This document summarizes a study on brackish groundwater comingling in Texas aquifers. It reviewed applicable statutes finding no clear definition of comingling. Factors like water quality stratification, hydraulic gradients, and well construction can enable comingling. Assessments of the Gulf Coast, Eagle Ford Region, and Trans Pecos aquifers found potential for comingling due to multi-aquifer wells and water quality variability. Case studies provided evidence of comingling. A statewide ranking identified 10 high-risk aquifers based on cross-formation completions. Future policy guidance on assessing comingling potential in brackish settings was recommended.

1. A Presentation by
TAGD Regular Business
Meeting
Coauthors
TWDB Project Manager
Brackish Groundwater Comingling
TWDB Contract No. 2000012442
February 8, 2022
Van Kelley, INTERA
Steve Young
Clark Griffith
Daniel Lupton
James Pinkard
James Golab

2. Study Objectives
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Work was funded by the TWDB – Contract No.
2000012442
Document a scientific assessment of brackish
groundwater comingling issues statewide and with a
focus on select aquifer/regions.
Inform future policy development regarding the
concept of comingling.

3. Outline of Presentation
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 Review of Applicable Statutes and Codes
 Physical Framework for Comingling
 Assessment of Potential for Brackish Groundwater
Comingling in Select Aquifer/Regions
−Case Studies
 Statewide Assessment of Potential for Comingling
of Brackish Groundwater
 Future Considerations

4. Task 3: Review of Statutes and
Terminology
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5. Task 3: Comingling
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Only definition of comingling in the statutes and
codes reviewed was 16 TAC 76.10(16)
Comingling – the mixing, mingling, blending or
combining through the borehole casing or annulus
or the filter pack of water that differ in chemical
quality, which causes quality degradation of any
aquifer or zone

6. Task 3: Comingling
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In this definition two conditions must be met for
comingling to occur:
−mixing occurs between waters of different chemical
quality and
−mixing causes degradation to an aquifer or a zone.
16 TAC 76 has no definition of degradation

7. 16 TAC §76.10(16) Allows for
Interpretation
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 There are two concepts in the definition of
comingling that are subject to interpretation:
Meaning of “differ in chemical quality”
Meaning of “degradation”

8. Task 3: Pollution
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“Pollution” defined in 16 TAC 76.10 (42)
−The alteration of the physical, thermal, chemical, or biological
quality of, or the contamination of, any water that renders the
water harmful, detrimental, or injurious to humans, animals,
vegetation, or property, or to public health, safety, or welfare,
or impairs the usefulness or the public enjoyment of the water
for any or reasonable purpose. (same as 30 TAC 331.10(86) –
UIC)
This definition is consistent with much of the
groundwater protection code in that the definition
considers groundwater use and protection thereof

9. Task 3: Takeaways
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Our interpretation of 16 TAC 76 is that comingling
intends to prevent contamination of useable water
with water that is either a human or environmental
health risk.
Mixing of groundwaters that maintains use may
not be considered degradation and therefore
comingling
− consistent with the definition of pollution (16 TAC
76.10(42))
Groundwater protection dependent upon end use
is consistent with the Texas Groundwater
Protection Committee Groundwater Classification
System and non-degradation policy goal

10. Factors Affecting the Potential for
Comingling
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 Stratification of Water Quality
− Caused by regional flow processes (TDS)
− Caused by specific deposits (Arsenic)
 Vertical Hydraulic Gradients
−Naturally Occurring (artesian conditions)
−Pumping Induced (depressurized zones)
Well Completion
− Screens intersecting undesirable (injurious) water
Drilling and Well Operations
− Well (period of non pumping)
− Borehole (time left open)

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Conceptual System

12. Zonal Flow in a Pumping Well:
Conceptualization

13. Ambient Flow in a Non-Pumping
Well: Conceptualization

14. Direct Information of Ambient
Zonal Flow in a Well*
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Vertical Flow within
the borehole
upward
downward
*Data collected by BESST Inc
* Interpretation by INTERA
Data from a well in
Comal County*
Courtesy of the
Edwards Aquifer Authority

15. Conceptual Takeaways
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Comingling is mixing of groundwater within a well
between zones such that one zone degrades
another aquifer or zones
−It does not apply to the mixing that occurs at the well
head from pumping.
Comingling could occur under non-pumping and
pumping conditions. However, the potential is far
greater under non-pumping conditions
The data required to delineate the potential for
comingling is rarely collected
−Zonal water quality as well as zonal flow rates under
non-pumping well conditions

16. Task 5 - Assessment of Select
Aquifer/Regions
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Develop data driven
assessments of potential for
comingling in the following
aquifers/regions:
−Gulf Coast Aquifer System
−Eagle Ford Region Aquifers
−Trans Pecos Aquifers

17. Task 5 – Assessment of Select
Aquifer/Regions
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Conditions that could cause brackish
comingling
−Presence of brackish groundwater
−Multi-aquifer completions
−Maximum head differences within multi-completed
wells
−Water quality (TDS) variability that could result in
comingling
Also looked at select well case studies

18. Task 5 –
Eagle Ford
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Task 5 –
Eagle Ford
Assessment
shows that that
there are many
wells completed
in more than one
aquifer (31 % of
wells)
Multi-Aquifer Completed Wells

19. Task 5 –
Gulf Coast
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Assessment
shows that that
there are 2,214
wells completed
in more than one
aquifer (3.4 %)
Multi-Aquifer Completed Wells

20. Task 5 –
Gulf Coast
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Assessment shows
that vertical head
differences
(gradients) for
aquifers co-
completed can be
significant
(average 43.6 ft
ranging 390 feet)
Maximum Head Difference in
Multi-Aquifer Completed Wells

21. Task 5 –
Gulf Coast
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Significant number
of wells with
screens straddling
the 1,000 mg/L
TDS boundary
Water quality
based upon
geophysical log
interpretation
(Young and others
(2016))
Wells Completed Across the
1,000 ppm TDS Boundary

22. Zonal Sampling - Gulf Coast
Aquifer - Pumping
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Well head TDS = 840 mg/L
Interval range = 419 to 1,393 mg/L
Courtesy of the
BESST, Inc

23. Zonal Sampling - Gulf Coast
Aquifer – Non-Pumping
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Pump interval
TDS = 419 mg/L

24. Zonal Sampling – Eagle Ford
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Courtesy of the
Evergreen UWCD

25. Task 5 – Assessment of Select
Aquifer/Regions
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Assessment shows that all three aquifer/regions
have potential for comingling to occur
The Gulf Coast Aquifer and the Eagle Ford
Regions have the highest potential for
comingling of the three focus areas
The case studies presented:
−Provide well-specific evidence of potential
comingling and the mechanisms that may cause
comingling
−Provide insight into the types of detailed
characterization data that requires collection to
evaluate comingling

26. Task 4 – Statewide Ranking of
Comingling Potential
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Desktop study of statewide
ranking of each aquifer for
potential for comingling
The method provides a
hierarchical ranking of aquifer
Ten aquifers were ranked to
have a high potential
The highest-ranking are:
−multi-aquifer/formation
aquifers with a
−high percent of cross-
aquifer/formation completions

27. Future Considerations
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 Regarding brackish groundwater production zones
defined by the TWDB, we recommend that the
TWDB, GCDs and other stakeholders define
minimum standards regarding the assessment of
the potential for comingling in brackish aquifer
settings.
 We recommend that the Groundwater Protection
Committee (which includes WWD/PI) provide some
guidance with regards to application of the
comingling statute in brackish aquifer settings

28. Questions / Input
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Van Kelley
vkelley@intera.com
512-569-0689