"I've Got a Sinking Feeling..." - TWDB's Statewide Subsidence Risk Evaluation Study - Mike Keester
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TWCA Mid-Year Conference 2018
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"I've Got a Sinking Feeling..." - TWDB's Statewide Subsidence Risk Evaluation Study - Mike Keester
- 1. “I’ve got a sinking feeling…” TWDB’s Statewide Subsidence Risk Evaluation Study TWCA Mid-Year Conference Michael Keester, P.G. June 14, 2018 Project Funded Through TWDB Contract Number 1648302062
- 2. Outline • Project Objectives • Aquifer Subsidence Risk Assessment Methodology • Summary of Subsidence Vulnerability Evaluation • Subsidence Prediction Tool • Questions
- 3. Project Objectives • Consideration of subsidence is required per Texas Water Code – Management plans – Permitting – Desired future conditions explanatory report • Texas Water Development Board – Identify and characterize areas within Texas’ major and minor aquifers that are susceptible to land subsidence related to groundwater pumping – Create a tool for stakeholders to use for characterizing subsidence risk – http://www.twdb.texas.gov/groundwater/models/research/subsidence/subsiden ce.asp
- 4. Aquifer Subsidence Risk Assessment Methodology • Aquifer lithology and distribution, thickness, and compressibility of clay layers within the aquifer • Amount and timing of water-level changes • Lowest historical water level (that is, preconsolidation level)
- 5. Clay Distribution and Thickness • Submitted Drillers Reports Database “WellLithology” table • Used keywords to identify clay intervals (for example: Gumbo, Blackland, Sticky) and partial clay content • Mapped calculated total clay thickness Keyword Multiple on Clay Thickness SAND 0.5 SANDY 0.5 SHALE 0.75 SHELL 0.75 ROCK 0.25 CLAYEY 0.25 SND 0.5 SD 0.5 SILTY 0.75 SILT 0.75 SLT 0.75 GRAVEL 0.5 STONE 0.25 CALICHE 0.5
- 6. Clay Compressibility • Very important consideration • Typically not available • Applied standard ranges of values Lithologic Material Compressibility (β), psi-1 Plastic Clay 1.8 × 10-3 to 1.4 × 10-2 Stiff Clay 9.0 × 10-4 to 1.8 × 10-3 Medium Hard Clay 4.8 × 10-4 to 9.0 × 10-4 Loose Sand 3.6 × 10-4 to 6.9× 10-4 Dense Sand 9.0 × 10-5 to 1.4 × 10-4 Dense Sandy Gravel 3.6 × 10-5 to 6.9 × 10-5 Rock, Fissured/Jointed 2.3 × 10-6 to 4.8 × 10-5 Rock, Sound Less than 2.3 × 10-6
- 7. Water Levels • Primarily used GAM simulation results – Transient calibration water levels for preconsolidation level – DFC simulations for future water-level declines – Trend based on relatively recent conditions • For aquifers where GAM results could not be used (such as the Hueco-Mesilla Bolson and Blossom aquifers), measured water levels used 0 50 100 150 200 250 300 350 400 450 1980 1990 2000 2010 2020 2030 2040 2050 2060 2070 DepthtoWater,feet Year Measured and Simulated Water Levels SWN: 6053516 – Evangeline Aquifer Measured Water Levels Simulated Water Levels
- 8. Preliminary Aquifer Subsidence Risk Matrix • Qualitative assignment of a quantitative value of risk • Develop classes and class values for each risk factor • Rank risk factors and assign weights • Values calculated on a well-by-well basis and normalized to a value between 0 and 10 (inclusive) Subsidence Risk Factor (Weight) Subsidence Risk Factor Class Class Value Clay Layer Thickness and Extent (6) Regional Extent – Greater than 300 feet 5 Regional Extent – 200 to 300 feet 4 Regional Extent – 100 to 200 feet 3 Regional Extent – Greater than 0 to 100 feet 2 Local Extent or No Clay 1 Clay Compressibility (5) Plastic Clay 3 Stiff Clay 2 Hard or No Clay 1 Aquifer Lithology (4) Unconsolidated Clastic 4 Consolidated Clastic 3 Carbonate/Evaporite 2 Igneous 1 Preconsolidation Characterization (3) Current Static Water Level Less than Historic Low Water Level Plus 25 Feet 3 Current Static Water Level Greater than Historic Low Water Level Plus 25 Feet and Less than Historic Low Water Level Plus 50 Feet 2 Current Static Water Level Greater than Historic Low Water Level Plus 50 Feet 1 Predicted 50-Year Water Level Decline based on Trend (2) Greater than 200 feet 5 Between 100 and 200 feet 4 Between 50 and 100 feet 3 Between 0 and 50 feet 2 Less than 0 feet 1 Predicted DFC Water Level Decline (1) Greater than 200 feet 5 Between 100 and 200 feet 4 Between 50 and 100 feet 3 Between 0 and 50 feet 2 Less than 0 feet 1
- 9. Subsidence Risk • Risk assessed on a well-by-well basis • Aggregate statistics calculated for each major and minor aquifer • Each aquifer categorized as having high, medium, or low risk
- 10. High Subsidence Risk • 7 aquifers identified to have a high subsidence risk – 5 major aquifers – 2 minor aquifers • Primary factors in common for high subsidence risk – Unconsolidated clastic – Thick clay sections Pecos Valley Hueco-Mesilla Bolsons Ogallala Brazos River Alluvium Yegua-Jackson Carrizo-Wilcox Gulf Coast
- 11. Gulf Coast Aquifer System Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 1.4 to 3,645 feet 2 Clay Compressibility Estimated based on lithology Plastic Clay 3 Aquifer Lithology Kasmarek and Robinson (2004) Unconsolidated Clastic 4 Preconsolidation Characterization Preconsolidation and static water level from transient model calibration and final MAG simulations -353 to 798 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels – Northern GAM: 1981 – 2021 (Wade, 2016); Central GAM: 2000 – 2020 (Goswami, 2017b); Southern GAM: 2000 – 2020 (Goswami, 2017c) Less than 1- foot decline 2 Predicted DFC Water Level Decline Difference in head as described in final MAG simulations Average 28 feet decline 2 Total Weighted Risk: 5.9
- 12. Gulf Coast Aquifer System – GMA 14 Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 1.4 to 3,234 feet 3 Clay Compressibility Estimated based on lithology Plastic Clay 3 Aquifer Lithology Kasmarek and Robinson (2004) Unconsolidated Clastic 4 Preconsolidation Characterization Preconsolidation and static water level from transient model calibration and final MAG simulations -228 to 450 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels – Northern GAM: 1981 – 2021 (Wade, 2016) Less than 1- foot decline 2 Predicted DFC Water Level Decline Difference in head as described in final MAG simulations Average 21 feet decline 2 Total Weighted Risk: 6.7
- 13. Ogallala Aquifer Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 560 feet 2 Clay Compressibility Estimated based on lithology Stiff Clay 2 Aquifer Lithology George and others (2011) Unconsolidated Clastic 4 Preconsolidation Characterization Preconsolidation and static water level: Head for 2017 from final MAG simulation 2,116 to 4,474 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from 1980 through 2012 from calibrated High Plains Aquifer System GAM Average 43 feet decline 3 Predicted DFC Water Level Decline Difference in head for 2062 from initial head from final MAG simulation Average 35 feet decline 2 Total Weighted Risk: 5.2
- 14. Ogallala Aquifer – Permian Basin UWCD – Howard and Martin Counties Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 141 feet 2 Clay Compressibility Estimated based on lithology Stiff Clay 2 Aquifer Lithology George and others (2011) Unconsolidated Clastic 4 Preconsolidation Characterization Preconsolidation and static water level: Head for 2017 from final MAG simulation 2,349 to 2,909 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from 1980 through 2012 from calibrated High Plains Aquifer System GAM Average 1 foot decline 2 Predicted DFC Water Level Decline Difference in head for 2062 from initial head from final MAG simulation Average 9 feet decline 2 Total Weighted Risk: 4.7
- 15. Trinity Aquifer – Williamson County Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 860 feet 3 Clay Compressibility Estimated based on lithology Hard Clay 1 Aquifer Lithology Kelley and others (2011) Carbonate/ Consolidated Clastic 3 Preconsolidation Characterization Water level from end of transient model simulations (Kelley and others, 2014) 463 to 1,035 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from transient and final MAG simulations (Kelley and others, 2014; Beach and others, 2016) Average 33 feet rise 1 Predicted DFC Water Level Decline Difference in head from final MAG simulation (Beach and others, 2016) Average 44 feet decline 3 Total Weighted Risk: 4.4
- 16. Trinity Aquifer – Williamson County Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 860 feet 3 Clay Compressibility Estimated based on lithology Hard Clay 1 Aquifer Lithology Kelley and others (2011) Carbonate/ Consolidated Clastic 3 Preconsolidation Characterization Water level from end of transient model simulations (Kelley and others, 2014) 463 to 824 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from transient and final MAG simulations (Kelley and others, 2014; Beach and others, 2016) Average 47 feet rise 1 Predicted DFC Water Level Decline Difference in head from final MAG simulation (Beach and others, 2016) Average 61 feet decline 3 Lower Trinity Total Weighted Risk: 4.4
- 17. Trinity Aquifer – Williamson County Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 315 feet 4 Clay Compressibility Estimated based on lithology Hard Clay 1 Aquifer Lithology Kelley and others (2011) Carbonate/ Consolidated Clastic 3 Preconsolidation Characterization Water level from end of transient model simulations (Kelley and others, 2014) 653 to 880 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from transient and final MAG simulations (Kelley and others, 2014; Beach and others, 2016) Average 15 feet decline 2 Predicted DFC Water Level Decline Difference in head from final MAG simulation (Beach and others, 2016) Average 27 feet decline 2 Upper Trinity Total Weighted Risk: 5.5
- 18. Trinity Aquifer – Williamson County Subsidence Risk Factor Variable Data Source Value 3rd Quartile SRV Clay Layer Thickness and Extent SDR lithology table 0 to 376 feet 4 Clay Compressibility Estimated based on lithology Hard Clay 1 Aquifer Lithology Kelley and others (2011) Carbonate/ Consolidated Clastic 3 Preconsolidation Characterization Water level from end of transient model simulations (Kelley and others, 2014) 628 to 1,035 feet mean sea level 3 Predicted Water Level Decline based on Trend Trend in simulated water levels from transient and final MAG simulations (Kelley and others, 2014; Beach and others, 2016) Average 18 feet rise 1 Predicted DFC Water Level Decline Difference in head from final MAG simulation (Beach and others, 2016) Average 22 feet decline 2 Middle Trinity Total Weighted Risk: 5.2
- 19. Subsidence Prediction Tool • Uses same method as MODFLOW SUB-WT package • User inputs site specific information • Provides subsidence risk value and potential subsidence based on inputs
- 22. Subsidence Prediction Tool 2010 2020 2030 2040 2050 2060 2070 0.00 0.05 0.10 0.15 0.20 0.25 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 SubsidencePredictions(ft) DrawdownComparedtoBeginningYear(ft) Year Drawdown and Subsidence Prediction Drawdown Compared to Starting Water Level (ft) Minimum Predicted Subsidence (ft) Maximum Predicted Subsidence (ft)
- 23. Subsidence Prediction Tool – Example • SDR Tracking Number 429169 • Evangeline Aquifer • 127 feet clay
- 24. Subsidence Prediction Tool – Example
- 25. Subsidence Prediction Tool – Example
- 26. Subsidence Prediction Tool – Example
- 27. Subsidence Prediction Tool – Example
- 28. Subsidence Prediction Tool – Example Default Water Level Trend 1 Foot per Year Decline
- 29. Subsidence Prediction Tool – Example
- 30. Summary • Subsidence risk is not isolated to the Gulf Coast Aquifer System • Valuable information for the TWDB, GCDs, and GMAs to meet statutory requirements • Subsidence Prediction Tool provides a means for quantifying potential effects of pumping on subsidence – User friendly – Flexible – Easily modified to meet varying needs
- 31. QUESTIONS “I’ve got a sinking feeling…” TWDB’s Statewide Subsidence Risk Evaluation Study TWCA Mid-Year Conference June 14, 2018 Mike Keester, P.G. Mike.Keester@LREWater.com (512) 962-7660
Editor's Notes
- Intro Thank TWDB Acknowledge co-authors Blanton & Associates Wet Rock Geoscience GLS Solutions High level evaluation
- GCDs are required to consider subsidence Many lacked the data, tools, etc. to provide much more than a qualitative consideration Some GCDs simply stated that subsidence consideration was not applicable The TWDB sought to give the GCDs something more to hang their hat on The project also resulted in a tool stakeholders could continue to use for future evaluations
- We began by developing a means for considering subsidence risk in every aquifer Aquifers had varying lithologic and hydrologic characteristics We were looking at a high level and wanted to use a consistent methodology Presented the methodology to the TWDB early in the project to obtain feedback and discuss alternatives
- For clay thicknesses, we used the SDR database Statewide coverage Easily automated for the project and future evaluations Less labor intensive than geophysical log analysis We found there are a lot of ways that a driller may describe clay
- Clay compressibility is extremely important, but not typically known Used reported information to assign lithologic description
- Risk values are based on judgement and feedback from the TWDB Weights were assigned in order of importance of each factor on potential subsidence
- High risk: SRV >= 4.7
- No surprise, the Gulf Coast Aquifer System has a high risk for subsidence due to groundwater pumping Some areas with very high reported clay thickness Relatively small predicted water level decline
- Zooming in to look at GMA 14 where we are right now The SRV increases to 6.7 The increase is due to an increase in the number of wells with a reportedly thick clay interval Other factors remain similar to the values for the Gulf Coast Aquifer System as a whole
- Saturated clay thickness relatively small 35 or more feet of predicted water level decline But what about a more local area such as the Permian Basin UWCD?
- Lower SRV Thinner saturated clay thickness Much less predicted water level decline
- Let’s look at a very local example The Trinity Aquifer extends from Oklahoma through central TX Overall, the Trinity Aquifer has a SRV of 4.5 correlating to a medium level of risk Looking locally, the overall risk is similar in Williamson County (4.4) Reportedly thick clay in some areas Differences in water level trend and MAG run water level change However, GMA 8 adopted DFCs for subdivisions of the aquifer correlating to the Upper, Middle, and Lower sections
- Lower Trinity: SRV = 4.4 (653 wells) Most wells in the SW portion of the county
- Upper Trinity: SRV = 5.5 (34 wells) Reported clay is typically thick
- Middle Trinity: SRV = 5.2 (518 wells) Reported clay thickness is relatively high Many of the wells in the northern area have SRV > 6 Change in aquifer conditions within county Why important since it is unregulated Informative for local residents and groundwater users even though not a regulated area Part of GMA 8, Still need to address subsidence as part of the consideration
- To accommodate the various aquifer conditions across the state, we used the equations of the SUB-WT package in MODFLOW However, we did not include time delay in the calculation One goal was to make it as simple as possible, while also giving users some flexibility Tool relies on some macros and custom functions written in VBA Allows flexibility within the tool Allows for easy customization and use in other applications
- When first opening the tool Input values will be blank you’ll likely want to select an aquifer
- Once an aquifer is selected, the fields populate with default values for the aquifer Though populated with default values, any of the blue cells can be manually changed Gray boxes are drop-down menus with specific selections Selecting a different option, will update default values and re-calculate risk Orangish cells are calculated based on the user input values These cells are protected to prevent accidental changes However, if a user wants to inspect the calculations the password to unprotect the cells is provided in the user manual Since the tool resets itself when you open it, there is an option to turn that feature off Will then keep previous entries Won’t have to start over Just uncheck the box prior to saving and closing
- The tool also provides a chart of the predicted drawdown and subsidence Predictions based on user input values and calculations Reminder, does not include time delay for subsidence
- Look at a specific well example The well is located within GMA 14 in Waller County
- Initial information is from GAM thickness, clay thickness from SDR
- Change Aquifer Thickness to Well Depth of 353 feet Risk does not change, because factors used in risk assessment have not changed Calculated specific storage values do change Results in an increase in predicted subsidence Next, we can update the current water level
- Change GAM estimated water level to SDR level of -2 feet MSL No significant changes You’ll notice that the water level trend is 0
- Calculated trend from GAM simulation is so small that there is essentially no effect Change trend to -1.0 ft/yr Results in some increased subsidence prediction
- On the left is the predictions with essentially no change in water level On the right, we see the effect of an increase in the rate of decline
- Change from Plastic Clay to Stiff Clay Calculated specific storage values decrease Results in decrease in the risk value along with a decrease in the subsidence prediction This is just a portion of the options in the tool There are several other features of the tool that users can explore