Management Evaluation Model for the Edwards Aquifer


Download Management Evaluation Model for the Edwards Aquifer


Preview text

LP 91-08
Management Evaluation Model for the
Edwards Aquifer
Texas Water Commission
Aplil 1991

Management Evaluation Model for the
Edwards Aquifer
by Joseph L. Peters. Ph.D.
and Scott T. Crouch M.S.
LP 91-08 Texas Water Commission
April 1991

TEXAS WATER COMMISSION

B. J. Wynne, III, Cha/nnan

John E. Birdwell, Commlaslona,

Cliff Johnson, Commlaalon.,

Allen Belnke, Executive Dlreclo~

Authorization for use or reproduction of any original material contained in this publication. i.e., not obtained from other sources. is freely granted. The Commission would appreciate acknowledgement.

Published and distributed by the
Texas Water Commission Post Office Box 13087 Austin, Texas 78711
ii

TABLE OF CONTENTS

Chapter

I. INTRODUCTION

1

Purpose and Scope of Investigation.

1

Physiographic and Hydrologic Setting.

2

Acknowledgements.

4

II. HYDROGEOLOGY . . .

5

Stratigraphy.

5

Structure

6

Hydrology . .

6

III. MATHEMATICAL BASIS FOR COMPUTER MODEL.

13

IV. MANAGEMENT EVALUATION MODEL FOR THE EDWARDS

AQUIFER . . . . . . . . . .

....

15

Modifications to PLASM Program.

15

Model Development . . . . . .

18

Historical Data . . . . . . .

20

Calibration and Verification.

22

Model Output.

22

V. MODEL APPLICATION.

25

VI. LIMITATIONS AND RECOMMENDATIONS.

29

SELECTED REFERENCES.

31

APPENDICES

Appendix A - Data Entry Procedure . . . . . . . .

33

Appendix B - Computer Code Listing

.

41

Appendix C - Explanation of the HISTORY.DAT File.

59

Appendix D - Output for Example Applications . . .

63

iii

LIST OF FIGURES

Figure

1. Location of the San Antonio Region, Edwards

Aquifer, Physiographic Regions, and

Drainage Basins That Contribute Recharge

3

2. Correlation of Cretaceous Stratigraphic Units

7

3. Geohydrologic Section of the Edwards Aquifer.

10

4.

Model Grid for the

for the Edwards

Management Aquifer.

E

va
.

l

u
·

ati
·

o.n

Mo
..

de
.

l

·

19

5.

Plot of the Water Level Management Concept A

for

the

Index
.··

W.

e

l

l

.

F

r

o

m
·

27

6. Plot of the Water Level for the Index Well From

Management Concept B

·· . .·

28

LIST OF TABLES

Table

I. Recharge Distribution . . . . . . . . .

21

iv

INTRODUCTION
Purpose and Scope of Investigation
The purpose of this report is to document the development of the Management Evaluation Model for the Edwards aquifer and provide users of the model with the necessary information to simulate a variety of management scenarios.
The management of the Edwards aquifer has been discussed on an ongoing basis for many years and continues to be a topic of discussion in the San Antonio region, as well as other areas. The Texas Water Commission became involved in late 1989 as a mediator/ negotiator in discussions with the regional interest groups.
The water level in the J-17 index well, located at Fort Sam Houston in San Antonio, declined to 622.7 ft above Mean Sea Level (MSL) during the summer of 1990, which was the lowest level recorded in the well since the "drought of record" in 1957. The J17 index well, an observation well, has been used to monitor water levels in the Edwards aquifer since the early 1960's. The CY-26 observation well, which was located near J-17, was used as the index well from 1933 until 1963.
The low water levels enlivened the discussions and emphasized the need for a comprehensive plan to manage aquifer development and use. As a result of the these discussions, many different

management concepts were suggested by different concerned parties but no means were available to compare them. This created the need for a tool capable of evaluating any proposed management plans. The model was developed to meet this need and to provide a means for evaluating the relative impact of different management scenarios on the aquifer.
Physiographic and Hydrologic Setting
The Edwards aquifer in the San Antonio region lies within two physiographic provinces, the Edwards Plateau and the Gulf Coastal Plain. The catchment area for most ·of the recharge lies within the Edwards Plateau, where recharge takes place to the Edwards-Trinity aquifer. Ground water in the Edwards Plateau moves southeastward and, unless withdrawn, discharges as springflow near the southern end of the plateau. The resulting streams flow southeastward across the eroded surface of the Glen Rose Formation, until reaching the updip outcrop of the Edwards (Balcones fault zone) aquifer, where recharge into the aquifer takes place. The water then flows downdip toward the confined portion of the aqui fer, where water movement is principally to .the east and northeast. Major discharge points are located in the eastern areas of the aquifer at Comal Springs and San Marcos Springs,
The aquifer extends for approximately 180 miles and varies in width from 5 to 40 miles (Figure 1). It's eastern and western boundaries are marked by ground-water divides in Hays County and Kinney County, respectively. The northern boundary occurs along
-2-

:.- ....
~ ()
a:: w >

I
I
575'

'.',
TEXAS

-r.,. _ _.: Q

1<)

i. ••.•.••..• _-_.__ •••. J. ....

20 !.__..

30 MItES

o

10

:10

'0 KilOMETERS

EXPLANATION

-~--

IM:I RECHARGE AREA--Modified from Puente, 1978

BOUNDARY OF FRESHWATER PART OF EDWARDS AQUIFER

LINE SEPARATING UNCONFINED ZONE TO THE NORTH FROM THE CONFINED ZONE TO THE SOUTH, JULY 1974
- " - BOUNDARY OF DRAINAGE DIVIOE

660' SPRING ElEVATION

Location map
Figure 1.-Locationof the San Antonio Region, Edwards Aquifer, Physiographic Regions, and Drainage Basins that Contribute Recharge

NOTE:

Balcones Escarpment separates the Edwards Plateau
from the Gu" Coastal Plain. Catchment area lies
within the Edwards Plateau and yields surface runoff to streams that cross the recharge area of the Edwards aquifier in the San Antonio region Modified after Maclay and Land, 1987

the faulted outcrop of the aquifer and generally coincides with the Balcones Escarpment. The aquifer extends south to the "badwater" line, an arbitrary boundary represented by a total dissolved solids (TDS) concentration of 1,000 milligrams per liter (mg/l). South of the "bad-water" line, TDS values increase rapidly as transmissivity values decrease. Acknowledgements
Acknowledgement is extended to Mr. John E. Birdwell, Commissioner, Texas Water Commission and Mr. James Kowis, Assistant Division Director, Water Rights and Uses Division, Texas Water Commission for their interest and support in the development of a ground-water management tool for the San Antonio region of the Edwards aquifer.
The model is based on the BASIC version of PLASM (PrickettLonnquist Aquifer Simulation Model) .
Appreciation is also extended to the Texas Water Development Board for providing access to computer data files containing hydrologic information for the Edwards aquifer. The Edwards Underground Water District was instrumental in providing historic data for the Edwards aquifer.
-4 -

HYDROGEOLOGY
Stratigraphy
In the San Antonio region, the Edwards aquifer consists of Lower Cretaceous reefal, shallow marine, and lagoonal deposits. The Edwards aquifer is underlain by the Glen Rose Formation, which is a limestone, in part shaley with minor amounts of dolomite. The upper confining unit is the Del Rio Clay. The stratigraphic nomenclature for the area was first developed by R.T. Hill (1891) and was later modified by Rose (1972). Rose (1972) elevated the Edwards Limestone to group status and subdivided it into two formations; the lower formation was designated the Kainer Formation, and the upper was named the Person Formation.
Within the San Antonio region are three distinct depositional provinces; the Maverick Basin, the Devils River Trend, and the San Marcos Platform. The Kainer, Person, and Georgetown Formations are found on the San Marcos Platform and are stratigraphically equivalent to the Devils River Limestone, found in the Devils River Trend. The West Nueces, McKnight, and Salmon Peak, Formations in the Maverick Basin are also stratigraphic equivalents to the
.5"

Preparing to load PDF file. please wait...

0 of 0
100%
Management Evaluation Model for the Edwards Aquifer