Groundwater Flow and Transport 

over Larger Volumes of Rock:

Cross-Hole Hydraulic

and Tracer Testing

USEPA-USGS Fractured Rock Workshop

EPA Region 2

14 January 2014

Claire Tiedeman and Allen Shapiro, USGS
 Identify flow and transport 

pathways and barriers between 

boreholes. 

 Use of this info with geologic 

framework helps identify 

locations of connected high-K 

fractures and lower-K rocks.

 This characterization data is 

important to developing the site 

conceptual model.

 Quantitative analysis of test 

data yields estimates of K 

values.

Purpose of Cross-Hole Hydraulic Testing

(also called aquifer testing, pump testing) 

Cross-Hole Hydraulic and Tracer Testing 2

Fast

Response

No

Response
 In fractured rocks, 

hydraulic responses 

can travel long 

distances in short 

times.

 Drawdown will not 

necessarily decrease 

with distance from 

pumped well.

Expectations: Cross-Hole Hydraulic Tests

Cross-Hole Hydraulic and Tracer Testing 3
 Well spacings and open intervals

 Because of high degree of heterogeneity, difficult to predict distances 

over which permeable fractures are connected, prior to drilling wells. 

 Bedding-plane-parting fractures and karst features likely wellconnected over longer distances than fractures in crystalline rocks.

 Use multiple criteria when selecting locations of new wells – value for 

characterizing contamination as well as for hydraulic testing.

 Multilevel wells important for understanding vertical connections.

 Testing

 Monitor water levels in as many wells as possible.

 Pump & treat system provides excellent opportunity for using 

remediation activities to further characterize hydrogeology.

Designing Hydraulic Tests

Cross-Hole Hydraulic and Tracer Testing 4
 Procedure:

 Shut down pump in one well of the P&T 

system. 

 Monitor water-level rises in obs wells.

 Start with relatively short tests (run test 

during the day, with overnight recovery)

 Repeat for all wells of system

 Advantages

 No additional contaminated water 

withdrawn

 Relatively short tests limit effect of 

shutdown on offsite contaminant 

migration

Using Existing Pump & Treat System for 

Cross-Hole Hydraulic Testing

Cross-Hole Hydraulic and Tracer Testing 5
Well Shutdown Testing in 

Sedimentary Rocks: An Example

Cross-Hole Hydraulic and Tracer Testing 6

1st Day: Shut down 45BR

2nd Day: Shut down 56BR

3rd Day: Shut down 15BR

rain
24BR open to bed ‘301’

Well Shutdown Testing in 

Sedimentary Rocks: An Example

Cross-Hole Hydraulic and Tracer Testing 7
 In heterogeneous 

fractured rock aquifers, 

analytical solutions for 

estimating K or T from 

hydraulic tests have 

limited applicability.

 Need to use numerical 

model (e.g. MODFLOW) 

so that heterogeneity can 

be properly represented.

Analyzing Cross-Hole Hydraulic Test Data

Cross-Hole Hydraulic and Tracer Testing 8
 Process of developing and calibrating 

gw flow model helps advance the 3D 

hydrogeologic conceptual model. 

 Enables consistent synthesis of site 

characterization data – geology, 

geophysics, hydraulics.

 Provides a tool for understanding the 

3D effects of changes to the flow 

system, such as increased pumping.

 Can be updated as new data collected

 Starting point for transport modeling

Value of Modeling for Interpreting 

Hydraulic Test Data

Cross-Hole Hydraulic and Tracer Testing 9

Hydraulic Conductivity Representation

Simulated Drawdown
 Valuable for identifying:

 Paths for relatively rapid contaminant transport

 Less permeable volumes of rock where slow advection and 

diffusion likely dominate transport

 Interpreting hydraulic test data:

 Incorporate heterogeneity! Use geology! 

 Presence of permeable high-angle fractures might be inferred 

from data, but can be difficult to identify their locations

 Tracer testing provides more definitive characterization 

of transport processes

Cross-Hole Hydraulic Tests in 

Fractured Rocks: Final Thoughts

Cross-Hole Hydraulic and Tracer Testing 10
Cross-Hole Hydraulic and Tracer Testing 11

Application of Tracers Tests for 

Decision Making at Sites of 

Groundwater Contamination

 Hydraulic testing monitors fluid pressure responses (hydraulic connections)

 Groundwater velocity or processes 

affecting chemical residence times and chemical

dilution are difficult to infer from hydraulic

information alone (especially in fractured rock)

 If information on residence time and dilution is needed. . . conduct a test 

that provides a direct measure of these quantities 

~ ~

K

v h

n

  

 Groundwater velocity, v

 Estimates of fracture porosity (n) from mechanical considerations are unreliable

 Hydraulic conductivity (K) is usually an order of magnitude estimate

 What are the errors in estimating the hydraulic gradient (h) in highly heterogeneous aquifers ?
Cross-Hole Hydraulic and Tracer Testing 12

Characterizing Residence Time and Dilution

Designing the injection of remediation amendments

CV

OC Conc. (micrograms per Liter)

Continuous pumping

Feasibility of 

bioaugmentation in 

strata between 36BR 

and 15BR. . .

What is the travel time 

and dilution from 36BR 

to 15BR? 

Hydraulic communication 

between monitoring 

locations is a prerequisite 

for conducting tracer 

studies. . . 
Cross-Hole Hydraulic and Tracer Testing 13

135 feet

Bromide concentration - February 2008

(7 months after injection)

Tracer injection Pumping

Characterizing Residence Time and Dilution

Designing the injection of remediation amendments

0.00

0.02

0.04

0.06

0.08

0 10 20 30 40 50 60

conc (mg/L)

time (days)

simulated

observed

Simulations demonstrate slow 

advection (low K) is responsible 

for mass retention at greater 

depths in dipping beds

Naval Air Warfare Center

West Trenton, NJ
Cross-Hole Hydraulic and Tracer Testing 14

Processes Affecting Chemical Migration

Estimating parameters governing matrix diffusion

Characteristics of breakthrough 

curves can be used to identify the 

significance of chemical processes

Simulations for different 

effective diffusion coefficients

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

BRP-03 15BR

Fill

Regolith

Fractured

Bedrock

Inject

Withdraw

ALTITUDE, IN FEET

DISTANCE, IN FEET

B B'

Storage of tracer solutions 

at land surface

BRP3

10-2

10-1

100

101

1 10 100 1000

Bromide Concentration (mg/L)

Elapsed Time from Start of Tracer Injection (hours)

0.00

0.20

0.40

0.60

0.80

1.00

0 200 400 600 800 1000

Fraction of Bromide Mass Recovered

Elapsed time from start of tracer injection (hours)

BRP3

15BR

71BR 70BR

25BR

90 ft

Tracer

Injection Pumping

Bromide Concentration vs. Time at 15BR Bromide Recovery at 15BR

Naval Air Warfare Center

West Trenton, NJ
Cross-Hole Hydraulic and Tracer Testing 15

Tracer Test Design

 Tracer tests can be conducted over various distances (provided hydraulic perturbations are observed 

between locations)

 Single-hole tracer tests can also be designed (point-dilution, push-pull, etc.)

 Tracer tests can be designed over hours, days, months or years

 Various types of tracers are available to quantify processes of interest (inert, reactive, varying free-water 

diffusion coefficients, dissolved gases, bacteria, colloids, microspheres, etc.) 

 Maintaining the geochemical signature of the ambient groundwater (oxic, anoxic, fluid density)

 How much tracer mass must be added to register and interpreted a response ? Preliminary estimates of 

dilution and attenuation are difficult to derive in fractured rock (In some cases, tracer tests are not run; 

they are re-run!) 

 Effect of fluid density in structured media

 Designing injection apparatus at land surface, and apparatus in injection and monitoring boreholes–

maintaining geochemical conditions of ambient groundwater; volume of fluid in boreholes may be large 

relative to fracture volume; borehole volume may dilute tracer responses

Tracer arrival at monitoring 

wells (> 5 km from tracer 

injection) in the Madison 

Limestone
Final Thoughts

Controls on Fate, Transport, and Remediation of VOCs 16

 Tracer tests can provide a direct measure of fluid velocity, chemical 

dilution and attenuation processes

 Tracer tests provide valuable information to (1) conceptualize processes 

affecting fate and transport of contaminants, (2) design and implement 

remediation strategies 

 Tracer tests in fractured rock are likely to be successful under hydraulically 

stressed conditions, provided the hydraulic perturbation can be monitored 

in cross-hole tests

 Single-hole tracer tests are used primarily to estimate in situ process and 

identify local parameter values. . .less reliable in estimating groundwater 

velocity