard (Clark and 

Fritz, 1997), which approximates the 14C activity of wood grown 

in 1890 that was relatively free of CO2 from fossil-fuel sources. 

A discussion of how the measured 14C are used to calculate the 

apparent ages can be found in the section “Paleowaters.”
Dissolved Gases 17

Major Ion Chemistry as an Indicator of 

Groundwater Evolution and Travel Time

Analytical results of major ion chemistry sampled from 

groundwater from wells developed into the Chicot, Evangeline, 

and Jasper aquifers can be characterized as predominately 

calcium-bicarbonate waters (fig. 7). The sediments have been 

described as containing limestone, calcite (CaCO3), dolomite 

(CaMg(CO3)), and gypsum (CaSO4*2H2O) (Popkin, 1971), 

which is consistent with producing predominantly calciumbicarbonate type waters. The relatively shallow wells and even 

the deeper wells to the north and east of Houston are near the 

updip end of the aquifer, so water is relatively dilute and has 

generally homogeneous composition.

Sites where the water is characterized with a slightly 

different signature indicate there may be other factors or 

processes influencing the major ion chemistry. Water from 

three wells (TS-60-44-805, well 3; TS-60-45-513, well 15; 

and TS-60-45-114, well 25) developed in the Chicot aquifer 

yielded very low concentrations of major ions in comparison 

to water from the other wells developed in the Chicot aquifer 

(fig. 7). This composition may indicate recent, minimally 

evolved recharge water, dilution occurring from mixing with 

a source of localized recharge, or both. Two downgradient 

wells in the Jasper aquifer (YW-59-64-206, well 17; and 

TS-60-55-313, well 41) yield elevated concentrations of 

sodium and low concentrations of calcium; such composition 

is typically characterized by cation exchange process (fig. 

7). Cation exchange occurs as water moves downgradient 

and exchanges dissolved calcium in water for dissolved 

sodium held on clay mineral surfaces. Such composition is 

typical of long groundwater flow paths and with older water 

that has had long periods of contact with the clay minerals 

(Szabo and others, 2006).

Dissolved Gases

Dissolved gas concentrations, as well as the ratios of 

dissolved gas concentrations, can be used to calculate the 

recharge temperature of a groundwater sample because the 

solubility of gases in water varies as a function of temperature 

(Plummer and others, 2004). Determining the groundwater 

temperature is important because concentrations of CFCs (upon 

which groundwater ages are determined) are based on Henry’s 

law solubility, which in turn is dependent on the groundwater 

temperature at the time of recharge and also altitude (pressure) 

at the location of recharge. Land-surface altitudes at all sites 

were used for recharge altitudes because actual recharge 

locations were unknown, but because the deposited sediments 

were part of a delta complex aggrading at the margin of the 

Gulf resulting in the relative flat topography of the study region 

that allowed for the assumption of minimal change in altitude 

with time and distance and therefore the current land-surface 

altitude at each sampled site, was a reasonable approximation 

of recharge altitude. Gas concentrations and therefore 

calculated recharge temperatures and groundwater ages 

have little sensitivity to estimated recharge altitudes over the 

relatively small range in the altitude of the study area (Plummer 

and Busenberg, 2000).

Most dissolved gas samples contained little excess air 

(0.9 to 5.8 cubic centimeters [cm3]) in groundwater samples 

collected from the Chicot, Evangeline, and Japser aquifers 

(Oden, 2011) (0.9 to 3.1 cm3 in samples collected from the 

Evangeline and Jasper aquifers, appendix 4). The small amount 

of excess air within the samples is consistent with diffuse rather 

than focused recharge. Diffuse recharge occurs over a large area 

as water from precipitation infiltrates and percolates through 

the unsaturated zone to the water table. Focused recharge 

occurs at discrete locations such as along a preferential pathway 

or location (Nimmo and others, 2002). Focused recharge can 

occur rapidly, resulting in larger quantities of excess air (more 

than about 6 cm3) being trapped in aquifer pores and dissolving 

into the groundwater under recharge conditions.

Mean recharge temperatures determined from DG ranged 

from 10.6 to 23.5 °C in groundwater samples collected from 

the Chicot, Evangeline, and Jasper aquifers (Oden, 2011). 

Modern recharged waters should be within a few degrees of 

the mean annual temperature for the region. The mean annual 

temperature for Houston Intercontinental Airport over 30 years 

(1971–2000) was 20.4 °C (National Oceanic and Atmospheric 

Administration, 2010). The median recharge temperature at all 

sites was 15.7 °C (table 3). Field water temperatures measured 

at the time of sampling were typically higher than the recharge 

temperature determined by using DG, with a median difference 

of 8.0 °C (table 3), indicating that groundwater at the time 

of sampling probably recharged at lower temperatures a 

considerable time ago than it did at the time of this study (2008 

and 2011). Another factor affecting the temperature of the 

groundwater was the likelihood that most recharge in the region 

might occur in winter and early spring, which are the coolest 

times of the year. In addition, groundwater temperature was not 

measured in situ, and the water might have warmed during the 

process of pumping it to the surface for sample collection.

One site located in Waller County (YW-59-64-206) had an 

anomalously high mean recharge temperature of 51 °C, which 

is a mean recharge temperature that is not plausible. Water 

collected from this well also contained an anomalously high 

concentration of methane and low concentrations of the other 

gases. Water from this well has most likely undergone a more 

complex evolution than other groundwater in the study area, 

and recharge temperature could not be determined accurately 

from the nitrogen and argon gas concentrations in this sample. 

Nitrogen and argon gases might have been stripped from the 

sample by exsolvation of the methane during sampling (U.S. 

Geological Survey, 2011a, b). The DG data from this well 

with a mean recharge temperature of 51 °C was eliminated 

from the analysis, and the aforementioned minimum, 

maximum, and median recharge temperatures (table 3) do not 

include this outlier. 
18 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Bayou

15

17

23

24

27

28

31

34

38

40 39

42

43

44

6

32

11

18

25

30

41

36

37

3

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K 

COUNTY

HA

R R I S COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES COUNTY

SA

N JACINTO COU

NTY

W

ALKER COU

NTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

Aquifer data from Strom and others, 2003 a,b,c

and Kasmarek and Robinson, 2004

TEXAS

Study area

Location map

ANIONS

20 16 12 8 4 4 8 12 16 20 (meq/l)

Chloride

Calcium

Magnesium Sulfate

Sodium

Bicarbonate

0

MILLIEQUIVALENTS PER LITER

CATIONS

17 - Well developed into the Jasper Aquifer

(17) - Map Identifier

11 - Well developed into the Evangeline Aquifer

(11) - Map Identifier

 6 - Well developed into the Chicot Aquifer

(6) - Map Identifier WATER CHEMISTRY DIAGRAM - Shows major

chemical constituents in milliequivalents per liter

(meq/L). The diagram can be used to compare and

characterize types of water. 

Figure 7.

Figure 7. Water chemistry in the Chicot, Evangeline, and Jasper aquifers in April–May 2011, Montgomery, Walker, and Waller 

Counties, Texas.
Groundwater Age 19

Groundwater Age

Modern Environmental Tracers

Groundwater ages are a measure of the time since the 

water entered the saturated zone and was isolated (through 

additional recharge) from the atmosphere to set the “age” 

(time elapsed since recharge) of the water. The age of 

groundwater is estimated from concentrations of dissolved 

chemicals or isotopes used as environmental tracers (such 

as CFCs, SF6, and 3H) and comparison of the equivalent 

atmospheric concentration of each tracer to the atmospheric 

input signal for that tracer. Factors affecting the isolation 

time can be porosity of the unsaturated zone, recharge rate, 

and magnitude of water-table fluctuations (Plummer and 

Busenberg, 2000).

Groundwater ages commonly are termed “apparent 

age” because the ages are modeled by using simplifying 

assumptions regarding transport processes that may affect 

the concentration of environmental tracers (CFCs, SF6 and 3

He/3H) in the water that was recharged and isolated from the 

atmosphere (Plummer and Busenberg, 2000). The simplest 

and most common transport assumption in groundwater age 

dating is piston flow, in which the constituent concentration 

is not altered by mixing or dispersion from the point of entry 

to the measurement point in the aquifer. Determination of 

groundwater ages by using a piston-flow model is likely an 

oversimplification because mixing and dispersion often occur 

during groundwater flow (Rupert and Plummer, 2009). The 

effects of mixing and dispersion tracer concentrations are 

beyond the scope of this project, and piston flow is assumed 

for estimating apparent groundwater ages.

Paleowaters

14C is typically reported as percentages of modern 14C 

concentrations (Clark and Fritz, 1997). When 14C is used as 

a tool to estimate groundwater ages, the carbon is considered 

to represent the total DIC in the groundwater. The DIC in 

most groundwater flow paths will interact with the carbonate 

minerals and sources of 14C-free deep CO2, resulting in a 

decreased pmc and apparent groundwater ages to be older than 

actual. To achieve more accurate estimated groundwater ages, 

the 14C pmc can be corrected by using a geochemical reaction 

path model which accounts for the sources and sinks of carbon 

along a flow path (Plummer and others, 1983; Plummer and 

others, 1991). 

Chemical and isotopic data were interpreted by using 

the NETPATH model (Plummer and others, 1994) to adjust 

initial 14C activities for geochemical reactions. NETPATH 

uses equations of chemical mass balance, electron balance, 

and isotope mass balance to define possible net geochemical 

reactions between the initial and final water along a specified 

groundwater flow path. Geochemical reactions are constrained 

to occur among reasonable reactant and product minerals and 

gases, and they are consistent with the observed geochemistry 

and available isotopic data of the groundwater. Geochemical 

reaction models are then solved as an isotope-evolution 

problem (Wigley and others, 1978), accounting for isotope 

fractionation along the flow path to predict the isotopic 

composition at the end point in the reaction, including 

adjustment of the initial 14C activity for geochemical reactions 

(Plummer and Sprinkle, 2001).

 Plummer and Sprinkle (2001, p. 144–145) described 

traditional and NETPATH approaches to radiocarbon dating 

of dissolved inorganic carbon (DIC) in groundwater for the 

purpose of determining adjusted 14C radiocarbon ages:

Traditionally, hydrologists have applied some of the 

well-known inorganic adjustment models (Ingerson 

and Pearson 1964; Mook 1972; Tamers 1975; 

Fontes and Garnier 1979; Eichinger 1983) to DIC 

of water from a single well to estimate adjusted 14C 

ages. The approach is illustrated in [figure 8A]. This 

approach is well suited for geochemical systems 

undergoing relatively simple water–rock reactions, 

such as carbonate-mineral dissolution, gypsum 

dissolution, Ca/Na [calcium/sodium] ion exchange, 

CO2 gas dissolution, and isotope exchange between 

soil CO2, calcite, and DIC during recharge. Wigley 

[and others] (1978) present Rayleigh distillation 

and isotope mass-balance models to predict 

isotopic evolution in carbonate mineral water 

systems where both dissolution (incoming carbon) 

and precipitation (outgoing carbon with isotopic 

fractionation, including gas evolution) reactions 

occur, and they propose a general approach for 

constructing radiocarbon adjustment models for 

evolutionary waters. 

Table 3. Summary of recharge temperatures and field water 

temperatures for wells sampled in and around Montgomery 

County, Texas during March–September 2008 and April–May 2011.

[°C, degrees Celsius]

Temperature 

type

Sample 

count

Minimum

temperature

(°C)

Median

temperature

(°C)

Maximum

temperature

(°C)

Recharge1 45 10.6 15.7 23.5

Field2 64 20.6 23.7 31.1

1

Inculdes 2 wells (TS-60-55-710 and TS-60-53-516) resampled for dissolved gases in 2011.

2

Includes the field water temperatures for the 20 wells resampled in 2011.
20 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

A. Traditional Ao (model)

Ao (model)

Aobs (measured)

B. NETPATH

And (calculated)

Aobs (measured)

B

B

A

∆t (years) = In 5,730

In 2

Ao

Aobs 

∆t (years) = In 5,730

In 2

And

Aobs 

Reaction model

A + reactants B + products

Ao And

A = B Traditional adjustment model

 approach in NETPATH

Ao Initial 14C composition, calculated

Aobs Measured 14C 

And Adjusted for chemical reaction

 but no radioactive decay of 14C

A Initial well

B Final well

ln Natural logarithm

∆t Adjusted age (years)

EXPLANATION

Figure 8. Conceptual approaches to radiocarbon dating of dissolved inorganic carbon (DIC) in groundwater:

A. Traditional. B. NETPATH (modified from Plummer and Sprinkle, 2001).
Groundwater Age 21

NETPATH (Plummer [and others] 1994) 

incorporates the modeling approach of Wigley 

[and others] (1978) and can be used to construct 

14C-adjustment models for complex hydrochemical 

systems that cannot be treated by previous DIC 

adjustment models. The approach is shown in [figure 

8B]. By combining carbon mass-balance equations 

with Rayleigh distillation equations for all incoming 

carbon sources and all isotopically fractionating 

outgoing carbon phases, the initial 14C (Ao) is 

adjusted for the modeled geochemical reactions [And

in figure 8B]. NETPATH includes most of the wellknown adjustment models that can be applied to the 

initial water to obtain estimates of Ao. In radiocarbon 

dating of DIC in groundwater using NETPATH, the 

traditional adjustment models are usually applied 

to the initial water only, where, in the recharge 

portions of aquifers *** the geochemical reactions 

are usually relatively simple. In general use, the 

initial and final waters are defined separately in 

NETPATH. NETPATH is then used to describe the 

geochemical reactions that reproduce the chemical 

and δ13C isotopic composition of DIC in the final 

water. This, in effect, develops a separate adjustment 

model for each water analysis. The adjustment 

is applied to the initial 14C to compute the 14C 

expected in DIC at the final well, as if there were 

no radioactive decay (And) [figure 8B]. The adjusted 

no-decay 14C activity is then used with the measured 14C activity to compute travel time from the initial 

to the final point. Further details of radiocarbon 

dating applications in NETPATH are given in 

Plummer [and others] (1994). 

 Plummer and others (1994, p. 25) explained that 

“NETPATH considers nine options (adjustment models) for 

definition of Ao or the initial 14C along the flow path.” Three 

of the models were considered to represent the conditions 

that defined the initial 14C (Ao TDIC), 1. Ingerson and Pearson 

(1964), 2. Fontes and Garnier (1979), and 3. Eichinger 

(1983). The model of Ingerson and Pearson (1964) assumed 

a carbonate dissolution model to estimate Ao TDIC based on the 13C data for the inorganic carbon system. The model of Fontes 

and Garnier (1979) used a two-stage evolution of recharge 

waters that accounts for the dissolution and isotopic exchange 

of carbonate minerals with CO2 in the unsaturated zone and 

isotopic exchange with carbonate rocks in the saturated zone. 

The Eichinger (1983) model developed an isotope-exchange/

mass-balance model for equilibrium isotopic exchange for 

introduction of soil CO2 into the water and equilibrium 

exchange between dissolved inorganic carbon and the 

carbonate rock.

To simulate geochemical evolution and interpret carbon 

net mass balances, NETPATH calculated mass transfer 

of phases for calcite (CaCO3), dolomite (CaMg (CO3)), 

gypsum (CaSO4*2H2O), Ca/Na ion exchange, and CO2-gas, 

mixing between the Chicot and Evangeline aquifer waters, 

groundwater and isotopic exchange of carbon between 

dissolved inorganic carbon as bicarbonate and CO2 gas, and 

carbon derived by dissolution from solid calcite and dolomite 

source in the aquifers. Carbon fractionation factors for the 

inorganic 13C used the approach identified by Mook (1980) 

and Mook and others (1974). A delta (δ)

13

C of 0.0 per mil 

(‰) was assumed for the carbonate minerals. An initial δ13C 

of -25‰ was used for the soil-gas CO2 assuming an “open” 

system where soil gas reservoir supplied CO2 to groundwater 

(Clark and Fritz, 1997; Kendall and McDonnell, 1998). The 

initial 14C activity of 100 pmc was used for the Ao for the 

soil-gas CO2. Each net reaction was checked for consistency 

between the observed 14C and the calculated δ13C as a way 

to calibrate the models representing results from each well. 

The net difference between the observed and calculated δ13C

was generally less than plus or minus 1‰, although in a 

few cases it was not possible to constrain the model to this 

small of a difference between the observed and simulated 

values. Table 4 shows the initial percent modern carbon 

(Ao) along with calculated (NETPATH output) and observed 

(laboratory measured) values for δ13C and 14C and the adjusted 

ages calculated by using NETPATH and the dissolution/

precipitation of minerals used in the models along individual 

flow paths, and the percent mixing for models where mixing 

was used with composition from more than one initial well to 

calculate the mass transfer of phases along a flow path. The 

sensitivity of the models to the assumed δ13C for the carbonate 

minerals was evaluated by using input δ13C of -1.0‰ and 

1.0‰ for one model from each aquifer to determine the 

uncertainty of the adjusted ages (table 5). Generally there was 

very little sensitivity to the carbonate mineral δ13C within this 

limited range of input values. 

All three of the models were constrained by calculating 

δ13C close to the observed δ13C. Calculated reaction used to 

adjust 14C ages for any well simulated in our modeling efforts 

represented the overall mass transfer of phases calculated 

between the initial water and water from a particular final 

well rather than the mass transfer that would be calculated 

incrementally between successive wells. Although the wells 

were along generalized flow paths, the calculated mineral 

mass transfers were actually independent of flow path 

(Plummer and Sprinkle, 2001). For sites with adjusted ages 

that included mixing models involving waters from two 

separate initial wells, the selection of the second well or 

water used to mix along a flow path was selected on the basis 

of an assumption that the water analysis from this site best 

represented the mixing condition that may be occurring along 

the flow path. Table 4 provides information on the wells used 

to adjust for the individual 14C ages along the generalized 

groundwater flow paths.
22 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Table 4. Summary of adjusted carbon-14 ages and mineral mass transfer in the Chicot, Evangeline, and Jasper Aquifers, Montgomery 

County, Texas.

[A0, initial 14C composition; 14C, radio-isotope of carbon; 13C, stable-isotope of carbon; Ca, calcium; Na, sodium; Ca/Na, ratio of sodium to calcium; CO2, carbon 

dioxide; mmol/kg, millimole per killigram; >, greater than; --, not determined]

Aquifer Station name

Map 

identifier 

(figure 9,

table 1)

Wells (by map 

identifier) 

or rainwater 

used to adjust 14C ages 

A0

Initial 

percent 

modern 

carbon 

calculated1

14C

Percent 

modern 

carbon 

observed

14C

percent 

modern 

marbon 

calculated

13C

(per mil) 

observed

13C_calc

(per mil) 

calculated

14C

adjusted 

ages in 

years before 

present1

Chicot TS-60-45-114 25 -- -- 101.53 -- -- -- Modern

Chicot TS-60-45-513 15 25 to 15 88 82.80 96.80 -21.10 -23.20 Modern

Chicot TS-60-51-811 37 25 to 37 88 66.57 75.47 -16.97 -17.01 900

Chicot TS-60-51-814 38 25 to 38 88 36.52 66.00 -15.00 -13.80 5,000

Chicot TS-60-54-805 6 25 to 6 88 93.35 73.04 -17.17 -17.10 Modern

Chicot TS-60-51-809 36 25 to 36 89 75.52 82.17 -17.70 -18.70 650

Evangeline TS-60-35-503 11 rainwater2 to 11 98 59.75 61.84 -15.87 -15.46 1,200

Evangeline TS-60-35-907 32 11 to 32 63 1.02 54.60 -11.80 -12.83 33,000

Evangeline TS-60-37-909 34 11 to 38 to 34 47 17.72 5.06 -10.87 -9.79 8,000

Evangeline TS-60-43-511 18 38 to 18 58 28.39 19.97 -7.28 -8.23 900

Evangeline TS-60-45-716 28 11 to 18 to 28 36 2.54 36.00 -9.88 -9.70 26,000

Evangeline TS-60-52-199 39 11 to 18 to 39 46 0.29 45.49 -6.89 -8.30 42,000

Evangeline TS-60-51-599 40 11 to 18 to 40 37 0.75 37.00 -8.07 -7.20 32,000

Evangeline TS-60-53-516 30 25 to 30 89 86.90 93.00 -21.40 -21.70 Modern

Evangeline TS-60-55-710 23 11 to 23 63 44.30 57.70 -16.51 -16.10 2,700

Jasper YU-60-28-802 24 rainwater2 to 24 98 11.67 57.41 -14.87 -12.45 13,000

Jasper TS-60-36-410 31 24 to 31 59 6.06 10.44 -12.04 -13.40 18,000

Jasper TS-60-36-812 44 24 to 44 59 0.62 10.80 -12.55 -13.77 37,000

Jasper TS-60-42-206 43 24 to 43 59 7.88 8.60 -11.30 -11.10 16,000

Jasper TS-60-35-703 42 24 to 42 59 5.75 9.90 -11.53 -12.70 19,000

Jasper TS-60-43-514 27 24 to 27 59 2.16 9.69 -11.59 -12.34 27,000

Jasper YW-59-64-206 17 24 to 17 59 0.03 4.25 -4.68 -5.44 >42,000

Jasper TS-60-55-313 41 24 to 41 59 0.35 11.67 -13.65 -14.74 42,000
Groundwater Age 23

Table 4. Summary of adjusted carbon-14 ages and mineral mass transfer in the Chicot, Evangeline, and Jasper Aquifers, Montgomery 

County, Texas.—Continued

[A0, initial 14C composition; 14C, radio-isotope of carbon; 13C, stable-isotope of carbon; Ca, calcium; Na, sodium; Ca/Na, ratio of sodium to calcium; CO2, carbon 

dioxide; mmol/kg, millimole per killigram; >, greater than; --, not determined]

Aquifer Station name

Map 

identifier 

(figure 9,

table 1)

Wells (by map 

identifier) 

or rainwater 

used to adjust 14C ages 

Initial 

well 1

percent 

mixing

Initial 

well 2

percent 

mixing

Calculated mass transfer

Calcite

(mmol/kg) 

water

Dolomite

(mmol/kg) 

water

Gypsum

(mmol/kg) 

water

Ca/Na 

exchange

(mmol/kg)

water

CO2

(mmol/kg) 

water

Chicot TS-60-45-114 25 -- -- -- -- -- -- -- --

Chicot TS-60-45-513 15 25 to 15 -- -- 1.30 0.12 -- 0.50 4.71

Chicot TS-60-51-811 37 25 to 37 -- -- 0.88 0.12 0.74 0.56 --

Chicot TS-60-51-814 38 25 to 38 -- -- 1.29 0.12 -- 0.46 -0.87

Chicot TS-60-54-805 6 25 to 6 -- -- 1.27 0.13 -- 0.53 0.72

Chicot TS-60-51-809 36 25 to 36 -- -- 0.55 0.11 0.62 0.54 --

Evangeline TS-60-35-503 11 rainwater2 to 11 -- -- 2.11 0.09 0.96 0.68 --

Evangeline TS-60-35-907 32 11 to 32 -- -- -- 0.26 -0.99 0.24 -1.41

Evangeline TS-60-37-909 34 11 to 38 to 34 85 15 0.09 -- 0.79 -0.35 --

Evangeline TS-60-43-511 18 38 to 18 -- -- 2.20 0.51 -1.55 -0.05 --

Evangeline TS-60-45-716 28 11 to 18 to 28 28 72 -- 0.02 0.40 0.82 --

Evangeline TS-60-52-199 39 11 to 18 to 39 58 42 -- -- -1.07 0.40 -2.48

Evangeline TS-60-51-599 40 11 to 18 to 40 33 67 -0.40 -- -- 0.49 -2.02

Evangeline TS-60-53-516 30 25 to 30 -- -- -- 0.20 2.01 1.79 1.30

Evangeline TS-60-55-710 23 11 to 23 -- -- -2.64 0.08 1.14 -0.32 --

Jasper YU-60-28-802 24 rainwater2 to 24 -- -- 2.01 0.15 0.11 0.59 -1.46

Jasper TS-60-36-410 31 24 to 31 -- -- 0.46 0.04 -0.27 0.24 --

Jasper TS-60-36-812 44 24 to 44 -- -- 0.16 0.10 -0.17 0.32 --

Jasper TS-60-42-206 43 24 to 43 -- -- 1.52 0.04 -0.81 0.85 --

Jasper TS-60-35-703 42 24 to 42 -- -- 0.77 -0.00 -0.34 0.53 --

Jasper TS-60-43-514 27 24 to 27 -- -- 0.76 0.08 -0.41 0.64 --

Jasper YW-59-64-206 17 24 to 17 -- -- 7.79 -0.11 -3.32 5.73 --

Jasper TS-60-55-313 41 24 to 41 -- -- -0.12 -0.14 0.69 2.03 --

1

These are averages based on results from Ingerson and Pearson (1964), Fontes and Garnier (1979), and Eichinger (1983.)

2

Because an upgradient well was not sampled for modern 14C, rainwater used in lieu of the upgradient well. 
24 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Table 5. Sensitivity analysis for modeled carbon-14 adjusted ages for selected wells within the Chicot, Evangeline, and Jasper 

aquifers, Montgomery County, Texas.

[A0, initial 14C composition; 14C, radio-isotope of carbon; 13C, stable-isotope of carbon; Ca, calcium; Na, sodium; Ca/Na, ratio of sodium to calcium; CO2, carbon 

dioxide; mmol/kg, millimole per killigram; --, not determined]

Aquifer Station name

Map 

identifier 

(table 1)

Wells used 

to adjust 14C 

ages

(map 

identifier)

Calculated mass transfer

A0

initial percent 

modern carbon 

calculated1

14C

percent 

modern 

carbon 

observed

14C

percent 

modern 

carbon 

calculated

13C

(per mil) 

observed

13C_calc

(per mil) 

calculated

 0.0 per mil carbonate phase

Chicot TS-60-45-114 25 15 to 25 88 82.80 96.80 -21.10 -23.20

 -1.0 per mil carbonate phase

Chicot TS-60-45-114 25 15 to 25 88 82.80 96.70 -21.11 -23.2

 1.0 per mil carbonate phase

Chicot TS-60-45-114 25 15 to 25 88 82.80 96.77 -21.11 -23.18

 0.0 per mil carbonate phase

Evangeline TS-60-35-907 32 11 to 32 63 1.02 54.60 -11.80 -12.74

 -1.0 per mil carbonate phase

Evangeline TS-60-35-907 32 11 to 32 63 1.02 54.6 -11.8 -12.83

 1.0 per mil carbonate phase

Evangeline TS-60-35-907 32 11 to 32 63 1.02 54.6 -11.8 -12.65

 0.0 per mil carbonate phase

Jasper TS-60-42-206 43 24 to 43 59 7.88 8.60 -11.30 -11.10

 -1.0 per mil carbonate phase

Jasper TS-60-42-206 43 24 to 43 57 7.88 8.6 -11.3 -11.23

 1.0 per mil carbonate phase

Jasper TS-60-42-206 43 24 to 43 61 7.88 8.6 -11.3 -10.7
Groundwater Age 25

Table 5. Sensitivity analysis for modeled carbon-14 adjusted ages for selected wells within the Chicot, Evangeline, and Jasper 

Aquifers, Montgomery County, Texas.—Continued

[A0, initial 14C composition; 14C, radio-isotope of carbon; 13C, stable-isotope of carbon; Ca, calcium; Na, sodium; Ca/Na, ratio of sodium to calcium; CO2, carbon 

dioxide; mmol/kg, millimole per killigram; --, not determined]

Aquifer Station name

Map 

identifier 

(table 1)

Calculated mass transfer

14C

adjusted 

ages in 

years befor 

present1

Calcite

(mmol/kg) 

water

Dolomite

(mmol/kg) 

water

Gypsum

(mmol/kg) 

water

Ca/Na 

exchange

(mmol/kg) 

water

CO2

(mmol/kg) 

water

 0.0 per mil carbonate phase

Chicot TS-60-45-114 25 Modern 1.30 0.12 -- 0.50 4.71

 -1.0 per mil carbonate phase

Chicot TS-60-45-114 25 Modern 0.18 0.06 -- 0.05 4.71

 1.0 per mil carbonate phase

Chicot TS-60-45-114 25 Modern 0.18 0.06 -- 0.05 4.71

 0.0 per mil carbonate phase

Evangeline TS-60-35-907 32 33,000 -- 0.26 -0.99 0.24 -1.41

 -1.0 per mil carbonate phase

Evangeline TS-60-35-907 32 33,000 -- 0.26 -0.99 0.24 -1.41

 1.0 per mil carbonate phase

Evangeline TS-60-35-907 32 33,000 -- 0.26 -0.99 0.24 -1.41

 0.0 per mil carbonate phase

Jasper TS-60-42-206 43 16,000 1.52 0.04 -0.81 0.85 --

 -1.0 per mil carbonate phase

Jasper TS-60-42-206 43 15,000 1.52 0.04 -0.81 0.85 --

 1.0 per mil carbonate phase

Jasper TS-60-42-206 43 16,000 1.52 0.04 -0.81 0.85 --

1

These are averages based on results from Ingerson and Pearson (1964), Fontes and Garnier (1979), and Eichinger (1983.)
26 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Environmental Tracer Data and 

Apparent Groundwater Age

Modern Environmental Tracers

Of the 40 samples collected March–September 2008, 

17 were from wells completed in the Chicot aquifer, 13 from 

wells completed in the Evangeline aquifer, and 7 from wells 

completed in the Jasper aquifer (table 1). Of the 37 samples 

collected for modern environmental tracers from wells 

completed in the Chicot, Evangeline or Jasper aquifer, water 

from 15 wells was datable by these modern environmental 

tracer methods (table 6) and were used for calculating recharge 

estimates presented later in this report. The remaining 25 

samples collected in 2008 were not datable because either the 

concentrations of the modern environmental tracers measured 

were less than the expected reasonable concentrations within 

the datable range (apparent age older than 1950s), or the 

apparent ages were unrealistic when compared to nearby 

better constrained estimates of apparent groundwater ages, 

and three wells initially identified as Evangeline aquifer were 

later determined to be in the Burkeville confining unit. The 

samples obtained from the seven wells completed in the Jasper 

aquifer system had detectable concentrations of modern age 

tracers, indicating the groundwater was theoretically datable; 

however, for six of the wells the groundwater was reportedly 

too old to be datable based on excess 4He and little to no 

detectable 3H in the samples, indicating there may have been 

a (1) low-level contamination of the equipment or sample, 

(2) poor well construction, (3) poor well development, or 

(4) any combination of these factors. Twenty wells from 

which groundwater samples were not datable by the modern 

environmental tracer methods in 2008 were resampled in 2011 

for 14C.

The hydrogeology of the study area needs to be taken 

into consideration when evaluating the apparent groundwater 

ages. There are numerous interbedded silt and clay layers 

(figs. 4 and 5) within the water-bearing sands of the Chicot 

and Evangeline aquifers. Multiple sand and clay layers that 

were relatively thin were identified by using the geophysical 

techniques described in the “Methods” section (fig. 5). In 

seven of the logged wells, clay lenses ranging from a few feet 

to tens of feet thick were identified in the unsaturated zone 

or near the land surface and might impede the downward 

movement of water from the surface to the aquifer. The 

borehole geophysical cross sections made from the few data 

points collected for this study show that on a local scale, 

groundwater flow paths, although complex, might have a 

larger lateral extent than previously discussed by Popkin 

(1971) and Kasmarek and others (2010). These interbedded 

clays create localized confinement in the Chicot aquifer which 

Table 6. Apparent groundwater age derived by using samples collected during March–September 2008 from wells completed in the 

Chicot, Evangeline or Jasper aquifer, and the environmental tracer used for apparent age determination, Montgomery County, Texas.

[4He, helium-4; 3He/3H, helium-3/tritium; CFC-12, dichlorofluoromethane; SF6, sulfur hexafluoride]

Map identifier 

(fig. 1). Station name Aquifer Sample date Sample time Apparent age (years) Tracer

1 TS-60-53-719 Chicot 3/12/2008 1415 73 3He/3H

2 TS-60-53-720 Chicot 3/13/2008 1410 27 CFC-12

4 TS-60-45-413 Chicot 3/19/2008 845 62 4He

5 TS-60-45-414 Chicot 3/19/2008 1215 23 3He/3H

7 TS-60-54-806 Chicot 3/24/2008 1200 71 3He/3H

10 TS-60-54-807 Chicot 3/26/2008 900 32 SF6

12 TS-60-44-806 Chicot 3/27/2008 1230 48 CFC-12

14 TS-60-26-208 Jasper 4/21/2008 1400 62 CFC-12

16 TS-60-34-301 Evangeline 4/22/2008 1330 88 4He

19 TS-60-63-110 Chicot 4/24/2008 1530 50 CFC-12

20 TS-60-37-309 Chicot 4/28/2008 1200 44 4He

21 TS-60-54-702 Chicot 4/30/2008 1030 32 3He/3H

22 TS-60-44-212 Evangeline 7/10/2008 1224 825 4He

25 TS-60-45-114 Chicot 7/18/2008 1135 35 3He/3H

30 TS-60-53-516 Evangeline 8/18/2008 1530 2,092 4He
Environmental Tracer Data and Apparent Groundwater Age 27

might mean the groundwater is older than the age range that 

can be determined with the environmental tracers used in this 

report. The Evangeline aquifer is considered a confined aquifer 

because of the difference in hydraulic head compared to the 

overlying Chicot aquifer. Where a confining unit separating 

the Chicot and Evangeline aquifers is absent, leakage or flow 

between the two aquifers is possible (Gabrysch and Coplin, 

1990; Kasmarek and Strom, 2002); however, even in the 

absence of a discrete confining unit separating the Chicot and 

Evangeline aquifers, the Evangeline aquifer is considered 

confined because of the effects of the many interbedded 

clays in the Chicot on hydraulic heads (Gabrysch, 1984). The 

heterogeneity of the aquifer is such that there are reduced 

vertical flows in the system caused by interbedded clay layers 

and considerable mixing and dispersion which may make 

application of the piston-flow assumption problematic in this 

system, as in many others (Rupert and Plummer, 2009).

 Most of the modern environmental tracer data indicate 

that the groundwater was recharged prior to the 1950s. 

Water of this age can be difficult to “date” with the modern 

environmental tracers used for this study in 2008. To date 

water samples by using 3He/3H, the samples generally need 

to contain at least 2 TU of 3H. Samples with 1 to 2 TU of 3H 

can be dated in favorable conditions, but samples with less 

than 1 TU are difficult to date even under the most favorable 

conditions (L. Niel Plummer, U.S. Geological Survey, written 

commun., 2009). On the basis of these criteria, samples 

obtained from three wells (TS-60-45-414, TS-60-54-702, and 

TS-60-45-114) with more than 1 TU of 3H are datable with 

the 3He/3H method. Samples obtained from two wells (TS-60-

54-806 and TS-60-63-110) with less than 1 TU appear datable 

based on other tracer data (CFC and or SF6) (table 7). Water 

collected from well TS-60-53-719 has less than 1 TU of 3H 

determined with the 3He ingrowth method and is considered 

datable at 73 years based on its 3He/3H data and on tracer 

data from nearby well TS-60-53-720 (table 6). Well TS-60-

53-720 is located about 15 ft from well TS-60-53-719 and its 

top screen is about 35 ft shallower. That well TS-60-53-720 

is shallower in depth and intercepts younger water than the 

deeper well TS-60-53-719 is consistent with the conceptual 

hydrogeologic model (younger water is shallower than older 

water). Samples with little to no 3H indicate premodern water 

(recharged before the 1950s) and thus would not be datable 

by either the CFCs or SF6 method. Samples with no 3H cannot 

be dated by using the 3He/3H method. Samples not datable by 

the 3He/3H method contained 3H ranging from 0 to 0.83 TU 

(Oden, 2011). 

The 3H in precipitation was measured in Waco, Tex., from 

1953 to about 1985 by the USGS, but there are no recent 3H 

precipitation data in the Texas region (International Atomic 

Energy Agency, 2010). The measured 3H concentrations in 

precipitation from Waco were computationally “decayed” 

for each year from end of 3H precipitation data collection in 

Waco to time of sampling (March–September 2008) to project 

expected concentrations in precipitation. This computed 

3

H decay series was compared to 3H in precipitation data 

collected in Georgia during 1986–2007 at about the same 

latitude. On the basis of this comparison, precipitation in 

Texas (1986–2007) would be expected to contain 3H ranging 

from 2 to 5 TU. Assuming the atmospheric background 

concentration of 3H (prebomb) was similar or lower than the 

modern values of about 2 to 5 TU, prebomb water would have 

a 3H concentration of about 0.3 TU or less if recovered from 

the aquifer (Robert L. Michel, U.S. Geological Survey, written 

commun., 2010).

In samples datable by the CFC, SF6, and 3He/3H methods, 

estimated age dates are generally within 5 years between the 

different tracers (table 7). The apparent groundwater ages in 

samples collected from two of five wells completed in the 

Chicot aquifer with apparent ages determined from more than 

one environmental tracer were more than 40 years. On the 

basis of other tracer data, only a minority of samples collected 

from wells completed in the Chicot or Evangeline aquifers 

represent groundwater young enough for SF6 to be detected. 

Table 7. Comparison of apparent ages determined from environmental tracers (helium-3/tritium, dichlorofluoromethane, and sulfur 

hexafluoride) for groundwater samples collected during March–September 2008 from wells completed in the Chicot aquifer in 

Montgomery County, Texas.

[3He/3H, helium-3/tritium; CFC-12, dichlorodifluoromethane; SF6, sulfur hexafluoride; --, not determined]

Map identifier

(figs. 1, 11) Station name Aquifer Sample date Sample time

Apparent 

3

He/3H age 

(years)

Apparent 

CFC-12 age 

(years)

Apparent 

SF6 age 

(years)

5 TS-60-45-414 Chicot 3/19/2008 1215 23 29 30

7 TS-60-54-806 Chicot 3/24/2008 1200 71 48 --

19 TS-60-63-110 Chicot 4/24/2008 1530 49 50 --

21 TS-60-54-702 Chicot 4/30/2008 1030 32 36 29

25 TS-60-45-114 Chicot 7/18/2008 1135 35 25 31
28 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

The detection of SF6 in all 40 samples obtained from wells 

sampled in 2008 (Oden, 2011) might indicate a terrigenic, 

naturally occurring source of SF6 (Busenberg and Plummer, 

2000). An admixture of a small fraction of “young” (less 

than 60 years old) water at the top of extended well screen 

intervals is another possible explanation for the detection 

of SF6 in all samples. Lastly, because blank samples are not 

collected and analyzed for any of the tracers, the possibility of 

slight sample contamination cannot be completely ruled out.

As with SF6, the CFC-11 and CFC-12 were detected in 

all samples and CFC-113 was detected in 33 samples (Oden, 

2011). Given the apparent ages of groundwater which were 

mostly more than about 60 years (recharged prior to 1950), 

the widespread detection of CFCs was not expected. By using 

multiple tracers, however, samples from four wells (TS-60-

45-414, TS-60-63-110, TS-60-54-702, and TS-60-45-114) 

completed in the Chicot aquifer yield apparent ages less 

than about 60 years (table 7), and the CFC-12 compound can 

be usable for groundwater of this age. The relatively small 

SF6 and CFC concentrations measured even in samples of 

groundwater recharged less than about 60 years ago might 

be artifacts of well installation and development (Busenberg 

and Plummer, 2010), poor annular seals (U.S. Environmental 

Protection Agency, 1994), or a combination of these factors. 

Additionally, there may be nearby wells with multiple 

screened intervals in the aquifer(s), allowing for leakage of 

younger water to the older reservoirs of water and providing a 

conduit for cross-formational flow. Cross-formational flow or 

short-circuiting pathways are well known from karst aquifers 

such as limestones in Florida (Katz and others, 2007) but 

have also been recently documented in sands and clays of the 

High Plains (Landon and others, 2008; Landon and others, 

2010) where heavy drawdown exists in a region in which 

wells penetrate and have open intervals to multiple aquifer 

units. Other possibilities are mixing, dispersion, preferential 

flow paths, or that the initial assumption of the piston-flow 

model is overly simplified for these aquifers.

With the lack of datable waters for the study, 4He was 

used in cases where other tracers were not viable (appendix 

5). Excess 4He was detected in samples from most wells 

(Oden, 2011). The helium accumulation rate for the area is an 

important variable in the 4He age calculations, and multiple 

independent samples measured for 4He concentrations are 

needed to determine the helium accumulation rate in a 

regional aquifer (L. Niel Plummer, U.S. Geological Survey, 

oral commun., 2010). With samples from only five wells 

datable by multiple modern tracer methods used in this 

report (table 7), the 4He accumulation rate could not be 

reliably estimated for the Chicot and Evangeline aquifers 

for this study. A helium accumulation rate of 1.9 x 10-10

cubic centimeters at standard temperature and pressure, per 

gram of water (cm3STP/g), which is similar to the rate in a 

previous study by Busenberg and Plummer (2000). In that 

study, Busenberg and Plummer (2000) applied the helium 

accumulation rate measured in the Delmarva Peninsula, a 

sand aquifer in the North Atlantic Coastal Plain (NACP), 

which is of similar composition to the Gulf Coast aquifer 

system (Trapp and Horn, 1997). A considerable difference, 

however, is that the NACP lacks both the occasional high 

levels of uranium enrichment found in the Texas Gulf Coast 

deposits and the somewhat U-rich volcanic ash layer interbeds 

(Kier and others, 1977; Cook, 1980). The use of this estimated 

accumulation rate is reasonable, as a first approximation, but 

may need to be revisited as more 4He concentration data and 

datable waters become available.

 In a few cases where multiple wells screened at 

different depths and aquifers were available in the same 

location, older waters occurred above younger waters. 

Such an observation cannot be explained by the piston-flow 

conceptual model. An age reversal with depth could reflect 

the effects of aquifer heterogeneity on groundwater flow 

paths or cross-formation flow through wellbores, which 

may account for this condition (Katz and others, 2007; 

Zinn and Konikow, 2007; Landon and others, 2008) as well 

as occurrence of preferential flow paths documented by 

Landon and others (2010). A detailed investigation into the 

occurrence of older waters above younger waters is beyond 

the scope of this report.

Modeling of Carbon-14 with NETPATH

NETPATH (Plummer and others, 1994) was used to 

calculate the 14C ages of the groundwater samples, calcite 

saturation indices of groundwater samples, and mixing of two 

potential sources of groundwater.

Groundwater geochemical analytical well results for 14C, 13C, bicarbonate, and selected major ions were used in 

the geochemical model NETPATH to evaluate net flow path 

reactions to improve estimates for apparent groundwater ages 

by using model results from Ingerson and Pearson (1964), 

Fontes and Garnier (1979), and Eichinger (1983). 

For 20 wells with samples not datable by modern 

environmental tracers in 2008, wells were resampled in 2011 

for 14C as an attempt to determine the age of the groundwater 

and estimate the rate of groundwater recharge to those 

wells. Apparent groundwater ages analyzed from water from 

selected wells screened into the Chicot, Evangeline, and 

Jasper aquifers in Montgomery County reflected the influence 

that thick accretionary wedges of interbedded sand and clay 

layers have on groundwater movement and residence times. 

Generally analysis for observed groundwater 14C activity is 

measured from wells developed into the aquifer by drawing 

water from different depths within the aquifer. Consequently 

the observed 14C activity in pmc may have represented a 

mixture of young and old groundwater withdrawn from 

unconfined and confined segments within the Chicot and 

Evangeline aquifers in Montgomery County. Adjusted 14C 

ages from water from wells developed into the Jasper aquifer 

were all greater than or equal to 13 thousand (ka) indicating 

relatively long groundwater residence times and that mixing 

does not have much effect on these wells. 
Estimated Rates of Groundwater Recharge 29

NETPATH-adjusted groundwater ages identified 

potential areas where local recharge may be occurring within 

the Chicot aquifer under water table conditions in contrast to 

areas where only slight amounts of recharge may be occurring 

and the groundwater may be under semiconfined conditions 

(fig. 9). Adjusted groundwater ages for wells (TS-60-45-114, 

well 25; TS-60-45-513, well 15; and TS-60-54-805, well 

6) yielded “modern” water with observed 14C activities of 

greater than 80 pmc (table 4). The adjusted “modern” age 

suggested that some recharge may be occurring possibly 

from the nearby West Fork of the San Jacinto River, other 

localized sources, or both (fig. 9). Adjusted groundwater 

ages for wells TS-60-51-811 (well 37), TS-60-51-814 (well 

38), and TS-60-51-809 (well 36) yielded slightly older water 

ages suggesting shallow groundwater may be mixing with 

deeper older waters. The adjusted age for well TS-60-51-814 

is 5 ka years, which is significantly older than apparent ages 

for wells TS-60-51-811 and TS-60-51-809, 900 and 650 years 

old, respectively (fig. 9), suggesting there may be a localized 

interbedded clay layer potentially creating semiconfined 

conditions near TS-60-51-814 or that pumping may be causing 

older deeper water to mix with the younger shallow water.

NETPATH-adjusted groundwater ages from wells 

developed into the Evangeline aquifer does not appear to 

move along a simple progression in age downgradient; 

however, localized mixing of water in the Evangeline aquifer 

with water from the overlying Chicot aquifer or possibly 

direct recharge from the land surface into the Evangeline 

aquifer was indicated by the considerable groundwater age 

variability. The possible effect of preferential flow paths 

through zones of varying hydraulic conductivity among the 

interbedded sand and clay layers was indicated. Groundwater 

from downgradient wells TS-60-53-516 (well 30) and TS-60-

55-710 (well 23) yielded adjusted apparent ages of “modern” 

and 2.7 ka respectively that represented a considerable 

decrease in apparent age from the upgradient wells TS-60-

35-907 (well 32), TS-60-37-909 (well 34), and TS-60-45-716 

(well 28) of 33, 8, and 26 ka, respectively. Wells TS-60-53-

516 and TS-60-55-710 are located near the West Fork of the 

San Jacinto River suggesting a relatively short residence 

time that would be consistent with local recharge and mixing 

of younger groundwater from the overlying Chicot aquifer. 

Groundwater from well TS-60-43-511 (well 18) yielded an 

adjusted age of 900 years before present, decreasing in age 

from upgradient wells TS-60-35-907 and TS-60-35-503 

(well 11), 33 and 1.2 ka, respectively (fig. 9). The decrease 

in the adjusted age at well site TS-60-43-511 suggested that 

the aquifer lithology is permeable in this area with recharge 

entering directly from the land surface or that there is mixing 

of young water from the overlying Chicot aquifer which may 

be induced from pumping. Well TS-60-43-511 is located in 

the general area of the boundary between the unconfined 

and confined portion of the Evangeline and Chicot aquifers, 

where mixing with waters from the Chicot aquifer is likely. 

Modeled 13C values for the observed and calculated values 

for TS-60-43-511 were comparable only when Chicot 

aquifer water from well TS-60-51-814 was used to adjust 

14C ages (table 4). 

NETPATH-adjusted groundwater ages from wells 

developed into the Jasper aquifer generally showed a simple 

progression in age as groundwater moved downgradient along 

the flow path (fig. 9). The Jasper aquifer is under confined 

conditions beneath the Burkeville confining unit, prohibiting 

any mixing from the overlying Evangeline aquifer. The 

confining unit for the Jasper aquifer is thick, continuous, 

and expansive in contrast to the Evangeline aquifer where 

confinement may be poor. Adjusted groundwater ages 

progressing from 13 to greater than 42 ka indicated that 

generally the groundwater in the Jasper aquifer is older, 

deep groundwater compared to the Evangeline aquifer. In 

addition, the density of producing wells with open screens to 

multiple zones yielding large volumes of water in the Jasper 

aquifer was smaller than that in the Evangeline, which is the 

most used aquifer among these two; therefore, the potential 

development of preferential flow paths by overstressing 

the aquifer is somewhat less likely in the Jasper than 

the Evangeline aquifer.

Estimated Rates of Groundwater 

Recharge

Recharge is possibly one of the most difficult parameters 

of groundwater to quantify because it cannot be measured 

directly and therefore has to be estimated by using multiple 

methods (Delin and Falteisek, 2007). Several methods of 

estimating recharge, including the use of the concentration of 

environmental tracers emphasized in the study, are available 

and each has advantages, disadvantages, and limitations 

(Scanlon, Healy, and Cook, 2002). Groundwater age 

distributions with vertical groundwater velocities can provide 

reasonable estimates of recharge (Solomon and Sudicky, 

1991). The interpretation of environmental tracer data can 

be complicated by multiple factors that affect concentrations 

along groundwater flow paths, such as degradation, sorption, 

mixing, and transport through the unsaturated zone (Solomon 

and Cook, 2000). These factors cannot be readily avoided 

when planning a sampling program as they are often local 

in nature and no simple surrogate variable can be used as an 

indicator that might show where the effects are greatest.

To estimate the groundwater recharge rate for the Chicot, 

Evangeline, and Jasper aquifers by using the environmental 

tracers, the specific hydrogeologic properties of the aquifer 

(eq. 2 and 3) needed to be evaluated as indicated in a 

conceptual model of idealized flow through an unconsolidated 

aquifer (fig. 10). Water enters the aquifer (R) and moves 

along a flow path to a depth (z). If the depth (open interval 

of the well) and the apparent age of the water from that open 

interval (t) are known along with other properties, a recharge 

rate can be estimated. Recharge estimates in the unconfined 
30 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Bayou

15

17 (>42,000)

23

24 (13,000)

27(27,000)

28

31

34

38 (5,000)

39 (42,000)

40 (32,000)

42 (19,000)

43 (16,000)

44

6 (Modern)

32 (33,000)

11 (1,200)

18 (900)

25

30

41

36 (650)

37 (900)

•

•

•

•

•

•

•

•

•

•

•

•

•

•

• •

••

•

•

•

• •

•

3 (ND)

43 (16,000)

38 (5,000)

23 (2,700)

•

•

•

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

Well developed into an aquifer 

—Map identifier and adjusted 

carbon-14 age in years before 

present

Chicot aquifer

Evangeline aquifer

Jasper aquifer 

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K 

COUNTY

HA

R R I S COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES C

OUNTY

SA

N JACINTO COU

NTY

W

ALKER COU

NTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

Aquifer data from Strom and others, 2003 a,b,c

and Kasmarek and Robinson, 2004

TEXAS

Study area

Location map

(18,000)

(37,000) (8,000)

(42,000)

(2,700)

(Modern)

(Modern)

(Modern)

(26,000)

(see tables 1, 8, 9, and 10)

Figure 9. Adjusted carbon-14 ages from wells developed into the Chicot, Evangeline, and Jasper aquifers in Montgomery, Waller, and 

Walker Counties, Texas.
Estimated Rates of Groundwater Recharge 31

A

B

Flow

path

x x1

(C)

z

H

R

Flow path

xo

x

(A)

z

H

R

R Recharge;

H Thickness of the unconfined Chicot aquifer or unconfined zone of 

the Evangeline or Jasper aquifer, in feet (A) or thickness of the 

confined zone of the Evangeline or Jasper aquifer, in feet (B);

z Depth of sampling point within the aquifer, in feet.

x Width of the aquifer outcrop, in feet.

xo Distance from well to beginning of flow path

x' Distance from the well to the outcrop of the aquifer, in feet.

EXPLANATION

Figure 10. Conceptual model of idealized flow through an unconsolidated aquifer, modified from Cook and Böhlke (2000).
32 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Chicot aquifer and unconfined zone of the Evangeline and 

Jasper aquifers were determined by using the following 

equation from Cook and Böhlke (2000):

(2)

where:

R is the amount of recharge, in inches per year;

H is the thickness of the unconfined Chicot aquifer or 

unconfined zone of the Evangeline or Jasper aquifer, in feet;

ε is porosity, in percent;

t is the apparent groundwater age, in years;

ln is the natural logarithm; and

z is the depth of sampling point within the aquifer, in feet.

In the confined zone of the Evangeline and Jasper aquifer, 

ages using 4He accumulation or 14C were used to estimate 

recharge rates for the wells that were sampled. The following 

equation (Cook and Böhlke, 2000) was used to estimate 

recharge (R) in the confined zone of the Evangeline aquifer:

 ε ε R z

t

x H

tx

'

= + 

(3)

where:

H is the thickness of the confined zone of the 

Evangeline or Jasper aquifer, in feet;

xʹ is the distance from the well to the outcrop of the 

aquifer, in feet; and

x is the width of the aquifer outcrop, in feet and 

other variables were defined for equation 2.

Aquifer Properties for Estimation of 

Groundwater Recharge Rates

Porosity

Porosity is defined as the percentage of rock or sediments 

that are void of solid material. Porosity of the unconsolidated 

sediments is affected by the sediment texture (sand, silt, 

clay, and gravel), size distribution, and spatial distribution 

relative to the depositional environments (for example, 

channel bars, delta forests, and flood plains). Because of the 

heterogeneity present in the Chicot and Evangeline aquifers 

in Montgomery County, and the lack of recent empirical 

R H

t

H

H z = −

ε ln

data, a porosity of 23 percent (Noble and others, 1996) was 

used for the calculation of recharge rates in this report. For 

the Jasper aquifer, a porosity of 25 percent was used on the 

basis of information from The University of Texas Bureau of 

Economic Geology (University of Texas, 2012). It is important 

to note that porosity is linearly related to calculated recharge 

rate; consequently, estimated recharge is highly sensitive to 

estimated porosity.

Aquifer Thickness

At each well, the thicknesses of the unconfined Chicot 

aquifer, the unconfined zone and confined zone of the 

Evangeline aquifer, or the unconfined zone and confined zone 

of the Jasper aquifer at each well were determined by using 

aquifer altitudes as described in Strom and others (2003 a, b, 

c). The tops of the Chicot, Evangeline, and Jasper aquifers 

in the outcrop area were estimated by using land-surface 

altitudes (NAVD 88). During this process, it was determined 

on the basis of work done by Strom and others (2003b) that 

three wells (TS-60-35-503, TS-60-35-504, and TS-60-35-505) 

assigned the Evangeline aquifer designation in the USGS 

NWIS GWSI, were more likely screened in the Burkeville 

confining unit. The uncertainty of the aquifer designation 

could not be adequately resolved; therefore, these three wells 

were not considered for recharge estimates of the Evangeline 

aquifer in this report. Two wells stored in GWSI were coded 

as Evangeline but based on Strom and others (2003a, b, c) 

were determined to be in the Chicot (TS-60-37-309) and 

Jasper (TS-60-35-907) aquifers, respectively, and were treated 

as being screened in these respective aquifers.

Distance Measurement

In addition to the porosity and thickness of the aquifer, 

the distance from the well to the outcrop of the aquifer (x') 

and width of the aquifer outcrop (x) are required to calculate 

recharge rates in a confined aquifer as described previously 

(eq. 1 and 2, fig. 10). An approximation of xʹ was done by 

measuring along the estimated flow path from the well to the 

outcrop area. An approximation of x was done by measuring 

from the edge of the outcrop to the terminus of the aquifer, 

generally along a straight line northwest to southeast. 

Approximations of xʹ and x were made by using geographic 

information system (GIS) software.
Estimated Rates of Groundwater Recharge 33

Recharge Estimates for the Unconfined Chicot, 

Evangeline, and Jasper Aquifers

In the unconfined aquifer system, depth in the aquifer 

was calculated as the top of the screened interval below land 

surface datum minus the water level below land surface 

datum at the time of sampling. Three water levels were 

not obtained at the time of sampling in 2008 (TS-60-63-

110, TS-60-54-702, and TS-60-45-114), so water level was 

estimated from the regional water-level altitudes in Kasmarek 

and Houston (2008), and two water levels were not obtained 

at time of sampling in 2011 (TS-60-51-811 and TS-60-51-814) 

and were estimated from the regional water-level altitudes 

in Johnson and others (2011). One well (TS-60-34-301) was 

outside the mapped area of the regional water-level altitudes 

determined in 2008; based on water-level measurements by 

USGS within 4 miles (mi) of the well, water levels generally 

have varied a few feet, so the previously recorded water 

level was used (Oden, 2011). The depth in the confined 

aquifer was considered to be the altitude of the top of the 

Evangeline or Jasper aquifer minus the altitude of the top of 

the screened interval.

By using the recharge estimates derived from samples 

collected from 14 wells completed in the Chicot aquifer 

for which apparent groundwater ages could be determined, 

recharge to the Chicot aquifer ranged from 0.2 to 7.2 in./yr 

(table 8 and fig. 11). Recharge to the unconfined zone of the 

Evangeline aquifer from 1 well was 0.1 in./yr (table 9 and fig. 

12). Recharge to the unconfined zone of the Jasper aquifer 

from 1 well was 0.5 in/yr (table 10 and fig. 13).

Recharge Estimates for the Confined Zones of 

the Evangeline and Jasper Aquifers

On the basis of data collected from eight wells, estimated 

rates of recharge to the confined zone of the Evangeline 

aquifer ranged from <0.1 to 2.8 in./yr (table 9 and fig. 12). 

On the basis of data collected from 10 wells, estimated rates 

of recharge to the confined zone of the Jasper aquifer ranged 

from <0.1 to 0.1 in./yr (table 10 and fig. 13).

Uncertainty Analysis of the Recharge Rates

The uncertainty of the recharge estimates concerning 

changes in the input data (aquifer thickness, porosity, age, 

distance from outcrop, and width of outcrop) were evaluated 

to assess the effects that changing input variables may have 

had on the estimates. The values of the input data were varied 

within ranges deemed plausible for the Chicot, Evangeline, 

and Jasper aquifers in Montgomery County. For example, 

because aquifer thickness values from the literature are 

unlikely to be off by more than plus or minus 50 ft and 

porosity likely could range from about 20 to 30 percent 

for these aquifers, groundwater ages might vary by plus 

or minus 10 percent. 

The wells completed in the Chicot aquifer showed the 

most variability in the modification of input variables. The 

calculated recharge rates were most sensitive to changes in 

age and porosity, whereas they were least sensitive to changes 

in aquifer thickness. Decreasing porosity to 20 percent and 

keeping other input variables constant generally decreased 

the recharge rate at each site, whereas increasing the porosity 

to 30 percent increased the recharge rate. Using 90 percent 

of the selected modeled age (younger water) showed an 

increase in the recharge rate at wells completed in the Chicot 

aquifer, whereas 110 percent of the age showed a decrease 

in the recharge rate for wells completed in the Chicot 

aquifer (fig. 14).

When the same input variables (aquifer thickness, 

porosity, and groundwater age) were modified, the wells 

completed in the outcrop area of the Evangeline and Jasper 

aquifers showed less variability in the recharge rate estimates 

as compared to the Chicot aquifer, although the same general 

trends were observed in the variability of recharge rates for the 

Evangeline and Jasper aquifers (fig. 14).

In the confined parts of the Evangeline and Jasper 

aquifers, the thickness of the aquifer (adjusted plus or minus 

[+/-] 50 ft) showed changes in recharge rates that were 

generally in the few thousandths of an inch range. Although 

the same conclusion (few thousandths to few hundredths) 

tended to hold true for adjusting the well distance to the 

outcrop (which were varied +/- 1 mi) and width of the outcrop 

(varied +/- 1 mi), the effect on recharge was generally to a 

few hundredths. Porosity and age tended to have more effect 

on the estimated recharge rates for each well; however, the 

effects were not as pronounced as in the unconfined system. 

As with the Chicot aquifer, decreasing porosity to 20 percent 

decreased the estimated recharge rate in the Evangeline and 

Jasper aquifers, whereas increasing the porosity to 30 percent 

showed an increase in the estimated recharge rate (figs. 14 and 

15). In the confined system, decreasing the age of the water 

(10 percent for wells less than 10,000 years and decreasing 

5,000 years for well greater than 10,000 years) and increasing 

porosity showed a small increase in the estimated recharge 

rate (figs. 14 and 15). 
34 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Table 8. Apparent groundwater ages and recharge estimates derived by using samples collected during March–September 2008 and 

April–May 2011 from wells completed in the Chicot aquifer in Montgomery County, Texas. 

[in./yr, inches per year; 4He, helium-4; 3He/3H, helium-3/tritium; CFC-12, dichlorofluoromethane; SF6, sulfur hexafluoride; 14C, carbon-14; ND, not determined; 

NA, not applicable]

Map identifier 

(figs. 1, 11) Station name Sample date Sample time Recharge(in./yr)

Apparent age

(years) Tracer

1 TS-60-53-719 3/12/2008 1415 3.7 73 3He/3H

2 TS-60-53-720 3/13/2008 1410 4.8 27 CFC-12

3 1TS-60-44-805 3/17/2008 1300 ND ND NA

4 TS-60-45-413 3/19/2008 845 3.8 62 4He

5 TS-60-45-414 3/19/2008 1215 2.3 23 3He/3H

6 2TS-60-54-805 3/21/2008 1315 ND ND NA

7 TS-60-54-806 3/24/2008 1200 2.7 71 3He/3H

10 TS-60-54-807 3/26/2008 900 2.1 32 SF6

12 TS-60-44-806 3/27/2008 1230 1.0 48 CFC-12

15 2TS-60-45-513 4/22/2008 930 ND ND NA

19 TS-60-63-110 4/24/2008 1530 2.4 50 CFC-12

20 TS-60-37-309 4/28/2008 1200 1.9 44 4He

21 TS-60-54-702 4/30/2008 1030 3.4 32 3He/3H

25 TS-60-45-114 7/18/2008 1135 7.2 35 3He/3H

36 TS-60-51-809 4/18/2011 1025 0.9 650 14C

37 TS-60-51-811 4/18/2011 1100 0.4 900 14C

38 TS-60-51-814 4/18/2011 920 0.2 5,000 14C

1

Poor well construction, well not datable.

2

Well not datable, too old for modern tracer, not old enough for 14C.
Estimated Rates of Groundwater Recharge 35

Bayou

1, 3.7

2, 4.8

3, ND

4, 3.8

5, 2.3

6, ND

7, 2.7 10, 2.1

12, 1.0

15, ND

19, 2.4

20, 1.9

21, 3.4

25, 7.2

36, 0.9

37, 0.4

38, 0.2

Figure 11.

 

•

•

•

•

•

•

•

•

•

•

••

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K 

COUNTY

HA

R R I S COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES COUNTY

SA

N JACINTO COUNTY

W

ALKER COUNTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

Aquifer data from Strom and others, 2003 a, b, c,

and Kasmarek and Robinson, 2004

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

Chicot aquifer monitor well

—Map identifier and recharge 

rate, in inches per year (ND—Not Determined)

(see tables 1 and 8)

• 19, 2.4

0 10 20 MILES

0 10 20 KILOMETERS

Figure 11. Recharge estimates at wells completed in the Chicot aquifer in Montgomery County, Texas, during March–September 2008 

and April–May 2011.
36 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Table 9. Apparent groundwater ages and recharge estimates derived by using samples collected during March–September 2008 and 

April–May 2011 from wells completed in the Evangeline aquifer in Montgomery County, Texas.

[in./yr, inches per year; CFC-12, dichlorofluoromethane; 4He, helium-4; 14C, carbon-14; ND, not determined; NA, not applicable; <, less than]

Map identifier

(figs. 1, 12) Station name Sample date Sample time Recharge(in./yr)

Apparent age 

(years) Tracer

Well 

unconfined

or confined

16 TS-60-34-301 4/22/2008 1330 0.1 52 CFC-12 Unconfined

18 TS-60-43-511 4/20/2011 1140 1.0 900 14C Confined

22 TS-60-44-212 7/10/2008 1224 1.5 825 4He Confined

23 TS-60-55-710 5/3/2011 1640 2.8 2,700 14C Confined

26 1TS-60-53-210 7/21/2008 935 ND ND NA Confined

28 TS-60-45-716 4/26/2011 1200 0.1 26,000 14C Confined

29 2TS-60-42-902 8/1/2008 1400 ND ND NA Confined

30 TS-60-53-516 8/18/2008 1530 2.4 2,092 4He Confined

33 2TS-60-37-806 9/3/2008 1245 ND ND NA Confined

34 TS-60-37-909 4/25/2011 1100 0.2 8,000 14C Confined

35 2TS-60-59-102 9/4/2008 915 ND ND NA Confined

39 3TS-60-52-199 4/21/2011 1125 <0.1 42,000 14C Confined

40 3TS-60-51-599 4/21/2011 945 0.1 32,000 14C Confined

1

Well sampling point reconfigured by owner, unable to resample in 2011.

2

Unable to obtain permission to resample in 2011.

3

Final State well number assignment pending Texas Water Development Board.
Estimated Rates of Groundwater Recharge 37

Bayou

•

• •

•

•

• •

•

• •

•

•

•

16, 0.1

18, 1.0

22, 1.5

23, 2.8

26, ND

28, 0.1

29, ND

30, 2.4

33, ND

34, 0.2

35, ND

39, <.1

40, 0.1

Figure 12.

 

 

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

Evangeline aquifer monitor well

—Map identifier and recharge 

rate, in inches per year (ND—Not Determined)

(see tables 1 and 9)

• 30, 2.4

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K 

COUNTY

HA

R R I S COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES COUNTY

SA

N JACINTO COUNTY

W

ALKER COUNTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

Aquifer data from Strom and others, 2003 a, b, c,

and Kasmarek and Robinson, 2004

0 10 20 MILES

0 10 20 KILOMETERS

Figure 12. Recharge estimates at wells completed in the Evangeline aquifer in Montgomery County, Texas, during March–September 

2008 and April–May 2011.
38 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Table 10. Apparent groundwater ages and recharge estimates derived by using samples collected during March–September 2008 and 

April–May 2011 from wells completed in the Jasper aquifer in Montgomery, Walker, and Waller County, Texas.

[in./yr, inches per year; <, less than; >, greater than; CFC-12, dichlorofluoromethane; 14C, carbon-14; ND, not determined; NA, not applicable]

Map identifier

(figs. 1, 13) Station name Sample date Sample time Recharge(in./yr)

Apparent age 

(years) Tracer

Well 

unconfined 

or confined

13 1TS-60-26-901 4/21/2008 1030 ND ND NA Unconfined

14 TS-60-26-208 4/21/2008 1400 0.5 62 CFC-12 Unconfined

17 YW-59-64-206 4/27/2011 1100 <0.1 >42,000 14C Confined

24 YU-60-28-802 5/9/2011 1310 <0.1 13,000 14C Confined

27 TS-60-43-514 4/20/2011 1030 0.1 27,000 14C Confined

31 TS-60-36-410 4/15/2011 1000 0.1 18,000 14C Confined

32 TS-60-35-907 4/28/2011 1050 <.1 33,000 14C Confined

41 TS-60-55-313 5/11/2011 912 0.1 42,000 14C Confined

42 TS-60-35-703 5/12/2011 925 0.1 19,000 14C Confined

43 TS-60-42-206 5/12/2011 1040 0.1 16,000 14C Confined

44 TS-60-36-812 5/13/2011 940 0.1 37,000 14C Confined

1

Unable to obtain permission to resample in 2011
Estimated Rates of Groundwater Recharge 39

Bayou

•

•

•

•

•

•

•

•

• •

•

13, ND

14, 0.5

17, <.1

24, <.1

27, 0.1

31, 0.1

32, <.1

41, 0.1

42, 0.1

43, 0.1

44, 0.1

Figure 13.

 

 

EXPLANATION

Chicot aquifer 

Evangeline aquifer

Burkeville confining unit

Jasper aquifer

Jasper aquifer monitor well

—Map identifier and recharge 

rate, in inches per year (ND—Not Determined)

(see tables 1 and 10)

• 44, 0.1

£¤59

150 30

90

6

§ ̈¦45

156

§ ̈¦45

£¤190 75

Humble

Spring

Conroe

Huntsville

Lake

Creek

Lake Creek

Caney Creek

Spring Creek

Cypress Creek

West Fork

San

Jacinto

River

River

Peach

Bayou

 River

Winters

East

Fork

San

Jacinto

Luce

Creek

LAKE

HOUSTON

LAKE

CONROE

LAKE

LIVINGSTON

PO L K COUNTY

HA

R R I S COUNTY

A US T I N

COUNTY

WA S HI N G T ON

COUNTY

BR A Z O S

COUNTY

Magnolia The Woodlands

MONTGOMERY

COUNTY

WALLER COUNTY

GR

MI ES COUNTY

SA

N JACINTO COUNTY

W

ALKER COUNTY

19

Caney Creek

£¤290

Brazos

Houston

MADISON COUNTY

Trinity

River

GIBBONS CREEK 

RESERVOIR

105

LIBERTY

COUNTY

96°00' 95°45' 95°30' 95°15'

30°45'

30°30'

30°15'

30°00'

Base from U.S. Geological Survey 1:24,000-scale digital data

Albers Equal Area Projection, Texas Mapping System 

North American Datum of 1983

Aquifer data from Strom and others, 2003 a, b, c

and Kasmarek and Robinson, 2004

0 10 20 MILES

0 10 20 KILOMETERS

Figure 13. Recharge estimates at wells completed in the Jasper aquifer in Montgomery, Walker, and Waller County, Texas, during 

March–September 2008 and April–May 2011.
40 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Estimated recharge rate, in inches per year

Apparent groundwater age

Groundwater age + 10%

Groundwater age - 10%

Porosity = 0.20

Porosity = 0.25 

Porosity = 0.30

Aquifer thickness + 50ft

Aquifer thickness - 50ft

Final modeled recharge rate

Chicot aquifer

Evangeline aquifer

Jasper aquifer

EXPLANATION

Sampled wells (map identifier)

Apparent groundwater age, in years

10

100

1,000

10,000

100,000

0

2

4

6

8

10

1 2 4 5 7 10 12 19 20 21 25 36 37 38 16 14

1 2 4 5 7 10 12 19 20 21 25 36 37 38 16 14

Figure 14. Uncertainty analysis of recharge rates for unconfined wells screened in the Chicot, Evangeline, and Jasper aquifers.
Estimated Rates of Groundwater Recharge 41

10

100

1,000

10,000

100,000

0

1

2

3

4

17

17

18

18

22

22

23

23

24

24

27

27

28

28

30

30

31

31

34

34

39

39

40

40

41

41

42

42

43

43

44

44

32

32

Apparent groundwater age

Groundwater age + 10% or 5,000 years

 for ages > 10,000 years

Groundwater age - 10% or 5,000 years

 for ages >10,000 years

Porosity = 0.20

Porosity = 0.25 

Porosity = 0.30

Aquifer thickness + 50ft

Aquifer thickness - 50ft

Final modeled recharge rate

Evangeline aquifer

Jasper aquifer

EXPLANATION

Estimated recharge rate, in inches per year Apparent groundwater age, in years

Sampled wells (map identifier)

Figure 15. Uncertainty analysis of recharge rates for confined wells screened in the Evangeline and Jasper aquifers.
42 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Limitations of Estimating Recharge Rates

Estimation of groundwater ages and recharge rates 

requires assumptions of the hydrogeologic properties of the 

aquifer system, resulting in uncertainty in these apparentage and recharge estimates. Despite the complexity of the 

aquifer system in Montgomery County, the estimates of 

groundwater ages and recharge rates are appropriate for use 

as a general guide in hydrologic investigations. For example, 

these estimates can be used for further investigation into the 

availability of the groundwater resources in Montgomery 

County and can be used as input parameters in groundwaterflow models in the area. Recharge estimates from this study 

are within the range of expected recharge values estimated 

by other methods within and near the study area (tables 

11 and 12). The following limitations of these results 

should be considered:

1. The hydrogeology in the study area is highly variable on a regional scale and may exhibit localized 

groundwater flow paths contrary to the regional flow 

pattern. Localized variations in sand thickness and 

spatial continuity might allow for preferential flow 

paths within the aquifer and decrease the apparent 

age of the water, consequently increasing the calculated recharge rate. If the apparent decrease in age 

is simply a result of diverted flow or recharge from 

the surface or somewhere else in a focused manner 

to the well point, then the focused flow is capturing 

additional recharge, but the overall recharge to the 

aquifer may not experience an actual increase on 

the regional scale. Understanding the localized flow 

paths and preferential flow paths is thus important.

2. Piston flow in each of the aquifers is assumed for 

the conceptual model of this system (fig. 10). This 

assumption may over simplify the flow system in 

areas within the study area where hydrologic connection with overlying aquifers is prevalent.

3. The calculation of recharge for confined and unconfined aquifers requires an estimation of the porosity. 

Although the assumed final average porosity value 

along the flow path used to calculate the recharge 

rates presented within this report is within the range 

of expected porosity for this aquifer material, localized variation in sand and clay percentages and in 

sand grain size could affect the apparent age of the 

water. A decrease in porosity estimation will lower 

the estimated recharge rate, whereas an increase 

in porosity estimation will raise the estimated 

recharge rate.

4. Concentrations of the modern environmental tracer 

were often at or near analytical detection limits, and 

such low concentrations can result in relatively large 

age-estimate errors.

5. The possibility exists for the mixing of waters of 

different composition and age through vertical connection or along preferential pathways. Mixing of 

groundwater may occur over long flow paths as well 

as long screened intervals in some of the wells used 

for this study. Dispersion may also affect environmental tracer concentrations given the heterogeneity 

of the aquifers.

6. The estimated recharge rates calculated for this study 

are specific to each well location, and any individual 

well estimate should not be extrapolated or inferred 

as a countywide average. Local variations in the 

hydrogeology and surficial conditions can affect the 

recharge rate at a local scale. 

7. The interpretation of environmental tracer data can 

be complicated as tracers move along individual 

groundwater flow paths. The complications resulting from additional independent variables can affect 

the tracer concentrations such as degassing, contamination, dispersion, sorption, transport through thick 

unsaturated zones, and aquifer-water interactions, 

including input of excess 4He.
Estimated Rates of Groundwater Recharge 43

Table 11. Recharge methods and estimates of annual recharge rates previously determined (Nolan and others, 2007, table A2) for four 

wells completed in the Chicot aquifer in Montgomery County, Texas.

[in./yr, inches per year; --, not determined]

Station 

identifier

Sample 

identifier Aquifer

Darcianpedotransfer 

method

(in./yr)

Chloride tracer in 

unsaturated zone

(in./yr)

Chloride tracer in 

saturated zone

(in./yr)

300825095274801 MW14 Chicot 4.80 0.24 0.04

301008095302901 MW13 Chicot -- 56.22 0.24

301220095305501 MW15 Chicot 2.87 0.51 0.43

301716095400501 REF03 Chicot 0.04 1.85 2.13

Table 12. Comparison of recharge rates determined in Montgomery County during during March–September 2008 and April–May 2011 

by using environmental age tracers with recharge rates from previous studies that used various methods in the Gulf Coast aquifer system 

in Montgomery County and adjacent counties in Texas.

[in./yr, inches per year; <, less than; 4He, helium-4; 3He/3H, helium-3/tritium; CFC-12, dichlorodifluoromethane; SF6, sulfur hexafluoride; 14C, carbon-14; 3H, 

tritium; Cl, chloride]

Sample 

collection dates Counties Recharge rate (in./yr) Method

March–September 2008

April–May 2011

Montgomery <0.1–7.2 Environmental age tracers (3He/3H, CFC-12, SF6, 4He and 14C)

Previous studies Counties Recharge rate 

(in./yr) Method

Popkin (1971) Montgomery 1.7 Transmission capacity

Ryder (1988) Montgomery 0–2 Groundwater model

Williamson and others (1990) Montgomery 0.00–0.66 Groundwater model - Predevelopment conditions

0.66–3.00 Groundwater model - 90 percent 1980 pumpage

Noble and others (1996) Harris, Montgomery, and Walker 0.0–6.0 3H interface method

Nolan and others (2007) Harris and Montgomery 0.04–2.13 Cl tracer in saturated zone

Scanlon and others (2011) Montgomery 0.8–4.8 Hydrograph analysis and chloride mass balance
44 Estimated Rates of Groundwater Recharge to the Chicot, Evangeline, and Jasper Aquifers in Texas, 2008 and 2011

Summary 

Montgomery County is in the northern part of the 

Houston, Texas, metropolitan area, the fourth most populous 

metropolitan area in the United States. A good understanding 

of the rate of recharge is important to water managers to 

help them ensure the amount of groundwater withdrawn 

from the Gulf Coast aquifer system in Montgomery County 

is sustainable. Accordingly, the USGS, in cooperation with 

the Lone Star Groundwater Conservation District, estimated 

rates of groundwater recharge in 2008 and 2011 to the 

Chicot, Evangeline, and Jasper aquifers at selected wells 

using a combination of different environmental tracers in 

Montgomery County. 

Groundwater is withdrawn from the Gulf Coast aquifer 

system, which comprises the Chicot, Evangeline, and Jasper 

aquifers; Burkeville confining unit; and the underlying 

Catahoula confining system. The individual sand and clay 

sequences of the aquifers composing the Gulf Coast aquifer 

system are not laterally or vertically continuous on a regional 

scale; however, on a local scale, individual sand and clay 

lenses can extend over several miles. 

In total, 36 sample sites representing 44 existing 

groundwater wells in or near Montgomery County were 

selected for sampling. Groundwater-quality samples were 

collected once from each of 40 wells March–September 

2008 and analyzed for chlorofluorocarbons (CFCs), sulfur 

hexafluoride (SF6), tritium (3H), helium-3/tritium (3He/3H), 

helium-4 (4He), and dissolved gases (DG). Groundwaterquality samples were collected once from each of 24 wells 

April–May 2011 and analyzed for major ions and carbon-14 

(

14C), and a subset of wells were analyzed for 4

He and DG. 

Modern environmental tracers CFCs, SF6, 3H, and 3

He/3H were used to estimate apparent ages of modern 

groundwater, which was defined for this report as water that 

was recharged and isolated from the atmosphere between 

1950 and 2008. The radioactive isotope of hydrogen (3H) 

was used to identify recent groundwater recharge or 

groundwater mixtures that contained some recent water. The 

environmental tracer 14C was used to estimate apparent ages 

of groundwater more than about 1,000 years before present.

Most DG samples contained little excess air (0.9 to 

5.8 cubic centimeters). The small amount of excess air within 

the samples was consistent with diffuse rather than focused 

recharge. Diffuse recharge occurs over a large area as water 

from precipitation infiltrates and percolates through the 

unsaturated zone to the water table. Focused recharge, which 

can occur rapidly, resulting in larger quantities of excess air 

being trapped in aquifer pores and dissolving under recharge 

conditions, occurs when recharge is focused at a preferential 

pathway or location.

The median recharge temperature of all sites was 

15.7 °C. Field water temperatures measured at the time 

of sampling were typically higher than the recharge 

temperature determined by using DG, with a median 

difference of 8.0 °C, indicating that groundwater at the 

time of sampling probably recharged at lower temperatures 

than current groundwater temperatures and most likely is 

premodern. Other factors affecting the estimated recharge 

temperature of the groundwater included the likelihood that 

most recharge in the region might occur in winter and early 

spring, which are the coolest times of the year. 

Groundwater ages, or apparent age, were determined 

by assuming piston-flow. For the piston-flow conceptual 

model, the constituent concentration was assumed not to be 

altered by mixing or dispersion from the point of entry to 

the measurement point in the aquifer. Most of the modern 

environmental tracer data indicated that the water at each 

of these wells was recharged prior to the 1950s, limiting the 

usefulness of CFCs, SF6, and 3H concentrations as tracers. 

Additionally, low-level concentrations of CFCs and SF6, as 

noted in several wells, may have been an artifact of well 

installation, well development, or poor annular seals for the 

well. Additionally, there may be nearby wells with multiple 

screened intervals in the aquifer(s), allowing for leakage of 

younger water to the older reservoirs of water, providing a 

conduit for cross-formational flow and biasing the resulting 

apparent age. In many cases, no modern tracer was usable 

at a well for the purpose of estimating an apparent age 

and was subsequently resampled for 14C as a tracer for 

older groundwater. 

Estimation of groundwater ages and recharge rates 

requires assumptions of the hydrogeologic properties of the 

aquifer system, resulting in uncertainty in these estimates. 

Porosity, a component in the calculation of recharge rate 

in the unconfined and confined zones of an aquifer, was 

estimated as 23 percent for the Chicot and Evangeline 

aquifers, whereas 25 percent was used for the Jasper aquifer. 

By using the recharge estimates derived from samples 

collected from 14 wells completed in the Chicot aquifer 

for which apparent groundwater ages could be determined, 

recharge to the Chicot aquifer ranged from 0.2 to 7.2 inches 

per year (in./yr). Based on data from one well, recharge to 

the unconfined zone of the Evangeline aquifer (outcrop) was 

0.1 in./yr. Based on data from eight wells, estimated rates 

of recharge to the confined zone of the Evangeline aquifer 

ranged from <0.1 to 2.8 in./yr. On the basis of data from one 

well, recharge to the unconfined zone of the Jasper aquifer 

(outcrop) was 0.5 in./yr. Based on data from nine wells, 

estimated rates of recharge to the confined zone of the Jasper 

aquifer ranged from <0.1 to 0.1 in./yr.

There is an appropriate use for these data, and there 

are limitations in their applicability for several reasons. 

Estimation of groundwater ages and recharge rates 

requires assumptions of the hydrogeologic properties 

of the aquifer system, resulting in uncertainty in these 

estimates. Despite the complexity of the aquifer system in 

Montgomery County, the estimates of groundwater ages and 

recharge rates are appropriate for use as a general guide in 

hydrologic investigations regarding the availability of the 

groundwater resources in the Montgomery County area. 

The hydrogeology in the study area is highly variable on the 
References 45

regional scale and may exhibit localized groundwater flow 

paths contrary to the regional flow pattern, which may allow 

for preferential flow paths within the aquifer and decrease 

the apparent age of the water, consequently increasing 

the recharge rate. For a conceptual model of this system, 

piston flow in the aquifer is assumed, which in the case of 

the Evangeline aquifer may be inaccurate at the local scale 

because of the direct hydrologic connection between the 

Evangeline aquifer with the overlying Chicot aquifer, and the 

possibility of mixing of the two waters. The calculation of 

recharge for both confined and unconfined aquifers requires 

an assumption of porosity, which was assumed to be uniform 

countywide. The estimated recharge rates calculated for 

this study are specific to each well location and should not 

be extrapolated or inferred as a countywide average. Local 

variations in the hydrogeology and surficial conditions 

can affect recharge rates at a local scale because localized 

variation in sand and clay percentages and in sand grain size 

could affect the apparent age of the water. 

Other limitations include the fact that concentrations 

of the environmental tracer were often at or near analytical 

detection limits, and at such low concentrations, the 

analytical imprecision is relatively large and thus can result 

in relatively large age-estimate errors. Furthermore, because 

there were relatively few wells available for sampling, sample 

collection could not exclude wells with long well screens, 

and mixing of groundwater might have been enhanced by 

long open intervals and deep wells; in addition, dispersion 

might affect environmental tracer concentrations given the 

heterogeneity of the aquifers. 

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