Updated December 17, 2023

The reliability of a groundwater availability model (GAM) is dependent upon the quality and quantity of field data that is available to inform the model.  Having completed an update to the GMA-12 Groundwater Availability Model (New GAM) in late 2018 that includes a robust surface water-groundwater interaction capability, it is now necessary to conduct field studies to provide the data upon which the calibration and the predictions of the GAM depend.

Fortunately we have had a recent Big Win with the Lost Pines Groundwater Conservation District in its permitting process. Aqua Water Supply Corporation was granted a permit at the December 13, 2023, meeting that included a Special Condition requiring it to install two surface water monitoring wells as a condition of the permit. And, in a breath of fresh air, Aqua’s General Manager and support team did not hesitate in agreeing to the condition. This, in contrast to the Lower Colorado River Authority (LCRA) that has been fighting the requirement to monitor surface water ever since its permit was granted in 2021 by appealing the permit, and the special conditions, in District Court.

Current Project to Install and Operate a Pilot Monitoring Site on Colorado River.

Guide to Conducting Field Studies

Final Report: Chapter 7, Field Studies to Investigate Surface Water-Groundwater Interactions

Fortunately, the consultants that developed the New GAM provided us with a guide to conducting these field studies.  Chapter 7 of the final report “Field Studies to Investigate Surface Water-Groundwater Interaction”  provides a work plan for designing and implementing a data collection and analysis study.  The study would be to assess the interactions by monitoring collocated groundwater wells and surface water gages.  The wells would be installed in the alluvium at varying distances from the river in a pattern that resembles the well array in Figure 1 from the final report.  In addition to alluvium wells, the monitoring program would include at least one well that intersects the river sediments and a few wells in the aquifer surrounding and beneath the alluvium.  Figure 2 shows a potential pattern that resembles the well array described above. At a minimum, continual monitoring would occur in the river gage and in the alluvium wells.  Continual monitoring would be preformed by probes capable of measuring at 15 minute intervals for at least the following three parameters: hydraulic head (pressure head or water level), temperature and specific conductance.

Figure 2 (Figure 7-2 in Final Report); Pattern of well installation.

Figure 7-1 is a Schematic of a comprehensive monitoring well network for field study. It has three required components, a river gage, alluvium wells, and aquifers wells.

Monitoring over a several-year time period will be necessary to both calibrate the monitoring array and gather data on the impact of new groundwater pumping that has, or will be, permitted in the associated aquifers. 

The New GAM (a numerical model) will be used to interpret the field data and to develop estimates of the exchange of water between the river, streams and the alluvium over time.   To complement and check the numerical predictions of stream gains and losses, a gain/loss analysis should be performed using assembled streamflow information.  The best option for performing gain/loss analyses will be to use the Lower Colorado River Authority’s expertise and Daily Operation Routing Model to estimate an upper and lower bound for stream gains or losses caused by flow between the stream and the alluvium. 

Monitoring over a several-year time period will be necessary to both calibrate the monitoring array and gather data on the impact of new groundwater pumping that has, or will be, permitted in the associated aquifers. 

Hydrograph separations would also be performed to investigate whether or not programs such as Base Flow Index provide biased results for the regulated portions of the Colorado River.  

Candidate Locations for Field the Field Study

A number of factors must be considered in identifying candidate sites to investigate the dynamics of water transfer between the Colorado River, the Colorado River Alluvium, and the associated aquifers.  The authors considered six sites in the lower Colorado River basin shown in Figure 3 (7-3 in Report). 

Figure 3 (Figure 7-3 in Final Report). Selection Sites.

Site 1, which is the site selected for a monitoring network in the Bastrop, TX segment of the Colorado River includes four river gages, overlies the Carrizo-Wilcox Aquifer, is  part of GMA-12, and its groundwater is managed by the Lost Pines Groundwater Conservation District.  In addition, the  Colorado River alluvium associated with Site 1 is mapped in Section 5 of the final report. 

Current Project to Install and Operate a Pilot Monitoring Site

A project is currently underway to install a pilot surface water-groundwater monitoring system at a site (yet to be determined) on the main stem of the Colorado River in Bastrop County.   Environmental Stewardship’s Steve Box worked with the Colorado and Lavaca Basin and Bay Area Stakeholder Committee (CL-BBASC) in 2017 to direct $75,000 in funding toward the project. Texas Water Development Board periodically receives funding from the State Legislature in support of Senate Bill 3 Environmental Flow Standards passed by the 80th Legislature in 2007.  These environmental flow standards relate to the development, management, and preservation of the water resources of the state.  

The 2017 project titled “Investigate Surface Water-Groundwater Interaction along the Lower Colorado River”was awarded to the Lower Colorado River Authority (LCRA).   Additional funding was provided by the Brazos River Authority and TWDB.  The scope of work was generally focused on installation, data collection, and evaluation of various monitoring wells and devices at the site of LCRA’s Lake Bastrop input/discharge facilities along the Colorado River in Bastrop County. Unfortunately that attempt was unsuccessful in completing wells in the appropriate aquifers and alluvium.  LCRA and Environmental Stewardship are now investigating other suitable sites on the main stem of the Colorado for the project. 

Early reports of the work were provides to the CL-BBASC in February of 2019 and to Groundwater Management Area 12 in August, 2019.  

Colorado River and Wilcox Aquifer Interaction Monitoring Network

For purposes of monitoring the impact of groundwater pumping from the Carrizo, Calvert Bluff, Simsboro, and Hooper formations of the Carrizo-Wilcox Aquifer only two suitable river gages are located on the main-stem of the Colorado River;  the gages at the river bridges on FM 969 near Utley and US 71 in Bastrop.  Three stream gages are suitable for monitoring the impact of pumping on tributaries in the county;  Wilbarger and Big Sandy Creeks near Elgin and Cedar Creek near Bastrop.  See Figure 4 and Table 1 below.   In Lee County, two stream gages in the Brazos River basin are suitable for monitoring the impacts of groundwater pumping in Lee and Milam counties on surface waters.

Figure 4. SW-GW Monitoring Gauge Locations. Colorado River Main (3 blue pins), Wilbarger, Big Sandy Creek (2 orange pins) and Cedar Creek (not pinned) tributaries to the Colorado River, Middle and East Yegua Creek tributaries to the Brazos River (2 Green pins)

Table 1.  Gauging Station Location in Bastrop and Lee Counties that are potential sites for monitoring wells/geoprobes:

COLORADO RIVER WATERSHED

  • Colorado River @ Utley – Site 5450
    • Location:   Hwy 969 bridge south of Utley, TX
    • Description:  Gauge located at river bridge in Colorado River alluvium with fluviatile QT terrace deposits associated with the Hooper and Simsboro outcrops on the North and the Hooper, Simsboro and Calvert Bluff to the South.   This location is above the confluence of both Wilbarger and Sandy Creeks with the Colorado River.  
  • Wilbarger Creek @ Elgin –  Site 5464
    • Location:  2810-2814 FM 1704
    • Description:  Gauge located on Wilbarger Creek in fluviatile terrace deposits with Hooper outcrop  on both banks.  Simsboro outcrop just down-stream on north bank at confluence with Colorado River.  Calvert Bluff outcrop below confluence. 
  • Big Sandy Creek near Elgin – Site 5473
    • Location:  2333-2361 N Hwy 95, Elgin, TX
    • Description: Gauge located on Big Sandy Creek with Calvert Bluff outcrop on both banks and Camp Swift Military Reservation immediately East.   Simsboro outcrop located on North and South sides of Calvert Bluff outcrop. 
  • Colorado River @ Bastrop – Site 5499
    • Location:  Hwy 21/71 bridge in Bastrop
    • Description:  Gauge located on bridge with alluvium and fluviatile QT terrace deposits associated with the Carrizo outcrop on the East bank.  Calvert Bluff outcrop just up-river associated with the Piney Creek watershed, Bastrop Lake, and LCRA Power Plant. 
  • Cedar Creek near Bastrop – Site 5521
    • Location:  581-599 FM 20, Bastrop, TX
    • Description:  Gauge located on bridge with alluvium and flluviatile QT terrace deposits associated with the Calvert Bluff and Simsboro aquifers on the north banks and the Wilcox Group (Ewi) sands on the south bank.

BRAZOS RIVER WATERSHED (Lee County)

  • Middle Yegua Creek near Dime Box, TX – USGS Station 08109700
    • Location:  East Hwy 21, E TX 21.
      • Lee County, Texas, Hydrologic Unit Code 12070102
      • Latitude  30°20’21”, Longitude  96°54’16” NAD27
      • Drainage area 236  square miles
      • Contributing drainage area 236  square miles
      • Gauge datum 295.40 feet above NGVD29
    • Description: Gauge in alluvium on the borders of the Sparta Sands and the Cook Mountain Formation.  Likely is hydrologically separated by geographic features from the Carrizo-Wilcox Group.   For our purposes might provide some useful information to confirm/refute communication with C-W Group.   
  • East Yegua Creek near Dime Box, TX – USGS Station 08109800
    • Location:   East Hwy 21, E TX 21
    • Description:  Gauge in alluvium on the borders of the Sparta Sands and the Cook Mountain Formation.  Likely is hydrologically separated by geographic features from the Carrizo-Wilcox Group.   For our purposes might provide some useful information to confirm/refute communication with C-W Group. Also located on Lee and Burleson County lines.  

Acknowledgement:

The text of the first two sections of this webpage (Guidance for Field Studies and Candidate Sites for Field Studies) were adapted directly from the Final Report authored by Steven Young, Toya Jones, and Marius Jigmond of INTERA. 

LINKS TO OTHER SITES and DOCUMENTS RELATED TO SW-GW MONITORING

Final Report (full report): Final Report: Field Studies and Updates to the Central Carrizo-Wilcox, Queen City, and Sparta GAM to Improve the Quantification of Surface Water-Groundwater Interaction in the Colorado River Basin (August 2017)

Final Report (chapter 7): Field Studies to Investigate Surface Water-Groundwater Interaction (August 2017)

Report to CL-BBASC on Current Field Study Project (February 2019)

Report to CL-BBASC on Field Study Project (Final Report, July, 2021)

Report to GMA-12 on Current Field Study Project (August 2019)

Groundwater-Surface water interaction (ES Webpage)

Groundwater Availability Model (GAM) Improvements for GMA-12 (ES Webpage)

The Role of Groundwater Availability Models in Managing Groundwater -Surface Water Resources (ES Webpage)

Colorado River-Simsboro Aquifer Connection (ES Webpage)

Steven Young, Robert Mace and CarlosRubinstein: Surface water-groundwater interaction issues in Texas

Final Report (Volume 1):  Groundwater Availability Model for the Central Portion of the Sparta, Queen City, and Carrizo-Wilcox Aquifers (September 2018)

Final Report (Volume 2): Groundwater Availability Model for the Central Portion of the Sparta, Queen City, and Carrizo-Wilcox Aquifers (September 2018)