Determination of Ellenburger Aquifer Sustainability

James Beach LBG-Guyton Associates

Paul Tybor HCUWCD

Southeast Gillespie County

Minor Aquifers

Ellenburger Aquifer

Facts

• Ellenburger is the primary water supply– Fredericksburg – Irrigation– Rural

• Complex geology

• HCUWCD (1987)

• Aquifer has limits

Objectives

• Better understand the Ellenburger aquifer (SE Gillespie County)– Update and refine model– Assess long-term availability – Evaluate aquifer impact– Provide predictive tool for appropriate

planning, management and regulation

1. Data Evaluation

• Hydrostratigraphy and Structure

• Water Levels

• Hydraulic Properties

• Well Production

• Streamflow

• Water Chemistry

• Precipitation

2. Model Refinement

• Model Extent and Boundaries

• Hydrostratigraphy and Structure

• Calibration Targets

• Recharge

• Hydraulic Properties

• Pumping Allocation

• Sensitivity Analysis

3. Groundwater Availability

• Use updated model to assess:– Long term availability– Impact of increased demands– Optimize production

Modeling Basics

Groundwater Flow Modeling

Model “Cell” or “Gridblock”

Cells “Communicate”

Groundwaterflow

What Goes on In A Gridblock?

Groundwaterflow

Gridblock Accounting

• PermeabilityPermeability

• Storage valueStorage value

• ThicknessThickness

IrrigationIrrigationreturn flowreturn flow

Exchange of Exchange of water with water with

neighboring neighboring cellscells

NaturalNaturalrechargerecharge

Water Water remaining remaining in storagein storage

Water removed Water removed from storage by from storage by pumpingpumping

Geology

Faults

Stratigraphy

Glen Rose

Hensell Sand

Ellen-burger

Layer 1

Layer 2

Model Area

Ellenburger

Hensell Sand

Surface Geology

Extent of Model

New Model

Old Model

Gillespie County

Topography

Model Area

New Model

Old Model

Gillespie County All Wells

Model Area HighCapacity

Wells

HensellThickness

Hensell Sand

0

25

50

75

100

125

150

175

200

225

250

Hensell Sand Thickness (feet)

Hensell SandHensell Sand Thickness (feet)

N

Ellenburger

• Wells generally <350 feet deep

• Very deep wells have not encountered significant permeability-porosity

• Assume Ellenburger thickness of 200 feet

Groundwater Flow

Fault

Hydraulic Properties

• 3 pump testsSpecific capacity: 13 – 79 gpm/ft

Transmissivity: 25,000-38000 gpd/ft

3,500-5000 ft2/day

Hydraulic Conductivity: 22-33 ft/day

Storativity: 1x10-4

Well Capacity

20 gpm

50 gpm

Well Capacity

Ellenburger Histogram

0

100

200

300

400

500

600

1.0 3.2 10.0 31.7 100.0 316.5 1000.0

Production (gpm)

Fre

qu

ency

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Frequency

Cumulative %

Fredericksburg Pumping

Monthly Demand

Precipitation

Ellenburger Potentiometric Surface

Source: HCUWCD

Hensell Potentiometric Surface

Source: HCUWCD

Ellenburger Hydrographs:confined and unconfined responses

Hydrologic Summary Near Old Wellfield

• Water Levels in Ellenburger near old wells historically trend downward until new wells come online in the 1990s

• Both Hensell and Ellenburger water levels are typically below river levels

• Recent HCUWCD gain-loss studies and historical TWDB gain-loss studies in the Pedernales near the old wells show losses

GM-00008

1460

1470

1480

1490

1500

1510

1520

1530

1540

1550

J-40 J-50 J-60 J-70 J-80 J-90 J-00

Wat

er L

evel

Long-Term Ellenburger Water Level Trends Near Old Wells

USGS Pedernales at Fredericksburg vs Groundwater Level Near Old Wells

1480

1490

1500

1510

1520

1530

1540

1550

1560

1570

1580

J-98 J-99 J-00 J-01 J-02 J-03

Date

WL

ft A

MS

L

0

50

100

150

200

250

300

350

400

Ped

erna

les

Flow

in c

fs

GM-00010 Level (Ellenburger)

57-50-116 Level (Hensell)

USGS FredericksburgApproximate Pedernales Elevation near Old Wells

Levels of Hensell and Ellenburger Near Old Wells are Typically Below River Elevation

PEDERNALES FLOW AT FREDERICKSBURG VS. MEASURED GAIN/LOSS AT BROWN LOCATION (near old wells)

PEDERNALES FLOW AT FREDERICKSBURG VS. MEASURED GAIN/LOSS AT BROWN LOCATION

y = -0.1771x + 0.8607

R2 = 0.5752

-25

-20

-15

-10

-5

0

5

0 10 20 30 40 50 60 70 80 90

Flow (cfs)

Gai

n/L

oss

(cf

s)

Brown Flow vs GL

Linear (Brown Flow vs GL)

Source: HCUWCD gain/loss measurements

LBG-GUYTON ASSOCIATES

Intermediate-Term Ellenburger Water Level Trends Near New Wellfield

R-00074

1490

1495

1500

1505

1510

1515

1990 1992 1994 1996 1998 2000 2002 2004

Wat

er L

evel

Hydrologic Summary Near New Wellfield

• New Wellfield is near the Ellenburger outcrop on the Pedernales

• Ellenburger water level elevations at this location appear to be buffered near the river elevation

• Recent HCUWCD gain-loss studies at the Goehmann location suggest that at Ellenburger water levels at or above the river elevation the Pedernales gains, below the river elevation the Pedernales loses

Ellenburger Groundwater Level Near Ellenburger Outcrop and Pedernales River at New Wells

1475

1480

1485

1490

1495

1500

1505

1510

1515

1520

J-95 J-96 J-97 J-98 J-99 J-00 J-01 J-02 J-03 J-04 J-05

Date

WL

ft

AM

SL

57-50-329 Level(Ellenburger)

Approximate Elevation of Pedernales near New WellsPedernales Gains

Pedernales Loses

Hydrograph of Ellenburger Well Near Outcrop

Gain-Loss Data Near Outcrop

Goehmann Location Gain/Loss vs 57-50-329 Level

-4

-3

-2

-1

0

1

2

3

4

5

1480 1485 1490 1495 1500 1505 1510 1515

57-50-329 Leve l (ft AM SL)

Go

eh

ma

nn

Ga

in/L

os

s (

cfs

)

Goehmann G/L vs 57-50-329 Level

Only gains/losses greater than 10% of flow have been included.

Approximate river elevation

Modeling Periods

Steady-state

period

Calibration- Verification Prediction

Wate

r Ele

vati

on

in

Well

1940 1990 2004 2060

Observed Water LevelModel Water Level

Pre-Developme

nt Time

Simulation Periods

Time Period Stress Periods

Length

pre-1940: steady state periodprior to major pumping

1 -

1940 – 1990: calibration period 50 1 year

1990 – 2004: calibration period 180 1 month

2005 – 2060: predictive period ? ?

Path to completion

• Calibrate model

• Incorporate demands to 2060

• Simulate aquifer impacts