U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2016–3002

 February 2016

Prepared in cooperation with the San Antonio Water System

New Insights into the Edwards Aquifer—Brackish-Water 

Simulation, Drought, and the Role of Uncertainty Analysis

Key Findings

• Uncertainty analysis provided insights as to the reliability of the model inputs and model results. 

• Brackish water is not expected to move appreciably toward production wells near the transition zone interface if a repeat 

of drought-of-record conditions (1950–56) occurs.

• The model is reliable for predicting brackish-water movement near production wells. 

• The model is not reliable for predicting Bexar County index well J-17 water levels or for predicting flows at Comal and 

San Marcos Springs. 

• Many hydraulic properties of the Edwards aquifer, as well as recharge to the aquifer, remain highly uncertain, even after 

the model has been calibrated. 

The Edwards aquifer is an important water resource in 

south-central Texas, providing water for residents, businesses, 

and ecosystems. The aquifer is a highly complex karst system 

characterized by areas of rapid groundwater flow, faulted and 

fractured Cretaceous-age rocks, and multiple water-quality 

zones. Karst aquifer systems include soluble rocks such as 

limestone and dolomite that can convey tremendous amounts of 

water through dissolution-enhanced faults and fractures (fig. 1). 

Recent sustained droughts (2011–15) have heightened concerns 

about the possible effects of drought on this vital water resource.

The Edwards aquifer consists of three water-quality zones. 

The freshwater zone of the Edwards aquifer is bounded to the 

south by a zone of brackish water (transition zone) where the 

aquifer transitions from fresh to saline water (fig. 2). The saline 

zone is downdip from the transition zone. There is concern that a 

recurrence of extreme drought, such as the 7-year drought from 

1950 through 1956, could cause the transition zone to move 

toward (encroach upon) the freshwater zone, causing production 

wells near the transition zone to pump saltier water. There is 

also concern of drought effects on spring flows from Comal and 

San Marcos Springs (fig. 2). These concerns were evaluated 

through the development of a new numerical model of the 

Edwards aquifer.

San Marcos 

Springs.

Photograph 

used with 

permission 

from Gregg 

Eckhardt.

An example of some 

of the karst rocks of 

the Edwards aquifer. 

Photograph used with 

permission from Gregg 

Eckhardt.

What was the focus of the model? 

In 2010, the U.S. Geological Survey (USGS), in 

cooperation with the San Antonio Water System, began a study 

to simulate brackish-water movement within the Edwards 

aquifer (more specifically, the potential for brackish-water 

encroachment into wells near the transition-zone interface) and 

effects on groundwater heads and on discharge at Comal and 

San Marcos Springs under drought conditions. A new numerical 

model was constructed by using SEAWAT, a variable-density 

groundwater flow and transport code developed by the USGS. 

SEAWAT allows for more accurate simulation of effects of 

increased dissolved-solids concentrations (such as those seen in 

the brackish-water transition zone and saline zone) on density 

and groundwater flow. Using SEAWAT for this model analysis 

required the Edwards aquifer to be simulated with eight model 

layers, many more than were used in previous numerical models 

of the aquifer (Lindgren and others, 2004; Lindgren, 2006). This 

increased vertical discretization allows for better representation 

of the hydrostratigraphy of the aquifer (fig. 1), as well as 

improved simulation of variable-density groundwater flow. 

Model results are presented in terms of the upper, middle, and 

lower parts of the Edwards aquifer (fig. 1).
MAVERICK

BASIN1

DEVILS RIVER

TREND1

SAN MARCOS

PLATFORM1

HYDROGEOLOGIC

UNITS

Anacacho

Limestone

Austin Chalk

Eagle Ford

Group

Buda

Limestone

Upper

confining unit

Del Rio Clay

UPPER CRETACEOUS

Salmon

Peak

Formation2

Devils

River

Limestone2

Georgetown

Formation

LOWER CRETACEOUS

Cyclic and marine members (undivided)

Leached

member

Collapsed

member

Regional dense member

Grainstone

member

Dolomitic

member

Basal nodular member

Kirschberg

evaporite

member

Person Formation3 Kainer Formation3

Edwards aquifer

McKnight

Formation2

West Nueces

Formation2

Trinity

aquifer

Upper member of the

Glen Rose Limestone

Lower member

of the Glen

 Rose Limestone

Aquifer subdivision in the San Marcos Platform area4

I

II

III

IV

V

VI

VII

VIII

Erosional hiatus 

Very small

Very small

Anacacho

Limestone

Very small

Anacacho

Limestone

Very small

Moderate

Austin Chalk ModerateAustin Chalk Moderate

Very small

Eagle Ford

Group

Very small

Eagle Ford

Group

Very small

Small

Buda

Limestone Small

Buda

Limestone Small

Very small

Del Rio Clay

Very small

Del Rio Clay

Very small

Very small

Very small

Large Small to moderate

Large

Moderate to large

Moderate to large

Very small

Moderate to large

Moderate

Large

Moderate

Moderate

Very small

Very small

Moderate

Small

Small

STRATIGRAPHIC UNITSEdwards Group

SYSTEM

Upper

zone

Middle

zone

DEPOSITIONAL PROVINCE

1

Location shown on figure 2.

2

Lozo and Smith (1964).

3

Modified from Rose (1972).

4

Maclay and Small (1984).

Glen Rose

Limestone

Trinity Group

Glen Rose

Limestone

Glen Rose

Limestone

MODEL

LAYERS

1

2

3

4

5

6

7

8

Modified from Maclay (1995)

Descriptors "very small, small, moderate, and large" refer

 to relative permeability of stratigraphic units

Stratigraphic unit not included in the model

Edwards aquifer—Dotted where less certain

 Upper part

 Middle part

 Lower part

EXPLANATION

Figure 1. Correlation of Cretaceous stratigraphic units and hydrogeologic units, 

relative permeability, and model layers in the Edwards aquifer model area, San 

Antonio region, Texas (modified from Maclay, 1995, fig. 11). 

An example of variable-density flow using SEAWAT version 4. 

Modeling of the Edwards aquifer was completed 

in two phases: a calibration (history matching) 

phase and a predictive phase. History matching is 

the process of calibrating the model to historical 

real-world data, such as groundwater levels, spring 

flows, and dissolved-solids concentrations. Model 

input parameters, such as hydraulic properties and 

groundwater recharge, are adjusted during history 

matching to acquire a sufficient fit of the model output 

to the real-world data. The model fit was accomplished 

by using thousands of parameters that adjust aquifer 

properties and recharge into the aquifer, thousands of 

real-world data points, and the automated parameter 

estimation software PEST. Traditional measures of 

goodness of fit, such as one-to-one plots and correlation 

coefficients, indicated good agreement between 

observations and model-simulated equivalents with 

little or no bias over the range of observed real-world 

data (fig. 3) for the 1999–2009 time period. 

The second phase, the predictive phase, involved 

modifying the history-matched model to simulate 

conditions of drought-of-record rainfall during a 

7-year drought period (1950–56) combined with recent 

(1999–2009) pumping amounts. Recent pumping rates 

are much larger than pumping rates were during the 

1950s when far fewer people lived in south-central 

Texas. The main goals of the predictive phase were to 

simulate (1) dissolved-solids concentration change at 

25 production wells near the transition-zone interface 

from the beginning of the simulation to the end of 

the 7-year drought period, (2) the total volume of 

groundwater flowing from Comal and San Marcos 

Springs during the 7-year drought period, and (3) 

the groundwater level at well J-17 at the end of the 

7-year drought period. More details about the model 

development and history matching can be found in 

Brakefield and others (2015).

What does uncertainty analysis tell us about 

the Edwards aquifer?

Uncertainty analysis was applied to the new history-matched 

model of the Edwards aquifer because the parameters—which 

control aquifer properties, the spatial distribution of dissolvedsolids concentrations, rates of recharge to the aquifer, and 

predictions made with the model—all have substantial uncertainty 

associated with them and because of the inherent complexity of 

this karst aquifer. Considerable uncertainty exists in the spatial 

and temporal distribution of dissolved-solids concentrations 

throughout the Edwards aquifer. Additionally, marked differences 

exist between the amount of data available for characterizing the 

freshwater, transition, and saline zones; a relatively large amount 

of data is available for characterizing the freshwater zone, whereas 

few data are available for characterizing the transition and saline 

zones. The relative lack of data for the transition and saline zones 

contributes to uncertainty in those areas. Because the system is 

highly complex, few data are available in some areas, and models 

are known to be non-unique, uncertainty analysis was used to 

assess what was learned about these parameters through the 

calibration process.
UNITED STATES

MEXICO

1,000 mg/L

10,000 mg/L

1,000 mg/L

10,000 mg/L

1

2

3 4

5

6

7

8

9

10

0 10 20 30 KILOMETERS

0 10 20 30 MILES

29°

30°

31°

98°

99°

100°

Base from U.S. Geological Survey and 

City of Austin digital data, 2010

Albers equal area projection, 

Texas Centric Mapping System,

North American Datum of 1983

TEXAS

COAHUILA

LLANO COUNTY

BURNET

COUNTY

BLANCO

COUNTY

WILLIAMSON COUNTY

BASTROP

COUNTY

TRAVIS

COUNTY

COUNTY

HAYS

COMAL

COUNTY

KENDALL

COUNTY

GILLESPIE COUNTY

KERR

COUNTY 

EDWARDS COUNTY

REAL

COUNTY 

BANDERA

COUNTY

BEXAR

COUNTY

GUADALUPE

COUNTY

CALDWELL

COUNTY

GONZALES

COUNTY

WILSON

COUNTY

MEDINA

COUNTY

KINNEY UVALDE COUNTY

COUNTY 

MAVERICK

COUNTY 

ZAVALA COUNTY 

FRIO

COUNTY

ATASCOSA

COUNTY

LA SALLE COUNTY

DIMMIT COUNTY 

Lake

 Travis

Blanco River

Canyon 

Lake

Creek

Medina

Lake

Pedernales River

San

Marcos

River

Guadalupe

River

Nueces

River

Frio

River

Blanco

Creek

Dry

Frio River

West

Nueces

River

River

SanAntonio River

Cibolo

Creek

Leona

River

RIO BRAVO

Guadalupe River

Sabinal River

Dry Comal

Seco Creek

Hondo Creek

Colorado River

River Frio

Verd e Cr

Medina

Onion Creek

Barton Creek

Williamson

Creek

Bear Creek

Slaughter

 Creek

RIO GRANDE

NUECES RIVER BASIN

SAN ANTONIO RIVER BASIN

GUADALUPE RIVER BASIN

San Pedro Springs

San Antonio Springs

Hueco Springs

Comal Springs

Las Moras Springs

Leona Springs

San Marcos Springs

Barton Springs 

Bexar County index well (J–17)

MAVERICK BASIN

DEVILS RIVER

TREND

SAN MARCOS

PLATFORM

EXPLANATION

Regional-scale model area

 Catchment area

 Recharge zone (outcrop) of the Edwards 

 aquifer (modified from Puente, 1978)

 Freshwater zone of the Edwards aquifer

 Brackish-water (freshwater/saline water)

 transition zone of the Edwards aquifer

 Saline zone

1,000 mg/L

Depositional province boundary (modified from

 Maclay, 1995, fig. 2)

Model boundary

Line of equal dissolved-solids concentration, in milligrams

 per liter (mg/L) (A.L. Schultz, written commun., 2000)

Contributing area—Basin extent shown terminates at 

 downdip extent of Edwards aquifer recharge zone

 Major river basin boundary

 Stream basin or contributing area boundary

Groundwater divide

Spring—Tail points in direction of flow

Contributing area

 Nueces-West Nueces River Basin

 Frio-Dry Frio River Basin

 Sabinal River Basin

 Area between Sabinal and Medina River Basins

 (includes Hondo and Seco Creeks)

 Medina River Basin

 Area between Medina River Basin and Cibolo-Dry Comal Creek Basins

 Cibolo and Dry Comal Creek Basins

 Guadalupe River Basin

 Blanco River Basin

 Barton Springs segment contributing area (includes Barton,

 Williamson, Slaughter, Bear, and Onion Creeks) 

1

2

3

4

5

6

7

8

9

10

Figure 2. Recharge, freshwater, brackish-water transition, and saline zones of the Edwards aquifer, San Antonio region, Texas 

(modified from Lindgren and others, 2011, fig. 2).
550

500

450

400

350

300

250

200

150

710

700

690

680

670

660

650

640

630

450

400

350

300

250

200

150

100

50

0

Groundwater head, in feet

Spring discharge,

in cubic feet per second

Spring discharge,

in cubic feet per second

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Year

A. Comal Springs

B. San Marcos Springs

C. Bexar County index well (J–17)

EXPLANATION

Simulated

Observed

Figure 3. Model fit at A, Comal Springs; B, San Marcos Springs; and C, Bexar County 

index well J-17. 

Linear-based uncertainty analysis was applied to assess 

the uncertainty of model inputs, such as hydraulic properties 

and recharge, collectively referred to as “parameters.” Prior 

parameter uncertainty (prior to model calibration) was 

compared to posterior parameter uncertainty (after model 

calibration) to determine what was learned about parameters 

through the history-matching process. Parameter uncertainty 

is visualized through use of normal, or Gaussian, distributions. 

Prior parameter uncertainty for different parameter types (such 

as horizontal hydraulic conductivity, fig. 4A) is represented 

as a dashed distribution. Posterior parameter uncertainty for 

individual parameters within each group is represented as a 

shaded distribution. Reductions in widths of distributions or 

increases in the height of distributions, when compared to 

the dashed distribution of the prior, indicate a reduction in 

uncertainty (or increase in knowledge) about that parameter 

through history matching. Many hydraulic properties of the 

Edwards aquifer remain highly uncertain, even after the model 

has been history matched (calibrated) to real-world data.

Recharge is the driving force of 

groundwater flow within the Edwards aquifer 

but is known to have substantial spatial 

and temporal uncertainty. Results from the 

uncertainty analysis indicate that little is 

learned about recharge by history matching; 

recharge parameters still remain highly 

uncertain (fig. 4B). 

What does uncertainty analysis tell 

us about the reliability of the model in 

making a prediction, such as a repeat 

of drought-of-record rainfall (1950–56)?

The predictive model in this assessment 

simulated a hypothetical recurrence of 

1950–56 recharge conditions and higher-thanaverage recent groundwater withdrawal rates 

to provide insight into the potential effects of 

these conditions on predictions of dissolvedsolids concentration changes at production 

wells near the transition-zone interface (fig. 

5), discharges at Comal and San Marcos 

Springs, and groundwater levels at well J-17. 

Uncertainty analysis was also used to quantify 

the uncertainty of the predictions made with 

the model, similar to the uncertainty analysis 

applied to the model inputs. Predictive 

uncertainty can be thought of as an indicator 

of the reliability of the model to make a given 

prediction of aquifer response. The results of 

the predictive uncertainty analysis focused 

on the right tail of the Gaussian distribution 

(upper 95 percent credible interval) for each 

of the 25 dissolved-solids concentrationchange predictions to represent the expected 

maximum concentration change. 

Results from the predictive uncertainty 

analyses indicate that a minimal increase in 

dissolved-solids concentration (figs. 5 and 6) is expected at these 

locations during the 7-year predictive period. The uncertainty 

analysis also indicates that the model is reliable at making this 

prediction because the model inputs that control these predictions 

are well informed by the calibration dataset, causing the posterior 

uncertainty to be greatly reduced (blue bars in fig. 6) when 

compared to the prior uncertainty (gray bars in fig. 6). 

Predictive uncertainty results for total spring flows at Comal 

and San Marcos Springs during the 7-year period, as well as the 

results for water-level predictions at well J-17, were substantially 

different than the results for dissolved-solids concentrations 

changes at the production wells. The upper bounds of the 

95-percent credible intervals for spring flow at Comal and San 

Marcos Springs and water levels at well J-17 were 10 times larger 

than the prediction from the calibrated model, which implies 

that (1) the predictions of spring flows made with this model are 

not reliable and (2) model inputs that control these predictions 

are not well informed by the calibration dataset, even though 

the history-matching effort yielded model inputs that accurately 

reproduced spring flows and water levels at these locations during 
Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing density Increasing density Increasing density

Increasing density

FLUX MULTIPLIER PARAMETERS

PILOT POINT MULTIPLIER PARAMETERS

Posterior parameter uncertainty distribution for each 

 parameter within the group—The intensity of the 

 shaded area reflects the overlapping of distributions

 Aquifer

 Upper part of 

 the Edwards

 Middle part of 

 the Edwards

EXPLANATION

Prior parameter uncertainty 

 distribution—Uniform for 

 all parameters in this 

 group

 Lower part of the Edwards

 Trinity—Trinity aquifer

 flux into Edwards 

 aquifer

Trinity aquifer flux into

Edwards aquifer

Specific storage

Specific storage

Horizontal hydraulic 

conductivity

Vertical hydraulic 

conductivity

Horizontal hydraulic 

conductivity

Effective porosity

Effective porosity

Initial dissolved-solids

concentration

Initial dissolved-solids

concentration

Initial dissolved-solids

concentration

A

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Incre

pa

Incre

pa

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing logarithm of 

parameter value 

Increasing density Increasing densityIncreasing density Increasing density

Increasing density

FLUX MULTIPLIER PARAMETERS

THRESHOLD PARAMETERS SCALING PARAMETERS

Post

 d

 w

 o

 o

 Zo

 R

 B

 S

Prio

 d

 p

Dissolved-solids

concentration 

Dissolved-solids

concentration 

Annual recharge

Specific storage

Specific storage

Horizontal hydraulic 

conductivity

Horizontal hydraulic 

conductivity

Effe

Effe

Recharge (drier months) Recharge (wetter months)

Drier months (January–March, July, August, December)

Wetter months (April–June, September–November) 

High_thresh

Low_thresh

B

EXPLANATION

Posterior parameter uncertainty distribution for each parameter 

 within the group—The intensity of the shaded area reflects the

 overlapping of distributions

 

Upper part of the Edwards Aquifer

Prior parameter uncertainty distribution—Uniform for

 all parameters in this group

Recharge Zone

Figure 4. Prior (dashed) versus posterior (shaded) plots describing parameters of A, horizontal hydraulic conductivity for the upper 

part of the Edwards aquifer and B, annual recharge, drier-month recharge, and wetter-month recharge. The prior plots describe the 

uncertainty of the parameters before history matching to real-world data, and the posterior plots describe the uncertainty of the 

parameters after history matching. Prior and posterior plots for many more parameters can be found in Brakefield and others (2015).

Base from U.S. Geological Survey digital data

Albers equal area projection, 

Texas Centric Mapping System

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Active model area

Model extent

Recharge zone boundary (modified from Puente, 1978)

Approximate line of 1,000 milligrams per liter 

 dissolved-solids concentration (A.L. Schultz, 

 written commun., 2000)

Production well of predictive interest* and well number

Average annual groundwater withdrawals

 during 1999–2009 by well, in acre-feet

 22,812 to 13,007

 13,006 to 6,777

 6,776 to 4,074

 4,073 to 1,696

 1,695 to 611

 610 to 139

 138 to 0.000001

5

*Prediction of dissolved-solids concentration change during simulated 

 7-year (1950–56) drought of record conditions made at these production wells

UVALDE COUNTY

EDWARDS COUNTY

BEXAR COUNTY

KERR COUNTY

MEDINA COUNTY

FRIO COUNTY

KINNEY

COUNTY

ZAVALA COUNTY

REAL COUNTY

WILSON COUNTY

HAYS COUNTY

ATASCOSA COUNTY

BANDERA COUNTY

KENDALL

COUNTY

KARNES COUNTY

COMAL COUNTY

GUADALUPE COUNTY

GILLESPIE COUNTY TRAVIS COUNTY

BLANCO

COUNTY

MAVERICK

COUNTY

GONZALES

COUNTY

CALDWELL

COUNTY

100° 99° 98°

30°

29°

Area enlarged

29°30'

29°20'

98°30' 98°20'

24

22

21

9

11

13

23

6

19

17

16

14

15

18

12

20

25

3 5

4

1

2

7

8

10

Figure 5. Average annual groundwater withdrawals during 1999–2009, and the production wells of predictive interest near 

the brackish-water transition zone, San Antonio region, Texas.
Production well of predictive interest* (see fig. 5 for location)

*Prediction of dissolved-solids concentration change during simulated 

 7-year (1950–56) drought of record conditions made at these production wells

Upper bound of 95 percent credible interval—Change during

 the 1950–56 predictive model period

 Prior (before history matching) uncertainty of prediction

 Posterior (after history matching) uncertainty of prediction

EXPLANATION

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

Change in dissolved-solids concentration,

in milligrams per liter

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

Figure 6. Change in dissolved-solids concentrations at production wells during the 7-year period (1950–56). Gray bars come from the 

prior distribution (before history matching), and blue bars come from the posterior distribution (after history matching).

the history-matching period. This large uncertainty is likely 

attributable to the cumulative effect of upgradient conditions on 

flows at Comal and San Marcos Springs and water levels at well 

J-17. The large contributing area for these aquifer integration 

points includes many uncertain aquifer properties. These 

uncertainties, when combined with recharge uncertainty, result 

in reduced reliability to predict these quantities of interest.

This fact sheet is based on the following report: 

Brakefield, L.K., White, J.T., Houston, N.A., and Thomas, J.V., 

2015, Updated numerical model with uncertainty assessment 

of 1950–56 drought conditions on brackish-water movement 

within the Edwards aquifer, San Antonio, Texas: U.S. 

Geological Survey Scientific Investigations Report 2015–

5081, 54 p., http://dx.doi.org/10.3133/sir20155081.

References Cited

Lindgren, R.J., 2006, Diffuse-flow conceptualization and simulation of the 

Edwards aquifer, San Antonio region, Texas: U.S. Geological Survey 

Scientific Investigations Report 2006–5319, 48 p.

Lindgren, R.J., Dutton, A.R., Hovorka, S.D., Worthington, S.R.H., and Painter, 

Scott, 2004, Conceptualization and simulation of the Edwards aquifer, San 

Antonio region, Texas: U.S. Geological Survey Scientific Investigations 

Report 2004–5277, 143 p.

Lindgren, R.J., Houston, N.A., Musgrove, M., Fahlquist, L.S., and Kauffman, 

L.J., 2011, Simulations of groundwater flow and particle-tracking analysis in 

the zone of contribution to a public-supply well in San Antonio, Texas: U.S. 

Geological Survey Scientific Investigations Report 2011–5149, 93 p.

Lozo, F.E., Jr., and Smith, C.I., 1964, Revision of Comanche Cretaceous 

stratigraphic nomenclature, southern Edwards Plateau, southwest Texas: Gulf 

Coast Association of Geological Societies Transactions, v. 14, p. 285–306.

Maclay, R.W., 1995, Geology and hydrology of the Edwards aquifer in the San 

Antonio area, Texas: U.S. Geological Survey Water-Resources Investigation 

Report 95–4186, 64 p.

Maclay, R.W., and Small, T.A., 1984, Carbonate geology and hydrology of the 

Edwards aquifer in the San Antonio area, Texas: U.S. Geological Survey 

Open-File Report 83–537, 72 p.

Puente, Celso, 1978, Method of estimating natural recharge to the Edwards 

aquifer in the San Antonio area, Texas: U.S. Geological Survey WaterResources Investigations Report 78–10, 34 p.

Rose, P.R., 1972, Edwards Group, surface and subsurface, central Texas: 

Austin, The University of Texas, Bureau of Economic Geology Report of 

Investigations 74, 198 p.

By Linzy K. Foster and Jeremy T. White

For additional information, please contact:

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, Texas 78754–4501

http://tx.usgs.gov

Publishing support provided by

Lafayette Publishing Service Center

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20163002