central texas trinity aquifer – science and management marshall jennings and richard earl texas...
TRANSCRIPT
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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
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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
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Alluvium
Undivided
FortTerrett
Formation
SegoviaFormation
Hammett Shale
Sligo Fm.
Lowermember
Uppermember
SycamoreSand (in outcrop)
Cow Creek Limestone
Hensel Sand
NW SE
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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
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The Water Cyclein the Hill Country
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Groundwater Flowpaths
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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 %
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Past and Predicted (dry demand) Pumping
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County-by-County Pumping
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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
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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
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Pumping Distribution
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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
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Study Area and Grid
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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!
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Click on image to start animation
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Predicted Water-Level Declines
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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
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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
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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
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Congratulations to the
Texas Water Development Board
for successfully completing Phase I of the groundwater availability modeling program!