groundwater flow and transport over larger volumes of rock: cross-hole hydraulic and tracer testing...

Groundwater Flow and Transport over Larger Volumes of Rock:

Cross-Hole Hydraulicand Tracer Testing

USEPA-USGS Fractured Rock WorkshopEPA Region 2

14 January 2014

Claire Tiedeman and Allen Shapiro, USGS

Cross-Hole Hydraulic and Tracer Testing 2

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)

FastResponse

NoResponse


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


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 well-connected 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


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withdrawn Relatively short tests limit effect of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


Well Shutdown Testing in Sedimentary Rocks: An Example

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


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


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

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


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 chemicaldilution are difficult to infer from hydraulicinformation 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

~ ~

Kv 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 (Dh) in highly heterogeneous aquifers ?


Characterizing Residence Time and DilutionDesigning the injection of remediation amendments

CVO

C Co

nc.

(mic

rogr

ams

per L

iter)

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. . .


135 feet

Bromide concentration - February 2008(7 months after injection)

Tracer injection Pumping

Characterizing Residence Time and DilutionDesigning the injection of remediation amendments

0.00

0.02

0.04

0.06

0.08

0 10 20 30 40 50 60

co

nc

(m

g/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 CenterWest Trenton, NJ


Processes Affecting Chemical MigrationEstimating parameters governing matrix diffusionCharacteristics 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 15020

30

40

50

60

70

80

90

100

110

120

130

140

150

160BRP-03 15BR

Fill

Regolith

FracturedBedrock

InjectWithdraw

AL

TIT

UD

E, I

N F

EE

T

DISTANCE, IN FEET

B B'

Storage of tracer solutions at land surface

BRP3

10-2

10-1

100

101

1 10 100 1000

Bro

mid

e C

once

ntr

atio

n (

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

Fra

ctio

n of

Bro

mid

e M

ass

Rec

over

ed

Elapsed time from start of tracer injection (hours)

BRP3

15BR71BR 70BR

25BR

90 ft

TracerInjection Pumping

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

Naval Air Warfare CenterWest Trenton, NJ


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

Controls on Fate, Transport, and Remediation of VOCs 16

Final Thoughts

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

groundwater flow and transport over larger volumes of rock: cross-hole hydraulic and tracer testing...

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