U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3057

September 2018 Printed on recycled paper

Length = 2 m

Data-logging

multimeter GPS

receiver

Raft

Electrode

Electrode

TEXAS

GULF OF MEXICO

Study area

Guadalupe River

Prepared in cooperation with the Guadalupe-Blanco River Authority

New Insights Into Surface-Water/Groundwater Exchanges in

the Guadalupe River, Texas, From Floating Geophysical Methods

In south-central Texas, the amount of streamflow in the 

Guadalupe River is a primary concern for local and downstream 

communities because of municipal, agricultural, wildlife, 

and recreational uses. Understanding the flow paths and rates 

of exchange between the surface water in the river and the 

groundwater in the underlying Carrizo-Wilcox aquifer is vital 

for understanding the water budget and streamflow variations. 

In areas where the Guadalupe River crosses the CarrizoWilcox aquifer outcrop, the surface-water and groundwater 

exchanges are not well characterized. Traditional methods 

to measure these interactions, such as measuring differences 

in surface-water flows at different locations to infer gains 

and losses between the locations, are not feasible along this 

stretch of the Guadalupe River because of upstream dams that 

cause large daily fluctuations in streamflow. Consequently, the 

U.S. Geological Survey, in cooperation with the GuadalupeBlanco River Authority, applied geophysical methods in 

an exploratory study to identify reaches of the river where 

streamflow gains and losses (surface-water/groundwater 

exchanges) might be occurring. 

Electric Resistivity Tomography and

Self-Potential Profiling

The study integrated two geophysical methods—floating 

electric resistivity tomography (ERT) and floating gradient 

self-potential (SP) profiling (Ikard and others, 2018). These 

geophysical methods measured the voltages between two or 

more electrodes that were separated by fixed distances and 

submerged in the river near the water surface. The ERT and SP 

profiles were measured along 

approximately 15 kilometers of 

the Guadalupe River in 2016 

by towing instruments behind 

a boat or attaching them to a 

kayak (fig. 1). These geophysical 

methods, signal processing 

procedures, and resulting datasets 

are described in detail in Ikard 

and others (2017, 2018). Voltages 

measured by ERT electrodes 

can be related to the subsurface 

geology and electrical resistivity 

(Reynolds, 1997; Griffiths, 

1999), whereas voltages 

measured by SP electrodes can 

be related to water flowing 

through the geologic materials 

(Nyquist and Corry, 2002; Revil 

and others, 2017).

Figure 1. Geophysical measurement equipment and typical streamflow conditions in the 

Guadalupe River, south-central Texas, during the self-potential survey, September 2016.

[GPS, Global Positioning System; m, meter. Photograph by C. Hartman, U.S. Geological Survey] 
Summary of Findings

The ERT profile identified differences in the subsurface 

hydraulic properties beneath the stream channel bed by mapping 

differences in electrical resistivity that likely correspond to 

locations of fine and coarse materials (Griffiths, 1999). In the 

upstream part of the study area, the ERT profile indicated that 

the Carrizo-Wilcox aquifer was present beneath a few meters 

of alluvium in the streambed, whereas the downstream part of 

the study area transitioned to more alluvium and channel sands 

(fig. 2). The shift in the ERT profile occurred approximately 

where the Carrizo-Wilcox aquifer transitions from an outcrop 

to a subcrop (in other words, is no longer present at the 

surface) and likely at the beginning of the Reklaw Formation 

(Baker, 1995). 

Numerical modeling of the SP profile identified subreaches 

where surface-water/groundwater exchanges were likely 

occurring (figs. 2 and 3). Where the SP profile was positive, 

numerical modeling indicated that the SP profile represented 

a net gain of water to the stream at that location. Conversely, 

where the SP profile was negative, numerical modeling indicated 

that the SP profile represented a net loss of water from the 

stream into the alluvium and channel sands or into the deeper 

aquifer. The net gain or loss of streamflow in the river channel is 

most likely the result of a combination of both regional ambient 

groundwater exchange (where water flows to or from the 

regional aquifer) and localized hyporheic exchange flow (where 

water flows into and out of the alluvium and channel sands in the 

streambed and flood plain within relatively short distances). 

The largest surface-water/groundwater interactions 

appeared to be occurring where the stream flows over 

the Carrizo-Wilcox aquifer outcrop. Specifically, the SP 

profile in the first kilometers of the study area (subreach 1) 

exhibited a strong negative signal (as much as approximately 

−70 millivolts [mV]), indicative of downwelling of water 

into the subsurface (figs. 2 and 3). At the upstream portion of 

subreach 2, the SP profile switched to a strong positive signal 

(as much as approximately +50 mV), indicative of upwelling of 

water coinciding with the Carrizo-Wilcox aquifer outcrop. The 

apparent upwelling of water to the stream lessened in subreach 3 

(the SP signal was generally less than +10 mV) as the channel 

becomes more dominated by alluvium and channel sands 

downstream. The SP profile was relatively neutral in subreach 3 

and then increased again at the end of subreach 4 (as much 

as approximately +30 mV), possibly indicating the transition 

into the Reklaw Formation, which is the next downstream 

hydrostratigraphic unit (Baker, 1995; George, 2011). Numerical 

modeling of the SP profile also indicated that at some reaches the 

surface-water/groundwater exchanges might be more influenced 

by regional ambient groundwater exchange flows, whereas at 

other reaches localized hyporheic exchange flows might be a 

larger contributor. 

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000

0

−5

−10

−15

−20

50

0

−50

Profile distance, in meters 

 Depth below water 

 surface, in meters Self-potential, in millivolts

Upstream part of the study area Downstream part of the study area

A

B

Channel

bed

 Alluvium and channel sands

(transition into Reklaw Formation) 

Carrizo-Wilcox

aquifer outcrop

Carrizo-Wilcox

aquifer subcrop

Carrizo−Wilcox aquifer

 Alluvium

Subreach 1 Subreach 2 Subreach 3 Subreach 4

Log10 resistivity (Ω-m)

1.0

1.4

1.8

2.2

2.6

EXPLANATION

Figure 2. Self-potential (SP) and corresponding electric resistivity tomography (ERT) data collected along an approximately 15-kilometer 

reach of the Guadalupe River, south-central Texas. A, SP profile with identified subreaches. B, ERT profile showing a cross-sectional 

representation of the subsurface hydrogeology beneath the stream channel bed.
Subreach 4

Subreach 3

Subreach 2

Subreach 1 Carrizo-Wilcox aquifer outcrop Carrizo-Wilcox aquifer subcrop

Subreach 4

Subreach 3

Subreach 2

Subreach 1 Carrizo-Wilcox aquifer outcrop Carrizo-Wilcox aquifer subcrop

97°43'15" 97°45'00" 97°46'45"

29°33'15"29°31'30"

0 0.5 1 KILOMETER0 0.5 1 MILE

Base modified from U.S. Department of Agriculture (USDA) Farm Service Agency (FSA) National Agriculture Imagery Program (NAIP) 2016 1-meter resolutionGeographic Coordinate System projectionNorth American Datum of 1983 

EXPLANATION

Self-potential,  in millivolts

605040302010

0

−10−20−30−40−50−60−70

TEXAS

Study area

Figure 3. Self-potential (SP) profile indicating potential surface-water and groundwater exchange in the channel of the Guadalupe River, south-central 

Texas, September 2016. The SP profile is the same shown in figure 2A. 
Understanding the dynamics of streamflow gains and 

losses and the hydrologic connectivity of the stream with the 

flood plain and underlying aquifers is important for water 

resource managers to make informed decisions regarding 

the Guadalupe River. This study demonstrated that floating 

geoelectric methods can be a powerful tool for providing 

information (for example, identifying aquifer properties 

and locating areas of potential gains and losses in streams) 

in a more expedient and cost-efficient manner compared to 

traditional surface-water/groundwater exchange methodologies. 

Future studies could be designed to quantify the flow rates 

between the surface water and groundwater, as water exchanges 

can vary based on season, drought or flood conditions, and 

aquifer hydrologic conditions. Additional studies could also 

further explore the delineation between possible local and 

regional aquifer interactions. 

This fact sheet is based on the following publication:

Ikard, S.J., Teeple, A.P., Payne, J.D., Stanton, G.P., and Banta, 

J.R., 2018, New insights on scale-dependent surface and

groundwater exchange from a floating self-potential dipole:

Journal of Environmental and Engineering Geophysics,

v. 23, no. 2, p. 261–287, https://library.seg.org/doi/

abs/10.2113/JEEG23.2.261.

References Cited

Baker, E.T., 1995, Stratigraphic nomenclature and geologic 

sections of the Gulf Coastal Plain of Texas: U.S. Geological 

Survey Open File Report 94–461, 34 p., 8 pls.

George, P.G., Mace, R.E., and Petrossian, R., 2011, Aquifers of 

Texas: Texas Water Development Board Report 380, 182 p.

Griffiths, D.J., 1999, Introduction to electrodynamics (3d ed.): 

Upper Saddle River, N.J., Prentice Hall, 576 p.

Ikard, S.J., Teeple, A.P., Payne, J.D., Stanton, G.P., and Banta, 

J.R., 2017, 14.86 km profiles of the electric and self-potential

fields measured in the lower Guadalupe River channel, Texas

Interior Gulf Coastal Plain, September 2016: U.S. Geological

Survey data release, https://doi.org/10.5066/F7CJ8CDH.

Nyquist, J.E., and Corry, C.E., 2002, Self-potential: The ugly 

duckling of environmental geophysics: The Leading Edge, 

v. 21, no. 5, p. 446–451. [Also available at https://doi.

org/10.1190/1.1481251.]

Revil, A., Ahmed, A.S., and Jardani, A., 2017, Self-potential: 

A non-intrusive ground water flow sensor: Journal of 

Environmental and Engineering Geophysics, v. 22, 

p. 235–247. [Also available at https://doi.org/10.2113/

JEEG22.3.235.]

Reynolds, J.M., 1997, An introduction to applied and 

environmental geophysics: West Sussex, England, John Wiley 

& Sons Ltd., 796 p.

—By Scott J. Ikard, J. Ryan Banta, and Gregory P. Stanton 

For more information about this publication, contact

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, TX 78754–4501 

For additional information visit https://www.usgs.gov/centers/tx-water.

Publishing support provided by

Lafayette Publishing Service Center

Any use of trade, firm, or product names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183057