Central Texas Trinity Aquifer –Science and Management

Marshall Jennings and Richard EarlTexas State University – San MarcosPresented at Texas Groundwater 2004, November 19, 2004; material from Mace, TWDB originally presented to

Hays County Commissioners Court in 2003

Purpose of Talk

• To overview aquifer science of the Central Texas Trinity Aquifer that includes 10 counties and 8 GCDs in Central Texas; the material is largely drawn from Mace et al (2000) or the Trinity Aquifer GAM

• To discuss aquifer management by GCDs and the Hill Country Alliance of GCDs

Alluvium

Undivided

FortTerrett

Formation

SegoviaFormation

Hammett Shale

Sligo Fm.

Lowermember

Uppermember

SycamoreSand (in outcrop)

Cow Creek Limestone

Hensel Sand

NW SE

Edwards Group

Upper Trinity aquifer

Middle Trinity aquiferGuadalupe

River

CanyonLake

Edwards aquifer(BFZ)

SE model boundary

No flow

No flow

A

A’

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

200

sea level

Spring flow

0 5 10 15 mi

Recharge

Cross-formational flow

Surface water-groundwater interaction

Groundwater flow

DrainPumping

Cross-section

The Water Cyclein the Hill Country

Groundwater Flowpaths

Recharge• Previously published values:

– Muller and Price (1979) 1.5 %

– Ashworth (1983) 4. %

– Kuniansky (1989) 11. %

– Kuniansky and Holligan (1994) 7. %

– Bluntzer (1992) (calc.) 6.7 %

– Bluntzer (1992) (est.) 5. %

– our analysis 6.6 %

– our model 4.0 %

• Conclusions:- Percentage likely varies with rainfall amount- For average conditions, 4 to 5 %

Past and Predicted (dry demand) Pumping

County-by-County Pumping

Groundwater Management

• Ten counties are in the PGMA; however Comal and Travis Counties do not have a local GCD

• Eight counties in the Trinity PGMA do have GCDs and by mid-year all of these will have management plans in place

• The GCDs have formed an Alliance which meets six times per year and coordinates activities

• Individual GCDs vary in their activities and focus

Groundwater Management

• All GCDs have a water-level monitoring program and some have QW program

• Several recording GW monitors are in operation including one in HTGCD that has GOES telemetry

• Data bases are not uniform—all should report to TWDB

• Aquifer test information could be better captured in common files

Pumping Distribution

Essentially all the science information from many years of careful hydrogeology work is entrusted to a “groundwater geek” such as Rob Mace….O boy!!

The Trinity Aquifer is modeled with a USGS-developed model called MODFLOW; the same model is used in several other Texas

GAMs

Study Area and Grid

GAM Simulation

• Mace et al (2000) describe a dramatic but realistic simulation for the Trinity GAM

• A power point “video” for the 50 year simulation is available

• The following slides show 1997 vs 2050

• My well in Hays County is projected to fall 100 – 150 feet by 2050!

Click on image to start animation

Predicted Water-Level Declines

Water Budgets Using the GAM

• 1975 was a typical year for the Trinity GAM region and a steady-state budget is available

• The following results for Hays County are similar for all ten counties but with varying quantities

• Wells in Hays County were only 700 AF/yr in 1975 and in the round-off error

Model water budget for the Trinity aquifer in Hays County

values in acre-ft/yr

upperTrinity

middle Trinity

lower Trinity

Hammett Shale

5,900

27,000

16,100

12,400

8,000

8,900

5,500

?

?

2,500

8,400

Water Budgets Using the GAM

• Large volumes of water are moving through each County each year but only a fraction is available to wells ---- if rivers & springs are to continuing flowing; Hays has about 52,000 AF/yr available.

• Groundwater pumpage can come from a fraction of new recharge and from net water moving through Hays County --- perhaps a maximum of 8,000 – 10,000 AF/yr is available

Congratulations to the

Texas Water Development Board

for successfully completing Phase I of the groundwater availability modeling program!